Does anyone know what is the lowest amount of cells per inch (CPI) that's legally allowed in a hi flow cat?

Why not just run two high flows in paralell, thats equivilent to half the restriction of a high flow can and they will probably last about twice as long.

Why not just run two high flows in paralell, thats equivilent to half the restriction of a high flow can and they will probably last about twice as long.

Becuase then instead of spending $200 on 1 cat which is what he wants, he has to spend $400 on 2 cats very stupid idea.

Why not just run two high flows in paralell, thats equivilent to half the restriction of a high flow can and they will probably last about twice as long.

Mmmm would cost a lot more than needed :nuts: For the street it has to be 200cpi or greater cat, get a metal substrate one probably wouldnt be a huge difference in power in a magna between a 100 and 200cpi cat anyway

considering there is no measurable power diffference with a gutted cat, i woudl say that its unilkely you could tell the difference between 100 and 200cpi.

Id got the 100cpi if i had the option and price was rediculous, and make sure as TIMMYC said, its a metal substrate cat not ceramic.

Id get the 100cpi because you need a cat for legalities sake, but when is a smog test going to be done to a magna? noise every now and then, but not emissions.

;1042473']Becuase then instead of spending $200 on 1 cat which is what he wants, he has to spend $400 on 2 cats very stupid idea.

Less resistance/longer life at the trade off of inital cost I guess.

Id get the 100cpi because you need a cat for legalities sake, but when is a smog test going to be done to a magna? noise every now and then, but not emissions.


It will happen. I think Penny Wong and Peter Garrett will get their heads together to try and buy green support, and vehicle emission will be on the agenda.

They have been doing smog tests as part of rego checks in some states of America for 20 years.

When I turbo'd my wagon to get it engineered it had to be taken to Sydney for a smog test. Havent quite finished it , so it hasnt been done yet.:iough:

There are high flow metal cats available on Skylines Australia. 3 inch flanges though so you'll need reducers welded on. Very minimal restriction though ;)

I have just bought myself a 3 inch 200 cpi metal cat. Even though my actual question hasn't been answered a bit of external research came up with the answers.

It will happen. I think Penny Wong and Peter Garrett will get their heads together to try and buy green support, and vehicle emission will be on the agenda.

They have been doing smog tests as part of rego checks in some states of America for 20 years.

When I turbo'd my wagon to get it engineered it had to be taken to Sydney for a smog test. Havent quite finished it , so it hasnt been done yet.:iough:

Well thats wher ei get the term smog test, as opposed to our emissions tests. The US have had them a while as you know. They wont bring in annual, bi-annual smog tests until they bring in yearly/bi-yearly warrant of fitness tests for cars (im in qld btw) At this stage once a car is rego'd, as long as it never runs out it will never have to see the inside of a service station again in its life (although unlikely at some point) when this changes, maybe then, the smog test for emissions but not before than.

considering there is no measurable power diffference with a gutted cat, i woudl say that its unilkely you could tell the difference between 100 and 200cpi.



Since when mate ? The difference might not be much on a stock exhaust, but, the Car is "normally" the greatest source of restriction along the whole exhaust in alot of cars.

With the Magnas for example, its amazing how much they open up with Extractors + cat.

Anyway, theres no set limit for what CPI is allowed. Youll find most 100cpi cats are marked as race use only. This is to save the sellers ass as they will generally NOT pass an emissions test. This is a general comment, and depends on the car, fuel used etc etc etc.

The benefit over running a 100cpi test insted of a test pipe is simply the EPA fine. The fine for not running a cat is MASSIVE in nsw, and ill assume similar in other states.

Where is the info from as im very interested?

i suppose every one has an opinion about this but some hard facts would be good.

This is not recommended at all but if you can find super lead petrol it will turn the cat into dust in a tank of driving...lol found this out the hard way.years ago.. please don't do it as i had to shake out the left overs in the exhaust.lol just had to say it ..sorry.

As we need a cat or shell of one lets work out the best one to fit that's legal.. eg a shell is not! It makes some sense to get a 3 inch with adapters as it will flow better but then it comes down to air flow as it will expand and be squashed back to 2.5 inch...or there about.? will it be better or worse? than a std size to suit the exhaust piping.

im not an expert but i think this needs some investigating.

as far as two cats go why? unless you had a twin system and price is through the roof! they still only flow at a certain amount.

Since when mate ? The difference might not be much on a stock exhaust, but, the Car is "normally" the greatest source of restriction along the whole exhaust in alot of cars.

With the Magnas for example, its amazing how much they open up with Extractors + cat.

Anyway, theres no set limit for what CPI is allowed. Youll find most 100cpi cats are marked as race use only. This is to save the sellers ass as they will generally NOT pass an emissions test. This is a general comment, and depends on the car, fuel used etc etc etc.

The benefit over running a 100cpi test insted of a test pipe is simply the EPA fine. The fine for not running a cat is MASSIVE in nsw, and ill assume similar in other states.

no, sorry, the magnas stock cat is proberley the least restrictive part of the exhaust system.
the biggest gain is removing the crap tri flow rear muffler, in place of a straight through.
extractors are proberley the next best thing to upgrade, (good choice is the pacemakers) then the overall piping of 2 1/4" inch, with an upgrade to at least 2.5" inch system with a better piping path. only if my engine was not stock would i upgrade the cat, (assuming there is nothing wrong with the origninal)
as i recall when stacky was boosting his car, they could not push it past around 170KW@wheels, as they suspected the stock cat was to blame, a stock engine, you will not see any gain. i have yet to see anyone, show power gains on a dyno, or even a better torque curve, from a cat upgrade

steve knight who does the ecu flashing recommends doing the cat before getting the tune as there restrictive.

steve knight who does the ecu flashing recommends doing the cat before getting the tune as there restrictive.

This is exactly why i will be doing mine.

dont get me wrong, they do pose some restriction,
my next and last exhaust mod is the cat,
but id rather spend my money on other parts of the system first,
and getting a tune is a bonus and you will recieve further gains from this , but if we are talking about, adding a low res cat, on a stock car, with a stock system, with the stock tune. your not going to gain anything, and if it is, its tiny

dont get me wrong, they do pose some restriction,
my next and last exhaust mod is the cat,
but id rather spend my money on other parts of the system first,
and getting a tune is a bonus and you will recieve further gains from this , but if we are talking about, adding a low res cat, on a stock car, with a stock system, with the stock tune. your not going to gain anything, and if it is, its tiny

ok i understand that but were not talking about a stock car.

ok i understand that but were not talking about a stock car.

oh ok, no one posted that he had other exhaust mods, i was replying to the post about how the cat is the most restrictive part of the system, which it is not


the Cat is "normally" the greatest source of restriction along the whole exhaust in alot of cars.



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For the street it has to be 200cpi or greater cat


Even though my actual question hasn't been answered a bit of external research came up with the answers.

Dude i answered you question in like the third post in the thread.

Cat was a waaaay better bang for buck item than my pacies. The stock headers really aren't all that bad, they may not look pretty, but there's stuff all gain to be had by replacing them. The standard cat may flow well off the factory floor, but a few thousand kays worth of exhaust crap goes a long way to reduce flow. My standard cat, when taken out, was full of crap, it was so bad you couldn't see through it. That was after about 130~k on the clock.

Dude i answered you question in like the third post in the thread.

I needed to confirm what you said... As there wasnt anyone that backed what you said i went to an exhaust place and actually asked them. Sorry for not giving you the kudos.

Cat was a waaaay better bang for buck item than my pacies. The stock headers really aren't all that bad, they may not look pretty, but there's stuff all gain to be had by replacing them. The standard cat may flow well off the factory floor, but a few thousand kays worth of exhaust crap goes a long way to reduce flow. My standard cat, when taken out, was full of crap, it was so bad you couldn't see through it. That was after about 130~k on the clock.

yeah, replacing a blocked cat, thats different story

thank you LOWRIDER

The cat is the least of your worries in a stock system. Just about any cat of the same size will flow better with no to little kms on it compared to another, so comparing a stock cat to a new hi flow, is like comparing apples to oranges.

The headers on magna's arent the problem, like you said a good bit of gear just a touch ugly, its the secondary pipe/pipes that are the issue (from the headers to the cat) this is where good gains are found as the stock exhaust isnt well designed (hence the comment about the cat being the least restrictive part of the stock system) oh and i mean "not well designed" in regards to performance, as an exhaust system to ensure all gasses exit the rear of the vehicle under the noise limit and emissions standards, its perfect.

Ill adjust what I said above about cat being the biggest restriction. This was applied to the Sports/VRX mufflers. For the lower models the muffler is totally rubbish.

And, as usual, a tune will bring the most of out any mod. I dont recall seeing any real improvment in peak power with extractors without a tune,but there was some increase in the midrange. With a tune, the midrange got better, and there was an increase in peak power.

So what size cat is best for a 3rd gen Magna 2''---2.5''---3''---3.5''???

So what size cat is best for a 3rd gen Magna 2''---2.5''---3''---3.5''???

I ended up putting a 200 cpi 3 inch Magnaflow on. It suited the 2.5 inch system i have and it instantly picked up power.

So what size cat is best for a 3rd gen Magna 2''---2.5''---3''---3.5''???

It all depends on the rest of your exhaust system. If stock, then a 2.5" is going to be plenty.


I ended up putting a 200 cpi 3 inch Magnaflow on. It suited the 2.5 inch system i have and it instantly picked up power.

Same here, with a similar set-up: Extractors - 3" cat - 2.5" pipework. Instantly felt the difference.

so the 2 1/2" piping maybe a difference compared to the stock piping?
hmmm looks like its time for new piping

It all depends on the rest of your exhaust system. If stock, then a 2.5" is going to be plenty.



Same here, with a similar set-up: Extractors - 3" cat - 2.5" pipework. Instantly felt the difference.

Pacemakers then cat (2.5") or (3") and then 2.5" pipe to a straight through muffler.?

Pacemakers then cat (2.5") or (3") and then 2.5" pipe to a straight through muffler.?

What have you now? Pacemakers, stock cat plus 2.5" piping or are you starting from scratch?

What have you now? Pacemakers, stock cat plus 2.5" piping or are you starting from scratch?

Pacemakers are on the way and the cat
is the part that i'am up in the air about (2.5") or (3")
but the 2.5" pipe to a straight through muffler are on the car.

Go the 3".

Go the 3".
Thanks mate.:happy::happy:

I run a 200cpi 3" on my Raptor SC. The only hope the 100cpi units have is if they run LONGER cell bricks or larger bricks with an increased frontal cross section. They've been trying hard to get the 100s legal for about 3years now. Haven't heard anymore on that from our suppliers.

Running a 1/2" larger cat than the surrounding pipe usually gets the same/similar job done on a road car.

On a related topic: We had a 3L 24v in at the muff shop that was running like a 3cylinder: the tab that divides the stock header secondary pipes (in that large aborition at the pipe union under the sump) had come away and was lodged INTO the cat and had flattened most of the cat's inlet biscuit! So, um, how good is the oem header arrangement :doubt:

I run a 200cpi 3" on my Raptor SC. The only hope the 100cpi units have is if they run LONGER cell bricks or larger bricks with an increased frontal cross section. They've been trying hard to get the 100s legal for about 3years now. Haven't heard anymore on that from our suppliers.

Running a 1/2" larger cat than the surrounding pipe usually gets the same/similar job done on a road car.

On a related topic: We had a 3L 24v in at the muff shop that was running like a 3cylinder: the tab that divides the stock header secondary pipes (in that large aborition at the pipe union under the sump) had come away and was lodged INTO the cat and had flattened most of the cat's inlet biscuit! So, um, how good is the oem header arrangement :doubt:

DO you think that metalcat is the best one out there?

DO you think that metalcat is the best one out there?

I've got a metalcat but they're all so similar now I wouldn't say one brand is better than another. Metalcats are popular and seem ok from our experience.

Go the metalcat as they're an Aussie brand.

When I got a 3" cat on my Ralliart I thought I was doing the right thing. In retrospect, I should have gone with the 2.5" same as the rest of the exhaust system. Reason being, when the car was still cold, it would make that ricey, rattling sound from the exhaust because the cat was too big. The exhaust gasses "bounce around" inside the cat making the sound until it warmed up properly.

Just my thoughts.

Yes I went 2.5 after originally asking for a 3 inch.....the fitters advised against it even with future mods in mind.

how good is the oem header arrangement :doubt:

yes I can hear that apparition in the headers rattlings on mine under certain throttle positions, sounds like a tractor sometimes.

So 3 inch aint the go anymore.?

So 3 inch aint the go anymore.?

Not IMO. From the experience I had with my Ralliart, if I had it over again I'd just stick with the 2.5"

So 2.5 with 200cpi is the best go then?

Anyone got any experience of these? did the 2.5 get rid of that tinny sound?

So 3 inch aint the go anymore.?

All depends on how much power your engine is putting out in NA form and if you SC or turboed then 3" cat is the go.

Just remember that the further the gases go along the exhaust the less importance there is in having a big diameter pipe etc due to the gases cooling down. (hope that makes sense)

So 3 inch aint the go anymore.?
It is if you're running 12+ psi. And as Jason said.

So 2.5 with 200cpi is the best go then?

Anyone got any experience of these? did the 2.5 get rid of that tinny sound?

yeh i got a 2.5 inch full exhaust from the flex pipe, with a 100cpi, with no resonators, this was very raspy, no matter how i drove it, it was loud and raspy, ask anyone who lives in wa haha, then i got a resonator installed, this removed most of the raspyness, still there a little when the pipes are cold or every now and then.