Just found this on another forum:

http://www.mgf.ultimatemg.com/group2/exhaust/catalysts.htm

Makes interesting reading, OK the effects maybe higher and on a greater BHP engine, but still, not very impressive to say the least.

Ben

but they are NA cars

Very true, but would that have a major effect?

NA engines usually depend on backpressure more than FI engines

Changes in the exhaust pipework - changes in width, silencers, and the cat connection - change the impedance of the system to gas flow. Where there's an impedance mismatch, you get reflections in the system and pulses of exhaust gas moving in both directions - effectively, you can get pulses of low pressure at the manifold.

On an NA engine, the valve overlap is designed to use these low pressure pulses to clear the last combustion products from the cylinders before the inlet cycle. If you put in a cat replacement pipe, all your exhaust timing is probably confused.

On a turbo engine, it's immaterial, since the turbo blades chop up the gas flow beyond recovery; there you simply want the lowest back pressure you can get for maximum gas flow.

Though removing the cat is of course illegal on any current road car since you will not meet the emissions specs.

Not entirely correct, any Pre 1993 vehicle does not require a catalyst to pass an MOT as the emissions test are far less stringent.

Hence many RX7/GTO/Supra/300ZX owners buying 92 reg cars for this reason.

Are you sure about the back pressure being irrelevant on a turbo? I thought reducing back pressure put more strain on the turbo seals. I'm clueless myself; just repeating what I've read somewhere.

Irrelevant as far as the lack of back pressure affecting the flow of gas, but it comes at a cost as you say, eventually !

isanyone running with minus the back box just a straight pipe form the cat and an exhaust tip, worked wonders on my leon cupra tdi, helped spool loads better etc, after map etc of course, was guna ask this as thinkin of doing it,

My exhaust is loud enough, no silencers would be be very obvious to the police in terms of sound and looks, and that kind of noise is bordering on the antisocial

Akeme - of course, older cars have different rules. I meant Coupe vintage

Alan - on a turbo, the back pressure is immaterial to the *engine* as it doesn't scavenge the cylinders like an NA. But as others have said - and there's *plenty* of anecdotal evidence in the tech threads - if you *do* reduce the back pressure you need to modify the turbo seals to accommodate this. But that's a mechanical point, not an engine efficiency effect.

Decat a Supra TT, gain around 40Bhp. 'Nuff said?

Take the precats off a MK3 MR2 and you have an engine that isn't likely to destroy itself from the inside out (apparently the cat material and break off and get sucked back in through the inlet because it recycles exhaust emissions or something daft - for the US market). Really got to get round the ripping these out as soon as the weather improves!

How much does a Coupe decat gain then...does anyone know? I know it improves spoolup....

I've got a GT28r with a cat still....would I need another Live map if I got a decat pipe fitted?

Ben

Personally, i have de-catalyzed my own NA, i changed the original silencer with a smaller one passing (you can see through one side to another, not like the original) and the whole system is 65 mm, with a T16 supersprint (road legal) exhaust..

It's MUCH more better than before, the car has changed a lot on mid-high revs (from 3800-6500) and I gained something also on low revs...

Since I don't have MOT problems (:P) , i can still keep this configuratio.. otherwise i would have put a Brain 200 cells Kat..

Brasilian spec 16 NA (all they sold there) were all sold without cats. And with an inch more on the suspension...

Coupe monster trucks

You mean it was one inch higher?

Yes. (2.5cm) The roads in Brasil are of, um, variable quality.

Often you can cross the equivalent of a county boundary and find that the road you're on changes from absolutely smooth blacktop to something with twenty or thirty centimetre potholes... embarrassing if you're doing a hundred km/h when you hit them. I've killed a set of tyres on a one-litre baby Fiat in a thousand km.