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How important is exhaust back pressure?


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So.. I just bought a pair of SLP Loudmouth IIs for the Stang. That started me thinking about my '73 Camaro. I'm most likely going to try to replace the transmission in it early next year (hopefully my tax refund will be generous) The exhaust is all custom and welded up and that would be a perfect opportunity to put a better system on it.

Currently, I'm running Hedman Shorties to 2.5 inch pipes into 3 inch Flowmaster single chamber Delta Flow mufflers (Probably the equivalent to what they call the 10 Series off road mufflers but I can't remember the series number from when I bought them) then reducing to 2 inches for the tail pipes (because at the time, they didn't make the stainless tips I need in a bigger diameter) The mufflers are definitely staying.

Originally I wanted to do 3 inch all the way back, or at least from the headers to the mufflers. But I've heard different things about cars NEEDING to have some back pressure.

So here are my options: 1) Leave it at 2.5 inch with an X-pipe to the mufflers and reduce it to 2 inches behind the mufflers. 2) Go 3 inches to the mufflers with an X-pipe and reduce down to (preferrably) 2.5 inches (but maybe 2 inches depending on tip availability) behind the mufflers. 3) Go with 3 inches (and an X-pipe) all the way through.

The motor is built, but nothing radical. It's a SB 400 4 bolt main, 30 over (406) with a hot cam. It's *probably* running around 10:1 compression. Has a Holley 600 running into a generic Edelbrock aluminum intake and stock heads. (Honestly, I really haven't had the money to build much on the motor since it's been in there --- It probably only has, at most, 2500-3000 miles on it.

I don't pretend to be much of a mechanic... My skills mainly relate to 2 things 1) Taking it apart and putting it back together and 2) Researching the hell out of something before I execute it.

Edited by FUTURE_OF_GM
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I'm really leaning toward 2.5 right now.

I'm not making HUGE power, and I don't plan to make this thing into a strip car. Plus I need power down low (Since I'm running the 3:73 posi-trac AND 17 inch wheels)

Everything I've read says that 3 is for in excess of 600 HP or something that has to rev a lot. My car is neither.

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