High-flow cats?

[Merli]

So I remember people saying that some "3" high flow cats" were a myth and a waste of money, and don't flow more than stock ones...

What are the good ones to get? What about 4" ones? I want to keep a cat because I can't stand the stained rear bar or the smell of petrol fumes permeating the cabin...

What's the best and highest flowing cat available? Brands, and points of sale please!

Cheers guys!

[Abo Bob]

CES have flow tested all the ones available in Aus. but Trevor?? wouldn't tell me which one he found to be best and since I already have one I'm not about to buy another one from him.

I'm considering getting a proper High flow from Japan which Alistair from Powerplay assures me is way better then anything available here but will watch this thread with interest.

When I mentioned these to Trevor?? he said that he knew about them but only used Australian suppliers so hadn't tested any of them.

Prices being thrown around were $1500 plus...

[Evo_Lee]

Merli..off the top of my head...HKS, ARC, SARD, Mines (I think). ARC and SARD seems to be used widely here.

I've also heard the GT-R stock cat is just as good. Same story I've heard with my Evo...I'm still running the stock cat because unless your running a straight pipe for purely circuit racing the stock cat is fine for the street.

edit: 4" fark that's a big bloody cat...

[cereal]

isn't the 3" part just the size of the exhaust that the cat bolts on to?

so a 4" cat would only bolt on to a 4" exhaust?

[Roy]

What exhaust system are you running Merli?

It would be expensive, but have you thought about running twin 2.5" cats before your exhaust merges into the centre muffler...hence the question about brand of exhaust / front pipes you are using.

Think about the cross sectional area and subsequent volume of catalytic converters using this type of setup.

Ferrari 355s apparently dont make any more power when the twin cats are thrown away, but cars like the Mclaren F1 make 30-40hp more, so twin 2.5" cats may get you out of gaol and keep those nasty smells/gases at bay.

[benjiman74]

Using pi r2 i've calculated the flow potential or cross sectional volume of some common exhaust piping:

2.5 inch = 31.67cm2

3 inch = 45.603cm2

3.5 inch = 62.141cm2

4 inch = 81.072cm2

From this you can see 2 x 2.5 inch pipes or with reference to this thread "cats" could only flow the same as a single 3.5 equivalent. The reason manufacturers would use twin systems is due to clearance problems associated with a bigger single pipe.

With respect to the ferraris not losing power with the cats removed you must remember they are na and unlike turbocharged engines would benefit from some backpressure.

I reckon if you are going to run a cat, get the biggest you can get your hands on.

[knore]

CES have flow tested all the ones available in Aus. but Trevor?? wouldn't tell me which one he found to be best and since I already have one I'm not about to buy another one from him.

 

I'm considering getting a proper High flow from Japan which Alistair from Powerplay assures me is way better then anything available here but will watch this thread with interest.

 

When I mentioned these to Trevor?? he said that he knew about them but only used Australian suppliers so hadn't tested any of them.

 

Prices being thrown around were $1500 plus...

$1500 for a cat!!

ouch!

[Roy]

Using pi r2 i've calculated the flow potential or cross sectional volume of some common exhaust piping:

2.5 inch  = 31.67cm2

3 inch  = 45.603cm2

3.5 inch  = 62.141cm2

4 inch  = 81.072cm2

From this you can see 2 x 2.5 inch pipes or with reference to this thread "cats" could only flow the same as a single 3.5 equivalent. The reason manufacturers would use twin systems is due to clearance problems associated with a bigger single pipe.

With respect to the ferraris not losing power with the cats removed you must remember they are na and unlike turbocharged engines would benefit from some backpressure.

I reckon if you are going to run a cat, get the biggest you can get your hands on.

But where a re you going to fit a 4" cat under a GTR. I think you are more of a chance to to fit twin 2.5" rather then a flanged down 4" cat (which is physically bigger then 4") If you can fit a 4" in there, then great. At 4" though wouldnt you be buying a truck cat, im not sure if they would live with the exhaust gas temp that a petrol turbo car will eb throwing at them.???

As for the 355 / NA thing, thats a fair point, but the back pressure thing typically influnences torque and low rpm response more then wide open throttle / peak HP... and its here where there was no difference (It was an article on the 355 Cup cars)

One thing i will say is make sure your tune is about right before throwing on your new cat, otherwise the gases and exhaust temps (lean=high temps = dead cat, or rich = fuel = dead cat) reduce the life of your car. Im pretty sure mine is dead after only 6 months, im guessing becuase of the A/Fs my car has seen in this time. (its now aflame thrower on trailing throttle )

[benjiman74]

But where a re you going to fit a 4" cat under a GTR. I think you are more of a chance to to fit twin 2.5" rather then a flanged down 4" cat (which is physically bigger then 4") If you can fit a 4" in there' date=' then great. At 4" though wouldnt you be buying a truck cat, im not sure if they would live with the exhaust gas temp that a petrol turbo car will eb throwing at them.???']

You're right, the size is the prob but you only need a 3.5 not a 4 to flow the same as 2 x 2.5's. Maybe get a custom that is oval shaped so the total vertical diameter is at a min?

True, the most difference would be down low.

[B-Man]

I've heard the same thing about the Jap cats as abo bob

Good but expensive.

[Ronin 09]

UAS have round cats - maybe you can wedge two in between the dumps and the existing cat position?

[croat]

I have a full 3" exhaust andhave no clue as to what cat i have... I think it is a rectangular shape from memory. If you punch the cat out and just leave the casing do you certainly get the stained rear bar eventually? Also what would happen if you were to be pulled over for this in a EPA check or highway patrol? Can they tell easily?

[Dash]

Read a write up recently in a mag about nizpro vs aps kits for the xr6t.

There is a pic in it of the aps "unique cylindrical high flow cat".

At 3.5 inches id it could be what your looking for.Whether they would sell you one

seperately I dont know.

[Evo_Lee]

I have a full 3" exhaust andhave no clue as to what cat i have... I think it is a rectangular shape from memory. If you punch the cat out and just leave the casing do you certainly get the stained rear bar eventually? Also what would happen if you were to be pulled over for this in a EPA check or highway patrol? Can they tell easily?

Probably not, but over the pits and an emission tests...they will get you.

[Merli]

Yikes... This thread grew very quickly!

Lots of questions to answer...

Roy: I'll be using HKS dumps, HKS front pipes, , APEXi GT-Spec rear.

I don't really fancy chopping up the HKS front pipes to put two 2.5" cats in-line there, but I'll keep it in mind.

AboBob & EvoLee: I shall give CES a call and see what they can tell me. I know that APEXi and Mines publish the flow rates for their cats, and they're excellent flow rates, but for those prices, they'd better be... I'd rather not pay $1500 for a cat if I can help it

cereal: as far as I know, in relation to cats, the measurement is across the actual honeycomb material, so with a 4" cat, you can weld 3" flanges to it and bolt it onto your 3" system, but since the honeycomb structure is restrictive, flaring out to a 4" cross section honeycomb will still be less of a restriction than a 3" honeycomb section, even though your flanges are 3"

I have no idea whether a 4" cat will fit under the car...

I think the main point of contention was that the honeycomb material used in most of these aftermarket cats were inferior, and so even though they were larger (cross section) than stock cats, they didn't actually lessen the restriction, so they were useless.

[surfiemullet]

have you guys ever thought of getting a 3 inch cat and cutting the ends off so you have two four inch holes. iv heard of fellas doing this. if they have the same core it shold work.

[Merli]

This is more a discussion about honeycomb core qualities...

The rest of the system is 3"-ish, so putting 4" flanges on will achieve nothing.

I'm worried about the actual catalytic convertor honeycomb design and it's flow properties. Which brand has the best flowing one?

[Dash]

I have no idea whether a 4" cat will fit under the car...

Well check it out for yourself

http://www.airpowersystems.com.au/falcon/e...ust/exhaust.htm

Looks compact enough,as for flow rates they claim a 2psi drop across the cat.

[Merli]

Thanks for the link

"At maximum rated power 330 kws the back pressure is less than 2 PSI"...

330kw is too low, although that's the first time I've ever heard of cats rated by kw figures...

[Cubes]

I too have been looking in to this.

Up until recently every 1 I spoke to in AUS said they cannot get 3.5" cats.

Exhaust Technology have recently told me they are now able to get 3.5" Cats and they definately make a huge difference when making upwards of 300rwkw and over.

They are $380 each.

They have a www site that is listed on the quote they gave me but it doesn't appear to be up at the moment? Maybe its a type on the receipt?!?

www.exhausttechnology.com.au