DaveR

All the after-market exhaust manufacturers seem to agree that a quality stainless-steel backbox will not only release the boxer burble, but will also give you marginal BHP gains (up to 5bhp I believe).

Why is this? --> I can understand how centre section or downpipe once decatted will give an improvement (as you remove a substantial obstacle in the pipe after all), but why do you get an improvement with a backbox that has no cat in the first place?

just interested

DaveR

Andy McCord

Dave, its probably due to a decrease in backpressure over the OE back box

DaveR

Ok, would this be because the exit pipe is bigger? Or the entrance to the backbox has fewer kinks? I would guess so. --> but if it were that simple it amazes me that the cars don't come fitted that way in the first place.......

mutant_matt

I suspect it's a combination of the internal piping being straighter and less restrictive coupled with a large tail pipe. Cars generally don't come like this because then they wouldn't pass EU Type Approval. Aftermarket parts don't have to conform to the stringent noise limits of OE parts which is why you can't by an STi with a PPP, you have to have it registered and then have the PPP fitted......

Matt

DaveR

ah, now I see! that explains a few things

thanks for your comments lads

Leslie

Basically it is down to the internal design of the BB and how efficient the gas flow is through it.

Les

ALi-B

iTrader: (1)

Yeah, You have to look at the actual design of the sliencer.

Some use a straight-through design : - best for performance, worst for noise (or should that be best )

whilst other use use multiple chambers or use series of "U's" to give the silencing characteritics of a much longer silencer. But, alas are far more restrictive.

Henrik

iTrader: (5)

Is there an exhaust that is loud but does *NOT* increase performance? I'm looking for one to fit to the MY03 WRX, because of insurance reasons

DaveR

Hi Henrik,

instead of doing that, change your insurance company!!! Any company wanting to charge you more because of the 'performance improvement' a better backbox will give you need a good spanking !

try Greenlight
or Adrian Flux

cheers
Dave

GaryScoobNCBR

tesco want 450 quid just to change the exhaust....but im too young for adrian flux

Krade

Dont tell em, I had Insurance with tescos on my "Standard 95 Wrx"
It had an afterburner back box, HKS induction, Sti wing, Oz wheels and nearly 300bhp

When I had the crash and they wanted to inspect it, took me an afternoon to change the wheels backbox and remove the spoiler and induction kit. Depends if you want to risk it or not I guess, Im now going with Liverpool Victoria who dont care about cat 1 tracker or any mods.

LG John

Krade you'd have had a difficult time doing all that if you were in traction in a hospital!!

scooby_si

gary try liverool victoria they r pretty mod friendly from my experiance
Si

Krade

yeah well if you ask the nurses nicely they dont mind getting thier hands Dirty

Oh L&V are over 30's only I belive.

ARM-Scooby

lol.

Attu

A P1 isn't exactly noisy but it only passed the EU type approval due to the EU super car rules which allow them to have an extra decibel.
Standard Imprezas arnt classed as Super cars no mater what we may think of them so they have to make less noise.

Andy

Pete Croney

Attu

There's a good story about that actually. They just couldn't get the P1 through the noise test and they couldn't get it to accelerate quickly enough to make the Super Car category.

Solution: Fit £2400 worth of forged magnesium wheels and the weight reduction (dead and centripidal) was enough tomake the sprint time, just.

Result: P1 type approved for the EC. Just a shame that the same wheels weren't fitted to all of them


With regard to backboxes, I posted a picture of the internals of an STi backbox a few weeks ago. The gas path is torturous. Since then I've cut open a UK99 backbox. You have to wonder how the gas ever gets out of that one. I'll bring the digi in tomorrow and post a pic up. Its quite something to see.

scooby_si

Pete just dont put the cut up one back on the car it's illegal to make an exhaust noisier lol
Si

mutant_matt

Krade,If you want to de-fraud insurance companies and take the risk of being insured, that's your lookout but I think you should a) keep that to yourself and b) not advise others to risk not being insured There are enough un-insured drivers out there already without encouraging others to do so....

Pete, can you post pics up of both? That would be interesting!

Matt

DaveR

Yes, I would like to see those pics too.....

thanks
DaveR

MadMark

I have just put the Prodrive backbox on my 03 WRX and I can't believe the difference! Lovely burble and so much more free reving and responsive! I would guesstimate 10bhp extra - no problem.
The gas flow is so much better - the standard backbox pushed out strong pulses of gas in a compressed jet at idle (felt with my hand)- whereas the Prodrive box just has it wafting out ....

scooby_si

L&V over 30's for imports is my understanding as they wont let me upgrade to JDM at the minute but otherwise a damn healthy price on nonstandard UK scooby
Si

[Edited by scooby_si - 7/9/2003 2:46:49 PM]