4"DP/MP and 4" exhaust >3" exhaust (qtp)
 

So i currently have 4" dp> 4" oval magnaflow muffler >4" round Mangna flow muffler. on a HBP. its insanely loud. I thought switching to the oval muffler would help but just changed the tone...

So right now im looking to do a 4" qtp cut out. so it would be
4" downpipe> 4" midpipe with resonator/muffler > 4" QTP reduced to a 3" with a racing beat single tip exhaust...
so when the cut out is open it will dump out 4" before the 3" piping.

would it be okay to run the 3" exhaust untuned? i was searching and came across on the evo forums people claiming that dumping right after the dp causes overboost and extra wear on the turbos because of more flow. this is the first time i heard of exhaust having that effect so now im worried about the results. will the back pressure of the 3" exhaust have significant effect?:scratch:

You should get it retuned after that.

The point of a cutout is to reduce exhaust restriction and therefore back pressure. So yea if you have a full exhaust and throw a cutout in (as it sounds like the EVO guys were talking about) they could have a boost creep if their wastegates couldn't keep up with the larger flow.

Your situation sounds different from the EVO guys situation. You already have a 4" exhaust all the way through and are tuned for it. Changing out the end for a 3" section and adding a cut out there shouldn't be much of a concern. When the cutout is closed you will have less exhaust flow and might run rich. Which isn't ideal but I doubt it would be enough to hurt anything. With the cutout open it should be pretty similar to your current 4" exhaust. This is assuming your cutout is after your resonator sections. If the cutout is right after the downpipe that's a different story. Bottom line IMHO it's a probably a good idea to get a retune, but I wouldn't be overly concerned about it.

In calculating the flow numbers at 1100 degrees (very conservative) and at 7000 RPM and a VE of 3.0 (300%) there really is no reason to have a 4-inch tube for the short run that the car needs; 3-inch exhaust system is adequate. Now stepping from a 4 inch tube to a 3 inch tube there will be a huge restriction due to the change in flow direction and concentric flow reduction to a point somewhat downstream of the transition (in fluid dynamics terms it's called the "vena-contracta"); which will realize a restriction less than the smallest pipe size. Assumed here is a high Reynolds number for fully turbulent flow (no other way to look at).

If you are after looks only, well, that's a different story....

well i definitely will be getting re-tuned, but i got a little scared "for the time being" rxtacy thanks for the help!
ttmott, that was the most scientific answer ive ever received and have no idea how to comprehend any of it hahha

im after noise reduction for one solution and performance for the other.

how to maps on the PFC work? is it possible to have it tuned for the 3" exhaust at 10psi and then at high boost tuned for 4?

Senor Bubbles,

you should have consulted with me before spec'ing out your exhaust mang...... i've posted a few times about my original full 4 inch unit. 4 in DP-->4 in res MP (circular magnaflow can)--> 4 in catback (another of the same magnaflows)

The car was LOUDDDDDD. It droned horribly on the highway, and on my initial trip down to DGRR back in 2006 poor Ihor rode shotty and almost lost his hearing. He hasn't ridden in the car since.

I eventually switched over to an Apex'i GT Spec rear section (3.75 inch with a large muffler) and it quieted everything down so it's tolerable but still sounds mean as a mother on track :D

Sorry for hijack but my question is similar to OP's question. I have 3.5" dp. Can I do it 3.5" all the way and still keep it quiet without losing hp? Or should I go 3.5" dp -----> 3" rest?

zomg. my car is soooooo quiet. i can now have civil conversations in the car with the windows down.................................... just did the RB single... so theory works. now ill pony up the cut out........................:(

yeha rich, but you know im too cheap to pony up for the apexi muffler lol.

also on a side note. the smell has gotten 100 times worse.

any updates with this and the cutout?

Remember heat makes atoms expand so they need more area like a downpipe and midpipe but when they cool you dont need as much area so you can run a 3" catback with no issues and will flow more than a full 3" system

negative, i copped out. too expensive, im rocking a racing beat single tip now.. i may decided to do this later on...