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Hi Guys,

I found it interesting that someone made an attempt to analyze the cooling efficiency of a VW aeroengine. I also found it interesting that the conclusions matched closely with the 100 mph climb speed many use to keep CHT under control.

Ref. https://www.mdpi.com/2504-3900/39/1/4/pdf

Wes

Good read. It's the evidence of what's happening in real life. But what can we do about it? Making cylinder heads with bigger cooling fins?

It's doubtful a custom designed cylinder head with larger fins is in our future. It's a demand thing. That is, unless you happen to be married to the daughter of the guy that owns a foundry. The fins simply must fit within the confines of the cooling tin found on VW cars.

That said, there are heads out there that have more than the typical 8 fins per head. The ones I chose have 9. The jury is still out however.....my engine won't start.

Wayne Clagg tried to make his VW deliver 100 hp at slow airspeed. He welded on fin extensions to provide better cooling and had some success, but the engine was never reliable in the long term. He claimed to see 80-100 degree reduction in CHTs.

"- Wayne Clagg installed a 2276cc engine in his CH 701, complete with a Valley Engineering 1.6:1 redrive and a 74" Powerfin prop. The redrive allowed him to get an estimated 100HP output from the engine (by running at higher RPMs). He had the (anticipated) problems with shedding heat from the engine. He brazed on the "big fin" mods to the head (which he claimed helped quite a bit), but the engine never proved to be reliable. "

https://www.kitplanes.com/building-on-a-budget/

AlexZ wrote:Good read. It's the evidence of what's happening in real life. But what can we do about it? Making cylinder heads with bigger cooling fins?

Without making changes to the engine, the only options for improving cooling are those that increase cooling airflow across the cylinder head fins. That is effected by inlet geometry, baffling, outlet geometry, and possibly some forced air solution(fans/pumps).

I couldn't improve on the AeroConversions baffling so that's not a consideration.

Some cowls (e.g. Vans) attempt to smooth the airflow into the upper plenum. But, the oil access doors on the front of my cowl preclude modification of the inlet.

I couldn't imagine a practical fan system. Scavenging exhaust energy in the form of an "ejector pump" has been studied but I'm not aware of that idea ever being applied in a practical manner.

So ..., that leaves the outlet geometry, both fixed and variable (i.e. cowl flaps). There are a lot of different examples out there. I wish I could find some good reading material on different cowl outlets and their advantages and disadvantages.

Wes

Interesting. I've found 70KIAS @ 2,900rpm gets me something sustainable in a climb. Pitch up a hair and lose the airspeed to 68 KIAS though and the back cylinders start heading north of optimal.

One relatively easy thing to try would be to model the top flow part they drew with what looks like an indentation between the cylinders to keep the air more in contact on the way to the aft cylinder. Some thin shaped aluminum sheet could be easily shaped up and attached to the inside of the cowl. Might try this before I try a cowl flap which I was going to do as the heat of summer comes on around here.

I couldn't imagine a practical fan system. Scavenging exhaust energy in the form of an "ejector pump" has been studied but I'm not aware of that idea ever being applied in a practical manner.
Wes[/quote


The exhaust energy scavenging system was employed in early 310 Cessnas I believe. As far as a practical fan system.....I'm going with two leaf blowers and an inverter to power them.

Hi guys,
This is a little off the topic however I have had difficulty getting an easy answer. I mounted my CHT probes adjacent to my upper spark plugs by screwing them onto the head about 3/8" from the spark plug. What I would really like to know is how much cooler the heat is than when measured from under the spark plug. My sensors that were meant to go under the plugs just wouldn't work. My CHT's run at around 230 degress f in cruise while rising to about 250 - 260 ish in the climb. These temps are excellent but I'm not sure how accurate they are. I enlarged the outlet on my lower cowl by around 40% from the plans due to most people complaining about high CHT's plus it gets hot in Australia. Soooo..where is everyone measuring their temps from?

Why couldn't you get the spark plug senders to work? I know there's a little step down to the plug seat on my heads that wanted to distort the sender, so I got some aluminum oil pan gasket washers to put between the plugs and head to eliminate that miss-match.

Duh! I didn't think of that! Mind you I was a little concerned with raising the plugs out of the combustion chamber too much, but I may have just been worrying too much about it. It just looked ugly the way the copper lug got bent so much. The way that I've got it at the moment has to measure a bit cooler, it's just how much cooler is what I'd like to know.