does anyone know how much bhp i will gain from removing the catalytic converter or if i will lose power from doing so?

This belongs in naturally aspirated really but I'm sure the mods will come and move it

Search the Naturally Aspirated section - a few people have tried it and said the gains are significant, others said they didn't notice a thing. It depends on the design of the de-cat, NA engines need a bit of backpressure and removing the cat gets rid of that (especially if you have a free flowing exhaust). So you would do better to keep the cat in place really, unless you get a properly designed de-cat that is narrower than the rest of the system, to increase the velocity of the air, improving torque. An equal sized de-cat is good for flow (so great on turbo cars) but crap for pressure (stick your hand over the spout of a tap while it's on - the water will flow MUCH faster )

I don't understand the myth of back pressure...

the only engine that really needs backpressure is a 2 stroke

A free flowing exhaust is better for any 4 stroke, even naturally aspirated, but it does rely on fluid dynamics, so a 4" straight through pipe will be too large for a small engine - not really pressure as such, just airflow


decat should give you 5 - 10 bhp

A [i]faster[/i] flowing exhaust will create more torque, whereas an exhaust that can move more air will create more power up at the top. So a 4" straight through would kill any torque the 1.5 ever dreamed about but a 2" pipe, with a 1.75" decat would be good

Topic moved

I have considered this a few times until i saw the price of the de-cat pipes, does anyone know of anywhere cheaper or possibly prices for custom ones.

http://www.jeroentje.nl/index.php?page= ... d=80104200

[quote="steven"]Topic moved

I have considered this a few times until i saw the price of the de-cat pipes, does anyone know of anywhere cheaper or possibly prices for custom ones.

http://www.jeroentje.nl/index.php?page= ... d=80104200[/quote]


That is advertised as a cat rather than a de-cat?

[quote="marcrs"]I don't understand the myth of back pressure...

the only engine that really needs backpressure is a 2 stroke

A free flowing exhaust is better for any 4 stroke, even naturally aspirated, but it does rely on fluid dynamics, so a 4" straight through pipe will be too large for a small engine - not really pressure as such, just airflow


decat should give you 5 - 10 bhp [/quote]


trust me, a 1.5 with no middle muffler and a small one at the back drives like sh1t - You loose all back pressure! So a 1.5 with a de-cat, no mid muffler and a small silencer at the back would sounds like a VTEC Civic but go like a snail on Viagra, trying to move but really not going anywhere quick.


For what its worth on a 1.5, leave the standard CAT, middle section and just get the rear box changed for a little noise.

But defo stick a decent induction system on it

trust me, a 1.8 GT with no cat breathes much more easily than with one, and revs easier...


weld a plate on the end of your tail pipe, just leaving a small hole - that will give you plenty of back pressure...see if your car goes better with it

I should think somewhere like powerflow or freeflow could make up a decat pipe relatively cheap? Or get hold of an old cat take the flanges off it and get a welder to stick a 1.75inch peice of tube in between, can't cost that much?

£80 from powerflow

i had the jerontje decat on my 1.5 for a bit but it killed the torque! can't really remeber if it was better up top, could have been, but for daily driving it was annoying as you had to rag it about more

have a decat on the mx5 think its roughly 2" all the way! exepct the manifold primaries? before the downpipe bit. and its much better than stock! (seems to work on the 1/4 times not tried dyno again yet) and you get nice pops now and again

although the mx5 is fully stainless where as the 1.5 was stock apart from backbox and decat

Kash and Murray have it spot on, fitting a de-cat pipe will only kill your torque, especially on a 1.5, makes it a bit louder too, if that floats your boat!

I've been thinking about this for a while though, every de-cat pipe I've seen on an F, matches the 2 inch downpipe diameter at the front end, runs 2 inch all the way along till it tapers to 1.5 inch right at the end. If you think about it, the CAT must create some back pressure or maybe its just gas flow, but I think if you had a de-cat pipe made up that tapered to 1.5 inches pretty much where the cat starts, you might keep the torqu AND gain some power!!!!

As I said I've been thinking about this for some time but I don;t want to waste £80 or so finding out

yep i am agreed with you Andy, i really dont want to waste 80 quid finding out. Maybe one day.l

My V6 got more energy after decat

Did you have it tested before and after? I know from experience that with some mods the improvement was in my mind and not on the car

[quote="andym323"]Did you have it tested before and after? I know from experience that with some mods the improvement was in my mind and not on the car [/quote]
Hehe.. I know what you mean, but it really got few horses. I can actually feel that, especially on low rpm's. And it's way nicer to drive that car everyday now

i went to pipecraft in basildon to get mine dont. it cost me 250 but he did me a whole new straight through titanium exhaust system. well worth it, sounds the nuts and you can feel the power difference

as far as i know, a decat that is SMALLER in diameter than the rest of you exhaust-system will improve torque.

nothing to base it on, just experience from guys from the dutch forums.

so, just kicking this one:

is it easy to make your own decat?

as long as you got a good stainless welder and some stainless pipe and stainless plate to make the joints

Stainless welder?

You only need a Mig welder, and some stainless welding wire

[quote="marcrs"]Stainless welder?

You only need a Mig welder, and some stainless welding wire [/quote]

thats what i ment lol

So stainless is the way to go? Was thinking of using a regular steel pipe..

oh and what size diameter? same as the rest of the exhaust?

use the same size as the rest of the exhaust as you dont want the pressure changing too much due to restrictions

hmhm, don't know the exact diameter of the standard exhaust, but was thinking of making it the same or a tiny bit smaller in case I can't get a pipe with the same size.

but a regular steel pipe will do too?

regular steel will do but if you want it to last get some nice stainless

hmm okay thanks! yeah you're right, but with a can of anti-rust coating and some heatwrap i'll get far enough

Could always buy mine for £60 and give it a go? I liked it and it gave me more power. However I did have the cat back widened, but the decat isn't as wide

think I can make one myself for less then that, but thanks for the offer

hm, anyone happen to know some details about the cat? length of the cat, etc?