Timing vs CHTs with PMAGs

[Randy]

Originally Posted by nigelspeedy

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

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

[Randy]

Originally Posted by FasGlas

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.

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.

[nigelspeedy]

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

[lr172]

Originally Posted by FasGlas

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.

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.

[FasGlas]

Originally Posted by lr172

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.

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.

[rph142]

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.

[nigelspeedy]

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...b-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

[FasGlas]

Originally Posted by nigelspeedy

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...b-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

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

[lr172]

Originally Posted by FasGlas

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 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

[FasGlas]

Originally Posted by lr172

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

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.