decat?

Gravelben · Post by **Gravelben** » Tue Sep 09, 2008 1:41 pm

Well I managed to knock a hole in my exhaust while doing some gravelbashing over the weekend. Looks like the hole is at the end of the cat casing where the temp sensor would be, but mine seems to have been disconnected at some stage - not just knocked off by the rock, the wire that would lead to the sensor is cut off some way back as well.

So I was thinking, given where the hole is would I be better off trying to find a suitable pipe to replace the cat section completely instead of just bodging something up, and does a decat give any real improvement in an MX5? I guess with the cut wire theres also the possibility that the cats been removed already, any useful thoughts?

poison · Post by **poison** » Tue Sep 09, 2008 2:57 pm

The insides may have been pulled out but rather unlikely due to the effort, usually they just cut it out and stick in a new section of pipe, quick and easy. I went to have mine taken out on my previous MX5, but the owner at the roll bar and exhaust place just laughed and said if it goes any faster it’s just the placebo effect and not worth the money although he gets lots of customers who swear by it. But in your case a straight section of pipe would be the quickest fix. Not sure how toxic all the stuff is inside if you start to pull them apart.

So this time Gravel-ben got Gravel-burn J

Post by **jif** » Tue Sep 09, 2008 5:14 pm

Hi Ben

may as well just straight-pipe it ... this may make things a bit stinky though, esp if you reverse, or go top down

When the miata magazine did an engine dyno they reckoned "less than 3 hp" improvement from a de-cat . So that's gotta be at least 2 hp right there !

cheers,
Jeff

SLYDIT · Post by **SLYDIT** » Tue Sep 09, 2008 5:34 pm

just make sure the green party dont hear about it or youll have a whole lot of sandal wearers around to your place wanting blood.

Post by **jif** » Tue Sep 09, 2008 5:57 pm

... or maybe tofu... not blood

seriously, mine stank like rotten eggs for quite a few months after the de-cat, then it went away and now it's just heady petrol vapours. esp when the engine's cold.

peter · Post by **peter** » Tue Sep 09, 2008 7:40 pm

I bought mine ten years ago with a leaky exhaust. Pit Stop in Grey Lynn suggested removing the cat along with the leaky bits, so I got them to put a 2 1/4 inch stainless steel pipe all the way to the back - it's still there. Slight improvement noted in power - something to do with less back pressure from the decatting and the bigger pipe.

And yes, good old Andy from Racefx added the dummy MR2-type exhaust for me, and good old Pitstop found me a second chrome pipe extension...

didn't notice no funny smells, must have been going too fast

Peter
DRGN1

Gravelben · Post by **Gravelben** » Tue Sep 09, 2008 9:14 pm

My exhaust looks like 2.25 inch pipe anyway? Apart from muffler etc obviously. Will have to find out if Cromwell even has a place that does exhaust work, unless I can find a pipe from somewhere that happens to fit the flanges for the cat.

Ian · Post by **Ian** » Wed Sep 10, 2008 2:42 pm

As a first step in my exhaust mod programme, I changed out the cat for a straight piece of pipe but soon tired of the 'rasp' at 3000rpm, so inserted a short resonator and sound was just a little louder than standard.
De-cating is, as mentioned, worth 2-3 hp, assuming the cat is in good condition. And no, on its own, 2 hp is completely undetectable, despite what anyone might say!! BUT, 2 hp here and 2hp there and 2 hp from over there when added together all add up and do help. Just as importantly,an aftermarket exhaust on a naturally aspirated MX5 is usually significantly lighter than the factory system. So you benefit both ways.........
Kiwi Undercar Ltd in Alex are exhaust specialists, not that I have used them.

Cheers, Ian

brownie · Post by **brownie** » Wed Sep 10, 2008 11:07 pm

above all else ,when you remove the cat make sure you get some money for it,They have a $100 .00 bounty for old cats thru most metal scrap dealers !!,so your exhaust shop is normally quite happy to dispose of it for you free of charge ,They send them back oversea's for recycling as they contain presious metals .
I understand early NZ NEW cars never had them anyway and 10 - 15 year old cats will be blocked to a certain degree because of our crappy fuel so the car is always going to run a little more freely without it.
Sounds like a good excuse to play to me.

Post by **jif** » Thu Sep 11, 2008 9:30 am

I wondered if that happened here... in the States people are having their cat' stolen by saw-wielding thieves ! It gets ~$200US at the scrappies there for a big SUV one ...

Growler · Post by **Growler** » Thu Sep 11, 2008 4:23 pm

I have taken the cat out. She only comes in at night for warm fresh milk...oh wait...wrong forum!

I have changed from the cat back. I put in an expansion box where the cat goes and replaced the big round muffler for a Holden Commodore one.
It is big enough not hinder the flow yet keep it generally quiet.
I went 2 1/4" right through with the exhaust piping. Was going to go 2 1/2 but the difference in cross sectional area was more than I expected. I wanted it to flow yet not lose back pressure.
I’m happy with what I have so far.
Next I will be fabricating a set of headers…once I know what sizing I want for the primary and secondary pipes!

madandy · Post by **madandy** » Thu Sep 11, 2008 9:34 pm

brownie wrote: I understand early NZ NEW cars never had them anyway and 10 - 15 year old cats will be blocked to a certain degree because of our crappy fuel so the car is always going to run a little more freely without it.
Sounds like a good excuse to play to me.

That's what I understood yet my 1990 NZ new has a cat

Gravelben · Post by **Gravelben** » Wed Sep 17, 2008 5:46 pm

Well I gave it to the local grarage today to whip the cat out and either gut it or replace with a pipe, but after finding the bolts on the cat had seized they decided that it would be more effort than it was worth and just welded up the hole instead. So no decat this time round but at least the leak is fixed.

sprsta · Post by **sprsta** » Thu Sep 18, 2008 7:22 am

i usually find when i pull the insides oit of a cat the exhaust sounds really tinny always better to replace with a resonator i think plus you can get some money back selling the cat

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