Dave Duncan

Looking for some advice on upgrading exhaust sections on my MY99 "classic".
Already fitted a wrapped H&S decat downpipe, but rest of system standard at the present time.
Other mods carried out include stage 2 Ecutek ecu, K&N panel filter and Samco hoses.
Now, I have located a decat mid-section( stainless, 21/2", make unknown) and wish to fit together with an appropriate back box.

However, as an "oldie", more interested in any additional power being produced quietly, I don't want to install a noisy backbox, especially if married up to a decat downpipe and centre section. I even found an original Scoobysport backbox too loud with decat downpipe and original mid-section!!

So, would it be crazy just retaining the original peashooter( as changing this alone on an otherwise standard system on this model seems to gain only about 3bhp)?
What I don't know is how restrictive it would be keeping the original backbox with the rest of the system fully decatted----would I be losing a lot more than 3bhp?

Opinions please?
Oh, and if the majority of respondents still believe it would be better to change the backbox, can you recommend any maker who can supply a quiet, free flowing backbox or is that a contradiction?
Would a standard sti backbox( non-stainless but with stainless outlet with logo) be of any benefit over the peashooter?

Thanks--Dave.

Jim_B

iTrader: (1)

I was under the impression that changing the backbox would liberate more like 8 to 10 BHP...

I did my exhaust on my MY00 piece-meal.. started at the back box ( Magnex 3.5'' round ) , then a resonated centre (Magnex again) and finally the downpipe (H&S open neck).

The car certainly felt much better with the back box replaced - much more free revving. In fact I'd say I felt the most difference from this change over the whole exhaust.

As far as noise levels - my system gives a reading of about 92dB on a static test [ i.e. 4500RPM, 45deg, 1/2 metre from the pipe ]. I think the Magnex is one of the quieter stainless back boxes.. Although the Sti / prodrive steel ones are quieter still.
If its the cabin noise you are keen to keep low - then perhaps you should look for a resonated centre box.

Hope that helps !

SlowBoy

iTrader: (1)

Biggest gains usually come from replacing the very restrictive (in terms of gas flow) downpipe. Back box has more to do with noise levels, imho.

The original SS system was made by H&S and is far from quiet. Not noisy like the Jap stuff, but you could easily tire of the noise level if you want something discreet. For quiet, try the Prodrive box which was fitted as part of the PPP (marked Prodrive across the top). I think this was made by Scorpion, so something very similar could be sourced independently of your local dealer. This has the benefit of being very subtle at idle and low revs, but produces a nice sporty note when you get it past 4k revs.

I don't think it would be 'crazy' retaining the original bb. If in doubt, my advice would be to keep an eye out for a used Prodrive box. If it doesn't suit, then you should be able to sell it on without it costing you more than a few quid. But defo do the dp first.

Best thing for you to do is get yourself to a local meet so you can check out exactly what's what from a noise point of view before spending any of your hard-earned.

chrome

I must be nuts.
I have gone from 3" DP to HKS hiper with unresonated centre (116db on the 45 deg' test as mentioned above) back to OE DP, Centre and BB..

god its going to be quiet..

SlowBoy

iTrader: (1)

You'll easily mistake your scoob for a Lexus and have to check the engines still running.

chrome

yup
at least the mrs wont moan

Dave Duncan

Thanks for replies to date.
To one respondent recommending a decat downpipe---see my original message--I already have a H&S fitted.
To respondent suggesting backbox replacement alone could be worth 8-10bhp--I have never seen such a high figure claimed as a gain for this mod. Most people seem to agree that the main benefit to a backbox change is to an improvement to the "burble/noise".
Even if a backbox change releases slightly more than my 3bhp estimate--maybe 5 bhp, that would be before modding the rest of the exhaust.
What I still don't know is how restrictive the existing backbox is going to be once the rest of the system is fully decatted. In this instance I could accept the potential bhp loss might be more than 5bhp but does anyone have any more info?

I realise that the Magnex and Prodrive backboxes have a reputation for being relatively quiet but would only consider if someone can come up with good info. on point made in last parag.

Thanks, Dave.

chrome

Dave- seriously investigate the revolution system!
It's high quality- performance biased- yet very quiet.
AlanG on here even had revo'added some more baffles for added 'comfort'
I suggest you drop him a PM or email

mw2655

on my my00 i had a magnex 6x4 bb, great sound but too loud for me and neighbours once i got the full decat.

i swapped it for a 2nd hand sti bb, quieter but no noticably difference in performance.

though psychologically slower as quieter

no idea on the bhp difference but probably not much.

cheers matt

ALi-B

iTrader: (1)

Would agree that an aftermarket backboxes only gives very marginal gains your looking at 5bhp at the very most...if your really lucky. The original one isn't that restrictive unless your running very high BHP (350+). (just sellers and manufactuers may try and make you think differently ). Decats do make a difference though

Dave Duncan

O.K.---mind made up. I'll ask my local friendly tyre/exhaust centre to fit my decat mid-section, hopefully over the weekend, retain original backbox for now and basically try to assess how car sounds and goes.
If satisfied I'll run with that combo. for now---if not I'll consider trying 2nd.hand STi backbox which I can source from another subaru independant specialist outlet in Perth.
Thanks--Dave.

Tim-H

I have tried, sebring, ; very quiet, very heavy , no better than standard for flow, prodrive, nice bit of kit , quiet, resonable flow, about 90 ish £ off ebay , (recomended) 2.5" / single rear box, good flow, 115db @6000 @ 2m (loud)
Don`t forget you will have to refit one of the cat`s for MOT, (unless you have a friendly mot`er :-)
Have a good one
Tim

DIPSY

Dave buy a pair of ear defenders or be an oldie who likes to make noise

Dave Duncan

O.K.---decat centre fitted last weekend. Fitter advised was going to be noisy but he had obviously never fitted a decat centre to a decat downpipe but retaining original backbox. Result---still very quiet, just slight increase in noise but now feels somewhat more responsive and good top end urge. Can recommend to any who have not tried this combo and are perhaps getting fed up with the boy racer reputation which seems to accompany modded noisy exhausts.
You don't seem to get modded, noisier exhausts among other enthusiasts with fast cars including Porsche, Mercedes owners and the stealth approach to modding in my opinion is preferable to the boy racer way, even if it saps that last 5bhp.
I admit however I still enjoy listening to a passing highly modded Impreza--its just that I could'nt live with the additional noise on a daily basis but each to their own so if you want to fit your loud exhausts, fair enough but count me out.

Next mod? Don't know but am considering headers but again care seems to be necessary going by posts on this subject re. manufacturers to be wary of and also torque at low revs may in fact be reduced but that's another topic.

Thanks to all who have responded to this thread, even Dipsy.
Dave.

Jay m A

iTrader: (2)

I, and others have found that replacing the headers made the car quiter than before.

Regarding the Backbox, Hayward and Scott can make you one as quiet or as loud as you like, give them a ring.

blueyzfr1

Hi,

I'm running a;

Scoobysport Group A downpipe
Std Centre Cat
STi backbox

Free flowing, quiet and MOT legal, just sold my Blitz Nur Spec R.
Power wise not a lot in it as I believe the downpipe is the most restrictive and I have a Group A downpipe...

Regards,
R.