Exhaust for an R33 M-Spec GTS-t Skyline

[HOTR33]

Hey All.. Very Simple Question..

Is there a great difference between a three inch and a three and a half inch exhaust? (Cat Back)

I've already got a four inch HKS pipe into the cat... But I've been told three inch would be plenty.. Any Idea's?

Cheers

[Blitz]

3 inch will be more than ample.

[Dawzey]

because of cat. 3 inch is enough by far

[surreal]

For most 'normal' people, a three inch does all that it is supposed to (less restriction). Anything bigger is for show or really, really high horse power applications (you do actually need some backpressure to run smoothly/properly for anything remotely like daily driving).

[JimX]

Forced induction cars don't need backpressure anywhere near as much as NA cars. Because the turbo forces more than 1 atmosphere of pressure into the cylinder, it's pretty well "sucked" out by the lower pressure atmosphere behind it.

It's also the main reason why extractors are more suited to NA cars. Not really because NA cars have more to gain from the extractors, but because turbo cars have less to lose from an inefficient exhaust manifold design.

I think 4" dump would be too big for a normal street-driven GTS-t and sap a little performance, but I think the same pipe would absolutely kill the performance of any normal NA car.

[Steve]

You cant have too little back pressure. Think about this, people spend alot of time and money getting the air in and OUT of the head as easily and quickly as possible - head porting, heat wrapping, etc - all designed to get the air out quicker. Back pressure will not help this, it will fight it, it will retain heat in the cylinders. What possible good does it do in the pursuit of hp????

The cat is a BIG restriction to the flow of the exhaust. If you ever get the chance have a look inside one - its scary.

I read a post by Sydneykid where he flow tested a 3" (or was it 2.5" - Sydneykid) pipe, and found no restriction to air flow up to around 600hp air flow (help again Sydneykid!) or more.

Another thing to consider is what mufflers/resonators are fitted to the exhaust. I originally fitted an N1 style exhaust - very loud but made boost very quickly and gave a good power increase.

I then changed to a Nismo exhaust with 1 muffler and 1 resonator and I noticed the car slowed down, and made boost later. Both systems were 3". Sorry I can't back this up with dyno figures.

My personal advice would be to get the biggest diameter, least restrictive exhaust that you can - it cant hurt! Then fit the highest flow cat you can find;) you'll love it.

[rev210]

The design of the exhaust is as tricky as you make it.

The most forgotten dimension by exhaust upgraders is the 'thermal' one (and it relates directly to exhaust gas speed). At the start of the turbo exit the gases have incredible heat (and therefore speed and pressure) ,as the gasses travel back the speed of the gas reduces as its energy is lost to the pipe and radiated as heat.

Here is where it gets hard. The cooler gases are best not put through overly large pipes as it can increase turbulence but, smaller pipes have a larger surface area to volume ratio to lose heat through!

In practice I have found that the largest diameter should be at the start of the turbine outlet, any larger and you create unnecessary turbulence. Factory turbo --- 3inch from memory. Think 'match porting' at this stage.

Next the cat needs to be as close to the turbo as is practical, better still lets forget you have one (they kill power).

The first 3inch muffler needs to be nice and close to the dump pipe and filled with stainless packing (fibreglass will usually die and turn the muffler into a marracca) Exhaust places directing you to fibreglass mufflers will never place a muffler very close to the dump for this reason.

The second muffler before the diff @ 2 1/2 inch and 2 1/4 over the diff into a slight megaphone shaped tail pipe that tappers gradually out to 4inches.

Wrap the dump pipe to the first muffler or cat (which has been ceramic coated) in heat tape. If you have the money coat or wrap the entire exhaust.

If you tell people to reduce exhaust pipe diameter as you go back on a street car exhaust , they look at you funny and think your a loon.No it's not about backpressure, it's about retaining a low pressure 'trail' all the way back to the tail pipe. Hi I'm certified !

[rev210]

I forgot to mention that the design is one that goes from 'hot' to 'cold' front to back and as that relates to high pressure to low pressure your home and hosed.

Then again if you can trap all the heat in then a uniform diameter of 3 inches all the way is the shot.

[Steve]

HOTR33 - you thought is was a simple question:D