Decatting a 911

[diddyman]

A friend of mine has a 911 (I think it is a Carrera 4S Cabriolet, but I may be wrong!)

He likes the pops and bangs I get from my car and I told him I suspect the only way he will achieve anything close to the sound I get is by decatting his car. He has already has a Sports Exhaust (which sounds like a drain pipe IMHO!)

Is decatting a 911 a simple task? Do a lot of Porsche owners do this?

Thanks in advance
Jim

[leosayer]

I don't think it's complicated but the car won't pass an MOT without a cat (unless it's pre 1993 like mine )

[gazzab]

But it wont sound like a flat plan V8 AJP engine. It will sound like a decatted porsche!

[diddyman]

leosayer said:

I don't think it's complicated but the car won't pass an MOT without a cat (unless it's pre 1993 like mine )

It's only 6 months old or so, so he is OK for a few years on the MOT, and can just stick em back in again for the test. (Like wot I does.)

gazzab said:

But it wont sound like a flat plan V8 AJP engine. It will sound like a decatted porsche!

How does that sound? Will it pop and bang? His sports exhaust sounds dreadful - like a very flatulent cow farting into a drain pipe. ANYTHING would sound better than that!

[lanciachris]

If he wants pops and bangs I would expect that as well as decatting hed have to alter the efi program so it overfuelled a bit, much like I would think a cerb does as stock.

[andy_ash]

My 911 has no Catalyst, although it was manufactured with one. It doesn't have to have one by law.

In my opinion it sounds better for it too. The other thing that I think makes a huge difference to the sound of a car is a Stainless versus a Mild Steel Exhaust. I always think a Mild Steel exhaust sounds far better. You do get problems with rot though!!!

I would have thought that all that popping and banging is going to do the insides of your engine a mischief in the end. I guess it's a bit different with turbo overrun, but I always wonder exactly how lean the mixture actually is, when one of these supercars goes bang.

Taken to an extreme, if you end up with detonation it's a bad thing!!!!

[AC79xxx]

andy_ash said:

I would have thought that all that popping and banging is going to do the insides of your engine a mischief in the end. I guess it's a bit different with turbo overrun, but I always wonder exactly how lean the mixture actually is, when one of these supercars goes bang.

I was lead to believe that the flames and popping was a result of over fueling where unburnt fuel ignites in the exhaust manifolds??

[diddyman]

andy_ash said:

My 911 has no Catalyst, although it was manufactured with one. It doesn't have to have one by law. In my opinion it sounds better for it too. The other thing that I think makes a huge difference to the sound of a car is a Stainless versus a Mild Steel Exhaust. I always think a Mild Steel exhaust sounds far better. You do get problems with rot though!!! I would have thought that all that popping and banging is going to do the insides of your engine a mischief in the end. I guess it's a bit different with turbo overrun, but I always wonder exactly how lean the mixture actually is, when one of these supercars goes bang. Taken to an extreme, if you end up with detonation it's a bad thing!!!!

He has some button somewhere on the dash that he presses and it goes from "one windy camel belching in a nissen hut" to "a whole herd of buffalo blowing bubbles in a waterhole". I think this was a Porsche modifcation on purchase.

Pops and bangs are, as Riccardo says, merely xcess fuel buring off on the exhausts. Has no direct effect on the engine. Just sounds great and makes people in the car behind you grin!

[andy_ash]

AC79xxx said:

andy_ash said: I would have thought that all that popping and banging is going to do the insides of your engine a mischief in the end. I guess it's a bit different with turbo overrun, but I always wonder exactly how lean the mixture actually is, when one of these supercars goes bang. I was lead to believe that the flames and popping was a result of over fueling where unburnt fuel ignites in the exhaust manifolds??

Well I think you're right. But the car has to be pretty highly tuned (i.e. quite lean) for the mixture to explode in the exhaust. Then you need some cycles without a spark or sufficient compression.

I suppose you could find yourself in this condition if you're valves were burned out as a result of having too little valve clearance, or maybe a whizzy cam of some kind.

I dunno. A nice exhaust note, even a straight through exhaust can sound nice. Big shoots of flame in a turbo, quite normal and possibly amusing.

Pops bangs and crackles, hmmm sounds sick (or going that way) to me.

[bumcrack]

diddyman,

Does it make any difference to either the Performance or the MPG?

[diddyman]

bumcrack said:

diddyman, Does it make any difference to either the Performance or the MPG?

I don't know on MPG to be honest, because mine's always been like that! I have heard that taking the cats of can reduce fuel consumption by about 10%. Most people I know with Cerberas take thew cats off as it does marginally inmprove performance (and they like the noise!) If I accelerate hard and then lift off fast I can get at least 2ft if not 3ft of flames from my exhausts (So sit well back!). Who cares if you only get 18 mpg instead of 20!!!!

[john purdie]

Diddyman,

Just seen you post.

I have a 1999 996 C4, I also would love to De-cat mine as a friend of mine has a Cerbie and god I love the sound and the burbling, It also gets more looks that mine when we are near a crowd of people, cant have that can we!

again I have had a few people look at removing the cats but it does seem quite a big job because of the way the cats are positioned if there is anybody out there that knows somebody who can do this or has had it done please let me know (I am near Brands Hatch)

[diddyman]

john purdie said:

Diddyman, Just seen you post. I have a 1999 996 C4, I also would love to De-cat mine as a friend of mine has a Cerbie and god I love the sound and the burbling, It also gets more looks that mine when we are near a crowd of people, cant have that can we! again I have had a few people look at removing the cats but it does seem quite a big job because of the way the cats are positioned if there is anybody out there that knows somebody who can do this or has had it done please let me know (I am near Brands Hatch)

Hi John

I'm waiting for someone to pop on here and say "Yes, I've done it"

No luck so far!

Even if it was a big job then I am sure he would do it. Surely a matter of removing a section of exchaust behind the manifolds and showing a new section of pipe in, me thinks...

[andy_ash]

If you were suggesting me. I didn't do mine. Not that it would have been that difficult, it's in a straight section of the exhaust under the nearside rear wing.

Four bolts out four bolts in. Jobs a goodun. I can't volunteer any information about the 996 because I have 911/930. I don't know anything really about the M64 and onwards.

I have an idea that the exhaust plumbing is a bit more complicated as you go forward in time. Having wrestled with my own Heat Exchangers and so on, I do wonder how they could have made it more complicated.

[ultra violent]

Pops and bangs result most relate to a lean condition. It tends to be a high rpm, low throttle area of the map. Here you clearly need very little fuel because you can power the car in that area, only really gets used as you take foot of accelerator at speed. Flames join the fun when you over fuel that same area.

As I have Motec I can dial in as much flame or pops & bangs as I like...

[andy_ash]

Cool.....

It's all down to personal taste in the end. I guess I must be cheap 'n' trashy

Does your management system have Knock Sensors. If it does have you used them. If you have, are they effective?

It's a cite cheaper than Ti Rods.