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Mr. Bradford wrote: You actually want the exhaust system to scavenge when the exhaust valve first opens, but want back pressure just before it closes. This helps suck the burned fuel out of the cylinder, and promote flow. just before the valve closes, you should actually be sucking the new fuel air mixture out of the exhaust valve. The back pressure then pushes this mixture back into the cylinder... The problem is that it only happens in a narrow RPM range. -------------- Correct. Which is why we paid attention to that fact as racers. You ended up "tuning" the header extension to a torque peak. The extension still does not effect the flow and/or backpressure as much as the size and length of the primary runners. I realize that this is not readily applicable to most of our cars, since we rarely put them in this situation. But my mind still runs on alot of those old principles that were instilled from that time. Header size/length, muffler style/size will have little effect on most peoples cars, therefore the "glasspack" muffler has become popular because of the sound. But for those that are building a larger engine, designed for more performance, then it starts playing into the picture. The backpressure does not push the fresh mixture back into the chamber, as much as it inhibits. This is also much more related to cam overlap and lobe centers. Thanks for pointing out that this theory is pretty much limited to high performance scenarios. Brian Schlepp 69 Fasty - "Purple Passion" with more growl than bite ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/