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Guys... c'mon. I know there are very few Supra owners who may have seen them intact and functional but trust me on this: The window trim IS A SEAL. Go buy one of the parts which is still available new and check it out. I've posted about this a few times in the past two weeks (probably all in response to the same person's posts... :P ) including details on how to more successfully reuse your old trim.

Grandavi, it's called "scavenging" not "back pressure". The velocity of the exiting air creates a vacuum which helps more effectively evacuate exhaust gases from the combustion chamber. For this to work properly the pipe mustn't be too large in diameter at any point. Removing turbulence and oscillations in the air flow help a great deal (accomplished with well designed collectors, as few bends as possible and a GOOD resonator); And the temperature of the exhaust charge must be kept from dropping significantly as it travels through the system lest it slow, increase in density and create a positive pressure obstruction. Ideally, you want gradually increasing pipe diameter (ie: 2.25" to 2.5" to 2.75" to a 3" exit). This applies to all engine setups except that with a turbocharged setup there is little to no scavenging to speak of between the turbo and engine while you are boosting.... here there is indeed back pressure. The larger the pipe the larger the cold charge of dense air you have to push out of the way to get things moving again... But you have a larger buffer for sudden increases in volume as you get with a turbo. Turbocharged setups aren't the exception to the rule but they bring other significant factors with them which minimize the significance of other certain factors.

I hope this makes sense.... half asleep here...