3" or 2.5" is there a big difference

Hi Guys


It me agine .... Ok for my Down pipe does it really matter if I get a 3" or a 2.5" down pipe

I have a 16g that I plan on turning up the boost on this year. I have a High flow Cat (3") and 3" Thermal Cat back all the way to the back.

Thanks Yasar

Now I don't know a lot about this since I haven't got my Apexi N1 yet, but you'd think 2.5" would be better, less chance of getting boost creap. And I know that RRE sells the downpipe that starts at 2.5" then goes to 3" at the cat.

Question for ya Yasar, how do you like your Thermal? I'm most likley getting a N1, but is the Thermal loud at all? annoying loud I mean...thanks

2.5" is fine for a 16g.

I don't think you need 3" turbo-back until you're pushing big boost with an 11sec goal.

According to Buschur, 2.5 crush bent piping will take you right into the 12s.

The 3" would give you slightly better top-end, but I'd be surprised if it's enough to notice. If you want a real-world comparison, put a 2.5" pipe up next to, say, a stock Honda exhaust. (:

Even my 2.25" exhaust seems to produce decent power, and I know that setup has seen 11's with a 16g (years ago).

True...but, bigger is always better in turbo exhaust land. I'm in the process of building a 3" turbo-back for mine, and I've still got the 14B. Mind you, I'll be running 19psi...but if you want high boost, you want big pipes.

Well yeah ideally you wouldn't want an exhaust at all on our cars, but when it comes to comparing 2.5" and 3" there really isn't that much difference on smaller turbos, especially on the stocker.

19psi on a 14b is nothing compared to the same boost on a 20g or bigger turbo. By going to a larger exhaust you're losing more bottom-end too, so that's something to consider.

It's not a question of which is better... it's about what you need or don't need. 3" is a lot more expensive than 2.5" and it's not necessary for a stock or slightly bigger turbo, no matter how much boost you're running on them.

Hey Ryan

My Thermal has a deep sound not wimpy...But keep in mind I still have the Stock Cat and Down pipe I'm sure it going to get a bit louder... A lot of the guys say my car sound nice ... I would buy Thermal gain if I had to

So far what I got is

It's not worth it to go to a 3" unless you have a 20G or bigger. I will also be slower off the line?

If I go with the 2.5 I should be OK... and not lose to much on the bottom end?

Yasar, I have 3" all the way. You want to take a look, have a listen and go for a ride (drive) in my car to see what the 3" Thermal etc is like, let me know. We can meet somewhere and you can see if you like it. Personally, I understand what everyone is saying about low end power etc but frankly, I am not sure if I would be able to tell the difference.

Like Darko says, not alot of difference with smaller turbo applications BUT, in years to come, pick up a used 20g for alot less, you will not be kicking yourself having a smaller exhaust. That is why I went 3". I have a 16g now, hopefully, in a few years, I can bolt the 20g on because all the fuel upgrades etc will be done already. That is why I went 3". Planning for the future. Send me an email if you want to have a listen. I would buy the Thermal again.

What is the price diff? I would go 3" (I did ). If you have 2.5" and you are not going fast enough, you will wish you had 3". Do it once, do it right!

I agree with Jon. Bigger is better. Turbos spool via pressure differential. A bigger exhaust will get that turbo spooling where the real power is.

Who cares what happens "down low"

The first couple feet of exhaust after the turbo is the most important part. Most turbocharged engines will make more power with a 3" downpipe and 2.5" exhaust, than they will with a 2.5" downpipe and 3" exhaust.

On the small turbos, there probably isn't as much of an advantage, but the larger pipe generally will help spool the turbo quicker in all cases...it may not give any more power than the 2.5" if the flow isn't needed. The general idea to using a large downpipe is that it causes turbulence in the exhaust to reduce the swirling effect of exhaust leaving the turbine. Swirling exhaust doesn't travel (linearly) as fast as the turbulent exhaust.

Lastly, behind a turbo, there is not a problem going with too large of pipe for the engine. The backpressure that's necessary for low end torque is provided by the turbo. Anything after the turbo just slows the turbo down. No exhaust would be preferred.

This is backwards from everything I've ever heard, and seems contradictory to your assertion that the best exhaust is no exhaust.

You want the smaller pipe where the exhaust is hot and the velocity is high (i.e. at the turbo side) and the larger pipe where the exhaust is cooler and the velocity is lower.

Think of blowing into the small end of a cone, then blowing into the large end of a cone. Which one is harder?

Besides, putting the smaller pipe downstream will cause a major flow problem when the pipe suddenly reduces from 3" to 2.5" and some of that exhaust smacks into "the wall" and slows eveything down.

Except you forgot what I said about the exhaust swirling on it's way out of the turbo...

All you need is one book...Turbocharging by Hugh McKinnes. Plenty of other books on turbocharging also discuss this same behavior with exhaust.

What you're saying about small pipe keeping the velocity up by keeping the temperature up right after the turbo really only applies to N/A engines. Also, I'm not saying a 3" downpipe to 2.5" catback is preferred. 3" all the way back would be better still, but the first upgrade to exhaust should be the downpipe.

You're also forgetting the Combined Gas Law. A high temperature gas takes up more space, and 'contracts' as it cools. So the big pipe right after the turbo still makes sense.

My 2.5" necks down to 2.25". Can't say I really notice any loss of power, but we'll see what I get with a 3" exhaust. (: