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Old 09-13-2021, 08:15 PM
FishStretcher FishStretcher is offline
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Join Date: Dec 2010
Location: Greater Boston
Posts: 1,136

I can only offer you a modest amount of guidance based on experience with other engines.

I did a fair amount of cooling work on Rotax 912 IS sport 100hp 4 cylinder aircraft engines. These were liquid cooled heads, but air cooled cylinders. And an oil cooler.

Crucially, they can operate for a long time at or near full power, like a marine engine.

For a 100HP engine (metric HP, but close enough for what we are discussing here) they were rated at 73.5kW for 5 minutes or 72kW continuous

Go to and look for the installation manual for details: d06847.pdf

They required 30kW of liquid cooling, 6kW of air cooling and 10kW of oil cooling for takeoff. Takeoff is the worst condition, especially for oil cooling because of this highest engine speed.

In my experience, this air cooling was largely unneccesary except at above 80% power.

So, if we think the GM 5.3 is more efficient than a Rotax 912 iS Sport, and it probably is, then an estimate would be 30kW of water cooling 6kW of air cooling and 10kW of oil cooling for every 100HP at the crank.

So for the estimate: for every 100 metric HP you make 73.5kW shaft power and need 46kW of cooling.

Again, the 5.3 may make less heat, it's a modern engine.

You say you cam make 400HP. That's at WOT, at maximum engine speed. Power requirements are very approximately related to the cube of speed. So at 80% speed, you will need very approximately 51.2% power. (and heat rejection). Or 200HP.

So at 80% speed at 200HP, you need 2x(30+6+10)kW cooling. Call that 92kW heat rejection.

You have 350,000BTU/h heat rejection. My online calculator and a couple of references make me think this is 102.5kW.

So. For a first guess- I think you might overheat running at full throttle at full engine speed. But you are probably fine for almost all operation.

This is just a guess- a slightly scientific guess with a reference, but just a guess. You could be OK at all speeds. I don't think this is a terrible place to start for cooling. But it doesn't seem overcooled.

Remember, this is just an opinion that you got on the internet, using a different engine as a benchmark.
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