De-Cat or Not to De-Cat

Alex 111s · Post by **Alex 111s** » Mon Jan 14, 2008 3:13 pm

My 111s has had a de-cat pipe fitted to a std. exhaust rear box. i've read that this doesn't actually do any benifit for the performance of the VVC. is this true? also, does it actually do any harm to the engine?

I'm considering re-instating the Cat and going for an aftermarket back box (Larini sport or possibly club sport) to save on the yearly re-fitting for MOT time. I don't want the exhaust to be too shouty, should i steer clear of the club sport?

all advise is welcome.

thanks

Alex

2F45T4U · Post by **2F45T4U** » Mon Jan 14, 2008 3:41 pm

this is a popular misconception in my opinion.

yes back in the day cats used to choke flow. but now not so much. or if there has been some over rich fueling and its melted it causing the cat to colapse

if the engine is heavily tuned over standard, then yes there will become a point where the engine is breathing so well that the cat becomes a restriction. but often its best left as it is. its also easier to get MOTs without lots of fraffing around.

BiggestNizzy · Post by **BiggestNizzy** » Mon Jan 14, 2008 3:53 pm

If memory serves me right and I await to be corrected, the VVC prefers the back pressure of the cat, and removing it causes a drop in power. probably by the same amount a "normal" car gains i.e. not very much

GregR · Post by **GregR** » Mon Jan 14, 2008 4:18 pm

VVC requiring back pressure from the cat was my recollection too

gorrie · Post by **gorrie** » Mon Jan 14, 2008 5:30 pm

Larini with cat in place is not too shouty mate... (clubsport with cat here

)

robin · Post by **robin** » Mon Jan 14, 2008 6:12 pm

Put the cat back on - performance is irrelevant - think of the people you're poisoning while sitting in traffic ...

I have no opinion on the back pressure theory, except that it won't be back pressure it needs. Exhausts make a difference for two reasons:

(1) If you had no exhaust at all (beyond a couple of straight pipes to help reduce turbulence) the cylinders would empty most efficiently, thus reducing the cost to the engine of forcing the gas into the manifold. Anything that restricts the exhaust will increase the cost of clearing the cylinder and thus sap power.

(2) If your exhaust has a restriction at the correct place, you get a pressure wave reflected back to the exhaust port; depending on the relative temperatures and pressures you can get a -ve or +ve pressure wave reflected. A -ve pressure wave timed to arrive at the exhaust port at just the right point causes "scavenging" which when combined with overlapping inlet and exhaust valves helps pull fuel/air mixture into the chamber at the top of the piston stroke.

In my opinion the cat is too far down the pipe for it to have any effect on scavenging and in any case the design of the standard exhaust manifold will dominate the exhaust behaviour in this respect.

That said, it's an oft-repeated statement, so perhaps there is some truth in it somewhere or another ...

Cheers,
Robin

Rich H · Post by **Rich H** » Mon Jan 14, 2008 6:22 pm

Would a long pipe not help with scavenge at lowrevs and a short pipe help with high revs? just like the inlet manifold?

a4drk · Post by **a4drk** » Tue Jan 22, 2008 5:58 pm

I've got a Stainless Cat Replacement Silencer box.
Its never actually been fitted as the person who made it never drilled a hole & welded the nut for the lambda probe to screw into.
I wasted £20 buying this on ebay & another £18 on an MIL resistor thing.
I might eventually get someone to weld a nut on it & try it out.
I know it won't give me any extra BHP...... i just want it to spit flames !

Here it is.......

robin · Post by **robin** » Tue Jan 22, 2008 8:14 pm

Yes and no - you need to tune to pipe to the point where it's going to make the most difference - smack in the middle of the cams overlap induced feeding frenzy I suspect ... at low revs, the cylinder fills and clears no bother anyway.

Mostly irrelevant on elise - you're stuck with one of three patterns really due to space constraints. Pipe diameter can make a big difference as well - too big and you cool/slow the gas too soon; too small and you choke the exhaust.

Cheers,
Robin

Rich H · Post by **Rich H** » Tue Jan 22, 2008 8:18 pm

Got a spare lambda boss somewhere if you want it.

a4drk · Post by **a4drk** » Tue Jan 22, 2008 8:46 pm

Cheers Richard
I've already got one.

I bought a 2nd hand Janspeed off ebay last year
It came with the cat & probe rusted onto it !
also came with the flexi downpipe.

So if anyone needs a standard downpipe or cat ........

David

Edin430 · Post by **Edin430** » Tue Jan 22, 2008 10:34 pm

Ali is holding onto my standard box and cat for me for my next MOT so if anyone is struggling give him a shout