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Discussion Starter #1
By de-cat I mean having a piece made up complete with flanges on each end so that it's a simple bolt on bolt off job to change at MOT time.

Must be worth 30bhp for not much cost...............?
 

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It'd be a large bypass and you'd have to do something about the lamda sensor

 

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Discussion Starter #3 (Edited)
Cheers for that.

Looks pretty straightforward to me - the flanges are even there! Lambda sensor just needs a hole/lug welding in presumably.

Anyone know what diameter the pipe is?

I know a great exhaust company who could definitely do this, but they can only do up to 2.5" diameter............

Another plus would be that it might make your cat last longer and delay the day when you have to spend over a £grand on it...........
 

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The cat has been removed on mine by the previous owner.

I doubt it's got 30bhp more than it did before. It certainly doesn't feel like it has.

I reckon you'd gain 1 or 2 bhp if any. Chances are with the standard exhaust flowing so well for a stock system you'll actually lose some torque due to the decreased back pressure.
 

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Discussion Starter #5
ste said:
The cat has been removed on mine by the previous owner.

I doubt it's got 30bhp more than it did before. It certainly doesn't feel like it has.

I reckon you'd gain 1 or 2 bhp if any. Chances are with the standard exhaust flowing so well for a stock system you'll actually lose some torque due to the decreased back pressure.
Mmmmmmmm...........interesting................

Thanks for your comments.

Anyone else with experience of this?
 

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Having said that mine is the only e34 M5 I've driven. It doesn't feel fast enough to have any power over standard though.

If anyone's local to me in West Sussex with a standard 3.6 I'll gladly do a back to back test with them. :D
 

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I've heard for gains as much as 30hp and more from removing the cat on other cars, so why would ours be any different?

Would be interesting to get your "de-catted" car on the rollers alongside a standard one, just to see...
:M5rev:
 

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Hi,

I de-catted my M5 about 10 months ago. There is absolutely no loss in torque or horsepower, which is contrary to what experts and BMW technicians say. In fact, together with other M5 owners here in Brazil, we have dynoed several M5s with and without the cats, be it 3.6 or 3.8. The ONLY difference is that after you pass the peak point on the S38´s torque curve, the line on the graph does not fall off as much as it does when the cats are on.

So basically, you are able to hold more torque at higher revs after you reach the engine´s peak torque. Also, there is a little bit of horsepower gain at high revs, but nothing more than 2-3bhp. What we have seen from the graphs is that there is ABSOLUTELY NO LOSS IN HP OR TORQUE through out the hp and torque curves.

The reason why you gain very little in terms of power when you take off the cats is that the exhaust on the M5 is extremely effecient already, so there´s not much room for improvement in this sector. The same applies to the induction on the car. In fact, the M5 will actually lose a bit of power without the stock induction system.

You can clearly feel that the engine "breathes" and accelarates a little bit easier once the cats are removed though.
 

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plisken said:
I've heard for gains as much as 30hp and more from removing the cat on other cars, so why would ours be any different?
Because it's already at a high specific output per litre for a naturally aspirated engine.

The only way you'll gain 30bhp just by removing a cat is on a turbo car that has a very restrictive factory exhaust. (Turbos love as little back pressure as possible).
 

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Rob Plummer said:
By de-cat I mean having a piece made up complete with flanges on each end so that it's a simple bolt on bolt off job to change at MOT time.

Must be worth 30bhp for not much cost...............?
From my experience you should gain at least 15 RWHP & 10 RWTQ w/o CATs :M5thumbs:
 

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kris said:
From my experience you should gain at least 15 RWHP & 10 RWTQ w/o CATs :M5thumbs:
Are those figures from the same rolling road on the same day?

i.e. did you take a before figure, swap the cats for the bypass pipes and then do another run on the same day at the same atmospheric conditions and temperature?

If so that's a superb result.
 

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ste said:
Are those figures from the same rolling road on the same day?

i.e. did you take a before figure, swap the cats for the bypass pipes and then do another run on the same day at the same atmospheric conditions and temperature?

If so that's a superb result.
Not the same day but the same atmospheric conditions
 

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Rob Plummer said:
By de-cat I mean having a piece made up complete with flanges on each end so that it's a simple bolt on bolt off job to change at MOT time.

Must be worth 30bhp for not much cost...............?
High!
I did just because of the gasoline quality here - i didnot feel no power increase actually.
Good luck :flag:
 

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I've noticed the guys claiming they've noticed a diference are in the US. Is it possible the US cars have a more restrictive factory cat? I know the US tend to get less power than the rest of the world, model for model due to their stringent smog laws. Does the US M5 have charcoal canister etc?
 

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Simonal said:
Hi Rob

One thing if you are still interested Supersprint seem to have a Kat replacement pipe listed in addtion to the standard replacement exhausts.

http://www.supersprint.com/homeuk.htm

May help.

Simon
"For racing use only" :hihi:

Sounds interesting... I bet it would actually
 

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Guys..

There is a disclaimer with the cat replacement front pipe from supersprint,
basically it will make your car louder and in some parts of the world you will get a ticket for noise pollution..
Thats why they say for race use only.

This info came from a UK distributor of supersprint exhausts.

Regards Andy S
 
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