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I am making the swap from nitrous to turbo and getting the Huron Speed T6 single setup but I will be having a 4" downpipe made out to a 4" Mufflex exhaust system. My question is that I will be putting a cutout just after the firewall and wonder if I will be giving anything up if I used my existing 3" cutout to save ground clearance or should I just buy a new 4" cutout? Engine is a forged LM7 bored out to LS1 specs and will likely be going with a billet S476. I would like to see at least 800rwhp and pretty confident that won't be an issue.
Ofcourse you would want an equal sized cutout as the rest of the pipe for best results.How much could the 3inch cutout hold you back ..who knows.Thats a answer you will find on a dyno and how well everything works together as a setup it could be fine most likely.You can always change it out later.Use what you have and see how it goes.
Take a backpressure measurement with all three conditions. Open downpipe, full 4" exhaust and with the 3" cutout open. Might be surprised! Only real way to know.
Btw, ForceFed86 ran a 60mm wastegate on his downpipe as a cutout. That's less than 2.5". The rest of the downpipe went straight into a fairly small muffler.
With the restricted exhaust and a 60mm gate, he didn't start loosing power until he was running 20+ PSI from what I remember, and was still around the 8s at that point.
Btw, ForceFed86 ran a 60mm wastegate on his downpipe as a cutout. That's less than 2.5". The rest of the downpipe went straight into a fairly small muffler.
With the restricted exhaust and a 60mm gate, he didn't start loosing power until he was running 20+ PSI from what I remember, and was still around the 8s at that point.
I saw a 70whp gain (also gained 1 psi) with the cutout open on GATE (10psi) on my single set up. But, it was an admittedly cobbled together and overly restrictive 3 inch exhaust 3 inch exhaust. It all really depends on the car.
Just put a good muffler on it like this one. It made basically the same HP as an open down pipe,well within about 5 hp on a 700+hp setup. If you had a 4" cut out, you might gain more but a 3" strait through muffler flows pretty much the same as a cutout. You can check out the design on the Borla Pro XS mufflers on their web site if your interested.
I saw zero noticeable improvement in trap speed going from a 2.5" glass pack and 60mm WG. VS an open 4" pipe about 14" long.
I literally welded around the body of this 2.5" muffler to a 4" pipe. the side going into the 4" pipe is necked down to 2.5" as well. So it was pretty restrictive (and nice and quiet).
Added a 60mm gate to it that opened around 1psi. (wasn't even positioned well).
Then tried this open 4" pipe. Picked up zero MPH at the same 20lb boost level on a 5.3 S475 combo.
IMO large exhaust systems (3"+) are a waste of weight and noise. Mufflers most of the time aren't needed either and are also alot of added weight for nothing. You can run a 2"-2.5" open crush bent pipe after the cutout and it cuts the noise levels in half. Run it to a small oem style muffler and you can cruise around with stock like noise levels. Then open the dump and make tons of power.
This is literally my entire exhaust system on my street/strip car. 4" down pipe, 4" cutout, and a 2" pipe. It's quiet enough that I can talk on the phone in the car easily and weighs very little. I added a tractor muffler to the 2" kick out and it was super quiet, but I've since removed that for weight savings.
Thanks for the info. I am going to do some street car class racing and sometimes it requires full exhaust so I already have the Mufflex system but might make something simpler to have two exhaust systems.
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What size cutout on downpipe? 
