Hey everybody, I joined here a while ago and have recently purchased a stablemate to my 90 STS, a 90 Eldorado touring coupe. I am planning to do some stock appearing mods to this one, first off the exhaust. I would like to do a true dual setup using the original square stainless outlets for a stock look. I have the engineering all figured out, but my question is about back pressure. Will the drop in back pressure affect how my engine runs/performs? Also, I am working on having a set of tube headers made by a friend of mine, but same question, would this have a negative affect on my engine performance? Thanks for any help!


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I see you have some truly high perfomance Big Block cars as well. You will know therefore that lowering your back pressure can indeed make more horsepower but the key is where. Often headers and duals change the torque curve and move the power higher in the rpm band resulting in poorer low end perfomance.

I suspect the idea is to make the car faster so reduced low end power may not concern you. Out on the highway you will be roaring!

 

Thanks for the info. That is along the lines of what I was expecting to hear. Do you think things would even out if I improved the intake setup on the car? I would like to retain some of the low end torque as that is what makes the 4.5 great, plus there is a 5 speed conversion down the line.

 

Others here have experience with Allante intake. It is probably hard to find but factory figures show it was rated at +20 HP over the one you have. You can also experiment with aftermarket systems like K&N. I also read that some guys see good gains with the Chevy BB 454 throttle bodies like the ones used on that vintage Suburban's when used on the Allante intake manifold. It all adds up. I think the 5 speed would allow you to keep the revs up where you can use them at launch as well.