Originally Posted by
Mr Cox
So what mods did you do to the exhaust and what blow down numbers did you give the Irvine?
With converted car engines one get much better (higher) blown down numbers without any mods needed to the liner, and tuned pipes will really make a difference there.
You only get that in airplane engine if you by the ones that are timed for tuned pipes. Novarossi will offer both types/timing for most of their engines, and they are very well priced nowadays. The build quality is also well above the OS airplane engines. It seem OS are only making an effort on their car engines...
You are very right on all accounts. The car engines are timed better for tuned exhaust due to a lot of reasons - mainly because car engines tend to operate far above most aircraft engines in rpm. Typical car engines have around 195 degrees of induction timing, 120 degrees of boost and transfer port timing, and 165-170 degrees of exhaust timing. Blowdown timing often fluctuates brand to brand, but 15-20 degrees is about normal. Believe it or not, the crankshaft often has more to do with power production and delivery than the liner porting. Changing the liner porting can make a little extra power, but often it just moves the powerband. The phrase I've heard used is porting to the pipe. The magic usually happens when you change your open and closing of the inlet port to open earlier and close later giving more induction timing and the real power adder is boring the crankshaft. My last two modded engines I did for 1/8 scale monster trucks had 9mm carb's (adjustable with different carb inserts for racing) and 8.5mm crank passages. Boring the crank to a bit over 10mm wakes them up and helps them breathe (run harder). The manifold has proven to be detrimental to how the engine revs and where the power comes in. Standard tubular manifolds do okay, but the power comes on stronger across the rpm range with a conical type manifold. Of course most of this is tailored to car engines, but some of most can be extrapolated to aircraft engines however there will be trade-offs if you go too far. Airraptor is right about getting the blowdown right - an easy thing to screw up. Get the induction too high and you can get blowback from the exhaust.