Okay, so I'm going to buy a 4" downpipe while the engine is being rebuilt in an attempt to quicken the spool a notch. (Note, the title is 4"ish because the turbine housing has a 3" v-band outlet, so I have to buy a 3-4" downpipe)

I had an epiphany when I was looking at options for the downpipe, though... it's gonna be loud. It is already too loud. ****.

So here's my idea. Right now this is my exhaust setup:
3" downpipe -> 3" blank cat -> A'pexi GT (has a presilencer built in)

What I'm thinking is this...

4" downpipe -> (cut out the blank cat) -> two of these: http://www.hopupracing.com/ma6rorase.html (4"IDx6BL"x12OL" version) -> A'pexi GT (piping varies, maximum ID is 3.74" if I remember correctly). So, in essence, I would have a full 3.5"+ exhaust system.

My goals from this setup would be to slightly quicken my spool -- keep in mind this is more of an upgrade-for-the-future when I go to a tubular divided manifold and turbine housing, I'm not expecting to significantly quicken my spool just by widening my downpipe -- while reducing the noise a bit by adding the two mufflers and tossing the cat. Would the possibility of flame-throwing arise from taking the cat out? I ask that because I don't want flames, haha. Not a fan of the whole flame-thrower burning the bumper thing.

Another concern would be boost regulation. Something tells me that I might run into some spiking problems if I go to an exhaust this wide. Granted, I'll have my AI by then so I won't be too worried about it, but it's still not a practice I would like to continue. For reference, I have a TiAl 40mm which holds boost perfectly at the spring pressure rate (11 psi) right now.

Thoughts?

 

My car is obnoxious with a 4inch dp(and 4 inch vband welded to turbine houysing) I am running a single magnaflow. I can keep boost at 9 psi with an hks standard though.

 

My car is also crazy loud with my 4" full exhaust. I also have 2 magnaflow mufflers and they help very LITTLE....

 

You have a T-2 and you want one 4" exhaust? You can easily split it in to two 2" or bigger and then run at least two cats preferably four, 17" silencer on each pipe then at least 17" exhaust on both sides.

I currently have a hight flow cat (which did nothing for noise, maybe 1db) and a Buddy Club exhaust which has a silence in the pipe and it literaly hurts my ears. I thought it would be quieter then my previous Border Racing "lol". I am currntly in the process of making my own custom exaust with at least 17" of silencer or as long as the pipe is strait (have to measure). I can have it custom made localy (posted on GoodfellaFD's thread) and then a Magnaflow muffler (was going to do Borla Turbo XL).

...Don't all 2nd gen's have dual exhaust?

 

Is it always loud or just when you get on it, how loud is it at idle and puttin around at 3000 rpm's?

 

I've been pondering the same idea...

Why use those short magaflow mufflers? when you can fit (99% sure it can be done) the magnaflow #12773 (4"\30"\36")

My plan (if I go with it) is to use two 12773, one as a pre silencer and one as a muffler. If its too loud, add a Apexi EVC.

Another option is to go with a custom dual exhaust, but that might turn out to be expensive and a PITA!

 

I think it's all relative. I have a 4 inch exhaust all the way back to the GT Spec Can. No other inline mufflers and I'm not bothered much by the sound. I've been pulled over once for something other than the exhaust/sound and the officer actually commended the sound.

 

I don't understand how going from a 3" to a 4" would have a noticeable impact on spool if you're no where near the max flow of a 3" pipe at that point. Have any of you guys that have decided this is the answer to all your problems done the math or gathered any data to see how much of a difference in the pressure ratio before/after the turbine there would be? Adding a 3-4" transition disrupts flow. If it is not gradual enough you may negate the 'benefit' of using a 4" pipe. Why 4", why not 3.50"?

Re. sound, this is something I battled with my 3.5" exhaust system. I spoke to Borla and Magnaflow about this. According to both, the larger the pipe the less efficient the resonator becomes because there is not as much interaction between the sound waves and the tubing walls. To make matters worse, there are far fewer muffler options in sizes above 3". Most of the resonators available in 3.5 and 4" seem to be geared more toward track cars than street cars. Everyone has a different comfort zone for noise. Personally, mine was at 3.5" with two full length resonators and an extra long tailpipe tip. Volume aside, I think the car sounded very nice. The bigger exhaust gave it a deeper more menacing tone. However, my reason for doing it was to get max hp at high rpm not to improve spoolup.

 

I'm not expecting noticable gains, it is merely the precursor to a divided tubular manifold and divided hotside.

The boost is about to be cranked up significantly as well, the last thing I want once I surpass 20psi is to have a bottleneck in the exhaust system. Just like everything else with these cars, I am buying myself headroom.

 

I am going with a 3.5" downpipe into a 3.5" midpipe that will neck into my Racing Beat Dual tip exhaust I think a tiny bit bigger than 3". I really wanted as livable an exhaust as possible and I would sacrifice performance for noise (hence the RB exhaust section). I am going with the longest Borla XR1 muffler that I could find. Is there room for more than one resonator in the midpipe?

I really don't want a droning car that hurts your head when the windows are up and you are cruising at 85 down the freeway. I have no want for my car to be like that. The whole reason I am getting the 3.5" DP and midpipe is because my engine/turbo builder requires it. I know post turbine back pressure is a bad thing, but I also like my hearing and this car will get driven almost on a daily basis. Anyone have any more ideas on what type of mufflers/resonators to buy for the midpipe?

 

Borla XR1 has a multicore version (3 small pipes inside one large one), that should help with a large 4" in/out but it may not fit as a presilencer but the oval would work as the muffler.

For you dual guys, you can do two Borla Turbo XL's dual tip which is similar to Racing Beat but i'm shure it's lighter.

If you guys are worried about weight, then you can go with my idea (you can tip me $1ea later ) Coast Fabrication 4" in/out ID and 6" OD which is about the same as Borla but you can get it up to 17" long or custom sizes for a presilencer and then you can specal order one even fatter for the exhaust, maybe 8" or 10" OD. They built the exaust for Mazda's JGTC car. Very nice shop, real fabricators not wannabe poser ripoff scammer asswipes (like shop behind M'Trix).

If you don't have a custom exhaust shop locally, i'd sugest A-Spec. The did a beautiful job n Goodfella's 4" exhaust. I asked them to do mine and they said no problem but I have to wait for the funds.

 

I have a t04r with .96 ar ptrim turbine housing sitting on a hks log manifold with a 50mm wastegate. I at first had 3 inch exhaust and went up to a custom 4 inch to dual 3 inch system. It did give better spool and power, but it also gave me horrible boost creep issues. It would hold whatever boost I set till 5k rpms, it would then go up a psi or two till 6k rpms at which point it would proceed to wrap the boost gauge by 7k rpms going right past 20psi. So depending on which manifold your using I would be prepared for boost creep.

 

If you want it quiet DITCH the GT Spec single!

I had a JIC Spec 90 (3 1/2" turbo back) and it was way way too loud.

Now I run the JIC Spec 90 downpipe (well first ~57" of pipe) to the RB ss 3" (80mm) exhaust rear section. It actually fits fight up once you trim a bit off the Spec 90 front and put a 3 1/2" to 3" reducer onto it w/ a RB 3" flange.

MUCH more quet.

2 mufflers in parallel make it twice as quiet and the TII is set up for it.




Now, if I were to do a custom exhaust for the TII, I would make one like the Knightsports dual and split into dual pipes right after the turbo and put in 4 presilencers and 2 mufflers.

Do dual 3" turbo back and it will spool so much better and be quieter and you can use regular 3" downpipes for first bends.


Just going from 3" to 3 1/2" makes a significant difference in spool.

Bigger pipe to help spool doesn't have to do with the flow potential of the piping, but rather the backpressure drop from the higher volume. You are maximizing the pressure differential between the sides of the turbo exhaust wheel to help spool.

 

Some good ideas in this post!

I've decided that I'm going to do a 4" downpipe to two long 3.5" Magnaflows to the A'pexi GT.

Keep in mind, I'm not looking for a quiet comparable to stock. My main concern is being so loud that pulling into my driveway wakes up the neighbors.

 

I beleive Borla has exhaust with different size OD on each side to accomplish what you want without buying a reducer. It's also possible that the multicore may be more effective since it has three smaller pipes inside of it.

You could also split your exhaust even if you didn't have oem duals originally. You can get a 4" Y-pipe to dual 4" exit (y-pipe used in reverse) then some reducers. I couldn't find any other version in 4" but it works out good because it spreads the pipe apart enought to either add a met' cat' on each one or a silencer on each one. Now you have not reduced velocity, you have improved it. For the muffler you can use (same as on my F150) a Magnaflow dual in/dual out to accept and secure both pipes, reduce noise, make the setup simpler and cheaper then two seperate ones. All that's left is adding some tips. My truck has dual 2.5" from a V8 with headers and intake and it's quieter then stock but then I have the deisel mufler that's like 24" long and you'd want the smaller stainless one about 17" long.

oh by the way, take a look at the Apexi GT. Does it not start out as 3" then step up to 3.5" pipe? So if you go with the above choice you posted you would be doing 4" to 3.5" to 3" to back to 3.5"? Why go to 4" DP when you reduce the pipe down the line to 3"?

 

This sounds like an interesting idea as well.

I was also thinking I could do this: from downpipe one 3.5" muffler -> split into two pipes, each with their own 3.5" muffler -> come back into the A'pexi GT as a single exit.

Would this comprimise flow?

PS: I cut the 3" section out of the A'pexi when I got it welded first, so it's only the bigger piping. Good catch though, that would have been a problem.

 

Just a note, the Apexi GT Spec is 3.75 inch piping.

 

Thanks, Rich. I'd heard conflicting stories about it and have never personally measured it.

Whatever the smallest ID portion of the cat-back is was cut out regardless... guess it was closer to 3.5". :o

 

I'm kinda worried about the noise on my setup as well.

4 inch dp to 4 inch mp (with magnaflow 6 inch round race muffler) to 4 inch catback (with another 6 inch round race muffler at the rear) to 5 inch tip.

I havent been able to drive it yet b/c of an insane exhaust leak. Have that fixed, now waiting on some damn ic couplers. It's definitely loud at idle.

What I'd really like to do is get an apexi-style ATS (active tailipipe silencer) to fit in my 5 inch tip (they only sell one that goes as large as 4.5 inch). Would be nice to have the option of installing it or removing it.

 

Rich you can always make one or have one made. I am sure sean would be up to the challenge

 

Exhaust gases condense as they cool, so having the larger pipe closer to the turbine makes total sense. If you wanted to maintain constant velocity of a cooling gas through a pipe, the pipe would taper substantially as it moved farther away from the heat source.

If you have a constant diameter pipe with a cooling gas flowing through it, the gas will slow as it cools because it is reducing in volume. And this reduction in volume is substantial if you think about how much these gases cool as they travel down the exhaust. The temperature reduces by half or more...

But in theory having a smaller rear section is not nearly as inhibiting as having a smaller DP, and reducing pipe size as you increase distance from the turbine makes more sense than the opposite scenario.

 

Yeah things expand as they heat, etc. but don't you think 4" to 3" is too big a difference? If he ended up with 3.75" or 3.5" then that would be more realistic. I'm not shure how tapering smaller will help anything since the vehicle is not pushed by exhaust. If the exhaust was completely open would that be worse then a tapered pipe? By using the 4" dp he allready got rid of the bottleneck. By splitting that and doubling the volume needed wouldn't the turbine see just as little back pressure as if it dumped directly to atmoshpere? Now since it has more volume then needed then adding cats and mutlicore mufflers won't slow down the velocity of the exhaust gases, correct?

... I proposed to split the exhaust to two pipes is because you could add multiple met cats but increase the airflow from 4" to 6" or 8" so the cats would not make any difference in flow. Also stepping down fom a single 4" to dual 2" or 2.5" would make silencers more effective and you could use twice as many of those as well. I am debating this route myself even though I have a 3" dp but I need to reduce noise. Driving with the screw in silencer that reduces the pipe to 1" is just bullpie.

I don't know, maybe you are getting into a scaveging effect by tapering down which is great when restricted but if there is enough volume then why would there be any need for scavaging to suck the exhaust and increas speed. Wouldn't the fastest flow be no restriction at all?

I looked at the similar situation with my truck. It was designed with dual headers and 4 cats both 2.5" and then Y'd together with 3" (i think, forget exactly). Same principal as you mentioned by tapering down several feet from the cats. Well if they would have went to a single 4" pipe it would have made more sense to me. So I experimented, cut off the Y and replaced it with an X then ran dual pipes to a dual in/out magnaflow. I increased torque, speed and milage.

 

GoRacer, if you're running a 3 inch exhaust, just get the magnaflow res mp from rx7store. It quiets the car down tremendously.

 

You are right, the best setup for a turbo is NO post turbine backpressure. Since the primaries on the exhaust have already come together there is no scavanging effect. If we had a V8 with a turbo on each side, where the two tubes merged we could see scavanging and pick up some torque, this is why an Xpipe or Hpipe crossover is beneficial, it allows one side of the engine's exhaust pulse to pull the other side's pulse along.

Our engines aren't set up that way so we are not going to see any scavanging benefits in exhaust other than perhaps equal length PRE turbine primary tubes on the header, and even the specifics of that are beyond me because I dont know the delay and firing sequence of each rotor's chamber.

My main point was that if a 4 inch tube tapered to a 3 inch catback a ways down stream, it would have a far less detrimental effect than if it tapered to 3" closer to the turbine. As far downstream as possible is the benefit, and the 3 inch tube at the muffler section will substantially reduce noise. Maybe I could do a dyno comparison side by side of a 3" racing beat and some other 3.5" exhaust (making my whole exhaust 3.5" all the way through)...I bet the difference in power an torque is there, but negligible when we taking into account the increase in volume. If noise is not of concern...go full 4" but I like my ear drums and I want to drive the car a lot.

 

I'd be worried about a baby crawling up into that(or maybe a cat) then I would about noise.