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Cool CHTs

Adam

Well Known Member
Guys I have an issue that most people don?t, my CHT?s run very cool this has been going on for many years and for the most part I?ve been pretty proud that I didn?t have an issue of high CHT?s. About two years ago I had a sticky valve I had taken care of it and the plane has been flying fine. When I was out at Oshkosh this year I met with Superior Engines and spoke with them for quite a while I told them of my issue with my low temps and their concern was that?s why I?m having a sticky valve as most of my temperatures never get above 300? in Normal straight level flight. Typical temperatures range from about 265? to about 295? cylinders 1 through 4, running 25 square at 4000 feet lean of peak at 78? outside temperature. So my question is what should I do, do, I close up the intake or do I close off the exit part of the cowling. Just a few bits of information this is an RV-8 with an I/O 360 fuel injected with a standard cowl the only thing that is different is I have an exhaust system that is 4 into 1.
 
Carl, my CHTs and EGTs read the exact ambient temperature when the engine is cold. Spoke to the manufacture and they said there is no adjustment, I really don't think its the probes.
Adam
 
I have seen two engines that were supposedly running cool, actual problem was they both had cht senders that did not have a spring forcing the probe to make contact with cylinder so they were just measuring dead air
 
Forgive me for possibly hijacking this thread, but can someone lead me to information on how to best raise the temperature of just one cylinder approximately 25 degrees? Cylinders 1, 2, and 3 are within 5 degrees of each other (340 -350F) but cylinder 4 is about 25 degrees cooler. Oil temp- 185-190 degs. While number 4 is coolest CHT, it simultaneously is the hottest EGT so I am thinking I don't want to lean it out.
 
dwrichy - I have the same CHT spread as you. 1,2,and 3 run 320-340, and #4 is always 25-35 degrees cooler. And #4 has the hottest EGT. O-360 Lycoming. I?m over 1,000 hours and it?s always been this way. Leaning doesn?t seem to change the relationship.
 
I have seen two engines that were supposedly running cool, actual problem was they both had cht senders that did not have a spring forcing the probe to make contact with cylinder so they were just measuring dead air
_


Thanks Jerry Ill check that!
 
Guys I have an issue that most people don?t, my CHT?s run very cool this has been going on for many years and for the most part I?ve been pretty proud that I didn?t have an issue of high CHT?s. About two years ago I had a sticky valve I had taken care of it and the plane has been flying fine. When I was out at Oshkosh this year I met with Superior Engines and spoke with them for quite a while I told them of my issue with my low temps and their concern was that?s why I?m having a sticky valve as most of my temperatures never get above 300? in Normal straight level flight. Typical temperatures range from about 265? to about 295? cylinders 1 through 4, running 25 square at 4000 feet lean of peak at 78? outside temperature. So my question is what should I do, do, I close up the intake or do I close off the exit part of the cowling. Just a few bits of information this is an RV-8 with an I/O 360 fuel injected with a standard cowl the only thing that is different is I have an exhaust system that is 4 into 1.

What fuel burn is this? How much LOP are you running? (perhaps for a separate discussion, LOP is usually not recommended above about a 24/2400 setting). Mixture has a powerful effect on CHT. I can run mine anywhere from 375 down to 300 depending on mixture.
 
I have seen two engines that were supposedly running cool, actual problem was they both had cht senders that did not have a spring forcing the probe to make contact with cylinder so they were just measuring dead air
_


Thanks Jerry Ill check that!

Also thank you Jerry, I will check that out also, but I am not too optimistic because the plugs in cylinder 4 are always black compared to the other three cylinders. I'm thinking I can wrap the cylinders from the top in such a way (fiberglass cloth held in place/saturated with RTV) that will bring the one cylinder temperature up. I believe the F1 guys at Reno do something similar to improve cooling efficiency, I would just need to neck the opening down a little more.
 
Whoa . . hold on a minute

Also thank you Jerry, I will check that out also, but I am not too optimistic because the plugs in cylinder 4 are always black compared to the other three cylinders. I'm thinking I can wrap the cylinders from the top in such a way (fiberglass cloth held in place/saturated with RTV) that will bring the one cylinder temperature up. I believe the F1 guys at Reno do something similar to improve cooling efficiency, I would just need to neck the opening down a little more.

I have my barrels (M1B 180 hp) wrapped to a 3" opening at the top, and unwrapped the 2 & 4 barrels to 5 " and there was zero change in the CHT balances. It is helpful to wrap up to the centerline though just for air volume management. I think wraps will affect piston temps more than anything else. Nothing we currently measure.

For CHT I would suggest review of the opening gaps under the head and barrels first, closing them slightly. Edit: My gaps are the James recommended (and others) 1" lower gap and 2.25 for the heads. Then, consider wrapping on the top of the fins. Use some aluminum tape first. Might be easier to just do two heads first to compare the balance - rather than all 4 and attempt evaluation using a flight to flight differences. Too many variables.

Kinda hard to understand your low CHT's with a parallel valve engine. Easier to fix than too hot.
 
Last edited:
Wrong Sensors

Hey Adam,

I would like to relay a story that might be of interest to you.

Last year a friend purchased a flying RV7a/360/FP Sens Propeller aircraft. Just before purchase the prior owner had dual HDX Dynons installed. The plane had stock baffles and stock Vans cowl.

The plane was delivered to my friend and after the transaction the data from the Dynon data log was down loaded. It showed that the cht's were running in the mid 250's f on the delivery flight in warm temps and high power/rpm settings. By this time we had seen the baffles (multiple leaks) and we were pretty sure that the temps were not as they were logged. Turns out the prior owner had kept some old sensors and spliced them to the new Dynon wiring---they were a mismatch. The sensors were replaced with a set made for the HDX and all is now normal with temp usually in the mid 300's. If you purchased this plane flying, is it possible that you also have mismatched probes? An easy way to test is to buy one probe that matches your engine monitor and see if the temp for that cylinder changes significantly.

If you built the plane and have matching probes trash this post.

Cheers,

db
 
49clipper

Don't worry about it. Mine has run at those exact temps for 10 years now. I called Lycoming twice about it, and their response was, You are a very lucky guy and your engine and cylinders will outlast you. Call me back when they get down into the 100-200f range". no issue at all. My probes have been replaced and checked several times. they are perfect (if there is such a thing as perfect).
 
My angle valve runs cool too. 300ish in high speed cruise. I have tightly sealed baffles with a tight fit plenum lid. It just cools really well. I intend to close off some exit space at some point, see if I can get these temps up to at least mid 300?s in cruise and reduced e cooling drag at the same time. If that works, I?m contemplating a few ideas for adjustable exit volume.
 
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