arozr600

it has a 305 TPI I want to run a cat back thats pretty loud at WOT but alright at idle. What do you guys think ?? how hard is it to do a true dual exhaust?? i would love to do that if i can make it work for under 500 bucks. Any help at all would be awesome. Thanks guys

Andrew

89junkyardgta

Doing a true dual exhaust on these cars is not easy. There just isn't a heck of a lot of space under there for clearance. It can and has been done, but it will be totally custom. A 305 and even a mild 350 should be just fine with a decent mandrel bent 3 inch exhaust. If you want more power, go to a set of headers and an upgraded y-pipe and cat, or in your case, cats. Actually, if those are stock cats, that might be the first thing to do before the cat-back. But don't go overboard unless you have a motor that needs more exhaust. Torque moves metal, and once you open up the exhaust so far, you will actually start loosing torque.

arozr600

thanks man i was thinking about doing a set of slp headers nd y pipe using the stock gutted cat nd running a 3 inch flowmaster with an 80 series muffler do you think it will have enough back pressure??

avro206

you don't need any back pressure.

89junkyardgta

Ok, firstly, to respond to avro06, I think 'some' backpressure is desireable, but then I have yet to properly drive my TPI car. I realize they are torque monsters, so perhaps a little less torque on the bottom is a good thing, but if you have no backpressure, you will loose torque. Arozr600, my opinion is that sounds fine. You may want to consider a set of DynoDon's headers, as I think they may surpass the SLP's as some of the SLP's seem to need a bit of modification to fit some cars. I really like the sound of the Flowmaster system, my brother has it on his '90 IROC. I think anything you do will be fine, as long as you don't try to stuff some sort of custom true dual 3" exhaust under the thing.

TA

iTrader: (1)

89junkyardgta is right, you do need back pressure, just a little in the right places, or you will loose both torque and horsepower. He is also right in that the Dyno Dons headers are superior to the SLP units, that are also out of production anyway. I have true duals, and it was very expensive, and with long-tubes I hit the header flanges on washouts. I love the sound, but there is no way to do it for $500, and have also had smaller engines put out more power on shorties/single exhaust catbacks than my 383 long tubes/true duals. The more I learn, the more I learn WHO to listen to.

TA

chesterfield

Well don't listen to anyone who says you need backpressure. You don't need pressure trying to push your exhaust back into the engine.

avro206

A high flow exhaust might give up a small amount of low rpm power. But the peak HP will go up--at a higher rpm.

It's a good trade off.

At least I think I am right....

1987ZTPI

Headman shorties, and a magnaflow catback is what you want.

Rolling Thunder

Well as far as exhaust go and pressure is concerned if there is no pressure there is no exhaust flow. So no matter what you do there will alwasy be some pressure. Now the question is how much is good? Theres really no right answer to this. To demonstrate my point when choseing any high performance part there are genrally to fundamental things to concider flow and velocity. Useing the analogy that a motor is an air pump the greater the volume of air you can flow as fast as possable will give you the most power however as you all know our motors arnt constantly running at 5000-6000 rpms. So a very large exhaust will generate alot of high rpm torque (also know as HP) because it has the ability to flow large amounts of air at a high velocity. Howery the flip side to that is at the low rpm it in essence flows too well. So for the given amount exhaust gasses will cause a low exhaust pressure that is generated by a motor running at a low rpm and it will not be able to cause the exhaust gasses to flow at a high velocity killing your low end torque. Now the other side to this is a small pipe will choke off a motor in the high rpms killing hp but will give better low end torque due to the higher velocities. Now all this being said knowing how big is too big and too small is too small is the key. For example a 305 with a single 3 in pipe may produce great high end hp but a 502 with the same exhaust will be choked off in the high rpms. So in conclution for a 305 with TIP which is normally only going to perform well in the low rpms get a 2.5 in factory style exhaust with a flowmaster or a magnaflow and also a high flow cat + headers and i think you will be very happy with the resulting performance.

MADMAXGTA

iTrader: (1)

I have a borla its a slip fit stainless exhaust,sounds good, looks good.Hawks makes a stainless y pipe for stock manifolds that works great with borla,slips right together.

89junkyardgta

Thanks, well stated.

Shinobi'sZ

This thing about back pressure always cracks me up. The less pressure against the crankshaft the faster it will turn up and the easier it will make tq. This is why windage trays were cut into the bulkheads on LS1s to vent and equalize crancase pressure. You think people would gain a better undrestanding about Volumetric Efficiency and the effects on a reciprocating mass. These aren't 2 stroke dirt bike engines that need an expanision chamber to generate tq and mid range power.

It's all about how easy the air gets into the motor and how well it gets out when it comes to efficiency. Put the biggest freaking exhaust on that you can and don't listen to anybody about back pressure on a V8 its rediculous.

When I had my 87 IROC I used to drive it up and down the interstate on California when I would come home on leave in the military. It used to barely get 300 miles to a tank of gas while cruising at 65 mph on the freeway (speed limit was 55 back then). I installed a test pipe for a while (removed the cat) and I was pleased to see my mpg go up to about 330 mile per 14 gallons...vs 285. I was also pleased that I could walk my buddies 86 Vette after removing the cat...from a stop. Whereas before I could not (it was almost even). So much for the back pressure theory.

89junkyardgta

Fine, whatever. Run open headers then.

IROCThe5.7L

https://www.thirdgen.org/forums/exha...-you-ever.html

arozr600

thanks alot guys i appreciate it alot for all the help

DARKmj16

iTrader: (6)

wondering when some one was gona post that link, read that thread it explains all bout back pressure. then you can choose which size exhaust you want.