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Discussion Starter #1
I’ve been reading everything I possibly can find on the down pipe sizing for twin s475’s and I know the common concenses is to use 5” od downpipes with that said I’m looking for help on how bad going 4”od with a mufflers from front mounted twin s475’s on my 489”bbc would hurt my combo. Here’s some info on what I’m building, it’s a 3300lb 25.5c cert 69 Chevelle street/strip combo with a 10.5:1 489”bbc with ProMaxx 320 heads and custom Isky mechanical roller cam ring gapped and bearing clearances for up to 15lbs of boost, twin s475 turbos, dual precision 66mm wastegates and dual 50mm tidal bov’s. I’m not looking to set the world on fire with it but I would like to see mid 8’s from it. I plan on doing rear fender exists on both banks and 4” would defiantly be easier to route and has a much better muffler selection.

Thanks,
 

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I don't think you will see any real drop in power at that level as long as it's a high flowing muffler design. I have used straight thru Magnaflows and Dynomax mufflers with good success on NA street cars up to around 700hp. The muffler is more of a concern than the 4" pipe is.
 

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Discussion Starter #3
Thanks for the reply, I decided on going 5” od after pricing 4” and 5” od tubing from a local auto truck and trailer company, I can get 5”od 16ga aluminized steel for $8.30 per ft and my engine builder can get me 5” dynomax bullets for $60ea.
 
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