Even More Observations on Engine Cooling

January 1996

Over the years, writing this newsletter, one of the most visited subjects has been that of how to cool the cylinder heads in an EZ. We have recommended almost every kind of fix you can think of. There have been numerous in-depth studies by homebuilders as well as here at RAF. The thing that has puzzled us has been the fact that one EZ will have excellent cooling, CHT's of around 350 degrees F in cruise and no more than 390 degrees F in a long climb, using the stock, plans baffling and cowling. Oil temperature might be 170-190 degrees F. Yet, another EZ, using exactly the same engine, cowling and baffling system, has trouble keeping his CHT's below 400 degrees F, and might see 450 degrees F in a climb, and his oil might reach 240 degrees F! How could this possibly be?

This kind of example has been reported to RAF many, many times over the years. That is why there have been so many articles and so many different "fixes" suggested in past CPs. As far as I can remember there has not been any mention of carburetion, that is, fuel flow to the engine at full throttle with mixture set at full rich. Depending on the main jet installed in any carburetor or fuel injector, this number can vary by several gallons per hour, and this fact on it's own, taking nothing else into account, can change CHT's by as much as 60 degrees F across the board! I recently experienced this myself on our Long-EZ N26MS. I put 1300-plus hours on my Lycoming using an Ellison throttle body, instead of a carburetor.

The jets in this unit were not changed over this period of 10 years. CHT's were perfect. A maximum of 400 degrees F in a long climb and 370 degrees F in cruise. Oil temperature ran between 200 degrees and 220 degrees F - a little high, but not bad. Then I had the engine and the Ellison overhauled to factory new specs. I re-installed the engine using the original baffling, except where it was cracked or damaged, but I reproduced new pieces of baffling exactly the same as the original. The engine ran incredibly hot, CHT's all above 450 - one reached 482 degrees F!

Knowing it was essentially a new engine, (I bought factory new cylinder assemblies) I expected it to run hot. I persevered, and flew it often, making small baffling improvements, even cutting more inlet holes in the cowling, top and bottom, with essentially no improvement. Ultimately, out of frustration, even desperation, I installed an Airflow Performance fuel injection system, with a calibrated maximum fuel flow designed for my engine at sea level. Frankly, I could not believe the difference! My CHT's dropped back to what I used to have and even a little lower. But fuel flow at full power was up a dramatic 17%! What happened here?

I don't believe there was anything wrong with the Ellison, I was always completely satisfied with it, and on my engine, before it was overhauled, it worked great. The Ellison has by far the best mixture control fidelity of any carburetor/fuel injection throttle body system I have flown. I believe it may be that my "old" engine (before overhaul) was not putting out full "book" power (indeed, this is quite probable) and therefore the Ellison worked well, because that was the engine it was tuned (bench flowed) for. When Mattituck overhauled the engine, it was rebuilt to new stock specs. It was not "hot-rodded," it has stock 8.5:1 pistons, stock cam, and I have been assured, it is a standard engine with the single exception of having been balanced by carefully selecting parts that weigh the same. In its present state my engine requires more fuel flow to cool than it did prior to the overhaul.

The whole point of this article is to alert you EZ builders/flyers that high cylinder head temperatures and/or high oil temperatures may not be curable by "improving" baffling, or increasing, the size of the cowling inlet or outlet. Keep in mind, the full power fuel flow may be the culprit. Get a copy of the Lycoming engine specification (a stapled collection of data sheets, including a fuel flow curve) for your exact engine, e.g. LYC 0-235-L2C; look up what your sea level, maximum power fuel flow should be, then check it in flight. This was easy for me since I have a digital, calibrated fuel flow meter on my instrument panel, and can read fuel flow real time, all the time.if there is a significant difference, one or two gallons per hour, get your carburetor checked/overhauled to the correct spec.

Keep in mind, engines used by homebuilders come from all sorts of sources, mostly used, and may or may not still have the original jets installed. Indeed, these jets get changed by the airframe manufacturers many times, based on cowling/cooling requirements for the particular airplane that they are installed on. 'Nuf said, just food for thought!

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