I have been surfing this forum for over a month and have run countless searches but I want some more specific answers (if anyone has them). First off, I have decided to put the Flowmaster American Thunder Cat-Back on mated to a High-Flow (probably Catco) Cat. I know the TBI only has a 2 1/4" exhaust from manifolds all the way back. I am probably going to go with the Edelbrock TES headers in the future. Now, I have heard about switching to a 3" exhaust from headers on back, but I have also heard that on the 305 TBI it looses torque which offsets any gains in HP. So, with that said, shouldn't I just use a 2 1/2" exhaust from the headers on back? Also, what Y-pipe should I use? Any help is greatly appreciated!

 

I have dynomax headers with a 2 1/2 y-pipe, no cat, and the 3" Edelbrock Cat back with dual 2 1/2s. I didn't notice to much torque loss, but the HP gain was well worth the money

 

you never lose torque by losing backpressure. That's a myth. You will gain hp and torque all around.

 

Yes, get rid of the crappy stock exhaust and you will notice a big SOTP difference.

 

I was told by a bloke at a muffler store that 3" is too big for my car (350 TPI) and that I would lose power is he just talking ?

 

Hes wrong. Everyone here seems to do quite well with the 3 inch exhasut.. now maybe true dual 3 inch pipe.. thats a bit over the top, but a 3inch exhaust is perfect for this car and helps it breath all that crap out.

 

3" is just fine, anything less would be restrictive.

 

im sure 3" pipe would work great, considering we are running single pipe setups on our thirdgens, well most of us anyways

 

Correct. Since your power band makes more power at higher rpm's than before you think you have lost low end. If you were to look at a dyno sheet you would see that the low end is very much there.

 

The headers can create pulse waves to help scavenge the exhaust, and pipe diameter, as well as pipe length and other factors play
into that and can effect power.

However, further back in the system, especially at the cat and after, the less back pressure
the more power everywhere. Anything else is an urban legend. But louder isn't necessarily less back pressure, and often the cat is not as restrictive as the muffler, so no power is gained by removing it.

Troy
So Cal

 

Backpressure = bad.

You CAN go too large on the pipe diameter. When exhaust gases are compressed (like in a 2.25" pipe) they travel faster then when they are not compressed (like in a 3" pipe). But if you have an overly-large pipe, the gases will not flow fast enough (because they aren't compressed enough) and will clog the exhaust system, which will cause a loss in horsepower and torque.

That's why Honduhs and other imports have very high-pitched exhaust notes - their exhaust pipe diameter is too small. a Friend of mine has a Mazda Protegé5 with a 2.5" header-back exhaust (no cat, nothing besides a muffle) and it sounds quite nice. It's "deep, warm and fuzzy!"......