I'm trying to design my exhaust system, which presently consists of only a 4" downpipe, and have a few conflicting design criteria that I need some help resolving since I'm not a fluid dynamics expert by any means.

I know that there is more of a pressure drop (manifested in this case in the form of undesirable back pressure) associated with doing ANYthing to a flowing fluid.

My conflict is this,
1) I want to have *relatively* quiet exhaust
2) I don't want any restriciton beyond an equivalent 4" exhaust

I've NEVER heard anything close to what I'd call quiet in straight 4". I think it's because the diameter is too large to force any sort of resonating or muffling to occur. Too much exhaust simply "sneaks" by the muffler walls without interacting, maintaining all of its sound.

My plan is this,
Run 4" pipe all the way back into a COLLECTOR MUFFLER that splits into dual 3" near the diff.
Then run each 3" to a RB dual tip.

I like how this idea sounds because the weight added compared to 4" all the way would be minimal since the dual 3" would be as short as possible. Additionally, the weight added by the extra muffler would be negated by the inversion required in the driver's side bottom portion of the tank to facilitate a muffler and heat shielding to be there. Also, dual 3" has a larger cross-sectional area than single 4", so the pressure drop (back pressure) added should be minimal while the sound reduction should be significant.

Please criticize away.
thanks
ryan

 

The main flaw that I see is your plan to run two 3'' pipes to the RB duel tip.

I thought the RB duel tip only had a single inlet... And I'm sure you don't want to run 2 duel tips (quad tips lol) nor could you fit it under the car. Unless you plan to modify the inlet to accept two 3' pipes then I don't think it will work. And on the same note you are gonna have to chop up a perfectly good exhaust anyway because i thought it was only sold as a cat-back.

I know this is just an idea of yours but realisticly you probably won't have a 4'' exhaust quieter that a 3''. plus I would think all those piping changes in the system would totaly defeat the whole purpase of a 4'' exhaust to begin with.

But I'm not a fluid dynamics expert either, just my $.02 so good luck.

 

Sorry, I guess I wasn't clear enough, I meant run EACH 3" to it's own RB muffler. I know it's not listed on their site, but they do sell just the mufflers.

I realize that it wouldn't fit under the car, that's why I mentioned inverting the driver's side lower part of the gas tank, so that it would fit under the car.

Thanks though, any input now that I cleared that up?
ryan

 

oops my bad, I didn't even notice that part. lol

But it still sounds a bit too complicated, do you have an idea how you would position the mufflers? stacked or side bt side. I'm just having a very difficult time visualizing the design in my head. Have you drawn up any pic of your idea?

 

They wouldn't be by each other at all. . .

One would be in the stock location, on the passenger side, and one would be in a mirrored location on the driver's side, hence my needing to cut, invert, and re-weld the gas tank on the driver's side to accomodate the second muffler.

It would be very much like any dual exhaust car, like the 2nd gen.
ryan

 

If you want quiet with big exhaust you need to run twin 2.5" exhaust systems after the downpipe. Use a pair of mufflers per system. Not cheap, but effective.

 

Whew... good that wouldn't be very pleasing to the eye lol. I'm curious to see what others have to say about it. Ive seen a photochoped FD with stock duals and it looks pretty good. There is just so little room to work wit under there unfortunately. are you gonnna be doing the work yourself or is a shop gonna be doing this? sounds like it will be quite the task.

 

1revvin7 do you have undercar pix of that setup?

 

The RB dual tip is not symetrical. you'd have to cut the tips from the mufflers and re-weld them to be a mirror image. A simple 3.5 DP to dual 2.5 tubes with a presilencer and mufflers like 1Rev said should do it and still out flow a standard 3" exhaust. You could use dual in/out presilencers and/or mufflers or two seperate presilencers and two mufflers or any combo of single muff, dual in/out or single in/out etc. Or even perhaps a single 3.5" in and dual 2.5" out (which is what I'll be running on my FC).

~Mike................

 

Do it myself. . .I don't think it'll be that bad actually. Between Burns' Stainless Vband clamps and a few welds, very little to it.

1Revvin7 the only problem with what you propose, is that that would be a more restrictive flow.

Single 4"
1 * (4)^2 * (pi/4) = 12.57 in^2

Dual 2.5"
2 * (2.5)^2 * (pi/4) = 9.82 in^2 = 78% of single 4" flow area

Dual 3"
2 * (3)^2 * (pi/4) = 14.14 in^2 = 113% of single 4" flow area

Again, I'm not sure how much that matters because fluid flow is not my forte. A 22% restriction seems significant to me though using dual 2.5".

Thanks for the input, any pics would be appreciated, and please keep the advice coming.
ryan

 

The exhaust is much cooler when it is reaching the dual part of your exhaust though, and therefore much smaller in volume (more dense). Having the 4" diameter at the downpipe and keeping it 4" all the way back, is almost like a river widening, it slows down. Ideally you would like a set tapper for the cooling gasses to maintain velocity, but that is impossible in such a dynamic setting as an engine running up and down a rev range.

I would say you would be perfectly fine switching from 4" down to dual 2.5" near the diff...by that time the gasses are SUBSTANTIALLY cooler than they are at the downpipe inlet. I would really like to see a volume analysis of exhaust gas as it cools. It would lend more or less credibility to my arguements.

I had similar requirements as you, however I only have a 2 rotor. So I went with a 3.5" downpipe, with a resonated 3.5" midpipe and it tapers into a standard RB cat back. I know the cat back is going to be a restriction I am just wondering if it is enough of a restriction to actually matter... That would take much more actual analysis, and couldn't be solved with speculation or standard static pipe area calculations.

 

Remember that energy equals mass times velocity squared and even though the gases are cooling it will take a force four times stronger to slow the gases down, combine that with the low pressure sound wave that would reflect by going to duel 3" (helping flow) verses the high pressure wave that would occur by going duel 2.5"(hurting flow) I would go with the duel 3" that would result in yet another low pressure wave as it exits the tail pipes (helping flow).

My plan is to go with a 4" down pipe with an inline 4" magnaflow muffler splitting into duel 3" magnaflow mufflers at the tail, I think this will yield a fairly quiet setup.

Justin Linder
www.linderpowersystems.com

 

You can get rid of that 4 inches inline Magnaflow muffler. I have the 4 inches downpipe and midpipe splitting into dual 3 inches Magnaflow mufflers and it is not as loud as I thought it would be and I actually like their sound. I used to have a 3 inches downpipe into dual 2.25 inches HKS mufflers and it was louder than my actual setup. So far I have dynoed the car with the actual setup at 537whp at 24psi. I couldn't get more because the transmission broke. As you can see this setup is quite capable of producing good numbers with reasonable sound. The mufflers I used are Magnaflow 14835 and all bends were mandrel bent.

 

thanks guys, time to warm up the credit card and the TIG.

Hopefully I'll be ready to start my thread detailing everything within a month.

again. Thanks
ryan