Hey everyone - I'm a newb to everything automotive and have questions regarding exhaust:

Firstly, note that my car is powered by the 4 cylinder Duke. When I installed my Borla 2" exhaust I replaced the cat with a straight pipe and wondered if it would be possible to remove that entire U-bend section and run a pipe straight from the exhaust manifold to the catback pipes. Here's what I mean:

Change this:

to this:

I apologize for the crappy image - made it just good enough to show what I mean.

However, I've been told that that configuration may reduce backpressure too much. First of all, is this true? If so, wouldn't the 3½" exhaust systems I've seen offered be equally problematic? If you can run pipes that wide, shouldn't you be able to run pipes that are significantly shorter, as well?

I won't pretend to know what I'm talking about... in fact, that's the whole point of this thread. Hopefully the "experts" here on PFF can shed light on exhaust systems for all newbs present and future.

On the same note (no pun intended), how much of a difference in power and fuel economy would result (estimate) from upgrading to different sized exhaust? For example, if I replaced my 2" Borla with some 3½" system, would I see major results?

Thanks in advance to anyone who replies!

Back pressure isn't much of an issue, but the big problem with what you what you want to do is that the exhaust is designed to be suspended from four points plus the tip. Two points on the forward part and two points at the muffler. If you remove one of those points you'll have 1/4 less springs holding up the exhaust and that will likely put more strain on the manifold. Those manifolds are stamped and welded assembles with a history of cracking, so extra stress is what you don't want more of.

JazzMan

quote Originally posted by JazzMan: Back pressure isn't much of an issue, but the big problem with what you what you want to do is that the exhaust is designed to be suspended from four points plus the tip. Two points on the forward part and two points at the muffler. If you remove one of those points you'll have 1/4 less springs holding up the exhaust and that will likely put more strain on the manifold. Those manifolds are stamped and welded assembles with a history of cracking, so extra stress is what you don't want more of. JazzMan

Thanks Jazz, + to you cuz you're always helping people like me out and putting up useful threads with cool ideas. We all appreciate it - your ratings are proof.

Mounting problems aside, would eliminating that U-bend improve performance at all (as opposed to a system with the U-bend and a straight pipe in place of the cat)? Also, any ideas on how mounts could be easily added to remove stress from that manifold?

quote Originally posted by timwdegner: Thanks Jazz, + to you cuz you're always helping people like me out and putting up useful threads with cool ideas. We all appreciate it - your ratings are proof. Mounting problems aside, would eliminating that U-bend improve performance at all (as opposed to a system with the U-bend and a straight pipe in place of the cat)? Also, any ideas on how mounts could be easily added to remove stress from that manifold?

Removing that bend would likely have no affect whatsoever on performance, the engine's ability to generate high velocity exhaust just isn't there. Now, if you had a 300HP SD4 in there it might be a different story.

JazzMan

I added a hooker header and a straight pipe out to a supertrapp muff. I had lost a lot of backpressure. It may not have added to performance, but it rev'ed a lot quicker, and the 1-2, 2-3 shift were a lot better. Sounded great too. May not have seen a diff at the track, but i swear i could feel it when i drove

Rob

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88 TTop coupe (CJB #95), 96 3.4DOHC/5 speed in progress

Light travels faster than sound. This is why some people appear bright until you hear them speak

quote Originally posted by timwdegner: For example, if I replaced my 2" Borla with some 3½" system, would I see major results?

Holy cow! What's on a 2.5 stock? 1.75" single exhaust pipe? 3.5" would practically quintuple your area! Maybe if you were planning on putting a cummins diesel in there down the line......

quote Originally posted by ryan.hess: Holy cow! What's on a 2.5 stock? 1.75" single exhaust pipe? 3.5" would practically quintuple your area! Maybe if you were planning on putting a cummins diesel in there down the line......

Yes, 1.75" sounds right for the Duke, stock. I'm not saying I'm interested in the 3.5 - just wondering what performance benefits would result from upgrading to that.

After dishing out plenty for my Borla it would be impractical to upgrade anyway!!

I don't think backpressure is really an issue, you're not talking about a high-horse engine, IF, and if you put this big fat pipe on your duke, the first thing you'll notice is a significant Drop in get-go off the line, I can almost guarantee you that. Unless your Cat ect is completely shot and clogged, you'll be wasting your money and making your engine inefficient.

what you must realize is that small pipe is small because there isn't a big volume of air to move. if you put that bigarse pipe on it the exhaust air will mostly just flounder around in there, and your engine won't Quite get all that exhaust out before the next ignition cycle. yer gonna lose lowend, and the topend isn't gonna be particularly grand, really, I'm 99% sure on that.

just my two cents.

quote Originally posted by Voided: I don't think backpressure is really an issue, you're not talking about a high-horse engine, IF, and if you put this big fat pipe on your duke, the first thing you'll notice is a significant Drop in get-go off the line, I can almost guarantee you that. Unless your Cat ect is completely shot and clogged, you'll be wasting your money and making your engine inefficient. what you must realize is that small pipe is small because there isn't a big volume of air to move. if you put that bigarse pipe on it the exhaust air will mostly just flounder around in there, and your engine won't Quite get all that exhaust out before the next ignition cycle. yer gonna lose lowend, and the topend isn't gonna be particularly grand, really, I'm 99% sure on that. just my two cents.

Thanks! That explains a good bit to me.

Now what I'm wondering is, why does there need to be backpressure? Why does the exhaust "flounder around" as you say, when the pipes are too big? What if some sort of fan was installed in the pipe, forcing exhaust through the system at a higher velocity? It seems to me that it would be much more efficient to have exhaust literally sucked right outta that engine during the exhaust stroke to lend the next combustion stroke more power (due to there being no already-burnt gases lingering in the cylinder).

Or, maybe I'm reading it wrong. Do I understand correctly that the velocity of gases through the exhaust pipes is what matters? I suppose, then, that larger pipes would lower the velocity while (theoretically) maintaining the same flow, just because they can...

Well, I guess I'm just wondering questions aloud. I just realize this should have gone in the Tech forum... well, please everyone, feel free to comment on my wandering thoughts!

Thanks.

The basic premise is that you don't get something for nothing, that's one of the basic fundamentals in physics. Another basic fundamental is that you always lose a little each time you change energy from one form to another. The total amount doesn't change, but some portion of it is lost as wasted heat for instance. So, with that in mind, lets look at the exhaust. If you stick a big fan to suck out the exhaust the power has to come from somewhere, and that's the alternator. It takes motor power to run the alternator, so you end up losing power overall. However, it is possible to design an exhaust that helps power for free, so to speak. When the exhaust valve opens and the piston squirts a shot of exhaust out the exhaust port that pulse of exhaust travels down the tube as a puff, or discrete blob of exhaust gas. When the exhaust valve closes the pulse continues to travel and since gas has mass, and therefore inertia, the pulse of exhaust effectively creates a "pulse" of vacuum behind it. If you design the primary tubes properly, not too big and no too short, you can actually uses this effect to create vacuum in the exhaust pipe so that the next time the valve opens it literally sucks the exhaust out of the cylinder. You ask, where does the energy come from to do this? It comes from the piston pushing the ehaust out the valve.

The idea is to reuse this energy to suck the next pulse out, sort of like with a pogo stick you're using the energy put into the spring by the down bounce to push you back up on the next stroke. The above explanation is very oversimplified and header design is a very complex science bordering on art, but from what I know that's the gist of it. If you go with a large diameter pipe you lose the flow rate, the diameter and flow rate are inversely proportional for a given amount of flow. You want a high flow rate, so going with a larger pipe only helps if you have a motor capable of creating more flow.

JazzMan

Jazzman makes good sense.

I have a 2.5" on my 2.8 L and that is the max recommended. I only went up to that because I was expanding the amount of air moving in and out of the engine with other intake and exhaust mods.

Strictly stock, the 2" pipes work well for the 2.8 L. So, it doesn't make sense to go bigger for a 2.5L engine.

Jazzman's explanation stands. I can only add that the speed of the exiting gasses is critical not the cross section of the pipe. Higher gas speed = greatest efficiency. If you look into boat racing technology, you will see they use "tuned" exhausts which means there is a certain length of pipe for every engine which is the highest efficiency. I don't know of anybody who has come up with a formula for the Fiero engines.

As for the CAT, any CAT that is old should be replaced because they do get plugged. If you don't need one for emissions, I would ditch it.

Arn

[edit] backpressure is a misnomer. You can't improve performance by constipating your engine

[This message has been edited by Arns85GT (edited 08-29-2004).]

Now I think I somewhat understand...........I thought about doing this to my '88 GT meaning doing a 2-1/2" exhaust. Will that make a difference? I really might take the 2-1/4" route if that's possible. Can someone explain what the difference in performance aside from the obvious (size wise that is).

Jess
88 GT T-TOP

When I chose to go 2.5" I was told either 2.25" or 2.5" would work on the engine with my mods, ported exhaust and carb.

There are better muffler choices with 2.5" and the slightly larger pipe gives a deeper sound.

Call me shallow, but pure performance only goes so far in my book.

I think if I ever go turbo, the 2.25" would not be enough.

Arn

Hey guys, thanks for your replies and very useful information!!

Dammit, Jazz, I would complain to Cliff about not being able to give you more than one plus.. but it looks like you don't need any more anyway. Thanks a lot, man.

Arns, It's funny you should mention the boats. I use Tuned Pipes on the model jets. With the right pipe length you can really improve the performance of most engines. Tuned properly, I have an engine with .82 ci displacement which will put out over 4hp.
It is for the model pictured below. I can't afford an engine swap, so I was going to roof-mount it on the Fiero :}

http://www.wordmark.ca/doc/tshark/pages/DCP_0006.htm

Seriously, I am glad this thread started. I have had the same questions abou the cars.

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Jay Brintnell
Owner / Design Lead
WordMark.ca Communications

quote Originally posted by Eclipse: Seriously, I am glad this thread started. I have had the same questions abou the cars.

I'm very glad to hear it - half the reason I asked the questions is I thought they might help other people out as well.

quote Originally posted by ryan.hess: Holy cow! What's on a 2.5 stock? 1.75" single exhaust pipe? 3.5" would practically quintuple your area! Maybe if you were planning on putting a cummins diesel in there down the line......

Same size pipe that's on the 2.8. Everything back to the muffler is 2 1/8", from the muffler to the tips is 1 7/8"