This isn't 3rd gen specific but the priciples will apply. I have an 87 TBI truck (LO3 equivalent). the muffler fell off the other day so it's time for an exhaust. I am going to change to a true dual exhaust and I want to know if a dual 2.25" will allow enough flow for the 330HO crate motor I want to install later. For now I will be running the stock manifolds (I know get headers, I wish I could but I have twins due very soon) so I think the 2.25" will be the best bet for now. I did price dual 2.5" but I'm afraid I won't keep enough flow velocity with that big a pipe on the puny 305.

What do you guys think? I'm getting the exhaust done on Thur so I have to decide by then.

 

I forget the exact figures, someone posted a chart awhile back with the specifics.

But since dual 2.25'' is nearly equivalent to single 3''. I would say unless that crate motor will push out more than 400 hp, you'll be fine with 2.25 duals.

 

Thanks Mark, I know Dynomax has a hp to pipe size chart, but I thought their requirements were a little large.
I'm not looking for the crate motor to ever see 400hp in the truck, If anything I was probably going to end up "de-tuning" it a little by installing the LT4 prod cam so I can eliminate the requirement of my EGR valve and have a roller cam.

 

well, id say dual 2.5" wouldnt hurt too much for the time being, and heres a portion of a flowmaster chart

suggested pipe sizes for street performance
300-350 ci- single 3" or dual 2.25" for 250 hp
single 3" or dual 2.5" for 300 hp

350-450 ci- single 3" or dual 2.5" for 300 hp
single 3.5" or dual 2.5" for 350 hp
single 4" or dual 3" for 400 hp