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Uneven cooling

cposer

Member
What suggestion is there to decrease the cooling of #1 cylinder and increase cooling to #3 cylinder? IO-360-M1B on an RV-7A.
 
What suggestion is there to decrease the cooling of #1 cylinder and increase cooling to #3 cylinder? IO-360-M1B on an RV-7A.
Let's assume your baffles are good overall, tight against cowl and tight against engine where it needs to be, without any major air leaks or other issues.

What is going on? Look at the cylinders. There is more fin area (deeper fins) on the exhaust side of the cylinder than intake. The engineers were trying to make engine more compact and get them closer. Obviously you need less cooling on intake side. The intake side has almost no fins on the side. This allows the jugs to be set closer and making overall engine length less.

On the LEFT bank Cyl #2 and #4 the intake is forward with reduced fin area.

On the RIGHT bank Cyl #1 and #3 the exhaust side with the deeper fin area is forward.

OK so obviously the #1 RIGHT FWD cylinder is getting the most access to cooling air entering the cowl. As well the deep fins are facing forward allowing more cooling air, and as stated a blocking plate is common practice to reduce airflow at the front of the #1 cylinder. That will get temps up. #2 Cyl (Fwd Right) is going to run a little warmer because the exhaust port is on the back side (next to #4) and has less cooling air. #2 has those shallow (almost non existent) fins on the intake side facing forward.

(NOTE: KEEP IN MIND, fuel cools the cylinders and uneven fuel distribution, typically more with carbonator setups than FI, will make a difference in CHT/EGT.)

The #3 Jug (Back Right) requires the "old washer trick". So the #3 jugs flat or shallow fins are on the back side next to cylinder baffling. Air can not get around the back side of the #3 jug because of restricted area and baffle bolted to the jug. Even though this is the intake or cool side of cylinder, there is less air going around the back of the #3 and to the bottom fins (that still need air). There is a threated boss for supporting the baffling with a fastener. Add a washer or two between the cylinder and baffle creating a gap on the back side of cylinder, promoting more air to go down and around the jug. Works like a charm.

Keep in mind the wrap around lower part of that baffle where the fins get deep again should be fairly tight with out lots of gap, especially on the edge, allowing air to just escape without doing the cooling work with lower fins. This can be a challenge to get right....

All these changes will make the cooling more even and a happy engine in the warm summer months. Cooling drag is a part of your drag. By managing the air through the cowl so it does max cooling without waste can increase your speed. When I say increase speed it can be less than 1 MPH if you already have a fairly tight efficient cooling system you are tweaking. However fixing a poor cooling system, leaky baffles, can add noticeable performance, as well as lower engine temps....
 
Lets quantify, please.

How uneven?

Mike hit the basics and that might be enough. The "washer" or opening up the back of #3 is good for about 20F (more or less depending).
 
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