Help - Is this a bad idea?

RV-4 with old style long cowl, O-320 with Carburetor.

I have been adding aluminum tape to the #1 & #3 cylinder top fins as well as an air dam in front of #1 in an attempt to increase the CHT's to match those of #2 & #4. Typically #3 is the coldest cylinder averaging 30-40 degrees F lower and is also the richest cylinder. #4 is the leanest cylinder.

#2 & #4 are withing 1-2 degrees of each other (Avg. 340) no tape, no air dam.

I have done all the sealing with silicone I can & baffles/seals are in good condition.

Pictured in this post is the current taping job which has increased #1 & #3 to be withing 5 degrees of the other side. My concern is it a bad idea to leave the tape in place? Will it cause other problems like hot spots on the cylinders/valves?

Is a 30-40 degree spread acceptable for a carbureted engine?

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Remove that tape

cause it is in a critical area. IMHO. Better to have the differences, and they aren't that bad.

RV6airplanePilot said:

cause it is in a critical area. IMHO. Better to have the differences, and they aren't that bad.

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I'm with Dale- It's a carb- so you are going to see spreads.
Don't try to raise Cylinder head temps like that.
Work on lowering the hot one first- the best you can-

As far as I know, there's nothing intrinsically good about "even" CHTs. The point is to lower all of them individually as much as possible. If you're left with uneven CHTs at the end of the day, so be it. Only try to raise a particular CHT if you think it will lower an even higher CHT elsewhere.

Why would you try to warm the cool cylinders instead of trying to cool the warm ones??

Mike S said:

Why would you try to warm the cool cylinders instead of trying to cool the warm ones??

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I have lowered the warmest cylinder (#2) by 15-20 degrees using the washer trick & sealing up the baffels as best I could.

If a 30-40 degree difference is no big deal, I'll probably just remove the tape.

I have been flying this airplane for 20+ years with steam gauges & a single CHT/EGT on #4 without any issues. This quest to even up the CHT's all started when I upgraded the panel to glass with 4 CHT/EGT probes. I never knew what the differences in the temps were before the upgrade. Maybe best to leave it alone?

rv4ator said:

I have lowered the warmest cylinder (#2) by 15-20 degrees using the washer trick & sealing up the baffels as best I could.
................Maybe best to leave it alone?

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We should try to lower CHT's where we can. (Never raise.)
I would take the tape off ASAP.... My quest is to lower my CHT's in climb as I have some go over 400 on a hot day- If you don't have that problem- count your cooling good and go fly!

(I should say in the case of the front baffle plate to redistribute air to the rear Cyl's- if it also raises an otherwise cooler front Cyl and lowers the target rear Cyl- then all he better.)

I'll post a minority opinion here..

There is often a reason to raise CHT's. If you have over cool cylinders, that is extra air you don't need to bring in to start with. Thus, more cooling drag. Also, in terms of pressure, restricting some flow in areas will increase your overall pressure, so maybe your high cylinders will come down. Maybe..

The downside to limiting it by your method may generate a very un-even temperature on the cylinder head, something you want to avoid for sure.

If I were you, I would spend more time cleaning up those baffle seals. You will see a much better gain in cooling efficiency when those work better. Although, still far from ideal.

Cooling is accomplished by pressure differential. High on top, low on bottom. Maybe you have abnormally higher pressure below your hot cylinders, which then lessons the amount of pressure differential, which lessons cooling efficiency. Baffle leaks around your inlets? Propeller area? All could cause that.

I am cooling over 240 hp, both cylinders and oil, with less than 20 square inches of inlet area.

THAT IS A BAD IDEA

PLEASE BEFORE FURTHER FLIGHT REMOVE THE TAPE

How are you sure which is leanest and richest? You may know, but it is not always the hottest or coldest as you may think.

Mixture controls the heat in part of the equation, the baffling and airflow around the engine bay controls the heat out part of the equation. What you are left with is the CHT.

Everything you think you know about how air moves under your cowls is generally wrong and the opposite of what you think.

DanH is worth searching for an read his posts about cooling air and baffles.

What you are creating is oval cylinders, for round pistons. When you get down to 175dF CHT's then start worrying about heating them up. Above 200dF there is nothing wrong.

Any time you do an experiment like this without SIX (6) thermocouples around each cylinder....you are sure to distort it and not know it.

Thanks for all the reply's. I was thinking that it might not be a good idea to leave the tape on. The consensus seems to agree. I will be removing the tape today.

Before the tape, I also tried a honeycomb air straightener on the bottom of the carburetor per idea from a post a few months back, but it did not have any effect on the CHT's.

www.vansairforce.com/community/showthread.php?t=97025

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Ok

Did it raise your static RPM at all?

No Change

I did not check the static RPM, but I could not tell any performance differences or lower temps after the honeycomb install.

Temperatures?

rv4ator said:

Thanks for all the reply's. I was thinking that it might not be a good idea to leave the tape on. The consensus seems to agree. I will be removing the tape today.

Before the tape, I also tried a honeycomb air straightener on the bottom of the carburetor per idea from a post a few months back, but it did not have any effect on the CHT's.

www.vansairforce.com/community/showthread.php?t=97025

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... Did this flow straightener have any effect on your EGTs?...

EGTs seamed the same

I do not run at WOT except on take off & climb. In cruise I suspect that the air straightener does not have that much effect because the carb throttle plate is partially closed which in turn is still disrupting the flow into the cylinders. I did not do any detailed testing, but after many flights at different conditions, I could not see any significant changes.