Quick question, I am replacing my exhaust on my 4cyl, I was planning on getting 2.5"pipes, a buddy tells me that the exhaust size will cause loss of torque with a higher top end and that I should stick with 2" or 2.25" for more torque. He's far from what I call an expert, just an average backyard idiot. But Just double checking on his facts.

What differences with the exhaust piping diameter make?

If you were talking tube headers for a free flowing engine yeah it makes a lot of difference.

The farther away you get from the head the less the diamater of the exhaust makes a difference. Your NOT going to connect to the heads anyways (you're still using your stock manifold) so you're not going to see much difference anyways. I'd stick with stock sizes just so you don't have to mess around with adapating up/down at different places to make it fit what you have.

As the exhaust cools the volume of exhaust gasses drops thus making the size difference most important at the head and a non-issue past several feet

Yes, the manifold is stock, or atleast stock for the crate engine that has been installed. He has access to an exhaust shop, labor is free and all I have to pay for is the stick of whatever diameter I choose. We have already got a 2.5" cherrybomb to replace the cat, just as my current exhaust already has. We plan to replace everything past the manifold.Would it be best to use 2.25" and use their exhaust expander to adapt to the 2.5" cherrybomb then?

Might as well step up to 2.5 right away but I doubt it will make any difference. Who knows? You won't be able to tell what made any differences since you are changing a bunch of things at once.

If you don't modify the stock exhaust manifold (which is 2" OD at the output flange), then putting bigger pipes on it won't help much, if at all. The exhaust sound might be a little deeper / louder, since there will be more room for the sound waves to bounce around in the pipes.

Also, if the exhaust piping is oversize, you can actually LOSE power, due to reduced scavenging efficiency.

Personally, I'd go with 2" OD pipes on a Duke. You can use reducers on the cherry bomb.

[This message has been edited by Blacktree (edited 06-14-2010).]

Ok, who do you know in town that does exhaust work?

Hmm? I'm about there..

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Currently in the middle of my 88 + 3800NA swap

quote Originally posted by aaronkoch: Ok, who do you know in town that does exhaust work? Hmm? I'm about there..

Two very flaky friends, I've been waiting in line two weeks and finally got ahold of him again today. I can probably get your exhaust done if you are patient. Can't guarantee the free labor that I am getting, but probably dirt cheap. Don't quote me yet, I still have yet to get mine done. Tomorrow (today considering it's 3 AM) sounds very promising for me though.

There is a bit of confusion about the science of exhaust, and that is understandable because it is complex at times.

But, basically, the primary pipes are the important ones for performance. After they collect, the pipe needs to be wide enough to dissipate the exhaust pulse so it doesn't telegraph back. Essentially if you don't have headers, you take your pipe, whatever size you choose, right from the flange. You weld the desired pipe size to the flange, or do a size converter at the flange.

If you want a deeper sound you just move up a size in pipe. The 2.5" will work just fine. Going more narrow after the collector isn't going to get you anything so you should enjoy the deeper sound. One thing though, if your pipe goes to more than 6' you need to break up the pulse with a muffler or resonator or the exhaust pulses will start conflicting with each other.

Hope this helps

Arn

you allso have to take the fact that your motor is in the back not the front....so you have less back peresure... most think thats good but at the same time it can hurt your motor.on my 84 2.5 fiero b4 i wreck'd it i had a 3" pipe with no cat with a fart can on it, it ran and sounded amazing if i still had my car i would probly go down to a 2 1/2" pipe with no cat, it would probly sound better.. cuase they motor has a bit of back presure... thats what i think i dunno if anyone agree's with me but Good luck man

[This message has been edited by lilnoobyfierokid (edited 06-15-2010).]

The exhaust pipe on a 2.5 really isn't going to make that much difference at all. If the motor was a Super Duty or some other high performance motor, then yes, it could make a difference. But on a stock 2.5, just go with whatever sounds the way you want it to sound.

i dunno if i agree with you top notch,in the crazy car world of today 4 banger's are the new thing. so if it does not matter then why do people run 5" piping on there rides now then? and just because its a duke and not a super duty does not mean you cant get alot of zoom-zoom for what it is. my 2.0 sunbird motor had 375 hp and i did that on a 998$ budget...took me awhile to find cheap parts and what not but i did it. and my 84 duke was pushing 180hp, b4 i wreck'd it. and theres a guy on the net, that tells you how to build 500hp iron duke motor.

Don't believe everything you read. A Duke cannot support anything close to 500 hp. The cylinder walls are too thin and the crank and journals too light.

Did you dyno your 180 hp Duke? The reason I ask is that to get there you'd have to do a pile of work and the engine would likely blow on you at some point due to the design flaws. I've not seen a 180 Duke but it might be possible with a careful and expensive build. Did you turbo?

The 2.0 Pontiac engine was an OHV and one version was turbo'd. Far different animal. Ecotec motors can be built easily to 500 hp.

A 3" pipe can technically work for the Duke if the primaries are correct, but, plumbing 3" pipe through the cradle is a chore. Without properly designed headers, it would likely not evaculate the exhaust fast enough.

As for backpressure, the term is misleading. You are actually referring to the negative pressure area behind the pressure pulse which sucks the next pulse down the pipe. Impeding the gas movement anywhere is a power robber unless you have your pipe volumes worked out. 2" pipe on a duke handles everything the engine can throw at it.

I'd stick with 2.5" as big enough but not too big.

Arn

[This message has been edited by Arns85GT (edited 06-15-2010).]

thanks for telling me the right terms and words im just a 20 year old back yard car guy, my iron duke was turbo'd yes only ran 6 psi and yes my duke was dyno'd. and yes my duke was very carefully build. the thing about my motor was is your right about it blowing i was so scared of that happening i never put the pedal to the medal inless i had to or when i saw this turbo'd mr2 on the highway(witch was really fun!!!) but then again anytime a mr2 and a fiero meet up on a highway, its allways pure war fare!!!! and now that i think about it, why dont people put more turbo'd 2.0 sunbird motors into fiero there a really good motor. and they never die!!!!

Here is a pic I robbed from Arn




THis is what I plan on doing, where the muffler he has there will be a cherrybomb. I notices that the cherrybomb is just 2"piping in the center with 2.5" adapters.

quote Originally posted by Arns85GT: One thing though, if your pipe goes to more than 6' you need to break up the pulse with a muffler or resonator or the exhaust pulses will start conflicting with each other.

Keep in mind that the exhaust is going on a 4cyl duke, it will wrap around and back up to the manifold adding extra length. Will I be safe with this setup? I want the 2.5" for a deeper sound, but if he can sell a stick of 2.25" for cheaper, so basically price is what is leading me at this point now that I know size will not make a preformance difference. Also, do you think this will be obnoxiously loud where others around me will be offended and/or cops will feel the need to write tickets? That is also something I am concerned about along with not sounding one of those &^%# &*^@% &@#%* RICERS!

the Cherrybomb is louder than the Magnaflow.

My system was plenty loud enough. The neighbours could not only hear me leave, but they could tell which way I was going, and when I was returning home. Be aware. However, it sounded absolutely incredible so no complaints from the neighbours.

http://www.megaupload.com/?d=90NRTKV4

Arn

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My '88 duke had a cherry bomb on it, wasn't that bad..

quote Originally posted by aaronkoch: My '88 duke had a cherry bomb on it, wasn't that bad..

But did it also have a muffler?

If it was like most it had a CAT plus the cherry bomb, but the cherry bomb gets louder with age because the fiberglass packing mats up and hardens. So while is isn't too much louder starting out it is much louder later on. Also, it does not have sound dampening so the cabin sound will be higher. The Magnaflow has stainless steel mesh with rock wool liner on the outside of the mesh so the outside of the muffler doesn't get as hot and the sound transfer through the case is less.

Arn

How obnoxiously loud it will be is hard to predict, the muffler rotted off my duke and it still wasn't very loud and i didn't have a cat either. i also did a hack job exhaust on my friends silverado with no mufflers no cat and duals and it was actually quieter than his old single exhaust with a glass pack, really just depends. I wouldn't put a glass pack on mine but i also don't want to sound like a half A$$ed ricer, before i started the 3800 swap i had new stock exhaust from the fiero store with no CAT and it had a good rumble, but it wasn't too loud or too quiet, it sounded like a classic sports car ( I loved to go down twisty roads with the windows down).

quote Originally posted by Shill: Tomorrow (today considering it's 3 AM) sounds very promising for me though.

And yet they flaked again. I'm about ready to give up.

After not being able to get ahold of him, again... Called around, explained what I wanted to a few shops, got rejected by most considering I have no CAT, nor do I feel like spending the obnoxious amount to get one. I even tried the plan of, "it's not a street car, it's my sand buggy on a fiero chassis." They still gave me an excuse of, it's illegal for us to do that, they claimed they will receive a $10,000 fine, and they will hand out my info to the officer and I will receive a $25,000 fine. I thought it was a bunch of B.S. So, I gave up with having an exhaust shop do it. and called some less known "performance shops" as they call themselves.

Here's my rant for the first place I tried to get in contact with

Called a place called SMC Customs, which specialize in mandrel bent exhaust and roll cages. Explained my situation, what I had and what I needed. got a quote for $50 per hour plus $30 for the stick of 2.5" exhaust that was needed and he could probably fit it in between one of his other projects tomorrow, which I assumed would only be an hour or two. Took my exhaust in to him personally and he quoted me $500 and it'd be a week and a half out and he couldn't guarantee that it'd fit unless he had the car there (even though I was giving him my old exhaust for measurements. I laughed at him and he said he could knock $100 off if it was a non mandrel bend. $400 was ridiculous when I was supplying everything but the tubing which he previously quoted at $30. That's over 7 hours of labor, just to bend my exhaust at his previously quoted prices. I was a bit angered with him at this point and I told him, LOOK, I HAVE EVERYTHING BUT THE PIPING, I even took him out to car and showed him the glasspack. and he knocked another $100 off of it for that and not having to do tips either... Why would anyone think it is okay to charge $100 for a $15 part? but he'd still need my car to do the work, which he was going to charge me to tow it to his place (only $25 for gas, but still ridiculous) Anyways, after that, he finally settled at a firm $200 for just the piping and the bends on a regular exhaust bender, and $300 for the mandrel bends. At this point, i could care less if he offers to do it for free. I don't really trust him after pushing me around like that with the price. For all I know, I can go to pick up my car and find out he changed the price yet again to something I can't afford, and now he is a proud owner of a new fiero.

End of Rant.

Other than that, called a good shop that some of the fiero guys go to alot for their work to be done, called Shelly automotive. The owner actually has a few fieros of his own, so he knows exactly what the exhaust should be and even has a vehicle there to match it to if needed. He will charge $50 for the pipe, and $100 for labor. Still unsure if he does mandrel bends or not, but it is an assumption because he has alot of other high tech equipment such as a dynojet. Hopefully all goes well for $150.

I still think this is a bit much considering my buddy can get it done for just the price of a stick of exhaust. Which I assume is really cheap.

Wish me luck tomorrow in my hunt for exhaust work and that it goes much better than today has.