Ive got a 89 Firebird formula with a 305 TBI. I need to change the stock exhaust and can pick up LT headers really cheap. I dont want to spend alot of money on this so i was just thinking of putting 3 inch piping from the headers straight into mufflers and having it dump out right before the rear axle. What kind of clearance issues will i be dealing with? is there anyone that has done this and can give me some tips on mufflers to buy?

 

Depends upon which LT's. If 1-7/8" primaries, your TBI probably wouldn't be too happy with you. If 1-5/8" primaries, not quite as bad.

However, think about it: Those with duals put H- and X-pipes in the system to make it "better". Especially with the X-pipe, what happens after the X isn't as important as the location of the X itself. Mixing the two sides is what provides the benefits, and a y-pipe with a larger pipe after the Y is just as good - at least at the typical street power level.

In the case of a TBI engine, there's no way dual 3" exhaust is going to work better than two 2-1/2" that're joined into a single 3". It simply won't "need" that kind of flow capacity.