For anyone whos had experience with both, or some direct dyno comparisons, is there a major power difference between a 2.5" turboback system, and a 3"? Basically the only reason I ask, is because I absolutely love the cat-back I have now ( Greddy SP ), and Id hate yo give it up, and it is 2.5 from the Y-pipe back. I was planning in running corksports 3"-2.5" downpipe, to a bonez 2.5" high-flow cat, and then the Greddy Cat-back. I could probably sell the cat-back and get enough from that to cover the price of a full 3" turboback system, but Itd kill me to give up the Greddy. The sound and appearance is great. But, if there will be a major power difference, I guess Ill have to do it. I will be running a t04-B hybrid turbo, streetport, FMIC, etc..

 

i full go for the full 3" if i were you. i think it would just flow alot better.

 

I was/am in the same boat as you...I sold my RB 3" turbo back and got a 2.5" RB dp/mp so I could use it with my Feed catback...

BTW what are youdoing about emissions,or don't you have any in Washington county?

 

Im in westmoreland county...yea I have emissions. I dont think Ill have a problem with them, Im keeping all my emissions equipment in tact. My N/A passed with flying colors with only 1 Bonez High-flow cat, which is what Ill be using with my Turbo as well. From my experience I dont think there is much of a power difference between a good high flow cat and a midpipe, so I chose to stick with the cat...

 

I am wondering the same thing, because I also don't want to get rid of my HKS catback because it looks and sounds really nice. The outlet on the stock turbo is only 2.5" so I don't really see how 3" would be better without making the outlet larger.

 

Its because the faster the exhaust gases can be vented,and the less backpressure there is, the faster the turbo can spool and produce boost. There is really no exhaust that is "too big" for a turbo. There is a point of diminishing returns though, which is why it isnt really necessary to have a huge exhaust on a nearly stock car.

 

Whatever you do my friend, do not ! I repeat do not sell
the Greddy SP Exhaust System. In my personal opinion that Greddy SP Exhaust System is a absolute beautiful work of art, it's the nicest exhaust system out there for the Turbo II, besides the Racing Beat Full Exhaust System. I myself wanted to do the following, get a Greddy SP Exhaust System, and buy the Racing Beat Downpipe, and Presilencer. And that would make a complete Exhaust System with the best of both worlds you got the RB Downpipe,and the presilencer, with the beautiful, and sexy looking Greddy SP Exhaust. The canisters on the SP are sweet, the quality,design, and style makes it a awesome exhaust system. I never understood why Greddy never made this system complete with a Downpipe, Mid Pipe, or a Pre Silencer like the RB Full Exhaust System. Dude everytime I used to see my friends Greddy SP Exhaust on his Turbo II I would just.

 

Thanks man..here are some pics of my Greddy to keep everone entertained









And as far as the RB sysytem, I had it on my turbo2, its nice, just doesnt do it for me asthetically like the Greddy does.

 

you really like that greddy system dont ya

i wish i could find some greddy EVOII mufflers i really really like the way those look and i hear they sound good.

 

Personally, I don't believe there is that much difference as far as evacuation is concerned. I believe the reason that most go with the 3" is that it will easily increase the boost, and therefore the performance, without other modifications.
I'm running stock (from the factory) cat back along with the Bonez 2.5" DP and hi flow cat. With the G force ecu it's right around 10psi WITH the stock intake system. Gotta say, runs super.
Quite a while ago I believe Vosko had even hit the high 12's in the quarter with a stock cat back.

 

Yes, I love it. If not so much for the looks (even though IMO it looks amazing) for the sound. Ive never heard a better sounding exhaust on a Rx-7 yet. Its not loud, just a mellow buzz. Id call it Ferrari-ish


Yea Im not sure it was 12s, I think it was somewhere in the 13s, but still impressive. I wouldnt think I would see alot of gains from a 3", unless I was running a larger turbo that the hybrid I am planning on using.

 

From what I understand 2.5" full exhaust doesnt become a restriction on a turbo car until past hp in the 300+ range, so if you're going for more than say around 350 plan on getting 3"

 

i was wondering the same thing. im working on my turbo project and was thinking of going RB 3" DP or 2.5". i want to keep every thing consistant. so if i go 3" DP it will be 3" all the way back. same witht he 2.5"

from what i understand the real difference will be the powerband.

3" will give you more high end power

2.5" more mid range.

 

The reason there is more boost is because there is much less restriction in the system. With a ported wastegate, boost does not increase, but boost response is better and lag time is decreased.

This is not entirely true either. With 3", you will see boost quicker than with the 2.5", and with a turbo car that is pretty much all that matters. Boost = power, obviously.

the area of the pipe is, of course, pi * r^2.

So 2.5" pipe has an area of 3.1415*(1.25^2), which is 4.9 in^2.

3" pipe, on the other hand, 7.1 in^2. The difference is ~2.15 in^2, which is a big difference in flow. (the area is not the only part of the equation, but it is a big indicator.

The outlet for the stock turbocharger is smaller than 3", but the increase from 2.5" to 3" piping is good because it allows the exhaust gasses to escape quicker.

I'd keep the GReddy SP, because it's nice ****, and it should flow very well (probably sounds really nice, too, doesn't it? ). You have a couple options - Keeping the 3" downpipe, you could have the y-pipe modded to have a 3" inlet, and get a 3" cat, or you could get a 3"->2.5" cat. I would try to keep the 3" dp, though.

victor

 

Corksport sells a downpipe that is 3" and reduces to 2.5" right before the cat. This is what I was planning on using. I figure it will have slightly more volume than a full 2.5" downpipe, which should help out a little.

 

Your right high 13's..........Oh well, what's a second!!!!!!

 

This just isn't true. The lower the post-turbo backpressure is, the faster boost will rise and the more power you'll make. I challenge anyone you prove a 2.5" exhaust makes more mid-range than a 3" on the same car.

Rxmfn7, if you're happy with the cat-back you have now then just keep it. There is no requirement for the exhaust to be the same diameter all the way through. Fit a 3" pipe from the turbo and connect it to the cat-back with a cone reducer. While this won't give you quite the same performance as a full 3" system, it's considerably cheaper than replacing the whole thing.

 

Its not the fact I cant afford to go for a 3" system, just the fact I really like my current cat-back I just dont want to be corking back considerable amounts of power with it , as opposed to what I could be making with a full 3". Thats why I asked though, I wasnt exactly sure if I would be making much more power at all?

 

You Bastard ! ! ! You want to sell them.