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CHT

scottmillhouse

Well Known Member
Stock RV-7A with new Lycoming IO-360, Hartzell CS prop, 10 hours time since new. Very tight baffling with no air leaks. Oil temperature at around 180 so I think I'm getting good flow and pressure. What are others seeing for their CHTs with similar equipment?

I'm on information overload. I'm finding it challenging to do Phase I testing with my new engine. I'm reading desired CHT of 360-380 and try not to go over 400 and never over 450. Lycoming tells me not to worry unless it gets to 450 and that 430 would even be ok at cruise. I'm routinely seeing around 415 and have to manage mixture, power and airspeed to keep below that throughout flight even at 5500 or 6500'. Other than descent the only way I can get below 400 is low power and sometimes even LOP. Forget any pattern practice. A hot takeoff today after a 1/2 hour fuel stop saw 430 and climbing 300' above ground before immediate power and climb reductions.
 
As mentioned above, check the ignition timing and make sure it is set correct.
Do all of your cylinder at that high CHT or one or two particulars? Tight baffling is somewhat a work of art and can always be improved, a wealth of info on this site.
If EI, you might want to limit the advance at this time and since you have 10 hours, I would think it would be safe to go higher up for a cooler weather. As your engine breaks in more and more, you ought to see better temp.

Good luck.
 
Standard Slick mags both sides. I will check timing.. assumed correct since new from Lycoming but? All CHT go high but #2 usually 10 higher, surprising since it is front. Also #2 EGT shows a lower trend. At 6500' full throttle did mag check. Smooth with all EGTs climbing consistently when switching mags. Also thinking fuel flow but checked Test Stand runs with Lycoming and they showed in higher range and flow usage rates appear consistent with Lycoming charts.
 
I would not assume the mags are timed correctly with the motor as delivered from the factory. So that is definitely worth checking.

But also, I don't know how many times I've helped owners that say "the baffles are tight" and I find a big gaping hole somewhere that was overlooked. Most often it is a leak path through the inlet diffusers that are glassed into the upper cowl. So......It can really help to get a fresh set of eyes on the baffles.

Anything close to 400F is abnormal.
 
I assume you installed the upper ramps? Are they sealed so air doesn't use them as a passage from the high to low pressure areas within the cowl?

If I was you, I'd check the mag timing, and if that doesn't fix it, take the airplane over to Moontown on Saturday, remove the cowl (at least the top), and ask some of the RV pilots to take a look.
 
I used all the tricks I could identify for baffles including glassing in the ramp ends and creating a backer plate with plate nuts so all rubber is 100% sealed to baffle aluminum. no gaps, sealed against top cowl checked with lights. All other gaps sealed. Air can only go down through the cylinders, oil cooler, air filter, and tubes to cool mags and alternator. As I said oil temperature runs cool. Highest ever 197, usually not to 180 until established in cruise after climb.
 
I don't remember what mine were (same setup as you...IO-360-M1B, Hartzell CS, mags), but I *was* told to expect higher CHTs until the engine is fully broken in, and IIRC, that was true. I'd have to go back and look at the logs in SavvyAnalysis to see what they were at the beginning vs. end of break-in period...if I get a chance, I will do that and let you know.
 
Scott,
There is a recommended modification for the baffles for the #2 and #3 cylinder for the (I)O360 engine. The fins on these two cylinder are shallow in the front and do not allow air to get the lower part very well. As mentioned there is a number of threads about this and DanH has a very nice set of pictures as how to create the gap.

This will certainly help your #2 cylinder CHT. Also, make sure your baffles that wrap around the cylinder (lower part) does no have gaps so air can skip easily. I have closed all gaps (on the sides) with red RTV.

As a reference, my IO360 ran in the 380 range during hot summer days and in the 320-330 in the cooler months.
 
Change the timing to 22 degrees until you have some more time on the engine. This was discussed just a few days ago.
I did this on non RV with 0 320 with 10-1 pistons based on recommendation from one of the best engine shops in US.
In my opinion you should get at least ten more hours before any sustained operation above 400 degrees.
Any indication that your piston rings are seated. Noticeable drop in CHT, reduction in oil consumption.
 
Looking through all my Savvy Analysis data I see that I have not had a notable cylinder head temperature drop yet. Actually the first two hours at 75%+ full rich circling the airport is looking cooler than my recent XC flights, closer to 400. Lycoming did get heads up to 440 in their test cell during their full power run portion. I've noted the lower baffle mods to provide customized flow to address a hot cylinder for future consideration. I'll probably spend time on that later but for now I just want to lower my CHTs across the board so that I'm comfortable continuing Phase I testing with slow flight, climb performance and pattern work. I'll check and then try retarding the timing to get heat down so I can run the engine harder without fear of redline CHTs.
 
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The VAF brain trust did it again. Left mag about 29 degrees with right at 25. Changed to 24. Hopefully that is the issue. I don?t know if Lycoming set it wrong or if it drifted in last 10 hours. Next flight will see if it is fixed.
 
Timing was the problem. Temperatures in 360-380 range now and peaks at 400-410 on take off but OAT was almost 90 here for take offs and 80 at altitude. All CHT are within 10 degrees now.
 
49clipper

You know the timing will drift as the points wear. And it will advance. There is also an AD out on that if the drift occurs within 5 hours of timing. Just for you reference.
Jim
 
A little goes a long way...

I have a YIO-360-A1B6 with dual Lightspeed Plasma III's.

I researched the old Lycoming SI/SB from the 1970's and I pondered the whole 25? vs 20? thing, 8.5:1 vs 8.7:1 CR and the odd -A1B6D variant that uses 25? -- I convinced Klaus to make me a 22? magnet ring for use with his system and I happily flew off my 40 hours.

What struck me was how hot things were when compared with my last engine (also an IO-360-A1B6). CHT's hit 400?F during initial break in and settled at ~380?F using best power fuel flows.

I recently had the opportunity to remove the prop (needed to be resealed -- another story) and I decided to go with "stock" timing -- I installed the 20? magnet ring. What a difference 2? in timing made! CHT's dropped by > ~20+? and lean of peak fuel burn also dropped almost 1 GPH.

Net-Net - Timing makes a difference...Check it, even if it's New In the Box from Lyco...
 
Timing was the problem. Temperatures in 360-380 range now and peaks at 400-410 on take off but OAT was almost 90 here for take offs and 80 at altitude. All CHT are within 10 degrees now.

Glad you got this figured out, 360-380 is a very good number of your engine specially in the warmer weather.

The mods for the baffles are also a good advise but it is more work specially now that they have installed. Why mess with it if you have good temp.

Good luck with your phase I
 
I am having similar high CHT issues right now on my O-360. I am 15 hrs into phase 1. The engine wasn't new, but the top-end was fresh. I have a plenum and have resealed that thing multiple times. At this point, I think it would hold water upside down...haha. I retarded the ignition 2 degrees as well to 23 (to go father I have to shift a tooth on the mags). Right now the engine only gets below 400 on decent or low-power even running well below peak EGT. Pretty much all operations are over 400. I have to climb out at 110+ to stay under 450. The thing is hot. Oil temps aren't bad, they peak at 215 or less after climb out. The engine doesn't really use any oil.

I am running out of ideas at this point. I am going to mess with the inlet connection from the plenum to cowl a little more, but I don't see much opportunity there.

I do have the vetterman 1.5" system with mufflers. I wonder if the possible additional restriction is causing problems.
 
Colin, remember a plenum is a sealing device, nothing more. Sounds like you've made sure it seals well, which is good.

There are other requirements for good cooling. You need good conversion of available dynamic pressure to increased static pressure above the engine. Although there are several contributors, it is mostly a function of inlet design. Good systems raise static by 0.8 x available dynamic pressure or more. In one set of pressure measurements, a certain popular cowl/plenum combination only came in at about 0.65.

Next you want all the air to pass between the hot parts for as far as possible. The big sin in this regard is aluminum baffle wraps which do not tightly conform to the tips of the fins. It allows air to flow out the sides of the wraps, rather than continuing between the fins all the way to an exit near the lower spark plug.

The intercylinder baffle plate should be silicone sealed to the case, etc.

It's common to find a major leak at the base of cylinders 3 and 4, aft side, where a tab on the rear baffle wall should have been bent to close the gap.
 
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Collin,

Something is not right.

You might want to mention your fuel flows. I?m no carburetor expert but I would verify the engine is getting enough fuel when ROP.

Carl
 
Dan,

Thank you for the advice. I checked out the inter-cyl baffle before, but I will take another look. When I fitted the baffles, I carefully bent the wraps to ensure they matched the cyl contour, i don't have any gaps that I could find, but will look again. I will take a close look at the rear tab of 3/4 as well. I have an older engine, so I do not have the casting line across the back of 3 snd front of 1, so I have not done the cutout mod to the baffles there.

Do you happen to have a good picture of a properly sealed inter- cylinder baffle I could see for reference?

Carl,
My fuel flow is in-line. 14+gph in climb. I can get a good 200+ egt rise when I lean at cruise.

Thanks,
Colin
 
Colin,

This just does not add up.

Based on your data, I would suspect timing but you ruled that out. Here you may want to check the timing setting with a light - just to make sure.

A local RV-7 guy was flying around for a long time with CHTs like yours and he did a boatload of cooling mods to try to fix it. His timing was verified several times by more than one person, but never with a light. The light check showed timing at 40 degrees (bad electronic ignitions, both sides). Ignitions fixed and his CHTs are now in the 360-380 range.

Carl
 
There is also an AD out on that if the drift occurs within 5 hours of timing. Just for you reference.
Jim

Actually I believe it is Service Letter L264A. Lycoming confirmed the mag was set correctly at the test cell run. When told about the timing drift in only 10 hours, Lycoming did not recommend flight with the mag and approved a warranty claim for a new mag. They felt the drift would continue. So far their distributor has been unresponsive on how to do a warranty swap so I'm flying and will check timing ever 5 hours.

For my installation I have noted that if I had the boost pump off during take off the CHT will climb faster and higher. I assume the engine pump alone can not pull the maximum fuel flow needed during a full power climb.
 
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Carl,

I verified TDC on #1 and it aligned with the TDC timing mark. I have check the mag-to-engine timing at least 3 times so far. The engine starts and idles fine, no kickback. Hard to imagine the timing could be that far off. However, it doesnt hurt to check.

If I had the harness timing order wrong I think the engine would have a lot of other issues. The mag harness is modified and running auto plugs. Everything else is stock. I do however have a hesitation around 1400rpm, which is on my list of things to solve.

I have calibrated my fuel flow sender. At last check it was within 0.1 gallon over 20, so I think the fuel flow is accurate. Since all 4 are similar temps, I think that rules out a sensor problem.

One thing to note, surface OATs here are 90-100 degrees right now. However, I have been flying early and high to stay out of the heat.
 
I was going to suggest what Dan has suggested. Look at your baffles at the bottom portion of your cylinder and make sure air can not scape easily on the sides and it is force to travel thru the fins all the way to the very end.
 
Colin,

While I have no issue with the recommendations on engine cooling checks, your numbers are still way out of line for an engine with the right timing and right fuel flow, regardless of potential problems on the engine cooling air side.

I suggest:
- Verify timing with a light.
- If good, do a couple of test flights with standard aviation plugs. Perhaps something weird is in play and the cylinders are lighting off early.

Carl
 
Colin,

While I have no issue with the recommendations on engine cooling checks, your numbers are still way out of line for an engine with the right timing and right fuel flow, regardless of potential problems on the engine cooling air side.

I suggest:
- Verify timing with a light.
- If good, do a couple of test flights with standard aviation plugs. Perhaps something weird is in play and the cylinders are lighting off early.

Carl
Colin,
How does one check timing with a light? Is this the same process as with a car? I have never done it with a plane and if it is as with a car, it will scare the *&%$ out of me to be so close to the turning prop.

I know many people do it, just never seen the process.
 
Mehrdad, Correct you do it like a car and it is very scary!

Helped a guy installing an electronic ignition and needed to verify timing at a higher rpm. Very,very scary and doubtful that I would ever do it again.
 
Colin,
How does one check timing with a light? Is this the same process as with a car? I have never done it with a plane and if it is as with a car, it will scare the *&%$ out of me to be so close to the turning prop.

I know many people do it, just never seen the process.

Look at the back of your flywheel the next time you have the top cowl off. The back has timing marks just like the front, just clocked in a different place. The reference line is the engine case joint. Easy to line up via timing light from behind the prop. DO NOT DO THIS USING THE FORWARD TIMING MARKS!

But - still a little scary. We take an extra precaution to put a strap around the guy doing this so that if he slipped he would not fall forward.

Amazing that this check has found more than one timing problem.

Carl
 
Have to make sure you have a very reliable helper. I have adjusted the carb on a running engine many times. Just dont end up in the sausage maker.

There is no way for me to test with standard plugs, but I will try a different heat range.
 
I was at the airport last night. I replaced the plugs with NGK BR8ES. I verified TDC marking on the flywheel and that the static timing is still 1 tooth after the 25 degree mark. All the plugs I pulled looked fine, nothing out of the ordinary. The bottom on #4 was a little oilier, but that is it.

I checked out all the baffling. There were a few very minor spots for improvement per Dan's comments. But we are talking a few tiny gaps here and there. Stuff that takes you from 389 to 385, not 430 to 380.

I re-terminated my EGT probes with proper K-Type thermocouple connectors. Hopefully I will have more reliable EGT data going forward to help with diagnostics. So far they have been unreliable.

Next step will be to check the timing with running. I have been trying to think through what combination of mag harness screw-ups by me would result in and engine that runs, but with abnormally advanced timing, nothing comes to mind.

I did have one other thought last night though. I do not currently have a cowl-to-nosegear intersection fairing installed (working on finishing and painting). The opening for the gear leg could be letting high-energy air into the lower cowl, particularly at high AOA. For the next flight I will tape it up and see if that helps.

The top-end only has ~13hrs at this point, so it is still breaking in, but temps should have started to drop. Maybe I just need to wait it out longer. As noted before, it doesn't use any oil. The only loss I have is a slow drip from the quick-drain that I will replace with this next oil change.

Other than that, I am out of ideas....
 
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I flew on Sunday morning with the new plugs and verified timing. Also fresh oil (was time), and taped-up nose gear slot. Some minor baffle sealing. Overall there was no noticeable improvement. Number 4 EGT was still wonky, going from fail to hot and all over, so I need to look at that one again. Maybe swap probes, but the rest were working.

In this picture I have been stable at cruise for a while. Power pulled way back. Leaned for best power (you can see FF is still pretty high). I would expect lower, but any more lean and the engine gets rough.

20190825-080840.jpg


Here is another shot from earlier with a little more power in

20190825-065007.jpg
 
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Colin,

Sorry - but the data demonstrates you are dabbling in the margins and not addressing the root problem. For that I still put timing/plugs at the top of the list of suspects - but perhaps something else is in play.

Shoot fire - if you want to run auto plugs swap out the mags for pMags. I hate mags as much as I hate aviation plugs.

Carl
 
Colin,

Shoot fire - if you want to run auto plugs swap out the mags for pMags. I hate mags as much as I hate aviation plugs.

Carl

I don't care for them either but PMAGs will certainly raise the CHT even more. Perhaps until the root cause has been identified and remedied, you might want to stick what has been known for a lower temp.

How accurate do you think your probs are? Is it possible that they are either misconfigured to give the wrong temp or just not accurate type probs?
 
Colin,

Sorry - but the data demonstrates you are dabbling in the margins and not addressing the root problem. For that I still put timing/plugs at the top of the list of suspects - but perhaps something else is in play.

Shoot fire - if you want to run auto plugs swap out the mags for pMags. I hate mags as much as I hate aviation plugs.

Carl

At the power levels posted above in cruise, i run 34* of timing and my summer CHTs are around 360. I don’t believe that iming is he culprit here. My examples are at peak, so Rop may be a few degrees hotter. If his timing is off, it is WAY off. Timing advanced far enough to raise chts more than 50* is going to come with a decent performance hit.
 
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SNIP If his timing is off, it is WAY off. Timing advanced far enough to raise chts more than 50* is going to come with a decent performance hit.

My exact point.

As a data point a local RV-7 had exactly the same CHT problem - finally traced to dual electronic ignitions that had timing at 40+ degrees BTDC regardless of RPM or manifold pressure (these were not pMags). The ignitions went in for repair and now his CHTs are normal.

With Colin?s mags, he has verified timing but not with a light. As with the above RV, timing was verified many times but never with a light. The light immediately showed the problem.

Carl
 
My exact point.

As a data point a local RV-7 had exactly the same CHT problem - finally traced to dual electronic ignitions that had timing at 40+ degrees BTDC regardless of RPM or manifold pressure (these were not pMags). The ignitions went in for repair and now his CHTs are normal.

With Colin’s mags, he has verified timing but not with a light. As with the above RV, timing was verified many times but never with a light. The light immediately showed the problem.

Carl
Your friend has ei and i agree that a light is the only reliable method. Not sure i agree that applies to a mag. Pretty simple to reliably time with static methods.

Larry
 
So to address the questions/comments:

I dont think the plugs are an issue. This plug setup is used on most EI setups and has been used on similar mag/auto wire combinations like mine before.

For the timing, it seems unlikely it would change on a mag like with EI. I wonder if I screwed up the harness and am off by a whole cyl. However, I cant imagine the engine would run at all or be very rough. The engine idles just fine, mag drops are good...

The CHT sensors are standard dynon bayonet, with the standard mapping and harness.

All I can think at this point is possible inefficiency due to inlet shape or flow characteristics. I may have some fun with tape and try some variations there.
 
Would be good to get a second set of eyes on the baffling/cowling. Sure seems like an airflow/volume/pressure/leakage issue. 415 at 75% cruise after 10 hours on the engine seems a good bit higher than normal for a 6. I certainly would not be concerned about running 20 hours at 435, beyond glazing of course, but would want to take steps to get quite a bit lower. Your running quite a bit higher than most similar aircraft, so this should be fixable.

Larry
 
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I don't have a lot of great pictures, but I will get more next time Im at thr airport.

Overall Penum

20190822-205608.jpg


#3 aft wrap
20190822-205339.jpg


#1 forward wrap
20190822-205317.jpg


Inlet (this is the area I think needs improvement)

20190822-204411.jpg


20190822-204348.jpg


20190822-204341.jpg
 
Pretty plenum, nice tight wraps, very good.

You mentioned needing work at the inlets. I'll venture two sins. The RV-6 inlet area is smaller than some other models, meaning inlet velocity is higher, which makes the system more dependent on internal diffusion for the conversion of dynamic pressure to static pressure. The plenum inlet ducts are generally well shaped, important for internal diffusion. The problems appear to be (1) upper seals which blow open rather than closed, and (2) aluminum backing strips which trip the flow. The result would be low upper plenum pressure. The proof or lack thereof would be found with piccolo tubes and a manometer.
 
Pretty plenum, nice tight wraps, very good.

You mentioned needing work at the inlets. I'll venture two sins. The RV-6 inlet area is smaller than some other models, meaning inlet velocity is higher, which makes the system more dependent on internal diffusion for the conversion of dynamic pressure to static pressure. The plenum inlet ducts are generally well shaped, important for internal diffusion. The problems appear to be (1) upper seals which blow open rather than closed, and (2) aluminum backing strips which trip the flow. The result would be low upper plenum pressure. The proof or lack thereof would be found with piccolo tubes and a manometer.

Dan I agree with your assessment. My next test was to remove those top baffle strips and backing strip and use duct tape from the cowl to the inside of the plenum sides and top. If that does not solve the problem, then I need to look elsewhere. Obviously, that would be a temporary solution for testing. The baffle material overlaps the inlet lip quite a bit. The shaped backing strip keeps it pretty rigid and tight against the inlet. I had hoped the labyrinth-like seal would be enough....looks like I was wrong. I think the flow-trip may be the larger of the problem than the seal to the inlet.
 
Dan I agree with your assessment. My next test was to remove those top baffle strips and backing strip and use duct tape from the cowl to the inside of the plenum sides and top.

Worth a try. The tape may rip on start up, so you'll probably need to do one trial start, then shut down for a look.
 
Worth a try. The tape may rip on start up, so you'll probably need to do one trial start, then shut down for a look.

I'll do a ground runup first to see. Ill leave the bottom flap as is and tape the rest. Ill leave the tape a little loose at the joint, so there is some play for the shake.

If that makes s solid difference, ill try it without the tape at all and compare.
 
Any concern about tape looking ok during the ground run then coming loose during flight and creating serious air flow issues (blocking a cylinder) before the plane can be landed?

Think I would come up with something other than tape for testing.....
 
I tested mine with tape that was a little loose at the joint. I saw little to no difference with the tape on or off.
 
This might seem pretty unlikely, but at this point with lots of details confirmed OK, lets look for zebras in the horse coral.

Is it possible the cht probes are the wrong kind (type J vs type K) or that there is a polarity swap somewhere in the harness?

Here is a test -- pull a cht probe out and put it in a small pot of water on a hot plate. When the water boils, you know the temp (depends a bit on your altitude) but it is a good end-to-end check that the cht probes are reading right.
 
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