I am going to replace the exhaust in my car. It is a TBI 305. I am plannig to do some mild upgrades to the engine: LT1 cam, 081 heads (ported) with new springs, modded TB, spacers, etc and a custom tune. I have a friend that installed a 3 inch catback with Hooker headers on his TPI 305. He says the car doesn't have enough back pressure, and thinks that a 2.5 inch would make the car take off better. I am doing Hooker SuperComp shorties, and wanted to know what size cat-back to get to maximize performance.

 

Most likely a speed density car. Increasing the flow threw off the PROM tables. Clean that up, it'll have more take off than ever.

TBI will suffer the same fate. PROM tuning.

Oh, yeah - go with the 3".

And do some PROM tuning. . .

 

Would i have that same problem on my 86 MAF TPI w/ 24lb injectors, ported &polished plenum, high flow runners & high flow vortec baseplate w/ vortec large port heads &358ci forged bottome end? with shorty hedders..

 

MAF = Mass Air Flow. It senses the mass of air flowing into the engine. It doesn't have to make any assumptions about the conditions surrounding the readings it is getting - like SD has to do.

 

You don't want back pressure in an exhaust system. You want the exhaust to be able to scavange. Go with 2 5/8" headers and the 3" catback system and you will be good to go. Like was said above you will want to reprogram for best performance. MAF is more forgiving.

 

Not trying to jack the thread or anything, but what do you think about this?

305 - 350 swap- Went for Exhaust quotes- Are they tryin 2 screw me??
OK- I did the famous swap in my 86 Iroc, I went with 1 5/8 primary Summitt shorty hedders w/ 2.5" collecters.

Ok- So i went to one exhaust shop here in town, took pictures of peoples FULL True Dual exhaust setups (from the hedders to the rear bumper) and told him which ideas i liked best and if he could do it.
He tells me - Well- I can do it for you, but its going to cost you $750 w/o the mufflers or tips b/c its going to be 10-12 hours of labor and we arent even open that long in one day.

and i asked him what he would reccomend- He then told me - Well, i can make you a custom 2.5" Y pipe from each hedder, run them together into a single 3" pipe back to a Flowmaster 3" single in 3" dual out muffler w/ no tips for $350.

I told him i'd talk to him later and walked out the door.

He also said to me that a true dual 2.5" setup ran in the stock location couldnt be done b/c it wont fit through the opening in the draw bar. ( I looked at the draw bar space in the back, I could pass a football through there )

And another place here in town told me 1. they cant do my setup b/c im not going to run cats. ( we dont test for emissions here, and there are so many vehicles in my town that dont run them, but they bein picky) and 2. if i ordered a full Borla , or Flowmaster system theyd put it on for me for $200..
and i was like - Umm- I think i'll save myself the $200 and put it on myself.
and then they said - Well, we can build you a dual rear section for $400 and you can put your own front on that way we arent liable for you running no cats. ( that is with me supplying the mufflers!!!)

SO- what do you guys think i should do?
Thanks

 

The pricing seems a little high, but around here there are about 8 shops within 25 miles.

As for the duals, that is what everyone I have talked to said: They can't do it/ it can't be done. I took pictures to one guy, and he said that he would be afraid to do it for me due to he didn't have the right equipment to mandrel bend and that he didn't know how much it would cost to do it. I like that answer, at least he was honest.

Another shop said you had to beat in the floor plan in the front...? Uhuh. There is a BMR upper panhard bar (draw bar) that increases the clearance quiet a bit, but it is $220. I think that I am going to get a Loud Mouth or Borla cat-back with Hooker Super Comp Shorties for mine. Might do duals with LTs in a few years for the S/C LS2.

 

lord how many posts are we going to have about "not having enough backpressure." Click the backpressure FAQ link in my sig, it'll let you know why your buddy thinks it's slower with a 3".

true duals can be done, but most shops dont want to do them. Anyone that tells you they aren't possible just doesnt want the hassle of doing it.

I'd recommend a nice single 3" system with your muffler of choice.