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Discussion Starter #1
Hi my 156 20v has 2 cats 1 precat/ and 1 large cat 175bhp.
Does anyone know how much power I would gain by decating.
 

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I removed my pre-cat. She spools faster but don't know about the power. And she sounds even better.
 

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I gained about 5bhp and 2mpg with faster spool up times and a nice noise to boot, well worth getting it done mate.
 

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20v has a centre cat similar to that on the 10v but also has a front cat built into the downpipe. The centre one is easy - just cut it out and weld in a plain pipe. The front one is expensive if you have to buy one but I'm going to get my son the demon welder to make one when we've got time.
 

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Has any one hollowed out both cats on a 20v
Hi,

It'd take some doing on the downpipe cat - its a coiled metal substrate. You'd have to open the can and remove it I reckon.

Not sure about the underbody cat, never looked inside it (although might be doing soon if I can actually get a front pipe sorted!)

@TOXIC: not bad really - check how much supersprint want (if they EVER release it) :)
 

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Discussion Starter #11
That's the 2 cats hollowed out about 2 hours work my Alfa booked in for it's mot on Monday and have a 100 mile drive. will let you all know the results.I am will be happy to get an extra 10 bhp 25 would be nice as then I would have 200 bhp we can al dream.
 

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That's the 2 cats hollowed out about 2 hours work my Alfa booked in for it's mot on Monday and have a 100 mile drive. will let you all know the results.I am will be happy to get an extra 10 bhp 25 would be nice as then I would have 200 bhp we can al dream.
Nice one! How did you do the downpipe one, just out of interest?
 

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Discussion Starter #16 (Edited)
A couple of bent chisels and some patience. Definitely no need to cut the precat open.
Does any one have any idea how much extra power I have gained.
 
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I always wondered about this but surely hollowing out a big box with a hole at each end and parallel ends is gonna be a turbulent beastie that still bothers flow of air a fair bit.

Seeing as it's just a nodding curved pipe I've no idea why someone doesn't sell a steel option for £30 -40 quid.

I'd much rather front was a plain pipe.
 

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A couple of bent chisels and some patience. Definitely no need to cut the precat open.
Does any one have any idea how much extra power I have gained.
ooh now thats food for thought next time i've got it off :)

I'll be interested to see what results you get now.

@mr-mac - yeah you'd think a decat would be fairly cheap. I think its the flanges and angles involved that add to the cost (or thats probably the reason a builder will give). The supersprint "coming soon" (for the last 5 years) one is made from "cut and shut" formed sheet so it's possibly not all that straightforward?
 

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Discussion Starter #20 (Edited)
Definite improvement with my double decat. sounds much nicer can now hear the turbo lovely smooth howl when accelerating much better throttle response.On my way to glasgow just before arrochar my rev counter was reading 3700 rpm,Was I in 3rd gear???(15.8mph per1000 revs) or 6th gear???(33.5mph per 1000revs).I was not in 5th 4th 2nd or 1rst gear near arrochar. At 3700 revs in 5th it would be a licence losing 102mph/(4th gear 81mph).We don't have any permanent speed cameras on the A83 I have seen a silver 5 series unmarked police bmw.
I would recommend a decat and I am hopeful I will gain a few extra mpg.
 
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