Old 06 January 1999, 05:14 PM
John Phillips
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Posts: 109

I have recently noticed my MY98 backfiring on changing of gears, revving the engine or coasting in say 2nd to a junction letting the engine slow the car down.
I have recently fitted a Scoobysport backbox and ramair filter panel.
Is this normal or is there cause for concern. It does sound good but if its not correct I will have to get it sorted.
Any suggestions would be appreciated.
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Old 06 January 1999, 11:29 PM
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Backfiring damages the internals of the cat.
The inside of the cat is a honeycomb ceramic element which is coated with the platinum/rhodium that does the work.
This element is the equivalent of having a crystal decanter in a tin can slung under the car.
The cat is therefore very unhappy with backfiring!!!!!
As to the reason for the backfire i understand that some aftermarket air filters cause the ECU to run a richer mixture (something to do with oil getting on the hot wire element of the air meter). Maybe this is the cause?
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Old 07 January 1999, 12:02 PM
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My guess is that you have a leaking exhaust joint. Almost certainly the one between the OE system and the new rear box.
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Old 08 January 1999, 09:07 AM
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This is probably obvious, but there's an important distinction between "popping" (which most Impreza's do on the overrun when driven hard), and "banging" (which is really backfiring and shouldn't happen, although it's quite fun on my non-fuel injected motorbike to scare peds occasionally ;-).

If you're seeing the latter, I'd get it looked at, for the reasons outlined by the others above.
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Old 08 January 1999, 09:22 AM
Pete Croney
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Location: Scoobysport, Basildon, UK
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The cause of this is un-burnt fuel igniting on contact with the exhaust manifold.

The sound comes out, when you fit a Scoobysport, Prodrive or other "straight thru" back box.

It is most prevalent when the car is being driven hard and the manifold is at its hottest, creating a popping and bubbling sound on "over run".

All turbo charged Impreza's do it, but the OE back boxes absorb the pulses that it creates, so you can barely hear it.

Enjoy :-)
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Old 08 January 1999, 10:04 PM
John Phillips
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Thanks for the replies guys. The backfiring is just as Andy and Pete have described it. A popping rather than blow your pants off, although the first time I heard it, that's what happened.
My mind is totally at ease knowing this grin inducing noise is ok.


Pete this exhaust has totally transformed my Scooby...cheers mate!!

[This message has been edited by John Phillips (edited 08-01-99).]
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