Jump to content
SAU Community

Recommended Posts

Feel free to flame if this has been made before, but I couldn't really find the answer I'm looking for.

Which would provide better performance and/or sound out of a 3inch 100cell high flow cat or a 2.5inch decat?

I can get my hands on a stock R33 GTS-T cat for free, which I believe is 2.5 inches and turn it into a decat, or I could spend some money on a 3 inch 100cell high flow cat.

Which would you prefer?

Almost forgot to mention I have a 3inch dump/down pipe and 3inch cat back, but a stock cat.

Edited by Dylaan
Link to comment
https://www.sau.com.au/forums/topic/367255-high-flow-cat-vs-decat/
Share on other sites

  • Replies 49
  • Created
  • Last Reply

Top Posters In This Topic

The step down to 2.5" for the gutted stocker would hinder it more than a fairly run of the mill 3" cat.

Obviously you realise no cat is going to flow better than A CAT of any type.. Yet there are nonsense legalities and heavy fines for running no cat.. And making it hard to pick (like putting a 3" pipe into the stocker) is a lot of work for reasonably small gains..

It has been covered, but I think this more than covers it for future and ease of searching. Just get a decent cat, save the hassle. Justjap sell decent highflow items for under $200.

GL

I know a decat is going to have more flow, but the highflow cat might have an advatage from having the bigger size? How about if I put some mesh inside the decat? would that still count as running no cat at all?

Buy a 3" straight pipe

Buy 3" cat shell

Cut ends off cat shell

Stick straight pipe in

Weld flanges onto ends

???

Profit

Could do, but as it is a daily I need it to be at a bit lower volume than a straight pipe. The car is suprisingly quiet at the moment, must be due to having countless mufflers and a stock cat. Could use a few more db before I get in any trouble for it.

Edited by Dylaan

My car is too low for them to see the decat to fine me for it anyway. :unsure:

As said above, I got a nicely made one for a similar price, can't go wrong.

3 advantages for me.

The cat was the lowest point and I was bottoming the f**k out of it. The decat gave me heaps more room.

Bit better flow.

Bit louder and slightly better exhaust note. :D

Edited by -Jimmy-

My car is too low for them to see the decat to fine me for it anyway. :unsure:

As said above, I got a nicely made one for a similar price, can't go wrong.

3 advantages for me.

The cat was the lowest point and I was bottoming the f**k out of it. The decat gave me heaps more room.

Bit better flow.

Bit louder and slightly better exhaust note. :D

your missing the point, a decat looks like a normal catalytic convererter, they can't tell unless the exhaust is dropped off and they look inside and the honeycombe cores are gone, or you get EPA'd and they check for noxious gases.

your missing the point, a decat looks like a normal catalytic convererter, they can't tell unless the exhaust is dropped off and they look inside and the honeycombe cores are gone, or you get EPA'd and they check for noxious gases.

No that would be a gutted cat. A decat is just a straight piece of pipe with flanges on it. Very easy to tell.

i see, in that case couldn't you just run a 'decat' with a catalytic shell? That would be next to impossible to tell.

That is the dilemma its self. The op is trying to decide whether a decat job (2.5 inch stock cat) is better flowing than a hi flow cat (3 inch)

Also, i know the fines for a decat are suppose to be quite hefty, but couldnt you plead innocence and try to pass it off as being damaged? Unless of course you have the 2.5 inch pipe running through the middle of it, in which case its quite obvious.

i see, in that case couldn't you just run a 'decat' with a catalytic shell? That would be next to impossible to tell.

That's what I was planning to do with the stock cat. Probably should have exaplined it better.

That is the dilemma its self. The op is trying to decide whether a decat job (2.5 inch stock cat) is better flowing than a hi flow cat (3 inch)

Also, i know the fines for a decat are suppose to be quite hefty, but couldnt you plead innocence and try to pass it off as being damaged? Unless of course you have the 2.5 inch pipe running through the middle of it, in which case its quite obvious.

Thanks, atleast you understand what I was trying to say :)

I still don't know what to do, if the cops saw a 3inch dump pipe and 3inch cat back but a stock cat, would they just assume it's been raped of it's inside?

That's what I was planning to do with the stock cat. Probably should have exaplined it better.

Thanks, atleast you understand what I was trying to say :)

I still don't know what to do, if the cops saw a 3inch dump pipe and 3inch cat back but a stock cat, would they just assume it's been raped of it's inside?

Tbh, i think it would come down to the officer. I have had mine sent to the epa (for noise) and the tester didnt even notice the dump pipe (it had factory front pipe though) He did however notice alot of other things lol. In my experience most cops arent too concerned until you have done something stupid in front of them (me, with my foor to the floor) or if you start being a smart arse, then they will without a doubt look for reasons. Their is however exceptions, where they will pull you over and look for anything they can. Most of the time though, the worst they can do is suspect modifications and send you out an epa notice, in which you just swap the parts back over. Also police will look for things like cannon mufflers and front mount coolers, so they can be a magnet for attention.

your missing the point, a decat looks like a normal catalytic convererter, they can't tell unless the exhaust is dropped off and they look inside and the honeycombe cores are gone, or you get EPA'd and they check for noxious gases.

Not true. A cat that is working stores a lot of heat compared to a gutted cat so all the epa need is an infra-red thermometer to check it's temp compared to the rest of the exhaust.

yeah >< i see now, had it in my head that turning it into the cat into a 'decat' was just taking out the honeycomb, not also welding in a straight 2.5/3 inch pipe though it.

Plenty of people just bash it out too mate.. Although im under the impression the pipe through the middle is the better option.

It's been said by others that driving behind someone with a decat/gutted cat is obvious because your eyes can get watery from those noxious gases

This is why i bought a hiflow- i heard stories like that too. Plus i had heard the fine is huge.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now



  • Similar Content

  • Latest Posts

    • The rain is the best time to push to the edge of the grip limit. Water lubrication reduces the consumption of rubber without reducing the fun. I take pleasure in driving around the outside of numpties in Audis, WRXs, BRZs, etc, because they get all worried in the wet. They warm up faster than the engine oil does.
    • When they're dead cold, and in the wet, they're not very fun. RE003 are alright, they do harden very quickly and turn into literally $50 Pace tyres.
    • Yeah, I thought that Reedy's video was quite good because he compared old and new (as in, well used and quite new) AD09s, with what is generally considered to be the fast Yokohama in this category (ie, sporty road/track tyres) and a tyre that people might be able to use to extend the comparo out into the space of more expensive European tyres, being the Cup 2. No-one would ever agree that the Cup 2 is a poor tyre - many would suggest that it is close to the very top of the category. And, for them all to come out so close to each other, and for the cheaper tyre in the test to do so well against the others, in some cases being even faster, shows that (good, non-linglong) tyres are reaching a plateau in terms of how good they can get, and they're all sitting on that same plateau. Anyway, on the AD08R, AD09, RS4 that I've had on the car in recent years, I've never had a problem in the cold and wet. SA gets down to 0-10°C in winter. Not so often, but it was only 4°C when I got in the car this morning. Once the tyres are warm (ie, after about 2km), you can start to lay into them. I've never aquaplaned or suffered serious off-corner understeer or anything like that in the wet, that I would not have expected to happen with a more normal tyre. I had some RE003s, and they were shit in the dry, shit in the wet, shit everywhere. I would rate the RS4 and AD0x as being more trustworthy in the wet, once the rubber is warm. Bridgestone should be ashamed of the RE003.
    • This is why I gave the disclaimer about how I drive in the wet which I feel is pretty important. I have heard people think RS4's are horrible in the rain, but I have this feeling they must be driving (or attempting to drive) anywhere close to the grip limit. I legitimately drive at the speed limit/below speed the limit 100% of the time in the rain. More than happy to just commute along at 50kmh behind a train of cars in 5th gear etc. I do agree with you with regards to the temp and the 'quality' of the tyre Dose. Most UHP tyres aren't even up to temperature on the road anyway, even when going mad initial D canyon carving. It would be interesting to see a not-up-to-temp UHP tyre compared against a mere... normal...HP tyre at these temperatures. I don't think you're (or me in this case) is actually picking up grip with an RS4/AD09 on the road relative to something like a RE003 because the RS4/AD09 is not up to temp and the RE003 is closer to it's optimal operating window.
    • Either the bearing has been installed backwards OR the gearbox input shaft bearing is loosey goosey.   When in doubt, just put in a Samsonas in.
×
×
  • Create New...