3in to twin 2.25in exhaust- (GT35) How much power/spool are we leaving on the table?
 

As title really.

I have a car here with a GT35 single turbo setup running a 3in turbo back system that splits to twin 2.25in near the rear diff.

I have seen many people saying even on a small turbo like a 35 there are gains to be had going bigger than 3in, but really, how much is to be gained?

Enough to warrent making a bigger system?

I'd be maybe more interested in spool/response/torque (or even lower EGTs) improvments than bhp, but of course that's nice too :icon_tup:

Would just a bigger downpipe and keep the current rest of system be a big help?

Needs to stay fairly quiet for track use, so the split to twin silencers is useful, and I don't want to spend a fortune making a complete new system for only small gains.

Thanks :)

I really don't see a problem with spool meaning it won't be noticeable. However your top end will be hurting. Along with back pressure, maybe even higher egts. Man 2.25 is small?

If part of your system is 3" I don't think it will be much more to do the rest in 3"???

Two 2.25" pipes have more area than one 3" pipe. Maybe additional friction comes into play, but swapping two 2.25" for one 3" pipe reduces the area of the piping

Assuming 0.063" wall thicknesses, dual 2.25" actually has more cross-sectional area than a single 3" does (7.08 sq. in vs. 6.49 sq. in.), however with the extra wall area there will be significantly more drag near the outside of the tubing with the dual 2.25" setup than with the 3". I think performance wise, dual 2.25" and single 3" would be similar, with the 3" edging it out.

More to the topic....is the motor ported? I believe with a ported motor you will see gains in response and horsepower potential if you were to move to a 3.5" downpipe and 3" downpipe-back. With the stock ports, you probably won't notice any real difference in response, but maybe in power potential.

Personally I run a straight 4" exhaust (modified A-Spec exhaust with a RandomTech cat) and I noticed great improvements in terms of response with my T04Z and street port over the previous 3" setup.

Cool I was going to replace my dual 3" exhaust with single 4" but not no more. I guess I already have a 5" exhaust flow!!

are you pushing 700+whp?

Not even close why do you ask?

Yes, street ports, nothing too crazy.

Sounds like we not losing out too much, but as the downpipe needs modifying a little, if we can make it bigger too, than sounds a good plan.

The engine is mostly about response and big midrange power, so that was my main concern, extra power up top would be a bonus though.

Because if you already have dual 3in but less than about 700bhp, why'd you want to change it?! Unless it starts as a single 3in, which makes a twin 3in later pointless.

Well my car came stock with duals and I want to keep it that way. Also I have a super rare RE Amemiya dual 3". I'm sure his reason for doing so is anything but pointless.

I was thinking of changing it because someone offered me $1500 for my system???!!!??! Daym??? But I changed my mind.

So from what you're saying these are the steps I should do to make my dual 3" system worthwhile:

1. Cut my y-pipe in half
2. Buy more 3" piping and weld it to each section of y pipe to make "true duals"
3. Buy 2 60 trims
4. Buy 2 wastegates
5. Make a twin turbo manifold(s)
6. Bolt on twin turbos
7. Attach each 3" exhaust pipe to each turbo.
8. Dyno at 700+whp

And yes I was actually thinking of doing this too.

I've found that when working with dual exhausts, it becomes tricky to calculate out just how large to go with each of the "daughter" pipes. See, the headloss equation states:
http://upload.wikimedia.org/wikipedi...3da4daa37f.png
where:
hf is the head loss due to friction (SI units: m);
L is the length of the pipe (m);
D is the hydraulic diameter of the pipe (for a pipe of circular section, this equals the internal diameter of the pipe) (m);
V is the average velocity of the fluid flow, equal to the volumetric flow rate per unit cross-sectional wetted area (m/s);
g is the local acceleration due to gravity (m/s2);
f is a dimensionless coefficient called the Darcy friction factor.

As the cross-sectional surface area of the pipe increases, the speed of the exhaust exiting the pipe will inherently decrease. You want to try to match as closely as possible the size of pipe for amount of flow coming out. Otherwise, you'll get more "headloss" and end up actually restricting the flow with a huge pipe. It's a common misconception with many people that the bigger pipe, the better.

I have a straight through 4" on my GT35R. I can get full spool by 2500rpm and be at 16psi. I run a 4" resonator and an Aero Trubine muffler. It sounds wicked...

^ sound clip?