We experimented with our cowl last summer to get better cooling during extended climbs.  As many know, the summer days in south central and interior Alaska does get hot from time to time.  

The engine has always run great, and with no cowl at all the coolant temperatures stayed below 195ºf during extended climbs.  Unfortunately, with the cowl we could hardly climb more than 800ft at a time without the temperatures reaching 220ºf.  At which time we level off to cool down.  We opted not to use a Naca duct on top of the cowl, but did open up around the spinner.   We also tried different mods such  added inlet louvers on the under side of cowl, and inlet louvers on the inlet sides of radiator cover.  None of this helped.  

Then, we put heat wrap around the exhaust, and added cooling louvers to the sides of cowl.  This seemed to lower the temps a little, but still not enough.  

Suspecting there may be to much air turbulence and restriction with air trying to get through the cowl by going around and under the flywheel, we removed around 2/3 of the cowls underside.   Doing this made a significant improvement by allowing a more direct airflow to the radiator, and likely allows more airflow around the engine as well.  

Believing there's still to much turbulence at the radiators inlet, we copied Mike Hille's 750 by adding an inlet air scoop to the radiator.  Unfortunately, we won't be able to test the effectiveness of this until next summer. 

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More photos of this mod may be viewed at facebook Experimental 750ES


Thanks for sharing this.   Based on what you and Jonathan have shared there seems to be some real value in working out a good solution for cooling with this engine and cowling combo.    I was wondering based on the picture, the opening of the radiator school is not larger than the outlet is it?   I dont have FB so be able to see the other pics.....

I might be worth setting up a pressure ga to the inlet of the radiator scoop to see how much air you are actually getting in to that.   

Either way, thanks for sharing, I look forward to hearing more about your trials and solutions, good or bad...


We have an abundance of information on our facebook page.  I thought you could still view facebook posts without being a member?  

send me a  link and Ill try.....



Thanks.    I clicked on the link and as luck would have it, the laptop I was using still had my wife's FB account so it opened it under her log in.  

I am not a Zenith builder.    And I am no expert, but it seems from what I have read, the exit of a coolant plenum, is as important as the inlet.  Most other aircraft have the outlet, or exits in a negative pressure location.  So it just seems that the outflow of this design where it is 90 degrees agains the airstream is a challenge right from the beginning.  But I know that some who have this set up, report no issues with the cooling, but others struggle.   

On my RV6A I followed a radiator plenum that others have had success with where the inlet is in a positive pressure area, and the exit is in a negative pressure area, and the exit is larger than the inlet. But...I am not flying yet so I cannot report how well it does or does not work.  But I can tell you that even with just the prop flow over the radiator, that I can run the engine at 20" of MAP with no over heating at all, just sitting on the ground.   

If I had a set up like yours, I would be tempted to refplace your Louvers with an exit in a negative pressure area, by way of having the opening facing rearward.    I bet it would not have to be large at all.   The width you have and probably only 2-1/2" tall would probably do well.   And yes the large opening in front of the flywheel, probably counter productive.  The inlet below the cowl by itself is more than large enough without the air from inside the cowl.    Just my 2 cents.  


I believe most aircraft using the Viking 130 do not have as much drag as a 750 STOL, and therefore they should have better extended climb performance before seeing high temperatures.  Our 750 has excellent STOL performance, but we're also looking for better extended climbs.  

As you can see from our earlier pictures, all air to our radiator had to flow around the flywheel and through the cowl.   Our new modification completely bypasses the cowl to allow an unobstructed high pressure area to the radiator.  By the way, our radiator outlet louvers do face aft.  



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