I have a stock 305 TBI and it needs exhaust bad! The Cat is shot and so is the rest of the system beyond the cat. My delima has to do with the size of the cat and cat-back I will be getting. I am either going to get the Flowmaster American Thunder 3" system (part# 17233), or the 2 1/2" intermediate system (part# 17234). I don't know whether to get a 3" cat through the 3" cat-back, or the 2 1/2" cat through the 2 1/2" cat-back. For now, I am keeping the manifolds and stock y-pipe. I know the LO3 looses low-end torque (the only good aspect of the engine) from headers w/3" system all the way back. I want the loud powerful sound of the 3" but not the loss of low-end torque. Now, who has this system? What do you guys recommend? Thanks.

 

I've got a 3" Flowmaster American Thunder, and I love it. It sounds great, and is quite loud. Probably helps some that I don't have a cat though. I've heard that you lose some power with that big of exhaust on the 305, but it seemed like it had a little bit more get-up-and-go the 1st time I drove it, but it wasn't that much.

 

I lost some low end after putting on headers with my 3 in exhaust. With just the manifolds it was ok though. Just my 2 cents

 

I just put on a Dynomax cat and catback. It does honestly feel a lot better over the stock crap that was on it. I bought the cat and cat back from Jegs for $260. I thought of Flowmaster, but everyone has it and I think it just sounds a little rednecky-- for me.

The Dynomax pipe dia is 2 1/2" (intermediate and tailpipes) compared to the 2 1/4" that was on my car. The 2 1/2 is big enough for the L03; you should gain a little low end power with it. I too retained the stock manifolds and y pipe (although I am looking a new y). My L03 is bored .030"+, so I have a tiny bit more power to start with.

The cat I bought is a direct fit 'hi flow'. It flows over 30% better than a stock one.

The muffler flows 260 cfm, I think, compared to the 150 cfm of a stock. It's not super loud, but it does have a nice low rumble to it at idle and a nice 'rasp' when you rev it to about 3500 RPMS. I am pleased with the system so far.

I made a post in the exhaust forum on some install tips.

Hope this helps.

 

I would keep the stock cat for now and just get any 3" exhaust system built for TBI cars. That way it will have the necessary reducer on it to attach to your stock cat. When you go with headers, it will be much easier to match diameters with less fab work. That is what I did. Now if you were replacing the enitre system, I would recomend getting a system from headers back for a 350TPI car. That way there wouldn't be any reducers and it would all be 3". You will not lose any low end power at all with a 3". You will make more power at every RPM point in the band. it may feel like you have lost power but that is just because of the power gain you feel up top. Get a 3" system and don't look back.

 

Hmmmm.. Thanks for those encouraging words Shifty. Now that you say that, it makes sense cause I did gain upper end power & that would explain the difference of feeling off the line. Thanks, you made a believer out of me

 

Yea it is wierd because your power band just widens a bit. For the same reason why people think LS1's have no low end. They are just blinded by its monsterous high end. But if you look at a dyno power curve you will see that the low end is very much there and much better than stock.

 

most people gain 20-30 rwhp from a header swap. IIRC they actually gain torque too. headers are a very good thing on our cars, im workin on getting mine in right now, boy what a b!tch.

 

Dynomax headers, 2 1/2 y-pipe into cut out (placed where cat used to be) and finally a 3" edelbrock cat back

the difference for stock is definitely noticealbe,

loss of low end is BS, I'm not affraid to run any LO3s around my area

 

I am glad this was posted, because this was my question. I am hereing things that go both ways.

I am running a stock LO3 and I plan on deleting the cat and just running a straight pipe from the Y back to the muffler. Are you guys saying to run 3 inch with a deleted cat or 2.5?

When the muffler fell off I lost some low end. I want it back dammit.