Big Block Exhaust Question - Impala Tech
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post #1 of 3 (permalink) Old 07-22-2009, 04:09 AM Thread Starter
Join Date: Jun 2009
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Big Block Exhaust Question

K guys, I have a question for you big block guys. I'm going to be setting up an exhaust on my 67' Biscayne, and for now I will be using the current y-pipe on my 307 (someone had one done at some point and is fairly decent in size, 2" pipes from collectors into 2.5 y-pipe) into a single inlet muffler with dual outlets and running 2 Z28 style tips exiting right before the rear tire on the drivers side. I have the tips setup in there now and they look pretty good IMO. In the future, I'm thinking this thing might get a Big Block. It'll be awhile but wondering if I could run a single 3" off a y-pipe on a big block or would it be too restrictive? The BBC would be a torque monster, not a high HP mill. I'm thinking maybe 350-400hp and looking for high 400's in torque (should pull my car trailer just fine lol) if I end up doing this. I'm thinking it should be too big of a deal, maybe lose a few on the top end but anyone else want to chime in? I know guys making 500+ on small blocks through a 3" y-pipe, but don't know too many with big blocks. I'd be going for a stock looking engine under the hood and would be using some cast hi-po manifolds on it. Thanks guys!
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post #2 of 3 (permalink) Old 07-22-2009, 09:48 AM
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The BBC is already exhaust restricted in terms of the short side radius in the head (since the motor is so wide the engineers have the exhaust running through an almost "U" shape inside a 1.76" port to get it moving down and out) that is why almost very aftermarket head raises the exhaust ports 0.3 to 0.6 inch. So I doubt if the 3 inch "Y" pipe would be any worse than the factory choke point. Of course the factory has been working on the exhaust side since the first square port hit the market in 1965 and going to a round port with the introduction of the open chambered combustion chamber in the 1968 L-88 second design aluminum ZL-1 head (3946074), finally going back to a square port with an added vane in the 'C' shaped port of the LS-7 1970 454 (part number 12363425) that never made it into production. 1974 was the last time a BBC was changed and they introduced the small "peanut" round intake port with the old style restrictive exhausts that have been on every BBC since 1975.

Big Dave
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post #3 of 3 (permalink) Old 07-22-2009, 07:09 PM Thread Starter
Join Date: Jun 2009
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Ya I was thinking a dual 2.5 into single 3" y-pipe would work, especially since I will be running stock manifolds anyways if/when I do it.
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