How to calculate glycol Radiator needed to cool your engine ???


This is a very interesting question, especially if your engine shows signs of overheating while inflight...

I built a CH701 and, for the engine package, I decided to go with an auto conversion done by a pro.

The engine is a Subaru EA-81 reworked to produce 105hp at 5600rpm. It came with its matched redrive and it works pretty good.

The preferred water cooler for this setup is a radiator from a diesel VW Rabbit/Golf but I was told that something else, much smaller, could be used. Even though the radiator I installed wasn't so small, I never knew IF it would keep the engine cool enough. Well sure enough, it didn't.

After the first few test hops, I started wondering how, without much trial and error, I could get the right radiator size...

Searching the Internet I came across this chart. It is NOT PERFECT but gets you a BALLPARK figure for sizing a radiator. Mind you, it is made for cars but aren't I using an auto conversion engine ???


So, I decided to test this chart with my engine and see what the numbers would look like.

Now, my EA-81 is 1800cc which is 100 cubic inch. So, as a starting point, we need 200ci of RadCore (2ci x 100 engineCI).

For my engine, I had to :


-ADD 0.2 for small engine in heavy car (engine working harder in plane)

-ADD 0.3 for no shroud

-ADD 0.2 for high outside Temps (high revving engine)

-SUB 0.1 for remote oil cooler

-SUB 0.2 for horizontal flow radiator


So, I had to ADD 0.4ci to the 2.0ci already given which would give us a 240ci radiator size. The VW Rabbit/Golf radiator has a 19 x 12 x 1 inch radcore which amounts to 228ci. Our calculation comes pretty darn close to the real life situation...

Now, my original radiator was 13 x 10 x 2 inch which comes to 260ci so why did it fail to cool my engine enough ???

Well, this radiator was actually 2 radiator core rows (1 inch each) one behind the other. This means that the 2nd core row is BEHIND the 1st core row and is receiving WARM air. It isn't cooling as much; only at 50% efficency.

This means our calculation becomes (13 x 10 x 1) + (13 x 10 x 1) x 50% = 130 + (130/2) = 195ci. 

So, for short bursts of full power use, it could of been OK but that's not what we're aiming for.


I had a HIGH performance radiator made using 16 x 10 x 1 inch for dimensions. It has 2 rows of radCore, so the calculation becomes (16 x 10  x 1) + (16 x10 x 1) x 50% = 160 + (160/2) = 240ci which should solve my problem!

I then decided to check the radiator on a Rotax 914 to see if the calculations would hold. So, the radiator I saw on a Gyro was (16 x 8 x 1) = 128ci which should be way too small...BUT wait, the Rotax 912/914 engines use AIR to partly

cool down their HEADS (fins) so it is fairly possible that the radiator they use should be that small.


Other things to watch for, in no particular order, when thinking of cooling the engine :


- use a remote OIL COOLER to keep oil temps low (you can always block air passages if too cool)

- make sure air coming in the radiator has plenty of space to get out (twice as much coming out)

- do NOT use a thermostat (they open too late - burn your engine)

- use FULL SYNTHETIC oil (20w50)

- use HIGH octane gas


DON'T QUOTE ME ONE THIS, do your homework and if you find better ways to calculate this, let us know.






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