Cylinder Head Temps, 5 degree's of separation, you kidding me!

[JonJay]

While my #2 Cylinder was out for the repair of a sticking exhaust valve, I decided to rework my ramps and forward baffling. I was never happy with how they originally came out so it was a good excuse and time to fix it right.

I am now flying again with just over 6 hours on they new cylinder and was pleasantly surprised to find that my cylinder head temps have a maximum separation of 5 degree's in cruise. I used to see 10 to 20 degree's and never worried about it, nor would I worry about it regardless. However, I did not expect to see these kinds of results;

25"/2500
55F. OAT
1500' alt.
150 ROP
185F. OT
CHT#1 = 349
CHT#2 = 349
CHT#3 = 353
CHT#4 = 348

I also find it interesting that these temps held pretty true, +- a couple of degrees, at 48F-60F. OAT. I always thought that OAT and CHT had a direct linear corelation but my data seems to not follow that while it may be direct, it is not 1 for 1. Same for oil cooling. Maybe someone can explain that to me.

I also firmly believe that while specific engines perform in the same realm of operating temperatures, there are differences. Some identically equipped machines run hotter and some colder. Perhaps the little details make the difference or it is just built in. Just my observation.

I think as a group we get a bit hung up on our baffling systems. However, with a little attention to detail, you can really increase the performance of the system.
I consider my rework a success.

[drill_and_buck]

Jon,

Can you share with us some of the mods you made to effect these changes. Did you take an iterative approach to tuning or was is a once and done?

[David-aviator]

Got to be something wrong with your indicating system.

[JonJay]

Originally Posted by drill_and_buck

Jon,

Can you share with us some of the mods you made to effect these changes. Did you take an iterative approach to tuning or was is a once and done?

Standard Van's setup with a 1" dam in front of Cyl. #1. This dam covers the cylinder head and is offset to cover the barrel as well. Pay close attention to seal up every nook and cranny with RTV. I had a 1/2" dam in front of cylinder #2 but it elevated that cht about 10 degree's above the others so I removed it.

I did modify the front ramp set up but I am not sure it has any affect on these numbers. The back side of the cowl inlet on the 6 angles down/inward. The standard set up is to bend the cooling ramp to match the inlet. I chose to lower the inside of the ramp to match the angle of the inlet so I did not have that bend. While the outer edge of the ramp is fixed at the original location off of the cylinder head, the inner edge is now down from its original location about 1 1/4". It does add more volume to the ramp area and a smoother transition for the air.

Please, I do not know if this had any affect or not and don't want people to rush out and change their ramp set ups to find it has nothing to do with it.

Mike - your building an 8. Very different baffle system. Still, close attention to sealing all the holes where air can escape pays back big.

[Bavafa]

Originally Posted by David-aviator

Got to be something wrong with your indicating system.

I don't doubt his numbers at all. My CHTs were about 10-12 degree apart from the hightest to the lowest. A few weeks ago, I reworked a portion of my sealing on the ramp of #1 Cylinder which was always the hightest and now I have 2 degrees of difference between the highest and lowest in cruise (I couldn't believe it myself) but multiple flight has confirmed it.

Now I am planning on cutting a bit of the dam in the front of the #3 to see if I can bring that down to even closer numbers.

[erich weaver]

ok, great, but I think you guys are going way overboard. Show me where a tighter CHT temperature range equates to better performance. Bart at Aerosport Power wasnt concerned about CHT temperature differences of 50-60 degrees when I questioned him about it. Besides, each temperature reading is from a single point on the cylinder; useful data, yes, but it certainly doesnt present the whole picture regarding the temperature profile over the cylinder.

Now donning flame-proof jock strap...

erich

[Neal@F14]

Originally Posted by Bavafa

now I have 2 degrees of difference between the highest and lowest in cruise (I couldn't believe it myself) but multiple flight has confirmed it.

Now I am planning on cutting a bit of the dam in the front of the #3 to see if I can bring that down to even closer numbers.

Don't touch a thing or you might jinx it. You've already achieved a holy grail of CHT spread there.

[Sig600]

Jon, notice any difference in performance?

[JonJay]

Originally Posted by Sig600

Jon, notice any difference in performance?

No, and I would not expect to see any. Unless you consider cooling to be part of that performance. I am seeing better cooling, meaning I can maintain higher power settings, steeper climbs, at higher OAT. I guess that means better performance overall.

Erich - I posted this as a simple observation as I was very surprised by the result. I am not sure who or what is going "overboard". I respect Bart very much, however, I would not accept 50 - 60 deg. difference in my CHT's as this seems very abnormal in comparison to data I have seen reported by the "fleet". Being limited in climb by one cylinder reaching 450deg while the others are below yellow doesnt sound optimal. I guess if you dont trust the CHT's as meaning anything then it doesnt matter.

I do think that if your CHT's are relatively close you probably have done all you can in sealing up air leaks between the cylinders and the plenum must be doing a pretty good job of pressurizing itself. Mostly, I think it is just plain lucky that they came in this close. But, I am not complaining about it!

[erich weaver]

Hold on there cowboy Im not saying CHTs are not useful, and if any CHT is at 450, then it deserves prompt attention. I was just questioning the value of time spent trying to decrease a temperature spread when its already quite good, thats all.

best regards
erich