Optimum Coolant / Operating Termperature.

Hey guys, happy Easter :)

I wanted to start a fairly broad discussion around coolant / running temperatures. There is no one-size-fits all answer I know, and I am interested in hearing your educated experience for different applications on the trade-off of cooling efficiency vs engine efficiency (turning fuel into torque). Fundamentally in terms of the cooling system efficiency, a higher delta-t to ambient enables more heat rejection, I'm interested if anyone can offer some insight into the trade-off to higher combustion chamber temps and potential lower knock threshold and reduced torque (on pump gasoline).

On paper, my thoughts are all else being equal and if the thermal load on the system is consistent, operating hotter increases the delta-t to ambient and the cooling system operates more efficiently, allowing an overall smaller cooling system. Im curious as I work fairly regularly on a projects which have compromised cooling system layout in the vehicle and I'm investigating using a compressor on the cooling system to allow the system to run at 2 bar and 110+ degrees. Having never tried this, am I interested whether the trade-off is likely to be a significantly reduced knock threshold? Interested to hear poeples thoughts on this. Again, this is on pump gasoline (98-99ron).

On the same token, who has dropped an operating temp and seen a good tangible improvement in torque e.g. moving from say a 90deg temp to 75deg? I have in mind knock limited forced induction engines of course!

Hi Alex,

Its an interesting conversation and as you mentioned each engine will react differently. Things to also take into consideration is engine tolerances/clearances which, depending how the engine is built, will also be a determining factor on what coolant and oil temps you want to run.

But putting that a side for now.

We did a lot of temp testing on one of our race engines, although it is a N/A 4 cylinder, we were attempting to pull every last bit of HP. We run this engine at 110c Oil temp and 70-75 water temp. This makes best power, at 100c water temp we lose around 3.5% in HP. I would assume mainly is charge temp increase.

I do find the majority of the race/track cars we tune (90% which are forced induction) make optimum power around 80-90c and up to 95. As you mentioned as you go up in temp you lower your knock threshold, you can remove timing for safety and this will increase temp even worse. You can then add fuel to reduce temp, this will only help so much until you really start to hinder your power output and engine feel (will start to feel flat and lazy).

With saying this, I find when you start getting up in temperature, it starts a viscous cycle (remove timing, add heat, gets hotter, removed more timing, more heat) and then your temps start to get out of control. Then add into the mix you're running a smaller cooling system which has less fluid for convection and a smaller radiator surface area for radiation. I think the battle you will find is keeping the cooling system stable and a consistent high temp, while still making optimum power. You could 100% make it run a hotter coolant temp and consistently, but I feel, on pump gas, you will lose a decent amount of power.