Decating a speed 6 easy option

[L88KED]

My engine is on the deck and im putting in all new racing green's hardened fingers and cams in her,
while its out and the exhaust is off, i was wondering if its possible to just pull out the cats in the exhaust and run it with out them. ?

Has anyone done it and if so what sort of differance does it make to both power and noise ?

cheers

[yzf1070]

L88KED said:

My engine is on the deck and im putting in all new racing green's hardened fingers and cams in her,
while its out and the exhaust is off, i was wondering if its possible to just pull out the cats in the exhaust and run it with out them. ?

Has anyone done it and if so what sort of differance does it make to both power and noise ?

cheers

Noise increase is certainly evident, power, I believe, is not so much (I think any gains felt are more in the mind).

If you will be running the engine with increased valve timing overlap (red rose or S spec), then you would be wise to leave the cats in place. Reason being your car will almost certainly fail the emmissins test come MOT time. If your cats are already shot and you want to test what the car would sound like decatted, then you can gut the existing cats by breaking up the honey comb substrate and pulling it out (wear a decorators breathing mask). If you decide on running the car with out cats I would recommend proper decat pipes for better flow than the standard decatted cans. If need replacement cats I really dont believe you can get better value than at www.cats2go.com the only difference I can see is that they do not have as heavy a duty outer heat shield skin, this is easily resolved if you really have a need to.

By the way if the car is a standard 4ltr then providing the engine is in a good state of tune, it should hopefully pass an MOT decatted, others on here have reported to have gotten away with it.

Good luck

[Buffoon]

Have enquired about having mine done and always get advised not to. Loss of back pressure is not a good thing. I believe there is a sports cat available if you want to go down the middle of the road route, but personally I'd wait until your cat is shot before spending money on it.

[nelly1]

Buffoon said:

...Loss of back pressure is not a good thing...

Erm, why?

Edited by nelly1 on Friday 26th September 10:15

[yzf1070]

nelly1 said:

Buffoon said:

...Loss of back pressure is not a good thing...

Erm, why?

Edited by nelly1 on Friday 26th September 10:15

A wild guess..... In theory, optimum performance N/A engines can have exhaust header lengths that are finely tuned so the back pressure pulses aid with scavenging in the cylinder and help draw in a good charge mix when the exhaust valve is on overlap with the inlet. Messing with the flow in the exhaust could reduce this effect. I understand that it is effective over a very short rev range and where gains can be made in one area they can be lost elsewhere. However I seriously doubt that this extremely time consumming development exercise was applied to the S6. Then again I could be talking bks and be totally off the mark. On a 4 stroke I have always thought that getting rid of the gases as quickly as possible gets rid of heat and reduces restrictions...better flow through the engine = better oomph.

I suspect Jools or Boosted LS1 can more appropriately explain.

[Buffoon]

nelly1 said:

Buffoon said:

...Loss of back pressure is not a good thing...

Erm, why?

Edited by nelly1 on Friday 26th September 10:15

Sorry, a bit thin on facts. Reduction in back pressure will of course improve potetnial power, but the loss of this pressure will draw more air through the intake, and on many cars this will result in overheated exhaust valves due to weaker mixture. Anything that may impact the speed six engine scares me.

Car needs to be correctly mapped to deal with this change, and from there I'm afraid my knowledge runs out. ECUs and Mapping are a black art to me. Now where's my hammer?

So in short, if you change what comes out you must ensure the appropriate change is made to what goes in.

[nelly1]

yzf1070 said:

nelly1 said:

Buffoon said:

...Loss of back pressure is not a good thing...

Erm, why?

A wild guess..... In theory, optimum performance N/A engines can have exhaust header lengths that are finely tuned so the back pressure pulses aid with scavenging in the cylinder and help draw in a good charge mix when the exhaust valve is on overlap with the inlet. Messing with the flow in the exhaust could reduce this effect. I understand that it is effective over a very short rev range and where gains can be made in one area they can be lost elsewhere. However I seriously doubt that this extremely time consuming development exercise was applied to the S6.

You're confusing pulse-tuning of the manifold with back pressure.

yzf1070 said:

...On a 4 stroke I have always thought that getting rid of the gases as quickly as possible gets rid of heat and reduces restrictions...better flow through the engine = better oomph.

Bingo! You don't see silencers on an F1 car for a good reason...

[yzf1070]

nelly1 said:

yzf1070 said:

nelly1 said:

Buffoon said:

...Loss of back pressure is not a good thing...

Erm, why?

A wild guess..... In theory, optimum performance N/A engines can have exhaust header lengths that are finely tuned so the back pressure pulses aid with scavenging in the cylinder and help draw in a good charge mix when the exhaust valve is on overlap with the inlet. Messing with the flow in the exhaust could reduce this effect. I understand that it is effective over a very short rev range and where gains can be made in one area they can be lost elsewhere. However I seriously doubt that this extremely time consuming development exercise was applied to the S6.

You're confusing pulse-tuning of the manifold with back pressure.

I am aware of the difference between the two, I was wondering if this was the perceived reasoning behind the advice being given. However, the other chap had a good point, would the decatting make the engine run significantly weaker mixtures? Is that why I have seen decat chips mentioned? I am sure there are folk out there who will have remapped a decatted S6 by now, is the fueling significantly weaker after a decat? Anyone suffered burnt exhaust valves as a result of decatting and not remapping?

[tail slide]

yzf1070 said:

nelly1 said:

yzf1070 said:

nelly1 said:

Buffoon said:

...Loss of back pressure is not a good thing...

Erm, why?

A wild guess..... In theory, optimum performance N/A engines can have exhaust header lengths that are finely tuned so the back pressure pulses aid with scavenging in the cylinder and help draw in a good charge mix when the exhaust valve is on overlap with the inlet. Messing with the flow in the exhaust could reduce this effect. I understand that it is effective over a very short rev range and where gains can be made in one area they can be lost elsewhere. However I seriously doubt that this extremely time consuming development exercise was applied to the S6.

You're confusing pulse-tuning of the manifold with back pressure.

I am aware of the difference between the two, I was wondering if this was the perceived reasoning behind the advice being given. However, the other chap had a good point, would the decatting make the engine run significantly weaker mixtures? Is that why I have seen decat chips mentioned? I am sure there are folk out there who will have remapped a decatted S6 by now, is the fueling significantly weaker after a decat? Anyone suffered burnt exhaust valves as a result of decatting and not remapping?

Ran my 3.6 engine for 10k miles decatted no remap. Didn't get that one on a dyno to check fuelling, but valves etc fine when engine came out for 4-litre and was checked, and I understand that scavenge effect has minimal effect on mixture even with the 3.6 or 4.0S high-overlap cam timing (apart from low rpm).

To drive, was tiny acceleration increase in top-end rpm according to data logger, and was slightly lumpy just above idle, smoothing of which I'm sure would have been main effect of a de-cat chip.

Edited by tail slide on Saturday 27th September 13:30