3 inch v.s. 2.5 inch full de-cat exhaust?
 

Hi!

have been going through the posts, couldn't find a specific answer to this.

which is better? the advantages? to go for a full decat on a 2.5inch or 3 inch pipings on a standard WRX (or maybe mildly modified).

any with both experiences?

thanks!

Hi mate,

I've got a 2 1/2" on my 05 WRX.

Basically it depends what other mods (and ultimately power) you have in mind. If you plan on keeping the std. TD04 turbo (with the right basic mods & remap good for a sensible 280-300 bhp) then a 2 1/2" system will give you VERY good spool up with hardly any lag, ie excellent drivability..!;)

If you plan on going for big power (ie 300+) then you should go for a 3" system as you're going to need a bigger turbo, injectors, fuel pump, possibly tubular (sp?) headers, remap etc... In this case you'd be better off selling your WRX and buying an STI as with that kind of power you will also need better brakes and suspension mods to make the most of it. Also a few gearboxes..!;)

IMHO I think a sensibly modded WRX using the TD04 producing somewhere between 270-300bhp is an excellent all round daily driver (and quick enough to see off the current crop of hot hatch with ease).;)

Thanks for that informative reply jasonius :)

While headers and up-pipe will influence turbo spool, the exhaust pipe diameter will not per se.
Choose the most efficient system you can find, generally the straightest and least restrictive. The TD04 can only flow a limited amount of air so it will probably perform equally well on either a 2.5" or 3" system, providing they are well designed.
Decatting your system will show immediate gains.

Harvey I thought it was something to do with back pressure effecting turbo responce..?

I'm probably spouting complete BS that I've picked up on SN, but I'm always willing to learn..;)

Jason

The way i see it is the 2.5" is more suited to midrange punch, 3 is more free flowing at the top end. But some say the 2.5 becomes a restriction at the 340bhp level anyway so if your aspirations go above that, 3" is the obvious choice.

Tony.

jasonius : I have read on here too that at low power levels you need back pressure in the exhaust system. On a turbo charged engine I am at a total loss to understand why any exhaust restriction could be beneficial and from personal experience I know the most efficient exhaust even at low power level (280 bhp) is the free est flowing.
Maybe just another Scooby myth but if one of the "experts" has a contrary view I would be very happy to hear it.

I'm with Harvey.
The freer the gasses can get in and out of the hot side surely the better? The spool will be better and boost will arrive sooner.

There are a lot of theories in Snet, that's ok, pity we dont have too many back-to-back real tests, but AndyF did some in Scooby Magazine some months ago?

A few meetings ago i raced against a Cossie with a Cosworth headed V6 and twin turbos. The car is run by Turbo Dynamics so they are close to the subject.

Each turbo had a 4'' down pipe and the quietest exhaust in the class.

It makes 550+ bhp...:norty:

I lost.

Graham.

Cheers guys,

Always pleased to be informed correctly..! ;)

Bloody SN myths... where's PS Lewis when you need him....NOT..!

J

But what about gas speed, you could have the best free flowing 10 inch exhaust on the planet, but as soon as the exhaust gas got into this huge space it would slow to a crawl. Possibly get flow reversal. I thought the idea was to keep the gas flowing at a high speed as possibly. So next to a completly matched exhaust system, the best one would be the smallest one which doesn`t restrict power . I don`t profess to be an expert, this is just information, I have read from various tuning books. Not the max power type I might add.


Jase.

Humm.. good point

Yes, I've read alot about that but I believe that is the case for natually aspirated engines. With a turbo bolted in the way, the turbo itself is the biggest restriction and ASAIK exhaust gas speed post turbo has no effect on gas speed pre turbo (unless restricted) so the least restrictive the better is my understanding on the matter.

Just to add pipe diameter pre turbo is another thread alltogether. It has been reported on here that too large an uppipe diameter can incease lag

This is really interesting cos Im trying to understand why the previous owner of my car (subaru mechanic) put a 2.2 inch freeflow zorst on ...

So I dont even have a 2.5 ... do you think he was looking for low power and thought that the smallest non restrictive exhaust would be better than say a 3inch ??

Hi

We use our own 3" system, when testing it was found the 3" bore size gave a better overall bhp and torque gain over a 2.5"
When fitted to a std V4 Type-R we gained 39 bhp and 42 lbft
We have used these on std cars and also cars running 350bhp+ without any problems.

Used by Harvey Smith and had customers of Andy Forrest buying them from us with positive results.
But some tuners prefer 2.5" for their ECU work - so it varies and its down to personal preference.

Hope this helps

Colin

Revolution

www.revolution.eu.com

I just changed from a 2.5" Ninja to a 3" Apexi N1.. the N1 actually has a 4"+ section just before the backbox and delivers the goods...:norty:

http://www.apexi-usa.com/graphics/products/1781.jpg

Stock ECU @ the moment and the boost actually changed from 0.9 bar to 1.0 bar (defi) after the system was fitted, Shocked me TBH :eek2: . Louder and decided to go for it before the PFC and 20g goes on but if I was not opting for bigger turbo I would simply stick with the 2.5". as it does the buisness. Ninja well priced and sounds great.

http://www.scoobyworld.co.uk/catalog...njacatback.jpg

Biggest gains for me were removing the downpipe cat & center decat... massive difference overall.:thumb:

Depends on your budget but a good quality 2.5" system would transform the car if it's go a stock system on. If your going for over 300bhp then conisder the 3". Just my 2p. :)

I bought a Revolution System and downpipe earlier this year and I was a little sceptical about their quoted power gains, but within 5 mins of driving I was blown away buy the throttle response (turbo spool up) and the top end power gain was excellent.
Gained 37horses and 38 LBFT on rollers (V5 STI) and turbo came in much earlier too.
It really did transform a fast Scoob into a fcucking screamer, so I would definately reccomend a 3".

The ideal design for a turbo car is high velocity and low pressure. So, the bigger the exhaust the better.

High velocity and low pressure don`t go together with big, as this will cause the gasses to slow down as they expand to fill the space forcing the air pressure to go up. Think of a river it runs faster when it`s shallow and runs slow when it`s deep. The trick is to get an exhaust that will give you the maximum exhaust velocity by making it small, without going beyound the maximum flow rate of the exhaust at the gas speed your aiming at, which is when it will become restrictive. So you need to look at the horse power your looking at producing, and go for an exhuast that will rated a bit over that to give you some leeway.

If you look at the back of something like a touring car they are not running huge exhausts. The reason is you get no benefit for the extra weight and cost, or possibly a slight decrease in performance, because of other reasons that would take to long to go into on here.

The exhaust gas speed in the headers and up-pipe are critical so too large a diameter can have a negative effect. Getting the diameters right for a particular set up is not easy but it is rewarding when it is achieved.
After the turbo, gas speed is not the criterion, free flowing, no back pressure is what is needed.
I remember the night I had the Revolution exhaust fitted to my STi 6 Wagon. I got the car three weeks before and chose the STi so I would not need any mods. It was standard apart from the new exhaust and as I drove back from Newcastle I was well impressed.

From actual testing on 2 litre engines up to almost 600 bhp I can assure you that a 3" system is the best option.

www.geocities.com/harveysmith3000

The only time you want high gas speed is on either side of the turbo to spool it quicker. Further down, as said, you want it straight, no boxes and big. The problem is then keeping it quiet. The best exhaust I've ever seen was on my car when it arrived from Japan - a 4" wide back box, 3" bore, no boxes, no joins and straight as it cold be.

Not a problem on scoobies, but I remember a guy losing loads of driveability by opening up his exhaust ports way out on a Cav Turbo which has a turbo *very* close to the head...