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

    • From there, it is really just test and assemble. Plug the adapter cables from the unit into the back of the screen, then the other side to the car harness. Don't forget all the other plugs too! Run the cables behind the unit and screw it back into place (4 screws) and you should now have 3 cables to run from the top screen to the android unit. I ran them along the DS of the other AV units in the gap between their backets and the console, and used some corrugated tubing on the sharp edges of the bracket so the wires were safe. Plug the centre console and lower screen in temporarily and turn the car to ACC, the AV should fire up as normal. Hold the back button for 3 sec and Android should appear on the top screen. You need to set the input to Aux for audio (more on that later). I put the unit under the AC duct in the centre console, with the wifi antenna on top of the AC duct near the shifter, the bluetooth antenna on the AC duct under the centre console The GPS unit on top of the DS to AC duct; they all seem to work OK there are are out of the way. Neat cable routing is a pain. For the drive recorder I mounted it near the rear view mirror and run the cable in the headlining, across the a pillar and then down the inside of the a pillar seal to the DS lower dash. From there it goes across and to one USB input for the unit. The second USB input is attached to the ECUtec OBD dongle and the 3rd goes to the USB bulkhead connected I added in the centre console. This is how the centre console looks "tidied" up Note I didn't install the provided speaker, didn't use the 2.5mm IPod in line or the piggyback loom for the Ipod or change any DIP switches; they seem to only be required if you need to use the Ipod input rather than the AUX input. That's it, install done, I'll follow up with a separate post on how the unit works, but in summary it retains all factory functions and inputs (so I still use my phone to the car for calls), reverse still works like factory etc.
    • Place the new daughterboard in the case and mount it using the 3 small black rivets provided, and reconnect the 3 factory ribbon cables to the new board Then, use the 3 piggyback cables from the daughterboard into the factory board on top (there are stand offs in the case to keep them apart. and remember to reconnect the antenna and rear cover fan wires. 1 screw to hold the motherboard in place. Before closing the case, make a hole in the sticker covering a hole in the case and run the cable for the android unit into the plug there. The video forgot this step, so did I, so will you probably. Then redo the 4 screws on back, 2 each top and bottom, 3 each side and put the 2 brackets back on.....all ready to go and not that tricky really.      
    • Onto the android unit. You need to remove the top screen because there is a daughterboard to put inside the case. Each side vent pops out from clips; start at the bottom and carefully remove upwards (use a trim remover tool to avoid breaking anything). Then the lower screen and controls come out, 4 screws, a couple of clips (including 3 flimsy ones at the top) and 3 plugs on the rear. Then the upper screen, 4 screws and a bunch of plugs and she is out. From there, remove the mounting brackets (2 screws each), 4 screws on the rear, 2 screws top and bottom and 3 screws holding in the small plates on each side. When you remove the back cover (tight fit), watch out for the power cable for the fan, I removed it so I could put the back aside. The mainboard is held in by 1 screw in the middle, 1 aerial at the top and 3 ribbon cables. If you've ever done any laptop stuff the ribbon cables are OK to work with, just pop up the retainer and they slide out. If you are not familiar just grab a 12 year old from an iphone factory, they will know how it works The case should now look like this:
    • Switching the console was tricky. First there were 6 screws to remove, and also the little adapter loom and its screws had to come out. Also don't forget to remove the 2 screws holding the central locking receiver. Then there are 4 clips on either side....these were very tight in this case and needed careful persuading with a long flat screw driver....some force required but not enough to break them...this was probably the fiddliest part of the whole job. In my case I needed both the wiring loom and the central locking receiver module to swap across to the new one. That was it for the console, so "assembly is the reverse of disassembly"
    • But first....while I was there, I also swapped across the centre console box for the other style where the AV inputs don't intrude into the (very limited !) space.  Part# was 96926-4GA0A, 284H3-4GA0B, 284H3-4GA0A. (I've already swapped the top 12v socket for a USB bulkhead in this pic, it fit the hole without modification:) Comparison of the 2: Basically to do the console you need to remove the DS and PS side console trim (they slide up and back, held in by clips only) Then remove the back half of the console top trim with the cupholders, pops up, all clips again but be careful at the front as it is pretty flimsy. Then slide the shifter boot down, remove the spring clip, loose it forever somewhere in the car the pull the shift knob off. Remove the tiny plastic piece on DS near "P" and use something thin and long (most screwdrivers won't fit) to push down the interlock and put the shifter down in D for space. There is one screw at the front, then the shifter surround and ashtray lift up. There are 3 or 4 plugs underneath and it is off. Next is the rear cover of the centre console; you need to open the console lid, pop off the trim covering the lid hinge and undo the 2rd screw from the driver's side (the rest all need to come out later so you can do them all now and remove the lid) Then the rear cover unclips (6 clips), start at the top with a trim tool pulling backwards. Once it is off there are 2 screws facing rearwards to remove (need a short phillips for these) and you are done with the rear of the console. There are 4 plugs at the A/V box to unclip Then there are 2 screws at the front of the console, and 2 clips (pull up and back) and the console will come out.
×
×
  • Create New...