CHT and gearing down to help cooling, myth OR fact, Get out your facts and scientific equations Options

[nditiz1]

It has been burned into my brain when I read it the first time that when going up a hill drop a gear (5th to 4th) to increase cooling thereby lowering the temps of the engine. Raby, Mark Henry, I'm sure others have stated the same thing.

I must admit my sample size is small (3 cars) AND 2 of which already had pre-existing issues with the engine health, but in all 3 scenarios when cruising on the highway going 75/80 dropping a gear down only momentarily lowers the temps only to shoot them back up possibly even higher as I am trying to maintain speed at a lower gear which requires more pedal. In each of the 3 cases I have been using a CHT over cyl 3. Two of which were a Dakota Digital and one was an aircraft spruce analog gauge. The one engine that was not plagued with less ponies was a rebuilt 2056 (by me). Maybe I built the engine wrong, I don't know. I guess I would like to hear what others have experienced. I am also not doubting the physics. More RPM = More fan turning = More cooling. Lowering gearing means the engine does not have to work as hard to pull up the hill. That last statement should have a caveat though. There must be a certain point of force vs gear ratio vs drag vs speed that would determine that staying in 5th would ultimately be more beneficial than dropping to 4th as far as work the engine is doing (probably a question for Phil - @superhawk996 )

So am I wrong in my limited testing and it does in fact shed heat driving on the highway 75-80 mph with ambient 85 - 90 or was it just a myth started because the physics seem to point that direction.

[gereed75]

It is indeed a very complex question with many variable parameters. If you really want to understand it better, I again suggest that you go to the world of aviation engines. The information developed by a group in Oklahoma is particularly insightful A good place to start is here

https://www.avweb.com/features_old/pelicans...8mixture-magic/

In the realm of aviation, they are generally not that concerned with burning valves or dropped valve seats. Detonation is the main concern You will have to wade through a lot of aviation oriented jargon, but it explains the relationship between mixture, CHT, EGT (directly relatable to mixture), timeing, power output etc. Remember - LOP refers to lean mixtures with EGT's below their peak value and ROP refers to rich mixtures with EGT's below peak on the rich side.

Problem is, with variable fuel injection (or carbs) constantly changing mixture, and without more instrumentation (AFR, EGT, CHT, MAP, knock sensors) you will never really know what is going on inside the combustion chamber. And in addition, an auto engine is a much more variable situation than an aviation engine, where variables change much more slowly and are pretty much constant for long periods.

So in our application you are basically left guessing and relying on the carbs or fuel injection to get it "right" and watching CHT as a rough, cumulative indicator.

The temp ranges you describe are pretty good and safe.