At what point do you need to run a 4 inch down pipe? Lets say the setup is a custom top mount set up, 2 liter b series. Looking for 500 on pump, and 600-650 on race gas. Is the 4 inch only necessary when your looking for like 800-900hp?

 

There are some cars in my area making a little over 700whp with a 3inch still. I'd say around 750whp is when you want to start thinking about it....

 

The way I see it, if you can fit it and it's not totally rediculous (like a 4" on a T25), run it.

 

Thanks for the info man.

 

x2. I wish I had done a 4" dp and tapered it down to 3" down the exhaust stream but ah well.

 

You can make 800whp on a 3" exhaust, doesn't mean its the most efficient.

 

I'm making over 500 on pump gas on a similar setup with a 3 inch.I would say its only wrth moving up to a 4 inch if the whole exhaust is 4 inch to make use of it and as long as the rest of the setup is capable of making the most power from the 4 inch downpipe.

 

I personally am making over 600whp here on 3" without any issues, one of my close friends makes a fuzz over 850awhp on his 2.0 liter 6765 evo with 3".

My 2 cents, 3" should be fine in your application. However if you can fit 4" and its not a bunch of added cost, go 4"... you probably would see a small advantage in peak power over 3".