snowcrash

Help!

Im deciding on weather to get a 3" or 2.5" system (full decat), can someone explain the advantages/disadvantages of each?

Cheers,
Simon.

snowcrash

there isnt any then??? LOL

small rodents can sleep in it?

Adam M

back pressure is entirely iirelevant once the turbo has spooled up.

ideally a 3" will therefore flow better and hence allow more power.. The downside being it is louder.

I would consider a twin dump 3 inch system such as the bpm or the scoobymania, combined with the equivalent centre section.

john banks

I did not gain a single horsepower changing to full 3" from a 3" downpipe which tapered to 2.5" for the joint to the centre at about 380 BHP. Not all 3" are noisy though.

snowcrash

the noisier the better

muhahaahhaahhaa anything i can do to **** of the neighbours

cheers.

[Edited by snowcrash - 7/25/2003 7:16:47 PM]

Adam M

john, just because a 3 inch system will allow more gas flow does not mean it was the limiting factor in terms of your car's peak power.

dowser

Question for me, if you do need a 3" system, is how far back does it need to go as 3"?

I really don't want a dustbin sticking out the back of my car, and assume as the gasses cool they need less room. Has anyone done any termperature readings along the exhaust system, and know the formula for working out what effect this has on required bore? Pavlo must know

Richard

Adam M

Does it matter?

a 3inch exit pipe is tiny, even the old scoobysport which is considered small is 3.5 inches.

Andy555

I was told by H&S with a standard STI you would loose torque if you went from a 2.5 inch intermediate pipe to a 3 inch pipe. Think their backbox has a 3.5 or 4.5 exit pipe but sure that the pipe inside is 2.5.

Told by H&S that 3 inch plus systems only really benefit a highly modifed scoob. My Jap import had a 3 inch plus Jap exhaust, and after changing to a 2.5 inch system it feels and performs a lot better. Not been prooven on a Rolling Road, but feels a lot quicker in all gears, maybe its just a more free flowing back box.

john banks

Indeed Adam, if it wasn't the limiting factor then something else was (octane IMHO), and I had changed everything else at that point. Not saying 3" is pointless, but 2.5" is quieter and easier for ground clearance, also cat friendly refitting for MOT etc.

snowcrash

i thought the less backpressure the better on turbo cars, therefore 3" full system is better than 2.5" or is this a myth?

john banks

I therefore expected a power gain. Obviously at 380 BHP the bottleneck was still somewhere else on my particular setup! Make of that what you will.

nom

3" is better than 2.5" for performance, period.
Spool-up can be heavily affected by the design of the DP, particularly the bit right up against the turbo. Like Adam, I think that BPM has pretty much as good a design as possible within the limits of an 'ordinary' road car (twin dump direct from the back of the housing (seperate flow), flows joining a long way back and smoothly). The Scoobymania ones pretty similar, I think? Never played with one
Whether you can make use of the additional flow of a 3" system is something else - if you're only running 300bhp then it's not going to make a lot of difference!
If you like noise, 3" is also going to be better. Manufacturers will drop the di of the pipe to 2.5" at the BB (eg APS in order to keep noise down for the oz regs), but at that point in the system it doesn't effect flow much as the gases are cooler (as Dowser said somewhere up there ^^^ - don't have any calcs to back that up though ).

john banks

Well the only difference between an APS system and say a Magnex then is that the Magnex goes to 2.5" for the centre pipe.

Oddly when I say I went from 2.5" to 3", the whole thing was 3" on the "2.5" system apart from the midsection and the flanges either side of it!

How many of the downpipes go to 2.5" well before the flange to the midsection? If so we are really talking midsection diameters since so many are hybrids? What is the temp drop along a midsection?

I am not going to be perverse and argue that 2.5" is better, just I was very surprised by my findings. I also mapped it again to see if any more was available, none was. Maybe I changed too many variables at once, there are a lot that can move inadvertently. Changing the downpipe to another of different design could affect the boost even with closed loop boost control.

[Edited by john banks - 7/27/2003 10:05:36 PM]