Exhaust Back pressure- De-cat?

[bhpjon]

Afternoon chaps, does anyone have a definitive answer as to power benefits /losses on a set of de-pre-catted exhaust manifolds. I have often fancied doing this for the noise, but i have read that the rover needs the back pressure that it has with the standard set up. Does anyone have back to back figures for pre-catted and de-pre-catted 500's please? all contributions gratefully recieved!

[IPAddis]

It's reputedly worth 5bhp if you fit flow cones to smooth the exhaust gasses. Back pressure is primarily provided by the main cat.

Ian A.

[GreenV8S]

No noticeable difference to noise or power, unless you're starting with blocked pre-cats. On average you can expect to get a power gain of a couple of bhp but rolling road measurements typically aren't accurate enough to show that reliably and it's *far* too small to be noticeable on the road.

Edited by GreenV8S on Monday 28th May 13:34

[bhpjon]

GreenV8S said:

No noticeable difference to noise or power, unless you're starting with blocked pre-cats. On average you can expect to get a power gain of a couple of bhp but rolling road measurements typically aren't accurate enough to show that reliably and it's *far* too small to be noticeable on the road.

Edited by GreenV8S on Monday 28th May 13:34

I haven't managed to find a question that you've not been able to answer Sir! Do you know if it's worth having the cones fitted?

[Sam Gamgee]

Will be having this done on mine....more to aid heat reduction under bonnet, really, without any "cones", I believe.

Dave.

[Nuggs]

I was advised that cones aren't required on the 400, but are a good idea for the 450s and 500s...

[GreenV8S]

bhpjon said:

Do you know if it's worth having the cones fitted?

I have them on mine because I thought they might help exhaust reversal at low rpm, doesn't seem to make a huge difference either way. People who have done without them have reported a slightly tinny sound to the exhaust, can't be sure this is due to the empty precat chambers but that's my suspicion.

[andy-w]

Jon,

If you fancy a dodge over to my house you can borrow my manifolds to see what it is like. I do not have any cones fitted though.
Take the main cat out as well....!!!

[thenick]

I wouldn't have thought that it made a loss by removing the cats, after all the engine originally ran without cats anyway! I would think that leaving the large bore pipes in place after a decat might cause some problems with gas flow though, hence the non reversal cones? - I have the ACT decat stuff and this has a much smaller diameter pipe all the way through so maybe this has a similar effect? There is a very noticable increase in the speed of the gasses coming out the back so this may be where the performance increase comes from? I did notice that the pre-cat engines were quoted with a slightly higher power output as standard... Thoughts??

Edited by thenick on Monday 28th May 19:44

[spend]

I cant see how it could possibly be viewed as a 'generic' improvement to have smaller pipes. I could only see this scenario working when very finely tuned to particular intake / heads and cam. I may be wrong but I just cannot make sense of the phenomena and the back pressure theory has always perplexed me... I am pretty sure it would be detrimental for Forced Ind though.

Dave

[bhpjon]

andy-w said:

Jon,

If you fancy a dodge over to my house you can borrow my manifolds to see what it is like. I do not have any cones fitted though.
Take the main cat out as well....!!!

Hi Andy, Glad to see all is going well with the build, with any luck you'll have it done by the winter!
I'm only considering it as i've got the motor in bits at the moment but when i asked a local exhaust engineering company to fit the cones for me, they said the pipe after the cones would have to be sleeved as well to keep the bore constant, otherwise you would have a virtual whirlpool as the gases would spill out of the cones into a much bigger space, cheers Bhpjon