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I am going to play around with my exhaust in my friends shop and I am wondering the effects of going from 2" Magnaflows at primary cats to 2.25" at Magnaflow x-pipe (in place of center cat) to 2.5" straight pipes (no resonators) to 2.5" mufflers (Flowmaster 40's)? Has anyone tried step increments before? I seriously can't find any posts, anywhere online, in any forum, about any car that has increased or decreased the pipe OD throughout the exhaust for sound effects.
Any theories?

Blizzy

iTrader: (-1)

I am not sure of the number of people who have done a stepped pipe diameter format. Perhaps there are a couple horses to be unleashed by such a system. However, the convention is 2.25" diameter pipe throughout.

2" pipe is potentially restrictive at the highest exhaust output ranges, although it would present a good sized pipe for normal driving circumstances.

2.25" pipe is the smallest diameter pipe that would be assured of offering no restriction through the entire rpm range. Of course, this is assuming the 2.125" pipe is not on hand. Perhaps 2.125 would be the perfect pipe diameter.

2.5" pipe will really kill your low end. Recall grade school geometry: A = (pi)*(r)^2. Increasing radius exponentially increases cross sectional area, which exponentially increases flow potential. However, with a 250hp engine, that flow potential is not actualized and exhaust gasses actually lose momentum as they scatter in such a big environment. You would pull hard from 5-6k rpms, but you would be lacking everywhere else.

Larger pipe allows fewer reflections of the sound waves within the exhaust pipe. Every time sound is reflected, its amplitude is diminished slightly. Thus, the larger pipe would offer louder exhaust tones.

If you are looking to wake the dead, 2.5" piping into Flowmaster 40's will do just that. If you are looking to maximize performance, 2.25" piping is the way to go. If you are a conservative driver, 2" piping would make your power more available at lower rpms.

Also, I am pretty sure I, myself, have given this same analysis a handful of times now. This information is available through search. On the other hand, information on a stepped diameter system is rather cutting edge. You will probably not find much.

With the system you described, I would expect your SC to lose low end torque, gain power from 5-6k rpms, and wake the dead with 40 series mufflers extending from 2.5" piping.

Sly Guy Yu

im goin 2.5" mandrel bent w/ xpipe and stock mufflers from the stock cats. no resonators

Skip Star

Great answer Blizzy.
And thanks for replying so fast too.
I'm hoping the cats and the 2" segment will keep my low end torque intact some. I wonder though... say your exhaust is 3 equal lengths, and you use 2", 2.25", and 2.5" (in that order)... is that the same as effect as using 2.25" throughout?
(As far as back pressure goes?)
Do you think it'll just end up having a mega-phone effect?
What if you reversed the order and diminished it instead?
A silencing effect?

Watch... I'll get it together and it will sound just like a trumpet.
I might have to borrow a video camera for this one...

Sly Guy Yu

having 2in in the front would just suffocate it, and make the car loud. If you want a little bit better than stock performance, and a loud car, that would be a decent setup.

Blizzy

iTrader: (-1)

http://www.carcraft.com/techarticles..._installation/

That is an article that comes to mind which briefly mentions variable pipe diameter within an exhaust system. Not sure what can be gained from it exactly.

You would certainly lose less low end torque than the person installing 2.5" pipe from the cats back with the system you have proposed, but I would still expect to lose a nice chunk of low end torque. The variation of pressures within your proposed system from different pipe diameters (and presumably crust bent pipe) may offer only marginal performance gains at best.

However, you will definitely hear the V8 more. If you have purchased a 40 series muffler, you are looking to be loud. I certainly expect that you will get your wish. You may sound like you are going a lot faster even if you aren't in reality.

Sly Guy Yu

2.5 +xpipe to the stock mufflers. That should give sufficient backpressure to not lose much low end.

Skip Star

Thanks guys.
Cool article too.
I think I'll go with 2.25" from the cats to the x-pipe to the Flowmasters.
My ideal exhaust would be low toned and subdued at idle, not too loud below 3000rpm (for normal excelleration), and just plain insane over 4000 (for turning heads). Can't really think of a good way to achieve this. If my system is too loud, I may try putting small straight through mufflers where the resonators were. Seems like that would cut low and mid rpm noise but still get crazy in the high rpm range. Maybe I'll write a program that can simulate sound propagation through different types of exhaust components. Oh wait. I only took basic computer programming 16 years ago in high school.
Any wiz kids out there want to make a couple grand?

Skip Star

Here's a question...
What is the stock OD of the exhaust pipe throughout?
I bought my two Magnaflow cats online and they are suppossed to be stock replacements. They are 2" in/out as quoted from their web page. So is the whole system 2"?

And look what I just found. This is interesting...

http://www.popularhotrodding.com/eng.../photo_19.html

Maybe I'll try 2.25" from x-pipe to 3.25" Flow Master muffers...

Lexmex

I have done some increases in my exhaust pipes over the years and I have documented the changes on page 3 of my site below...not sure if that will help, but I have noted my comments there.

Blizzy

iTrader: (-1)

Not really true.

2.5" straight pipe will flow at 521CFM. The standard convention is 2.3CFM for each hp of engine output. This means each section of 2.5" pipe would capacitate a 225hp. Thus, a dual 2.5" system could capacitate a 450hp motor. This is way too large.

This extra flow capacity is not needed. In fact, exhaust gases coming from your 250hp engine will not have the requisite momentum to make it down such a big pathway efficiently. Particles will scatter and velocities of exhaust flow will be less than what they could be in a better sized pipe.

Back pressure is really never a good thing. A small, 2" dual set up on the SC would supply good low end streetability not because of the back pressure in the system but rather because the pipe diameter capacitates the proper amount of flow demanded by our engine at low rpms. In fact, a mandrel-bent 2" exhaust with high flowing cats and mufflers would offer almost no back pressure until extremely high rpm's when the 2" pipe might not be sufficient for the exhaust flow required. Nevertheless, back pressure is not the issue. The proper pie diameter to capacitate the proper flow is the issue.

The smallest pipe that is ensured of presenting no back pressure at any rpm's in the power band is 2.25". 2.5" pipe would offer exhaust gasses too much room to roam and hurt low end performance. 2" pipe may hinder performance by not offering the requisite flow at the highest rpm ranges. In my opinion, I would rather have the power available in the rpms ranges that I am most often driving. 2.5" piping would only maximize its utility from 6-8k rpms...and that is not possible.

Blizzy

iTrader: (-1)

I would be very curious to see how the car would react to this. Because the pipe would be so big and so much more high flowing than what would be necessary, it may in essence shorten the perceived distance of your exhaust by the engine. Not really sure. Definitely would be interested in how the engine would react in terms of performance and sound.

That crazy idea about a program that would output different notes for different set-ups would be neat. However, with exhaust work being as much an art as a science, I just can't see that being realistically possible.

Your stepped idea is unique. I say go for it. See what it sounds like. If you hate it, change it out for something more conventional.

Sly Guy Yu

2.5" + xpipe + the stock muffflers. I'm running stock headers, so the 2.5" piping wont be used to its full potential. w/ the xpipe and the stock mufflers it should be even more restrictive, there fore if anything it willbe just a bit overkill.

Skip Star

Well as soon as UPS gets my cats here I can start! Early next week hopefully. And I'll update this post with our findings. Thanks for the info and ideas! - Skip

PS Quick question... Are the first two cats "pre-cats" and the center one "the cat" --- or --- are the first two "the cats" and the center one "a sub cat"?
I've seen it said both ways so many times I'm not sure of the correct terminology.

Blizzy

iTrader: (-1)

the two cats off the headers are primary cats. the center cat is secondary, but a cat nonetheless. I thought it was a resonator until about a week ago when I investigated further.