Ok guys this is not your average cummins but i was wondering about my exhaust size. with my increased fuel output these days i was wondering if there would be noticeable difference in 3'' to 4'' but before you guys say yes there is a few issues

This is on a chevy. i have a 3.5 inch downpipe to the factory 2nd gen pipe that runs to the bottom of the truck. now heres the interesting part. it goes from 3 to 4 to 5 inch stack. the 3 inch runs all the way to the box and a 4to 5 reducer hooks up the stack. now what im wondering is if i replace the 3 inch with 4 inch will it really make that much of a difference? the 3 inch is about 6 feet long. Ive already got the pipe laying around so i was just wondering if you think it would be worth it?
im prolly close to the 300 horsepower zone thanks guys

 

A restriction is a restriction....

 

chain is only as strong as the weakest link

 

I gained about 2psi on a stock truck going from 3 to 4". Noticed a slight gain in power not much but EGTs were lower.

 

I'd say a 3.5'' downpipe is good, run that to 4" all the way to the stack and I'm willing to bet your egt's will go down.
Your exhaust is only as good as the most restricting part.... In this case a 3.5" DP to 3 inch pipe may be your restriction.
If you have the pipe then I'd go for it and report back
The firewall comes down so low on a 73-87 chevy I'm guessin you don't have room for a 4" DP, not that it would make a big difference over 3.5.

 

Especially since the turbo outlet is only 3"

 

At one point I was still running 3" exhaust with the fueling in my sig. When I went to the 4" it made a drastic difference in how the truck performed. My turbo spooled quicker and smoother across the board. I'm able to hold my foot to the floor almost as long as I want (w/out a serious load), I can cruise at 80 up through 100mph w/out feeling as though the engine is wound out and something is chocking her out. I used to feel as though 70-75mph was max cruise speed, cause diesels can't handle the rpm. The 3" exhaust was restricting tons of flow with heavy fueling chocking out the engine and making me think it was just too many rpm's. Man was I wrong about how much flow affects hp. I knew it made a difference, but with heavy fueling it makes the difference between good hp #'s and low #'s. Just my $.02.