Timing vs CHTs with PMAGs

First off, this is relating to my RV7A, with IO-375 low compression Aerosport engine with dual Pmags. I also run the EI Commander that lets me see and change the timing etc. I do like the setup I have, and I wold not want to have it without the EI Commander.

Recently I called Bill Repucci and he walked me through the steps to retard the timing in an effort to get CHTs more in control as they would go past 400F very easily. We reduced the timing advance by shifting the curve 2.8 degrees and this adjustment did make a difference. Now that I have flown this way awhile, I think maybe even more retardation is something to consider. I will post a savvy analysis plot of my recent light here:

https://www.savvyanalysis.com/flight/1493062/db5110c8-30ea-4bfd-b93f-9cda52ffecd2

Too bad the EI Commander can't be part of the data log posted above. I did not take notes on the flight, but remember seeing 23 degrees advance at full power at 4,500' with 80F OAT. The curve will advance up to about 30 deg. up at altitude with lower MAP. 23 degrees does not seem like much advance but reducing it by the 2.8 degrees did help so maybe even less advance will help more? That is my question.

The 2.8 deg. adjustment made my CHTs more manageable for sure, but I still have to dump fuel through the engine to keep them below 400F in full power conditions down low as the logged data from the flight shows.

Running LOP brings the temps down very nicely of course.

Richening to 12 GPH will still get CHTs above 400F, take it up to 13GPH, and then they come into good control staying below 400F.

MY thought here is that pouring that much fuel into the engine is nearly the same thing as retarding the timing via slowing down the fuel burn. If I retard further, I should in theory be able to maintain the same performance at a lower fuel flow...

I wonder if my data is unusual or do others experience similar fuel flows to keep their CHTs under 400F?

I am interested in opinions on this. I was going to write this directly to Bill Repucci but realized others might benefit from the data and I am interested in others thoughts on this theory..


Randall Crothers
From Sedona, working in Northern California.

I have the same setup- low compression IO-375 and dual pmags in my RV-8 and don't see CHTs nearly that high using the standard setup. Looks like you're spending some time in NorCal, stop by KSQL sometime and we can compare.

Ditto

I also have the same setup, a low compression IO-375 with dual P-Mags and never see CHTs that high. ROP cruise burning 11.3 GPH at 7500 produces CHTs 340-350 max. CHTs in climb will stay below 380 and seldom get that high.

Something does not sound right. Perhaps a timing verification with a timing light would make sure you really have the timing set where you think it is.

Not saying you did, but are sure you set the pMag timing at TDC, not 25 BTDC?

Carl

High CHT is often an indicator of an engine problem, but you should not be adjusting timing to "tune" your CHT. Ignition timing is adjusted to optimize the combustion event with the goal of achieving the highest possible cylinder pressure at the most advantageous crankshaft angle. The CHT is simply the byproduct of that process. If your engine is correctly timed and making maximum power but can't shed the heat, then you need to look at cooling.

Step one however is to rule out the base timing as a factor. Make sure its right before you start playing with anything else.

We had issues with high CHT's on ours. TMX IO-360 dual P-Mags, MT prop, Silverhawk injection.

Ran with high temps for a while - put a dam on #1 cyl, adjusted the baffles and sealed them.

We then swapped to auto plugs and re timed the mags using the blow tube method - no retard, just standard timing.

Temps came down 50f.

Sadly, because we did multiple changes, we are no sure what made what difference.

Have another look, bench mark everything, see if it works.

Thank you all for the good advice and good data to compare with.

I will definitely double check the timing again and see what I find. It has been awhile since I looked there.

Randall

Thank you all for the good advice and good data to compare with.

I will definitely double check the timing again and see what I find. It has been awhile since I looked there.

Randall

Leaning in initial climb

Hi Randy.

My stroker motor also has dual PMags with the EICommander, the difference is that it has 10:1 compression. I use 0 shift and max 35 deg advance.

I see in your initial climb after take off you lean a little and the resulting EGT is higher by about 100F than what I assume is the full rich take off. If you FI is set up right full rich will be about 300F ROP so leaning like this puts you about 200F ROP which is probably a bit lean for high power and low altitude. Try leaning in the climb so the EGT stays the same as the full power sea level EGT. This will give you faster climb on less gas (than full rich) but will be rich enough to help keep the CHT in check. Using this method I end up about 100F ROP when I get to 10k. The other thing I see is that when you level off at 4,000' you are at about 80% power when you lean which is a pretty high power setting unless you are a long way LOP.

One thing I have noticed now that we are in the middle of summer is that I have to either climb a little richer than normal (~50F EGT) to keep the CHT around 375 where I like to see them or reduce the timing from the normal 36.4 deg to 35 deg. As toolbuilder says making power is fine but you have to be able to reject the heat. Hard to do when the OAT is higher and the delta t is lower or if the baffling is not in top shape and you are leaking air.


Cheers

Nige

You are not alone. I can only run 26 degs max on my dual pmags. There are plenty of threads and post about running pmags and having certain issues, high CHT's being one of those issues. Pmags have a very aggressive advance that's not adjustable, you can only adjust the max advance and the ramp shift. The best I can get to be within a usable CHT temp range is 0 shift and 26 degs max. If you have the EIC you can try changing the shift to -2 or -4 and that will pull the advance ramp back but not the max degrees. Also try bringing down the max advance some as well and see if you like the CHT temps. If you don't have an EIC you can remove a few degrees by retiming the pmags with the flywheel timing marks a few degrees retarded from TDC.

nigelspeedy said:

Hi Randy.

My stroker motor also has dual PMags with the EICommander, the difference is that it has 10:1 compression. I use 0 shift and max 35 deg advance.

I see in your initial climb after take off you lean a little and the resulting EGT is higher by about 100F than what I assume is the full rich take off. If you FI is set up right full rich will be about 300F ROP so leaning like this puts you about 200F ROP which is probably a bit lean for high power and low altitude. Try leaning in the climb so the EGT stays the same as the full power sea level EGT. This will give you faster climb on less gas (than full rich) but will be rich enough to help keep the CHT in check. Using this method I end up about 100F ROP when I get to 10k. The other thing I see is that when you level off at 4,000' you are at about 80% power when you lean which is a pretty high power setting unless you are a long way LOP.


One thing I have noticed now that we are in the middle of summer is that I have to either climb a little richer than normal (~50F EGT) to keep the CHT around 375 where I like to see them or reduce the timing from the normal 36.4 deg to 35 deg. As toolbuilder says making power is fine but you have to be able to reject the heat. Hard to do when the OAT is higher and the delta t is lower or if the baffling is not in top shape and you are leaking air.


Cheers

Nige

Click to expand...

Thanks for the info Nigel. I usually stop at Tehachapi for fuel on the way home to Sedona from my work in Northern CA. ,unless the weather is good to cross the Sierra's up near reno and stay up high for a non stop trip.

Looking back at my data, I stayed full rich up to about 1500', at full rich I gulp down nearly 17 GPH. I had my servo checked out as I thought it was too rich but they returned it to me saying it was operating correctly and within specs. Can you tell me what your fuel flow is at full rich?

I then leaned it down to 13 GPH for most of the remainder of the climb up to 4500, with some deviation as I was experimenting with the mixture and watching CHTs. Once established at 4500' I initially pulled it back to LOP with 8.5 GPH but wanted to get my speed back so after a bit I went back to 11.6 GPH which is pretty close to best power, 100F ROP, but the CHTs were climbing so I settled in on 13.3 GPH to keep my speed with CHTs staying in the low 390F range. (My TAS was 168 knots)

It would be quite interesting to compare my data charting from Savvy Analysis with another similar installation in similar flight conditions. Do you have a flight that you could share here in the same way? Any one else? This seems like a great way to learn more about operating these engines and trouble shoot installations via comparison with others.

"Try leaning in the climb so the EGT stays the same as the full power sea level EGT."

OK so to do this I would start out full rich as usual, but note the EGTs, and as they drop in temperature due to climbing up into thinner air, gradually lean to keep them at the same temperature as they were at full power and full rich at take off down low? OK so writing that question clarifies it for me. It is just not a technique I have tried before but will now that you have pointed it out. I think I have been reluctant to keep it running at what seems like excessive rich. I will give this a try but I do wonder if my fuel flow should be up at 17 GPH to start with... I have always felt the fuel flow was excessive, especially when I go full rich as I pull onto the runway and hear the engine kind of groan at what seems like excess fuel flow.

Before I do much more experimenting I do plan to get a timing light and verify where my timing is set without making any changes.

Always more to learn

Randall

FasGlas said:

You are not alone. I can only run 26 degs max on my dual pmags. There are plenty of threads and post about running pmags and having certain issues, high CHT's being one of those issues. Pmags have a very aggressive advance that's not adjustable, you can only adjust the max advance and the ramp shift. The best I can get to be within a usable CHT temp range is 0 shift and 26 degs max. If you have the EIC you can try changing the shift to -2 or -4 and that will pull the advance ramp back but not the max degrees. Also try bringing down the max advance some as well and see if you like the CHT temps. If you don't have an EIC you can remove a few degrees by retiming the pmags with the flywheel timing marks a few degrees retarded from TDC.

Click to expand...

FasGlass, thanks for another data point. It is nice to know I am not the only one having this experience. I won't adjust the timing again without first verifying for sure that it is where I think it is. Then I may pull it back some more and see what the effect is on my CHTs. I am also trying to compare my full rich fuel flow to others and see if it is as it should be.. I figure at 23 degrees at high power settings it is running less advance than a standard mag so the timing should not be an issue. Where I am set at present it will advance up to about 30 Deg at lower MAPs. I have a 2.8 deg. shift towards less advance in at this time. Max advance at lower power levels does not present any problems with my CHTs.

Fuel Flow on Takeoff

Randy,
In my aircraft the fuel flow during takeoff at sea level is around 17 gph so yours seems reasonable. At 5,000' leaned to my target EGT I still see around 14 gph.
Cheers
Nige

FasGlas said:

You are not alone. I can only run 26 degs max on my dual pmags. There are plenty of threads and post about running pmags and having certain issues, high CHT's being one of those issues. Pmags have a very aggressive advance that's not adjustable, you can only adjust the max advance and the ramp shift. The best I can get to be within a usable CHT temp range is 0 shift and 26 degs max. If you have the EIC you can try changing the shift to -2 or -4 and that will pull the advance ramp back but not the max degrees. Also try bringing down the max advance some as well and see if you like the CHT temps. If you don't have an EIC you can remove a few degrees by retiming the pmags with the flywheel timing marks a few degrees retarded from TDC.

Click to expand...

Lot's of folks running more agressive timing than 26 and not suffering cooling problems, myself included. As Mike stated, you need to look at the core cooling or other problem causing high CHT's for you. Dialing back the timing is a bandaid solution that is taking away power and efficiency when running at the lower MAP's.

lr172 said:

Lot's of folks running more agressive timing than 26 and not suffering cooling problems, myself included. As Mike stated, you need to look at the core cooling or other problem causing high CHT's for you. Dialing back the timing is a bandaid solution that is taking away power and efficiency when running at the lower MAP's.

Click to expand...

Without getting into all the prior threads, posts and data I'll just say I didn't have high CHT's until I installed pmags. Before the pmags I used Bendix mags than an ElectroAir EI without high CHT's. Pmags have been known to raise the CHT's, there's been many posts about this. It's more pronounced with higher compression engines of course but not uncommon. The only way I can keep my CHT's in check is to lower the max advance to 26 degs. I have a friend with the same setup as mine and was experiencing the same high CHT's, he sent his cylinders and pistons out for ceramic/teflon coating and since then dramatically lower CHT's and OT's. Some planes get it and some don't with pmags.

I recently replaced two bendix mags with Pmags on my 9.0:1 compression engine and have since been struggling with high cht's during climbs. All cylinders are 25-30 degrees hotter than before. I'm going to clean up the baffling before I adjust the timing.

Target EGT Climb

Randy,

Here is the link from a flight I did yesterday, of interest to you is the climb from time 10:00 to 25:00,

https://www.savvyanalysis.com/flight/1499130/7303a7e2-0f0c-4811-bbab-92812fb3312e

My home airport is at 4001' so you would expect higher fuel flow at sea level. I takeoff with WOT, Prop 2700 RPM and mixture full rich and boost pump on. If you plot the fuel flow against EGT on the top graph and PALT against the CHT on the bottom graph you will see the following:

The first reduction in fuel flow is the prop coming back to 2500 RPM from 2700 RPM with the throttle still wide open. The second reduction is the boost pump coming off. After that you will see little step reductions in fuel flow as I lean to about 1150 - 1200F EGT every couple of thousand feet, in-between you see a gradual reception in fuel flow which is the normal action of the fuel servo (for a great explanation of mechanical fuel injection see Dan H's article earlier this year in Kit Planes). You will see that the CHT's stay around 375F. If they get a little hotter I just delay the next leaning until they have cooled a little bit. If they are a nice and cool I lean a little more. You can see that I pretty high fuel flow all the way to 17,500'. Target EGT leaning is a technique not a law so if the CHT's are higher than you want, richen slightly, if they are cool lean a little more. If you can't climb like this with CHT's less than 400F the problem is probably not the ignition timing. In my engine with 0 deg advance shift and a max advance of 35 degrees I see the full 35 degrees by the time I am at 5,000' PA with WOT and 2,500 RPM.

Cheers

Nige

nigelspeedy said:

Randy,

Here is the link from a flight I did yesterday, of interest to you is the climb from time 10:00 to 25:00,

https://www.savvyanalysis.com/flight/1499130/7303a7e2-0f0c-4811-bbab-92812fb3312e

My home airport is at 4001' so you would expect higher fuel flow at sea level. I takeoff with WOT, Prop 2700 RPM and mixture full rich and boost pump on. If you plot the fuel flow against EGT on the top graph and PALT against the CHT on the bottom graph you will see the following:

The first reduction in fuel flow is the prop coming back to 2500 RPM from 2700 RPM with the throttle still wide open. The second reduction is the boost pump coming off. After that you will see little step reductions in fuel flow as I lean to about 1150 - 1200F EGT every couple of thousand feet, in-between you see a gradual reception in fuel flow which is the normal action of the fuel servo (for a great explanation of mechanical fuel injection see Dan H's article earlier this year in Kit Planes). You will see that the CHT's stay around 375F. If they get a little hotter I just delay the next leaning until they have cooled a little bit. If they are a nice and cool I lean a little more. You can see that I pretty high fuel flow all the way to 17,500'. Target EGT leaning is a technique not a law so if the CHT's are higher than you want, richen slightly, if they are cool lean a little more. If you can't climb like this with CHT's less than 400F the problem is probably not the ignition timing. In my engine with 0 deg advance shift and a max advance of 35 degrees I see the full 35 degrees by the time I am at 5,000' PA with WOT and 2,500 RPM.

Cheers

Nige

Click to expand...

This shows how aggressive the advance ramp is, as I posted prior. Move your shift back -2 or -4 degrees, this might help.

FasGlas said:

Without getting into all the prior threads, posts and data I'll just say I didn't have high CHT's until I installed pmags. Before the pmags I used Bendix mags than an ElectroAir EI without high CHT's. Pmags have been known to raise the CHT's, there's been many posts about this. It's more pronounced with higher compression engines of course but not uncommon. The only way I can keep my CHT's in check is to lower the max advance to 26 degs. I have a friend with the same setup as mine and was experiencing the same high CHT's, he sent his cylinders and pistons out for ceramic/teflon coating and since then dramatically lower CHT's and OT's. Some planes get it and some don't with pmags.

Click to expand...

I notice lots of the Pmag complaints are about CHT's in climb. I can't help but think that they have their advance coming in too quickly in the higher MAP areas. In my case (a custom advance map), I can see my CHT's increase only as I level off from climb to cruise, as my advance is pretty moderate until I am at cruise level MAP ranges. Mike's (Toobuilder) testing has shown the risk/reward just doesn't make sense in the higher MAP ranges. You'll be ROP here and there is limited to no benefit to timing greater than 25*

Yet one more reason people should be giving more consideration to open and flexible systems, like Ross' CPI system.

Larry

lr172 said:

I notice lots of the Pmag complaints are about CHT's in climb. I can't help but think that they have their advance coming in too quickly in the higher MAP areas. In my case (a custom advance map), I can see my CHT's increase only as I level off from climb to cruise, as my advance is pretty moderate until I am at cruise level MAP ranges. Mike's (Toobuilder) testing has shown the risk/reward just doesn't make sense in the higher MAP ranges. You'll be ROP here and there is limited to no benefit to timing greater than 25*

Yet one more reason people should be giving more consideration to open and flexible systems, like Ross' CPI system.

Larry

Click to expand...

I think a fully programmable map system is the way to go but then you have the problem of knowing what you're doing when programming it. More damage could be caused by bad programming then not. I'm not a fan of pmags ( no secret ), sorry I ever got involved with them but for the time being I just run the timing back and live with a few less ponies than to deal with high CHT's. Speed is not my problem. When the time comes I'll move into something more to my liking, I'm sure it will be programmable. Mike's been having good success with his CPI. I was going to work with ElectroAir on something new but that's not looking like it's going to happen anytime soon.

In my case, it?s not so much that I know how to program an ignition curve as much as I take a very conservative approach to testing. With CPI, you can emulate the timing of a magneto if you wish, and then take baby steps from there. There?s a big difference between programing in (or buying product with) a complete curve and hoping for the best vs. adjusting the timing incrementally from a single, known flight condition. And the ability to instantly ?undo? that adjustment with the flip of a switch as in the case of CPI makes it even less of a risk.

This is not to say that I think the ?locked down? curves of the other manufacturers are generally bad or unsafe, rather, I?m just trying to illustrate that the development of a custom tailored curve with CPI is safe and pretty easy if you use a little common sense.

Toobuilder said:

In my case, it?s not so much that I know how to program an ignition curve as much as I take a very conservative approach to testing. With CPI, you can emulate the timing of a magneto if you wish, and then take baby steps from there. There?s a big difference between programing in (or buying product with) a complete curve and hoping for the best vs. adjusting the timing incrementally from a single, known flight condition. And the ability to instantly ?undo? that adjustment with the flip of a switch as in the case of CPI makes it even less of a risk.

This is not to say that I think the ?locked down? curves of the other manufacturers are generally bad or unsafe, rather, I?m just trying to illustrate that the development of a custom tailored curve with CPI is safe and pretty easy if you use a little common sense.

Click to expand...

I agree completely, there's nothing but advantage with having the ability to adjust the curve that fits your needs... What I was referring to was the "If a little is good than more is better" approach that some people do have. I'm sure (us older guys) remember that guy that put 2 1050 cfm Holleys on his nearly stock small block Chevy. He found out quickly that more cfm does not equal more power, on the contrary it floods the engine.

Your approach of starting with a little, monitoring the data, doing minor changes and monitoring the data, etc, etc, is the right way of programming an EI. Power vs effect is a slow process to get the best performance with the least adverse effects to heat and possible detonation.

I knew a guy many years ago that installed a new Jeff Rose EI in his new Glasair. First flight, right after crosswind, his engine went into maximum detonation mode. Landed on the taxiway, lots of damage to the plane, he was lucky to have lived thru it. He later pulled the plugs to find all the electrodes were melted away. He claimed the ignition caused this problem but Jeff Rose found nothing wrong with it when tested. Later it was concluded that the installation was the probable cause. The MTH was installed 40 degrees advanced, not at TDC. Misread the instructions maybe?

Point being is, misprogramming could cause catastrophic effects. Having the ability to program at will is great but only if you know how. But it's not hard.

Agree completely on the "more must be better" trap. (I also completely relate to the huge carbs on a smallblock analogy) I think we see some of this with the "jumper in, jumper out" setting on the Pmags. Some run the more aggressive settings right off the bat because they think they are the ones that need it. I ran this curve on the -8 initially and after plenty of hours with no real issues, decided to pull it back a little. Speed stayed the same, CHT dropped a bit. That's a win in my book.

The beauty of the CPI is that you can simply advance the timing one degree at a time and observe the result. If you do it right you can observe the speed increase with advance, peak, and then fall off. Unless you are a masochist, you will not retain the "too advanced" setting because you can see with your own eyes that the engine will tell you what is required. No need to spin a fishermans yarn with how "big" your advance is - the engine needs what it needs.

But yes, with the power to alter your timing to ANY setting comes the responsibility to do it inteligently.

FasGlas said:

I agree completely, there's nothing but advantage with having the ability to adjust the curve that fits your needs... What I was referring to was the "If a little is good than more is better" approach that some people do have. I'm sure (us older guys) remember that guy that put 2 1050 cfm Holleys on his nearly stock small block Chevy. He found out quickly that more cfm does not equal more power, on the contrary it floods the engine.

Your approach of starting with a little, monitoring the data, doing minor changes and monitoring the data, etc, etc, is the right way of programming an EI. Power vs effect is a slow process to get the best performance with the least adverse effects to heat and possible detonation.

I knew a guy many years ago that installed a new Jeff Rose EI in his new Glasair. First flight, right after crosswind, his engine went into maximum detonation mode. Landed on the taxiway, lots of damage to the plane, he was lucky to have lived thru it. He later pulled the plugs to find all the electrodes were melted away. He claimed the ignition caused this problem but Jeff Rose found nothing wrong with it when tested. Later it was concluded that the installation was the probable cause. The MTH was installed 40 degrees advanced, not at TDC. Misread the instructions maybe?

Point being is, misprogramming could cause catastrophic effects. Having the ability to program at will is great but only if you know how. But it's not hard.

Click to expand...

In your case, I recommended it because you seem knowledgeable and possessing the intellect to do the research. I think Ross is now providing a pretty conservative timing map for his unit that will allow people to start with something safe and reliable. Or, as Mike said, start at a flat 25* and begin careful experimentation.

I do agree that each individual is not prepared to deal with the freedom of configurability, but it seems that many in the experimental world seem willing to do the research. That said, it only takes a couple guys to show up saying that 40* at 29" is awesome and some people may follow. That's the beauty of this forum. There is usually enough information so that, if you do the research and apply judgement, it's pretty easy to stay out of trouble.

Larry

Too much is not better

I did a recent test up at 17,500' and with WOT I get 15.5" MAP at 2,500 RPM. I saw very little difference (<5%) in specific range between 30, 35, 37 and 39 degrees advance. The difference in top speed was also only a couple of knots. This was 25F LOP. More testing to see how it affects down lower and ROP.
Cheers
Nige

nigelspeedy said:

I did a recent test up at 17,500' and with WOT I get 15.5" MAP at 2,500 RPM. I saw very little difference (<5%) in specific range between 30, 35, 37 and 39 degrees advance. The difference in top speed was also only a couple of knots. This was 25F LOP. More testing to see how it affects down lower and ROP.
Cheers
Nige

Click to expand...

Not sure if you meant to minimize this aspect with the word "only", but a couple of knots at cruise speed represents a fairly significant change in engine output. Plenty of us are almost willing to give up some body parts and certainly plenty of cash for a few more knots.

Only a couple of knots

I like going fast as well, the point I was trying to make was that a knot or two is not worth blowing the engine up for. In fact the highest advance 39.2 degrees did not yield the highest speed anyway.
Cheers
Nige

Copy. More advance does not always equal more power.

However, I do not believe that the optimum advance setting (optimum = most power) is destructive. There may be some exceptions, but the vast majority of our engines will be able to remain within their structural and thermal design limits without complaint.

Well, this is why there's no "One size fits all" in the EI mapping. There are so many variables in aviation. For the best performance and efficiency each pilot needs to build a map for his/hers needs. Altitude, MAP, humidity, RPM, fuel inconsistencies, mission usages and aircraft all affect the EI mapping. The only way to get your money's worth, per say, is exactly what Mike's doing. Figure out what your mission is for your plane and take small steps to build YOUR map.

All aircraft are designed with a compromise between stability and efficiency, the faster you can go the less stable it will tend to be. This is why a military fighter must have a computer flying it. That being said power vs speed is exponential, the faster you go the more power, exponentially, you need. The last few knots (since we were on that subject) is far harder to get to than the first 100 knots. This is why most see that same average speeds (max and VSI), then there's the few that push past the average. Barring weight, build and final trimming it comes down to horsepower. A few knots at WOT says a lot in the scheme of things.

Toobuilder said:

Copy. More advance does not always equal more power.

However, I do not believe that the optimum advance setting (optimum = most power) is destructive. There may be some exceptions, but the vast majority of our engines will be able to remain within their structural and thermal design limits without complaint.

Click to expand...

Just to play devil's advocate, I'll suggest that best longevity is found well away from any limits, rather than simply not exceeding them.

Remember, the original LOP mantra was running cooler at less peak pressure; with fixed timing, LOP delayed thetaP.

Being speed demons, we've twisted that into "How fast can we go on less gas?"

DanH said:

Just to play devil's advocate, I'll suggest that best longevity is found well away from any limits, rather than simply not exceeding them.

Remember, the original LOP mantra was running cooler at less peak pressure; with fixed timing, LOP delayed thetaP.

Being speed demons, we've twisted that into "How fast can we go on less gas?"

Click to expand...

Sure, but we know that the vast majority of the Lycomings we fly will live long, happy lives at 100% (sea level) power. That's our baseline. Any increase in altitude reduces power, and by the time you are at cruise altitude and LOP, even the most perfectly timed ignition event is not going to boost power to anywhere near the sea level baseline. Therefore, a well timed ignition will allow you to recover power lost to fixed magneto, but you are still well inside the limits of the engine.

Nigel, thanks for posting your Savvy Analysis chart from your test flight. Although your flight profile was very different than mine it is still useful for some comparison. Your chart has some interesting data up at the higher altitudes as I assume you were experimenting with running very lean.

I did as you suggested, plotting FF along with EGTs on one chart and Palt along with CHTs on the other. This ability to put operating parameters on a chart like this is a great tool.

One thing I take away from it is that I have been running much less fuel through my engine than you do. I suspect that if I richen my mixture to what you were running, my CHTs will come down considerably. My practice has generally been to keep it rich enough to achieve CHTs under the 400F mark, but it seems like alot of fuel to run through the engine.

Since you climbed up to about 17,000' there is only a brief period where we were at the same altitude but some comparison can be grabbed as you went through the altitude I leveled off at.

With your Palt at 4500' FF was about 15.5 GPH with CHTs 320 to 347
With my Palt at 4500' FF 13 GPH with CHTs 366 to 390
With my Palt at 4500' and LOP, FF was 8.5 GPH with CHTs 345 to 356

I have generally used LOP operations as kind of a band aid approach to keep my CHTs in check. I want to get to the point where i can actually use all the power available from my engine.

Yesterday I carefully checked that my Pmags were set up properly at Top Dead Center and they were spot on. BTW this is pretty easy to check with the spark plugs out. With the Pmags in setup mode I got cylinder 1 on the compression stroke and gently nugged the prop with my foot while watching both of the pmag led lights. They go from green to red at TDC. I was careful to get all the slack out of the gears etc during the check. I also double checked that I was using the TC mark and not the 20 or 25 deg. marks. All good. With such inexpensive spark plugs to use I went ahead and replaced them, why not and about $2.50 ea.? I had Iridium NGKs in, went with regular BR8ES with solid tops this time.

I have always thought my injection system dumps too much fuel at full rich on take offs. Especially up at my home airport elevation of 4500'. As I push the mixture to full rich I can hear the engine kind of groan and RPMs drop somewhat. I generally see 17 GPH at full rich and maybe this is ok for sea level power, not too sure about that. I need to develop a good technique for leaning before take off at higher elevations like where I live. I know it is simple to do but pushing that lever to the wall is an old habit.

OK so back to my original question or idea. Based on the idea that running a richer mixture has nearly the same effect as retarding the timing, why not go ahead and retard the timing and burn less fuel?

Is there another CHT cooling mechanism at work when going full rich other than slowing down the fuel burn which is same as moving peak pressures later in the cycle, ie retarding timing?

Now that I know for sure that my timing is what the EI Commander thinks it is, I can get on with some more testing. I will head up to 4500' and try 15 plus GPH and I would bet, based on my experience, that my CHTs will be similar to yours.

Then perhaps I will see if I can accomplish the same CHTs at a lower fuel burn with less timing advance.. I welcome comments and would love to see others' Savvy Analysis charts under similar conditions. Funny how it seems like posting Savvy Anaylsis data is like sharing some kind of intimate secret. I say go ahead and share them and we can all learn more.

PMag Timing v MAP

Hearing that some low compression engines struggle with high CHT with PMag and then that some high compression engines do just fine made me wonder if all PMag are equal.

There is lots of good info on how to do baffles and how to set the mixture at high power, so lets assume folks are all doing a reasonable job with these items. Could it be that some PMag introduce advance at a higher MAP than others? My thinking being that if more advance was introduced at some high MAP this could result in high CHT. If the advance on another unit was not introduced until a lower MAP and hence lower power CHT might be fine. I am not sure if there is a difference between units but though it might be interesting to investigate.

I did an experiment yesterday flying at 1,000'PA, which is as low as I can reasonably go in my local area. I set the prop at 2,500 RPM and the throttle wide open which gave about 29.1" MAP. I recorded the timing from the EICommander. In steps of about 1" MAP I reduced the throttle (leaving prop and mixture constant) and recorded the corresponding ignition timing/advance. I also noted the % Power calculated by the Dynon EMS.

Before starting I set the advance shift to 0 and the maximum advance to 36.4 degrees using the EICommander. Initially the advance was 28 degrees which I assume is the baseline plus any RPM advance. As the throttle is closed and the MAP reduces the advance is constant until 24.5" MAP where the advance increases up to the maximum of 36.4 degrees at 22.3" MAP. The ramp up of timing happens pretty quick so small throttle inputs were needed to capture each 1.4 degree step of ignition advance change.

While it is interesting to ponder just the ignition side of the equation I think I need to consider the power and likely mixture setting as well. Most literature I have read suggests that you don't want to lean until you are less than ~65% power, which is about 2500 RPM & 23" MAP (which you would get to with WOT at around 7,000'). At low alt and high MAP the mixture will likely be rich and you don't need any advance above ~25 degrees. So it seems to me that my PMag introduction of advance based on MAP is a little early, ~75% power. If the ramp up of advance based on MAP was delayed from 24.5" until 23" this would probably be less stressful on the engine (lower CHT with a bigger detonation margin) with little reduction in power when running ROP. 23" MAP would coincide with ~65% power where one would normally consider lean of peak operations and the increase in advance would be useful.

Anyone with dual PMag and an EICommander care to replicate the experiment and compare results?

[/URL][/IMG]

Cheers
Nige

Randy said:

Is there another CHT cooling mechanism at work when going full rich other than slowing down the fuel burn which is same as moving peak pressures later in the cycle, ie retarding timing?

Click to expand...

While ignition timing is often discussed as a means to move the EGT/CHT balance, as you elude to above, it is actually the peak pressure point that moves this balance and that is influenced by both the timing of spark and the speed of the fuel burn (air fuel ratio, MAP, etc.). Ross recently posted some good data on this.

In answer to your question, yes. Once you are richer than the stoichiometric (most noticeable after approximately max power), additional fuel (i.e. richer mixture) has a cooling effect on the burning charge (via latent heat absorption) and you will see that translate into lower CHTs, regardless of peak pressure.

Larry

nigelspeedy said:

Hearing that some low compression engines struggle with high CHT with PMag and then that some high compression engines do just fine made me wonder if all PMag are equal.

There is lots of good info on how to do baffles and how to set the mixture at high power, so lets assume folks are all doing a reasonable job with these items. Could it be that some PMag introduce advance at a higher MAP than others? My thinking being that if more advance was introduced at some high MAP this could result in high CHT. If the advance on another unit was not introduced until a lower MAP and hence lower power CHT might be fine. I am not sure if there is a difference between units but though it might be interesting to investigate.

I did an experiment yesterday flying at 1,000'PA, which is as low as I can reasonably go in my local area. I set the prop at 2,500 RPM and the throttle wide open which gave about 29.1" MAP. I recorded the timing from the EICommander. In steps of about 1" MAP I reduced the throttle (leaving prop and mixture constant) and recorded the corresponding ignition timing/advance. I also noted the % Power calculated by the Dynon EMS.

Before starting I set the advance shift to 0 and the maximum advance to 36.4 degrees using the EICommander. Initially the advance was 28 degrees which I assume is the baseline plus any RPM advance. As the throttle is closed and the MAP reduces the advance is constant until 24.5" MAP where the advance increases up to the maximum of 36.4 degrees at 22.3" MAP. The ramp up of timing happens pretty quick so small throttle inputs were needed to capture each 1.4 degree step of ignition advance change.

While it is interesting to ponder just the ignition side of the equation I think I need to consider the power and likely mixture setting as well. Most literature I have read suggests that you don't want to lean until you are less than ~65% power, which is about 2500 RPM & 23" MAP (which you would get to with WOT at around 7,000'). At low alt and high MAP the mixture will likely be rich and you don't need any advance above ~25 degrees. So it seems to me that my PMag introduction of advance based on MAP is a little early, ~75% power. If the ramp up of advance based on MAP was delayed from 24.5" until 23" this would probably be less stressful on the engine (lower CHT with a bigger detonation margin) with little reduction in power when running ROP. 23" MAP would coincide with ~65% power where one would normally consider lean of peak operations and the increase in advance would be useful.

Anyone with dual PMag and an EICommander care to replicate the experiment and compare results?

[/URL][/IMG]

Cheers
Nige

Click to expand...

Wow, that is a pretty aggressive ramping of the advance. Some food for thought on your theory above. I have a D120 EMS and early in phase 1 I found the MAP bouncing around substantially. The dynamic range was about a full inch or more at times. I had seen something similar when I built an electronic injection system for my Porsche, so I installed a restrictor (~ .040") in the line from the #3 cylinder to the MAP sensor and this gave me very stable readings.

I am guessing the Pmag is doing some smoothing on it's MAP sensors input. However, if they ramp 10 degrees over 1" of MAP and your MAP is dynamically oscillating over that 1" MAP range, I could see how there will be variability across installations. Based upon your numbers above, they are advancing 1* per .1" of MAP. I think they would struggle to get that kind of resolution on the MAP sensor unless they built a restrictor into their sensor.

Just food for thought.

Larry

Hello everyone..

I have been following this thread from the start.

There is a design flaw in the baffling/cylinder fin profile on many #2 cylinders.

The baffling fits closely to the cylinder and gas a "dam" in from of the fins. The fins are not completely open mid cylinder as well. The causes no airflwo to teh bottom of the #2 cylinder and also effects flow to the #4 cylinder.

I placed a washer in between my #2 cylinder and the baffling. This was after installing 2 PMAGS and noticing even higher CHTs than with my slick mags ~380 deg on #2 CHT in cruise.

Placing this washer has allowed my CHTS to run in the ~340-~360 deg range now.

Check your installation and see if yours is the same...

The aggressive advance on a pmag is a problem for some, not all. Moving the shift sometimes helps but not all. Adding the restrictor has made no difference on the high CHT temps, it does help to smooth the fast moving advance.


This is the curve of an ElectroAir, much smoother and less aggressive. When I ran an ElectroAir I did not see the higher CHT's. At higher altitude cruise the ElectroAir would advance as much as 40 degrees.

Just to make sure, is the "0" on the Y axis equivalent to 25 deg timing?

AX-O said:

Just to make sure, is the "0" on the Y axis equivalent to 25 deg timing?

Click to expand...

This represents the MAP curve. The ElectroAir is normally set to 18 degrees advance at around 1000 RPM. So not taking into account for RPM advance this would be 18 degrees plus MAP advance, "0" being at least 18 degrees.

Restrictor

I too put a 0.040" restrictor in the MAP line prior to the EMS sensor and the PMags.

Does the reduced heat from running rich really come from the latent heat of vaporization? Lets say peak EGT is at 10gal/hr and I run rich at 13 gal/hr, so 3 gal/hr is just being evaporated.
Gasoline has about 19,000 BTU/lb being burnt and only 150 BTU/lb evaporating.

10 gal/hr * 6lb/gal * 19,000 BTU/lb = 1,140,000 combustion BTU
3 gal/hr * 6 lb/gal * 150 BTU/lb = 2700 evaporation BTU or only 0.2% of the energy of combustion. I cant imagine that mechanism changes the CHT very much.

I see that the PMag MAP advance is about 4 degrees/ inch of MAP which is about twice what the Electroair example is. Certainly small errors in measuring MAP would have a big effect on timing with that ramp up.

Cheers

Nige

Nigel,

I flew my 7A home to Sedona and back over the weekend, about 4.5 hours each way from Redding, so I had ample time to experiment with the technique you mentioned, noting EGT at take off power and then adjusting mixture to maintain that EGT during the climb.

I can report with confidence that when I use this technique I simply do not have any issue with CHTs, they stay well below 400F.

The increase in fuel flow takes a little getting used to but it is nice to see that I can run rich and keep it cool using the technique mentioned. Such a simple technique it seems strange that I had not used it before... Now I wonder if I can put the timing advance back where it was, 2.8 deg. more, and keep the CHTs in check. I am not sure there is any real performance gains to be had from this as I did not notice any performance degradation from shifting the curve towards less advance.

I still think I should be able to run my engine at best power mixture in cruise, ie about 100F ROP without getting CHTs above 400F, that remains a problem for me.

With your setup can you run at best power mixture and keep CHTs under 400F?

For comparison, my cruise TAS at 8500' stays between 157 to 160 knots running LOP and burning a shade under 8 GPH. ( Timing under these conditions is 30 deg.) Going ROP I need to bump FF up to around 11 GPH to keep my CHTs in check and for this extra 3 GPH of fuel burn I gain right close to 10 knots airspeed. The cost for 10 extra knots of speed at $4 gas, $12 per hour. Seems a little steep...

FYI, I do not have the restrictor orifice in my MAP line that goes to the P Mags. Perhaps it could be affecting my CHTs with out me seeing any fluctuations in timing in the EI Commander? Maybe this could be the elusive cause I have been looking for, gotta try an orifice and see what happens.

Are Pmags a specific brand?

I'm new, building an RV-9A with O-320. Are "Pmags" a generic name for EI or a specific brand? Aircraft Spruce has an "e-mag p model ignition." Looking to have left mag electronic and right mag mechanical.

Specific not generic

Hi Rick,

The 'PMag' is a specific brand of electronic ignition. E Mag Electronic Ignition make the 114 series for four cylinder Continental and Lycoming engines.

Try: http://www.emagair.com/114-series/ for the company website.

Cheers

Nigel

nigelspeedy said:

Does the reduced heat from running rich really come from the latent heat of vaporization? Lets say peak EGT is at 10gal/hr and I run rich at 13 gal/hr, so 3 gal/hr is just being evaporated.

Nige

Click to expand...

Yes.

At stoichiometric (roughly peak EGT) the complete amount of fuel in the charge (air fuel mixture) will burn. Any richer than that and you have un-burnt fuel. The richer you are the more unburnt fuel. This un-burnt fuel absrorbs the heat, as any liquid does at it's vaporization point, from the burning air fuel mixture reducing it's overall energy (heat = energy). This obviously reduces the heat transfer to your pistons and cylinder head. This reduced heat/energy also reduces your ICP, increasing detonation margin.

There is more chemistry and physics involved, as at some point rich of Stoichiometric is best power. The additional un-burnt fuel helps to produce more power up to this point, but I can't remember the details of why. Also, up to this point the extra fuel is not really cooling the charge. Past this point, the extra fuel is reducing your heat output of the burning charge and marginally reducing power as well, due to the reduced heat/energy. I should also add that the richer mixture slows the burn rate and therefore moves your peak pressure point. This also increase detonation margin as well as cooler CHTs.

When rich of stoichiometric, the exhaust will have little O2 and varying percentages of hydrocarbonsm based upon how rich. When LOP, there will be very few hydrocarbons and varying percentages of O2, based upon how lean. This is how O2 sensors in the exhaust can determine the air fuel ratio at any point in time.

Larry

Thank you Nigel. Does the left mag - electronic and right mag - mechanical make sense? The EI guys want you to install two EI mags. Regards.

One or Two PMag

Rick,

I would do two PMags if I were in a position to do it again. I have had good support from the manufacturer. They have worked without fail in my application for 250 hours. Its really easy to set the timing of both PMag at the same time. If the amount of advance concerns you it is easy to hook up a laptop computer and limit the maximum and shift the curve (retard or advance the point where more advance is added), although not with the full authority that other systems offer. If you are looking for a system to do a grand ignition timing experiment look elsewhere. If you think you want something better than traditional mags (the ability to run auto plugs & wires, easy starting, internal power generation, limited control over timing) then they are a good way to go. Unless there is a big cost saving of a traditional Mag over the PMag I would get two PMag.

Cheers

Nige

The problem with the programming on a pmag is you can't change the curve. You can shift the curve and you can put a limit on the maximum advance but you can't change the aggressive MAP curve. That's where the problem is, it can be too much advance too quickly. It's too bad they didn't allow the curve to be changed. With all my experience with pmags I would never install them again on my plane. I have installed them for customers with mixed results.

I did two Pmags (144 series) right off the bat and and would do it again. 90 trouble free hours so far, super easy to time them. Sure you can't customize the MAP but you can't with a traditional mag either. I don't want to mess with timing other than a conservative advance and the other benefits of EI already mentioned.

Bevan

Now that we are shifting the subject to "one or two" I went from two Slick to one PMAG and one Slick and saw some positive improvements. Then after one year and building some confidence, I switched to two PMAG (114, V40) and have seen zero improvement. In some ways, it actually seems not as good but that is hard to measure so I am leaving it be.

Bavafa said:

Now that we are shifting the subject to "one or two" I went from two Slick to one PMAG and one Slick and saw some positive improvements. Then after one year and building some confidence, I switched to two PMAG (114, V40) and have seen zero improvement. In some ways, it actually seems not as good but that is hard to measure so I am leaving it be.

Click to expand...

The Pmag has too much advance at higher MAPs. When balanced with a mag at 25* this is not so bad. However, with two EIs, you can't run the same advance on both as you did when you had one balanced with a 25* mag. With 2 EI's, both need a lower advance setting at each step in the table. This is likely why you are losing power.

Remember, with a two spark setup, you have to consider the effect that the combination has on getting your peak pressure point to the optimum location. They really should have different tables for one Pmag vs 2 Pmags. My twin plug Porsche runs about 5+ degrees less timing than my big block Chevy with one plug.

larry