Firewall Forward

I posted this yesterday on Vansairforce.

"I hardly have a problem but am trying to operate with the CHT not more than 400F in the climb. At about about 135/140mph near gross I can make about 1400fpm with everything forward. (160hp O-320 MT 2blade c/s prop, P-mags.) After around 3000' I then have to change something or I break my self imposed 400F CHT limit. I would however like to be able to climb with everything forward to the stratosphere, or at least 10000'.

One solution is to get the speed above 150mph which of course cuts the climb rate. Another is to reduce to around 25 square and keep the same speed. The second is the more effective because I suspect the P-mags retard a little which more than compensates for the reduced fuel flow.

I dont have an oil temp problem. In fact next time the top cowl is off I will reduce the air flow to the oil cooler to get the oil temp up a bit leaving more air for the CHT.


I am wondering if I should cut some of the bottom cowl away. It is clear to me that with the 4-pipe exhaust there is less air outlet area than inlet area. This may well be the real issue but once its cut its gone for ever."

I had a number of interesting replies, though no one was convinced cutting the cowl was the solution, although several agreed the outlet should be larger than the inlet.







My idea is to cut straight across from the left hand pair of pipes until it meets the slightly deeper cut on the RHS.

















I guess when I posted this I was expecting to hear of others who had enlarged the outlet on a -4.