I have a 1991 firebird convertible 5.0 TBI. I want to replace the exhaust after the cat with a 3" flowmaster system.

My plan is to swap in a 5.7 TPI next winter. Does that exhaust swap over? also willa 3" system bolt up to teh stock cat? I dont want to change the y pipe and the cat now because it will have no back pressure then.


someone pease help me out on this 1.

thanks

 

For the work involved, you may find that swapping over a stock L98 isnt really worth the trouble and cost.

As long as its a smallblock, all smallblock parts are the same. the exhaust wont care about the difference between a 305 and a 350 as far as fit goes.

A 3 inch catback will bolt up to your stock y-pipe/manifolds and it will bolt up to any shorty headers that include a y-pipe for the most part.

 

if u do not have emmissions take the cat off and do a full axhaust like infernal said, externally, 305 adn 350 are identical.

 

I wouldn't waste time/money/effort on a 3" catback - you'll never need it. My 350 runs great on the stock 2.5 inch intermediary pipe. By the time the exhaust gases get to the cat, they have cooled and condensed, thus they neeed less "room" to flow the same amount. Everyone's 3" exhaust always scrapes driveways and such, especially if you ever plan on lowering the car. I'll race any 500hp car with big exhaust any day of the week and win, if I get to pick the road ... they won't beat me on bumpy roads since their exhaust tends to bottom out and fall off

If you have a 305 TBI - you really need to save cash for a motor if you ever want to satisfy your need for speed! Get headers if you don't have - they will bolt right up to a 350 later. Skip the catback - money poorly spent so you can say "I have it" when you'll never "use it".

 

i take it u have not seen a few members 3" setups on here then. Orr's is pretty legit.

 

I have a American Thunder Flowmaster cat. back with a high flow Magnaflow cat. Even though I am running the stock manifolds and Y pipe I still noticed a nice seat of the pants change,also it sounds like a totally different car. Mine is loud and aggressive. I would never go back to the old stock cat. or cat. back. I also opened up the intake with a open element and between the two there is a nice difference.
And when it comes to headers, thous of us that live in states that are strict about the visual smog inspection like CA can not run headers, so the only thing left is the cat. and cat. back .Working on and making changes/upgrades to my car is not only great for the car, I have allot of fun making those changes."IMO" I don't see any thing fun about running a stock sounding car. As far as the 350 gos you can swap it over at that time. Personally I chose a professional paint job over the 350 for now.Even though It will take me a "long" time before I can afford a 350 swap at least during that time I have a great sounding car.I would go to Summit Racing on line and search for what you need or better yet give them a call and ask for a part # and then you can just put that in and it will show up on their site so you can look at it.1-800-230-3030. Ask about their free shipping and mention that you want one of their free catalogs.

 

Regardless of what some have said, a 3" exhaust (including a 3" cat) is probably the best power releasing path for a TBI or LG4 car. I personally picked up 35 to 40hp on my LG4 while still using the stock manifolds with a new y-pipe, 3" cat, and 3" cat back exhaust.

I never had a problem with the 3" exhaust scraping on anything - at least until I put headers with the 350 that hung down low.

 

 

Make sure OP knows its still illegal to remove a cat. Even if you don't have to go for testing, that doesn't mean you can legally gut the emissions systems.

 

true. but hey what isnt illegal anymore lol

 

I will say I'm very happy with my Hooker Headers cat-back system. It has the 3 inch intermediate pipe and 2.5 inch dual tailpipes. I'm running manifolds at the moment and it doesn't drag. I love the aggressive sound and it did help with with my gas mileage too. I noticed a slight reduction in bottom end torque but it made up for it in the upper RPMs. Anything that makes your car flow better will help you. As long as a person doesn't go too big. There is a limit to whats needed and a certain amount of back pressure is still needed.

 

Catbacks don't affect engine back pressure or exhaust velocity. Or at least not enough to matter. By the time the exhaust passes through the Y pipe it has cooled off and slowed down. Catbacks are for sound, the power gains are negligable.

 

For some cars, that may be entirely true.

But for the majority of cars, a properly designed cat back will reduce back pressure and increase power. I picked up 20hp by upgrading from a 3" cat back to a 3.5" with no other changes.

 

My stock 305 picked up considerable power and better mileage with a 3" HO intermediate pipe and Flowmaster, losing the cat (no emissions laws out here). WC T5 and 3.23's put the motor in a sweet spot at 70 mph, 1850 rpm, exhaust just purrs, sometimes breaks 30 mpg on the highway.
I'm hoping Hedman #68470 headers next week won't mess it up by reducing back pressure. Building a 350 with Lunati cam, Patriot heads, Weiand intake to drop in this fall to breathe through same exhaust.

 

Rbjones, sounds like a good setup. Headers should help you even more. I think alot of people don't realize that with a free flowing system and with the more power added, you won't have to run the engine as hard during normal driving conditions. These cars can make good gas mileage if you let them. Mine makes 26mpg on the highway with 3.73 gears, but i need to put the 3.23s back in the them. My rpms at 70mph is 2400 and its really a pain going down hills at certain speeds when you actually have give gas to stay at the speed limit. My Hooker Header exhaust really made a difference and I can't wait to get my headers on to see the difference those will make.

 

Firehawk1988--That's really encouraging that your 355 makes 26 mpg, a very efficient setup. My 305 gets great mileage on long trips to the Rockies and the Ozarks but it doesn't fit the car anymore--the car looks and feels faster than a 305 can provide, so I'm building that 350 for it, stock appearing.
Crane Cams tech line told me the reason it gets good mileage is my crude computer making continuous slight adjustments to my Quadrajet and HEI, that putting in a good cam would confuse it, that it would be best to leave it all alone until I was ready for a 350, so I did.
FAST Fuels will burn me a new chip for my stock computer to use the same carb and distributor on the new 350, economy to 2500 rpm, power from 2500 to 5500.
I recommend you put those 3.23's in there--my car had 3.73's when I got it and it was out of breath by 50 mph with the original automatic. The 3.23's with the T5 overdrive is perfect for bottom end and midrange power and gives that great sweet spot cruise all day.

 

DO what you want for an engine, but if you dont need to meet emissions just get rid of the cat, your local muffler guy would be glad to run new exhaust back from the engine for cheap, choose what mufflers you want (one muffler is like having one n
*$) and your good to go. all new stainless looks so good anyway.

 

Actually my 355 was in my 89 truck. It was bored 30 over, stock short block(factory cam) with the 88 350 tpi heads(truck heads were the swirl port design that robs power), SLP siamiesed runners, adjustable regulator and a 52mm BBK throttle body. Its better then stock but its not very radical for the street so far.

I have another 350 sitting on the engine stand which has more compression and a bigger cam that i want to use, but with gas prices i'm happy with the 355 thats in it now.

My car actually felt like it hit a brick wall once it got to 4000rpms and at 55-70mph its geared too high with the 3.73s. The SLP runners and the exhaust made a huge difference, cut back the bottom end torque some and the engines pulls better after 4000. I don't race it(yet) and i don't run it hard on the street but i'll be happier when i swap the gears.

 

My stock appearing 350 will be 9.5 compression, about 350 hp so I can use regular gas and the stock WC T5, not the fastest car out there but plenty fast, dependable, efficient and economical. These headers will be an acceptable street style modification, nothing radical, and should breathe right. I'll add a duct from the back of the stock air cleaner to the HVAC plenum in the firewall, all under the stock hood, even paint the aluminum heads, intake and water pump the original flat black. There are so many LS conversions I want mine to look original. I also have a little street rod with an old style high revving, high compression 327 to really bust people's butts, but an hour in that is enough--nice to get in the Camaro and turn on the a/c.