The exhaust coming off my single that will be on soon is 4", I am not sure if I should do 4" all the way back, or if I should shrink it down to 3" after the down pipe?

The pipes have to be made, I have a HF cat that is only 3" inlet, so I'd have to shrink down to 3" to use the high flow, or just run straight pipe all the way back. I figure all the emissions crap will be off, who needs a cat... haha.

ANyway, juts need to know if 4" is too big or not...

THe turbo is basically a T78

 

Basicly its loudness vrs power. You should make more power with a 4" but its gonig to be really loud.

 

do i need some backpressure to help the spool time? or will back pressure have an affect on how fast my turbo spools?

 

Heres pic my 4'' compared with stock


Its loud but mostly because of my street port....

 

if you know the flow through the engine you can readily determine if a 4" exhaust will net you any significant gains using a basic pipe headloss spreadheet

my guess is you need to be pushing at least 400+ hp to see a significant gain, except at the very top of the rpm band

 

I'm running 3" downpipe, 3" midpipe, and Apexi GT catback almost 4" catback. No cats. I wonder if I switched to 4" how much of a difference it would make. Running no cats I figure would already be pretty free flowing.

 

I should be pushing 500 on stock ports... 550max at around 20lbs, but street will be around 13lbs somewhere around 400hp

 

damm what turbo? all on stock ports thats frigin crazy power on ther street i know i'd get in a lot of trouble with that setup

 

I am in favor of running a cat, since the car is so smelly without one. I also want my car to be relatively quiet, as in only 5 times as loud as a "normal" car, rather than 20 times as loud. I know those aren't popular sentiments on here, but after running a smelly, really loud car for a while, I am sick of it and want something a bit more comfortable.

It sounds like you would benefit from a 4" exhaust, so you might decide that it is worth it to run the larger size. I bet you could get one of those metallic cats (with a 4" body) and cut the smaller inlet and outlet off to put a cat in your 4" system. I believe that there are some 4" high flow ceramic cats available as well. You can also get mufflers for the 4" system, but they are likely to be loud ones. Perhaps with a cat and small muffler in the "midpipe" area of your custom exhaust, and a large rear muffler, you could create a very high-flow but livable exhaust system.

The cat and muffler options are more plentiful with a 3" system, though. And I am sure you can make it quieter than the 4" system. Unless the car is a dedicated track/drag car that you don't really drive on the street much, I would go for a 3" system. If you live in an area where the cops are quick to ticket loud, fast-looking cars (and especially if they will make you pop the hood looking for emissions compliance), the 3" system is almost certainly a better choice if you want to be able to enjoy the car on the streets.

-Max

 

unless you're running 500+ hp. or a 20b, running a 4" exhaust probably won't help you much. at 350 or 400 rwhp, you'll probably have a little less power and a slower spool time with a 4" rather than a 3", because of the heat that will be lost as the exhaust travels down the system. the 4" will flow more at a given temperature, but run the same volume of gasses through a 3" and it will retain more heat, becasue it moves faster, because it has less volume to fill in the pipe. i would go 3", because very few people on earth need a 4" exhaust, and they're running 650+ rwhp.

 

Just doing some more research on cats and found this... Random sells metallic cats with 3.5" and 4" in/out pipes according to their web site (http://randomtechnology.com/products.html).

-Max

 

I am running a Mitsubishi TD08 (Greddy uses the TD08 for their T78s), it isn't hard to push that much power... but I am only using 500+ for drag, street will be in the low to mid 400 section. Friend was pushing 500+ on stock ports with moderate boost, his setup is pretty close to mine.

I just dont want to have to make the downpipe 4" then convert to 3" - since the turbo exhaust is 4" I Thought I'd go straight through... but I run into the same problems you do with tires.. the bigger they get, the more expensive =P

 

Remember that the exhaust cools off a lot, so shrinking the diameter a bit after the downpipe might not be as bad as it seems. Keeping the velocity up with a smaller pipe might even help spool/boost threshold and low end torque.

-Max

 

alright, I Think I'll do the 4" to 3" conversion, i'll bolt on the highflow and have my midpipe to bolt on whenever... I'll let you all know how it turns out when it's finished =P

THanks