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.

[era vulgaris]

From 2014-16 I spent two years with a '75 914 that had a 2270 with a pair of DRLA40's, and a 4-channel digital MGL Avionics Stratomaster Velocity CHT designed for light aircraft. That car taught me alot about head temps. Along with the AFR gauge it had, it allowed me to really nail in the tuning of the DRLA's and the engine as a whole.

First off there are essentially two "zones". The crossover point between the two zones is between 60 and 65 mph. Below 60mph, air resistance and drag are not factors on head temps. Above 65mph, they heavily impact your head temps.

In the lower zone, the engine was coolest between ~2800rpm and ~4300rpm. You could get away with running a little below 2800 on a decline (maybe down to 2500), but if you started uphill then temps would slowly start to creep. The upper threshold was over 4k (maybe up to around 4500 depending on conditions), where sustained running would make the temps start to creep. Outside that window, head temp creep was directly analogous to how far outside the window you were: the further outside, the faster the creep.
In the "lower" zone I only ever used gears 1-4. 5th gear was verboten, and I'll explain that in a bit. My typical cruising head temps in this "lower" speed zone, usually cruising between 3k-4k were:
1) ~295
2) ~275
3) ~315
4) ~295
Those numbers are from memory and are not exact, but are in the ballpark. 1 and 4 were always very close if not the same, 2 was coldest, and 3 was hottest. There was usually 20-25 degrees between each step. I should also point out that with real time digital monitoring, the temps do not stay static. They are always moving around from second to second, but you get an idea of the average from the numbers they hover around.

However, once you cross the 60-65mph threshold things change. This is the speed where air resistance and drag begin to become real factors on the load the engine feels. My head temps were always about 30 degrees higher across the board going over 65mph on the interstate - with the top on. Take the top off and it would add another 15-20 degrees because the car's aerodynamic efficiency is now destroyed. In the "lower" zone below 60mph, it made no difference on head temps whether the top was on or off, but above 65mph it made a very real difference.
I never used 5th gear below 3000rpm, which on my car was 70mph, because head temps would slowly creep, even on flat grade running in 5th at say 2700 or 2800, which was like 67mph or so. And if I was going up hill in 5th at 3k, I'd usually drop a gear to 4th just to be safe. The 914, IMO, is best used as a 4 speed below 70mph. 5th should be saved for above 70 in all circumstances, unless you want to risk cooking your heads. Honestly I cringe every time I see a driving video on BAT where the seller is cruising around in 5th gear at like 60mph.
Also, above 65mph, the area of most cooling narrowed slightly to around 3000-4200. As with the "lower" zone, the further outside this window, the faster the temp creep, but with a sharper deviation.

But what was fascinating to me was how quickly temps could change. I'd be going down the interstate at 75mph with #3 cylinder at 355 degrees, and in the 10 seconds it takes to exit and sit at the stoplight at the end of the off-ramp, it would be back down to 315 degrees before I'd even come to a full stop.