For as long as I have been working with Corvairs, I have had a segment of ‘experts’ tell me their opinions about the Corvair exhaust such as ” It would make 30% more power with headers” All of these people were basing their misguided theories on ‘eyeball evaluations’ and the were just sure they were right. I knew they were wrong because I have testing on my side.
Below is a very interesting video showing how eyeball evaluations of exhaust systems are worthless. It shows a very potent 6,000 rpm V-8 on a dyno, in back to back tests where they flatten header tubes horribly, and it has next to no effect on the output. And that is on an engine making one and a half HP / cubic inch. The effect is even lower on engines like your Corvair flight engine.
Engine exhaust requirements depend on cylinder head design and camshaft design. Typically, low rpm engines like your flight motor, have modest cam profiles with short duration, to build torque. These engines are not punished for having the backpressure of a muffler, nor are they rewarded for having perfect free flowing balanced tubes. In our application, the systems we use are the correct balance of reasonable flow, matched to the cam profile, with the two critical factors: Low surface area and stainless construction to prevent it from heating the inside of the cowl, and having low weight and a stiff design that will not resonate and crack. The systems we offer are made of the best materials, with the best welding, to long proven designs. Sorry if reality offends the ‘eyeball evaluation experts’, but that is reality.
Get a look at some of our exhausts here: http://shop.flycorvair.com/product/3901-a-zenith-exhaust/
Read about our designs here: Stainless Steel Exhaust Systems
Above, An exhaust evaluation as part of an Electronic Fuel injection test on a 2,700cc Corvair in 2007. It is shown running at power on my dyno. The urethane wheel directly reads foot pounds of torque off the digital scale. Note the engine has headers on it, that could be swapped in minutes for other exhaust. The EFI allowed the air/fuel ration to be corrected to optimize the exhaust instantly at the twist of a knob, giving the fairest scientific evaluation of exhausts. The air / fuel ratio was read on a laboratory grade digital O2 system. The data conclusively showed that headers make very little difference on a Corvair, and EFI was not impressive either. Read more here: Testing and Data Collection reference page
4 Replies to “Eyeball Exhaust Evaluations”
William’s exhaust system has been on my engine since day 1. It still looks great, sounds great, and has been maintenance free.
That was fun!
William, The eyes don’t lie. Thanks for the link as it really helped a non-gearhead like me to be able to nod my head, smile knowingly, and walk away from the eyeball experts and think, “what a bunch of idiots”, instead of being fooled by pseudo-wisdom. Thanks.
Yes, people think exhaust design is a kind of “black magic” K.I.S.S. (keep it simple stupid) prevails. Cylinder volume has an effect on pipe length for proper scavenge but from there build em the way you like em. Keep in mind strength and ridgidity are your friend. I enjoyed the link and believe bashing the pipe creates a venturi and provides better velocity of the gasses thus better scavenge and in turn better cylinder fill. On a side note. As I write this I just saw the Mythbusters launch a 50 pound rocket 2900 feet powered with DOG S**t!! Naysayers be damned.