djtimodj

Im having a custom center section fabricated to mate up to my APS 3.5" downpipes and need some ideas on piping diameter.

Im going to still use my 5zigen rear cans that have a 2.5" inlets so i will be using tapered reducers in the system no mater what I do. My question is I can go for 3.5" DP > 3" dual center sections > 2.5" rear cans or Just go 3.5" Dp's and full 2.5" center out to the rear boxes?

The car will have no center silencers and only use the rear boxes so Im not sure if 3" piping would be far to noisey eg. >105dB ?

Im aming for 500whp on a dynodynamics so would the big down pipes and smaller 2.5" exhaust be ok with this power (14-15psi boost i would think) ??

I know most people go "bigger is better" but i dont want a a car so loud i can take it on the track etc etc.

Any input is great. Thanks.

deanfootlong

iTrader: (10)

bigger isnt better. when exhaust cools, it expands. hence, i personally wouldnt go with a 3.5 dp, to 3 inch mid section, to 2.5 mufflers. try a 3 inch all the way. that should definitely be plenty for your goal of 500whp on a DD.

thom000001

well when we finished my build I was still on stock exhaust for motor break in.

So 3in DP's into stock y-pipe, then stock mid, then stock muffler..........

car made 450rwhp on about 11psi of boost (somewhere in there on the boost just to see)......

But damn is it restrictive as he!!....thankfully I have a rather efficient turbo setup for that short timeframe lol.....

so yes it can be done, you will certainly give up efficiency and some power....

And for reference on restriction....I have AEM tru boost (so you set it to % of duty cycle on WG, not to a boost number per se), at same setting on boost controller to make that 11psi we pulled exhaust off (so straight dumped DPs). Made one pull on dyno and boost hit 24psi before Larry quickly let off (he said it would have gone a lot higher lol)....

Hope that helps,
Tom

str8dum1

iTrader: (11)

why not just go 3.5" from the down pipes back to the cans? cut off the 2.5" inlet and make it 3.5"

plumpzz

iTrader: (10)

In terms of hydraulics, going from 3.5'' to 2.5'' using high quality reducers will cause less frictional losses as compared to two half inch reducers especially in the turbulent regime.

http://www.freecalc.com/fricfram.htm

Remember you're dealing with a inviscid fluid, variable flow rate and low wall friction (carbon coating). The biggest causes of friction is poor mating at flanges, abrupt diameter changes and sudden cooling from expansion.

djtimodj

Thanks for the replys guys. Str8dum1, The internal dia of the exhaust cans is about 2.5" so cutting off the flanges and going to 3.5" i dont think ill gain much.

So far i think from what plumpzz has said im best going for the 3.5" DP then using a a large reducer 8" long to go down to 2.5" and just run that size to the cans should give me a nice smooth gas flow.

scotts300

iTrader: (46)

There has been a lot of work on piping diameters done on the VG30DETT with upgraded turbos, and it was found that a complete 3" exhaust - including dual 2.5" inlets, intercooler piping, etc. (compared to the stock 2" system) was beneficial over the 3" -> 2.5" system. There is also a max slope you want to keep when tapering down in exhaust size in the transition areas. This does get expensive, but it's the correct way to do it. I forget the rule of thumb, but believe it's something like 1/4" OD change per 6" in length. I still have the 3" --> 2.5" system, though it still makes over 650rwhp, there is more to gain if I switched to the complete 3" system.

The 3" system on my track 350 (N/A) with no cats but with a resonator measures at 104dB. It is loud, but your exhaust tip plays a role in that sound as well.

Sylvan Lake V35

iTrader: (23)

Mine goes 3.5" APS to 3" for about an 8" length then down to 2.5". I made well over 500whp with this set up. I plan to change it eventually but for now it works.

plumpzz

iTrader: (10)

Its typically 1.5X the radii change X 24 inchs for turbulent setups. So a 3.5 to 2.5 would be roughly a 18 inch conical reducer. Obviously 8inches is good enough since this isnt a real viscous fluid

IIQuickSilverII

iTrader: (13)

or ...op can use any exhaust he wants and just run SPs cutouts

djtimodj

I though of using the cut outs but the quick change in back pressure might make boost control/mapping a bit funny??

The pic's posted about show the APS DP's but it looks like they go from 3.5>3" on the outlet flange? Also If your DP's are 3.5" how do you get around the 2.5" flange mating up with the large one??

Last question guys! If the fabricator can fit me in this weekend then I wont have my big DP's on by then, so the exhaust with be 3.5" reducing to 2.5" but this center will need to bolt up to my 2.5" DP's i have on the car now?

I know this sounds like a load of crap but say for instance the stock cat out lets are 2.36" I think and you can bolt on an aftermarket y-pipe thats 2.75" inlets or bigger no problem? I assume this is the same for me just bolting the center up to my smaller DP's for the time being?

str8dum1

iTrader: (11)

Stock APS DP are 3.5" for the body but are only 3" at the flange. a 3" flange should bolt right to a 2.5" flange and not leak.

thom000001

y-pipe 3in inlets, 4in outlet...4in midpipe



Tom

Sylvan Lake V35

iTrader: (23)

I used a 3" flange on the exhaust and a short 3" pipe my fabricator reduced the piping to 2.5" to match the exhaust you can see how it necks down right after the flange, we didn't have a 3-2.5 reducer so he just made one from 3" pipe.

Here is another angle you can see the reducer he made on the cat back side of the DP.

djtimodj

Oh thanks for the replys guys. It looked like the APS had 3" outlets but i was not too sure. The fabricator im using makes race headers (http://www.ojzengineering.co.uk/ check out the website for sexy welds! lol..) and he's said he can roll me a nice reducer what ever size i want? I think 8" long for just a 0.5" dia reduction should be ok?

Tom stop posting up sexy exhaust parts!! I think the twin center sections im having made would cost about the same as your Y-Pipe so its more sensible (and cheaper!) to just stick with the true dual set up.

How come you wanted a Y-pipe set up? I know you run the gutted stck rear box is that the only reason? Oem looking?

plumpzz

iTrader: (10)

If you do a cone reducer you'd probably be the first to do it. Its really the best way to do it. Many would argue the cost/performance factor there, i say go for it.

thom000001

Yea pretty much.

Don't like the sound of about 99% of the aftermarket exhausts out there. They usually sound really good in one rpm range and REALLY BAD at every other rpm lol.

Plus I don't get hassled by the po-po!!!! And its not too loud, I can still listen to radio and talk on phone when driving normally.

Tom

ashtrojan2008

iTrader: (1)

i used a flowmaster venturi cone reducer(2.5-3). Mine isn't "reducing" however. The whole piece was about 10" long, but the reducer in it was only maybe 5"
I cut the straight ends off of it. Its going from my 2.5" downpipes to 3" exhaust.

ross