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Ok on the subject of backpressure. Backpressure increases low end torque while hindering your high end horsepower. The lower the backpressure the lower the torque. It doesn't mean that you will have NO low end torque.

In the case of dragsters, you aren't really talking about low end torque. When they launch, they are typically launching around 4000-6000 rpm. In that case you are just throwing all of the momentum of the engine at the rear wheels. Their low end torque is nothing like what you would want it to be if the car were a street car meant to be held at rpm's around 2000-4000. Drag cars are also working with 600+ hp & in most cases over 1000 horsepower.

I don't know how the decrease in backpressure will effect our 'Treps so I can't say whether or not what Valhalla 21340 is saying will actually hurt or benefit the car. I will say that 4" is WAY too big for even a 350cid Chevy, unless you are running it at rpm's reaching into the 10000 range. For our 3.5L engines I would say that 2.5 per side would almost be too much. What you should look at is the size of the exhaust ports where they exit the heads, measure the cross-sectional area & size your pipes according to 3 times the cross-sectional area you found. This should be a pretty close approximation to the optimized size.

If the pipes are too large the gases will expand in the pipes and actually slow down instead of speeding up. This could cause problems with hot spots in the exhaust system and make it wear out faster.

Ah and another thing, I would really like to see how you are going to fit 1 4" diameter tube under there, let alone 2.
 
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