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Right now I have a 4" downpipe that goes down and just past the trans. right after that the floor tends to drop down more than the DS does, so I lose some clearance. I was orig toying with oval 4" for a section, then back to round. Would I lose much if I went to 3.5" round, as opposed to the 4"-It will go all the way to the bumper. The oval gets pretty expensive. Running a vs 80 mm turbo, but its not a max effort deal, but dont want to cause back pressure problems either. Another idea was use 4" and keep it close to the floor, and wrap it. Car isn't slammed on the ground, but every bit helps.
I wouldn't think you'll have any backpressure issues until you get way up in power. Can you do what I did and make your own oval with some thick flat stock and a hydraulic press?
Why not weld in a cut out a short distance from the turbo, then uncap it when you want more power? I'm planning on having a stock location single 3" exhaust on my Formula when it's done, and I'm going to weld in a dump tube so the smaller exhaust is a non-issue.
I am in a similar boat and currently exploring an exhaust cutout that is boost activated. I am also waffling between 4” all the way back, 4” oval or just running dual 2” or 2.5” all the way back. Economically I think running dual 2” exhaust (side by side) would be cheaper than all the oval stuff.
I am in a similar boat and currently exploring an exhaust cutout that is boost activated. I am also waffling between 4” all the way back, 4” oval or just running dual 2” or 2.5” all the way back. Economically I think running dual 2” exhaust (side by side) would be cheaper than all the oval stuff.
mathematically a single 4” flows more than dual 2 or 2.5” pipe. I’ve never gotten to get something on the dyno and back to back test it or anything to see if it matters, so take it for what it’s worth.
I ended up with 4" all the way to the rear bumper, since my car isnt slammed on the ground clearance was good. I used a small straight thru muffler before the rear, and a large diesel muffler in front of the bumper. Getting it over the rear was tricky. Still louder than I prefer, once I take it on a longer drive I will see how it sounds. They make a "quite" diesel muffler, still straight thru, just diff design (and $100 more than the one I have)
, lol.
Not knocking it… I’m sure it sounds great. Looks nice too! Think about the total weight, cost, and time and effort into that system though. Weigh that VS a $120 4” dump up front and some crush bent 2.25” all the way back to a dinky OEM muffler. I get not everyone wants a quiet car… but damn. I’ve built some 3.5” systems in my day and it was challenging and expensive to say the least. Also was still crazy loud IMO with a typical “turbo muffler”.
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Exhaust downsizing. 
