Which down pipe should I get?

**Baron** · 04-05-2009

Hey guys Ive decided that Im going to do a down pipe as my next upgrade.So far I have gotten colder plugs and a 180* therm.I plan on not doing anything else to the exhaust except for the down pipe.I want to get rid of the U bend and I saw ZZP DP get rid of that.Would you guys recommend the 2.5" or the 3" for what Im trying to do.Should I also get it with the cat?Thanks in advance.

**Scimmia** · 04-05-2009

it would be nice to know what car we're talking about...

**Baron** · 04-05-2009

1998 gtp

**Bio248** · 04-05-2009

anyone that is sold a 2.5" downpipe is a fool. 3" all the way.

**Baron** · 04-05-2009

Originally Posted by Bio248

anyone that is sold a 2.5" downpipe is a fool. 3" all the way.

Even if Im just keeping the rest of the of the exhaust stock?

**Scimmia** · 04-05-2009

the 2.5" pipes are made for the L36, go for the 3"

**Bio248** · 04-05-2009

even an l36 should get a 3" pipe. 2.5" is just plain stupid.

**QwikGT** · 04-05-2009

i had a 2.5 catless downpipe and it worked fine on my car. i now have a 3 inch ZZP TOG downpipe because i got headers. there is no difference, a tune and a good 60 foot works wonders on these cars. what do you plan on doing with this car? better yet...how fast do you want to go and how much are you willing to spend?

**Scimmia** · 04-05-2009

Originally Posted by Bio248

even an l36 should get a 3" pipe. 2.5" is just plain stupid.

Yeah, because velocity is so overrated.

**Ragefear** · 04-05-2009

Bio, I am interested in your reasoning. At least back up your claims... I'm on scimmia's side with this... It really depends on whats going on with the car. slapping on random parts that aren't specific to one's application doesn't usually yield the wanted results...

personally, I am a fan of lower end torque. All really depends on the application...

**ROTTMAN** · 04-05-2009

Originally Posted by Bio248

even an l36 should get a 3" pipe. 2.5" is just plain stupid.

So have a 3" downpipe hook up to a 2.5" exhaust. Is that .5 extra from the manifold to the resonator really gonna make a big difference? Not trying to be a smart azz, just would like to know the facts. Even a full 3" exhaust on these cars doesnt seem logical but I would like to hear what you have to say in regards to that.

**2000GTP** · 04-05-2009

definitely 3 inch on gtp's.

**Bio248** · 04-06-2009

well, i take back half of what i said. realitically a GT is fine with a 2.5" downpipe, but it would help top end more to just have a 3" anyway. for an l67/l32 car to even debate a 2.5" downpipe is ludicrous because you need the breathing room.

anyone with an l67/l32 car that doesnt even think about headers is crazy with the cheap ones they have been coming out with lately.

**blueguy** · 04-06-2009

Originally Posted by ROTTMAN

So have a 3" downpipe hook up to a 2.5" exhaust. Is that .5 extra from the manifold to the resonator really gonna make a big difference? Not trying to be a smart azz, just would like to know the facts. Even a full 3" exhaust on these cars doesn't seem logical but I would like to hear what you have to say in regards to that.

The Downpipe and U-Bend is the most restrictive part of the exhaust system in the Grand Prix's...The .5" you talk about when the air is moving that direction really creates no sort of restriction that you would even notice. The stock catback is good for 300+ WHP and I bet I can count on both hands the people here that mare making more than that. I am heavily modded and have headers with a 3" Downpipe and 3" Custom Catback...I noticed no gain at all from the catback, just different a different sound. And again, I do doubt that it did a damn thing for my car but it never hurts to be able to get rid of air when you need too. That is why I love all the comments about dropping a pulley size and all I have is new plugs and a intake...it can take it right!?!?! Yeah...

Any car guy/gal should figure the better the car breathes the better it runs in all accounts...especially in boosted applications where getting rid of air is the key to making power. Running a 3" Downpipe on any car is a good idea, hell if I had money to throw away I'd like to see if my Mom's LeSabre would feel any different with a 2.5" vs. a 3"...since it a slow car to begin with why not try and see a difference...although I don't have money to burn on it so there goes that out the window...True, N/A cars probably don't NEED a 3" Downpipe...but is it going to hurt? No. Is it overkill? Some tend to think so but it is your car do what you like. I'd get the 3" now, that way if you ever turn to do a cam/rockers/HV3/HVTB/Top Swap...you won't have to worry about swapping to a bigger DP to compensate for the mods you have...

**Scimmia** · 04-06-2009

Originally Posted by bluegtp91

True, N/A cars probably don't NEED a 3" Downpipe...but is it going to hurt? No.

False. Exhaust velocity is what gives you the scavenging effect, giving you a lot more low end. Too big and you may gain on the high end, but you end up with no low end to get you there. The exhaust system needs to be balanced, which is why saying that every car should run a 3" DP is just ignorant.

**Ragefear** · 04-07-2009

Originally Posted by Scimmia

False. Exhaust velocity is what gives you the scavenging effect, giving you a lot more low end. Too big and you may gain on the high end, but you end up with no low end to get you there. The exhaust system needs to be balanced, which is why saying that every car should run a 3" DP is just ignorant.

thank you! Someone who understands and doesn't insist on just throwing the biggest exhaust available at the car.

An exhaust that doesn't scavenge is a motor that has to work harder. Depending on the type of boost and how much, it still can be a factor.

**blueguy** · 04-08-2009

Originally Posted by Ragefear

thank you! Someone who understands and doesn't insist on just throwing the biggest exhaust available at the car.

An exhaust that doesn't scavenge is a motor that has to work harder. Depending on the type of boost and how much, it still can be a factor.

I don't insist on throwing on the largest exhaust available. I understand his side of it. I am just stating that it is all personal preference...up to you whether you want a bigger downpipe or not...the market is there and you can buy it if you like. If you don't want to harm the back pressure of your N/A car then get a 2.5".

BTW...there was no Boost...we were talking N/A's last time I checked...

Originally Posted by Scimmia

False. Exhaust velocity is what gives you the scavenging effect, giving you a lot more low end. Too big and you may gain on the high end, but you end up with no low end to get you there. The exhaust system needs to be balanced, which is why saying that every car should run a 3" DP is just ignorant.

Just wondering...so you're telling me if you were to line up 2 GT GP's one with a 2.5" DP and one with a 3" DP...you're saying the outcome would easily be in the favor of the 2.5" DP'ed GT? I honestly doubt that your velocity claims make THAT much difference. The Torque the 3" DP guy loses from the larger Dp will make it up with less wheelspin. Same concept with a GTP with headers vs. a GTP with PEM's. Go test it and I'll side with you on your theory...

Some people would like more top end, others want torque. It is based on personal preference anyways IMO.

**98SilvermistGTP** · 04-08-2009

Since you have a GTP, go with the 3". If you dont get a cat, you will have to get an O2 simulator.

**crywolf** · 04-09-2009

Your exhaust is only as efficient as its smallest radius, whether it be in the down pipe, muffler, a bend or whatever. As for the rest of you debating what he should get. Why don't you wait to hear what he wants the car to do before you start saying such and such a pipe is stupid etc. We're supposed to be here to help people.