Exhaust system - 2.25" or 2.5" which size is better?

SpaceWalker

Hi Folks!

Here is my question.
Some people are saying that the pressure is more important than the size and 2.25" i better than the 2.5" exhaust system.
I've did quite a lot modifications:

Comp Cams Stage 1 - 2600-6800rpm
EGR Cheater
Balanced shaft removed
2.4 Manifold
4in1 custom exhaust manifold
K&N Panel filter
de-restricted air box
3" cold air intake pipe
Vectra C intake pipes
2.0 SC Throttle body

Currently i am with 2.5" exhaust system, but my car is not more than 200hp.
In that case would be better 2.25" exhaust system to keep better pressure in the pipes, or 2.5" for less pressure, but high capacity.
Opinions based on experience are more than welcomed.

Cheers!

Vlad

astra22bertoneauto

Sorry to go off subject Vlad

Did you notice much improvement after you fitted the 2.4 inlet manifold?

Getting back to the subject, have you uprated your injectors yet? Ive heard that standard ones have a limit of about 200bhp?!

SpaceWalker

Well....after the 2.4 the car was really good after 3500rmp, before that a bit passive. The problem is that i have 2.5" exhaust system before even implementing the cams and i am not able to decide was it a good call or not. I am checking in the performance exhaust dealers and they offer for this engine 2.25" which starts to think that the 2.5" is maybe to much for the car and i am loosing more hp and torque that gaining such.

Cobra, Scorpio...etc exhaust systems are 2" or 2.25" - what do you think?

sdbutler10

well i run a 2.25 in to a 2" as i have a quad system and that seems to be fine for me. i think 2.5 is to big unless you got a s/c

Blue_2_2

The size of the system should be dependant on the goal of the driver

2" is standard size
2.25" good compromise if the engine hasnt got uprated cams (2.5" without cams makes it feel flat at low rpm)
2.5" with cams/further engine work

Spacewalker your engine is ripe for a 2.5" exhaust
Except for the ported/polsihed head and 2.5" sports cat & matching milltek system we have (virtually) the same set up!

SpaceWalker

I totally forgot the head work (i also have port and polish of my head) and i do not have any cat at all (pre cat and cat are removed and replaced by resonators).
I was not aware that the stock system is 2" (50mm).

Blue_2_2

2" = 50.8mm

Has your car been on the rolling road yet?

SpaceWalker

I did only few timings - 0-100km for 7sec (G-Tech), 0-200km in 1200meters, and for my stupidity i've tested the top speed 240km (i am with 215-45-17 tires, the same score was on both GPS and mileage meter).

vocky

I would say roughly as a guide,

2" 145-170 bhp
2.25" 160-190 bhp
2.5" 180-250 bhp

Lightning_Coupe

I found that with cams, manifolds and a good quality sportscat that didnt hold the heat back like a cheap one, the backbox perf tube was getting very hot and melting with a 2.25" system. It was replaced under warentee, but it did it again within 100miles. Had the car checked out and it was fine, emissions were good, the cause was just too much heat from the fast gas flow in an unrestricted system. Stepped upto 2.5" and had no more trouble, in fact i'd say it felt torque'er and was actually quieter.

Pisoiul_Cel_Rau

for 160-190 hp, but pipe should be? and diameter and thickness?

I want to make a custom exhaust...need help

Pisoiul_Cel_Rau

so? nobody? 2,25" or 2,5"?? this days I want to make a custom exhaust, which is your opinion?

cavaddict

Well, i'm making mine up and am going for the 2.5". 😃

Pisoiul_Cel_Rau

're happy with it? you have pressure problems? I guess you don't have more than 190 HP

cavaddict

Well haven't actually finished it yet and am hoping to be in the 200bhp region with what i'm doing to the engine.

vectra22

im running a miltke centre section which is 2.5" with magnex 6x4 oval, tbh i think theres a few bhp gained

cavaddict

With exception to the aibox mod, my only other one is my £5 4-2-1 and i got 153.8bhp on the RR. This is a 2.5" and i'm hoping with the other mods i've got lined up she'll breathe a lot easier.

pottsey

cavaddict said:
With exception to the aibox mod, my only other one is my £5 4-2-1 and i got 153.8bhp on the RR. This is a 2.5" and i'm hoping with the other mods i've got lined up she'll breathe a lot easier.

good figures steve you pleased with your results ,if you come across any more £5 bargains and fancy knocking me a manifold up ,please dont let me stop you :thumbup:

cavaddict

Lol...yeah quite pleased with it. Sort of expect the exhaust to come into its own when the other mods are done really, but was good to see an increase anyways. Will keep em peeled for ya :geek:

SpaceWalker

I am using full 2.5 system from the down exhaust manifold till the rear muffler.
The car for sure is more quiet and also with better torque and acceleration.