[Ron1]

6 to 8 inches is plenty before the turn down. I looked at mine and it's about 3 inches before it makes the 45 degree bend. As far as back pressure, any, will rob power. What we should be talking about is velocity. The larger the exhaust, the velocity is absorbed by the larger pipe and slows things down. Does that create back pressure? I don't know if that can truly be defined as such. I'm not an expert but I do know this, the engine builders I have been around and some really good ones, believe that you want it freeflowing with VERY low or no backpressure. I think about it in very simple terms by visualizing what is happening inside that air pump. If it takes force to scavenge the exhaust, that is power being lost.
I think that this is why we use crank scrapers and windage trays, titanium keepers and valves, ultralite lifters and knife edge our cranks...to get weight out of the rotating assembly. It will RPM quicker and get into the power band quicker. The last thing I would want is to make some of the pistons work against a force that will keep them from reaching TDC. It can't be good for the motor.

But I have been proven wrong before...but let me leave you with a quote from David Vizard (Volume 1 ) "How to make HP: " Over the years I have heard many strange claims from so called experts. One I have heard proclaim more than once that a performance engine or any convential 4 cycle engine-needs some backpressure to optimize efficiency. That statement was obviously made by someone who had never used a dyno or had not carefully evaluated how exhaust systems flow affects performance. Let me assure you that by minimizing exhaust sytem backpressure, 999 out of 1000 times you will find more power. Keep in mind however that the air/fuel ratio and ignition timing are carefully optimized"

If you don't know who David Vizard is...well never mind.

Ron