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When Does A 5" Cat Become A Restriction

blue_vl_t

By blue_vl_t,
in Engines & Forced Induction

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blue_vl_t Rising Star

blue_vl_t

Posted
Posted (edited)

My r32 gtr, does not usually run a cat when taking the car to the track, i have a decat to bolt in its place, but this is a 3" cat and a 3" system, well some of it is 3" so it may as well all be

I'm currently making up a whole new exhaust which is a twin 3" into 4" front pipe with 3" HPI dump pipes, and a full 4" exhaust without any restrictions, Hks 4" muffler etc.

I do have in my possession a Matrix metal cat which is 4" pipe size and 5" through the body.

Wanting to know if anyone has back to back tested anything like this, will it be worthwhile me building a decat to test on the dyno and running at the track, or wil the 5" cat not be a restriction?

my mods and goals are:

rb26 stock stroke

700cc inj

pfc pro + Z32 afm's

2860-5's currently at 1.5 bar

HPI dumps

twin 3" into full 4" system straight through muffler

BP ultimate 98 octane fuel

looking for high high 300's rwkws, 400 would be nice :rofl:

any help appreciated

Edited by blue_vl_t
Stock e-Tune Contributor

Stock e-Tune

Posted
Posted

My r32 gtr, does not usually run a cat when taking the car to the track, i have a decat to bolt in its place, but this is a 3" cat and a 3" system, well some of it is 3" so it may as well all be

I'm currently making up a whole new exhaust which is a twin 3" into 4" front pipe with 3" HPI dump pipes, and a full 4" exhaust without any restrictions, Hks 4" muffler etc.

I do have in my possession a Matrix metal cat which is 4" pipe size and 5" through the body.

Wanting to know if anyone has back to back tested anything like this, will it be worthwhile me building a decat to test on the dyno and running at the track, or wil the 5" cat not be a restriction?

my mods and goals are:

rb26 stock stroke

700cc inj

pfc pro + Z32 afm's

2860-5's currently at 1.5 bar

HPI dumps

twin 3" into full 4" system straight through muffler

BP ultimate 98 octane fuel

looking for high high 300's rwkws, 400 would be nice :rofl:

any help appreciated

Room42 Proficient

Room42

Posted
Posted (edited)

why even run a cat? if they are going to get you they will for something else anyway

There is a massive difference between having no cat and any other mod as far as the law is concerned. You get a huge fine for not running a cat on the street. It is like bringing drugs to Bali. You could do it but when you get caught you get f**ked in the ass.

Edited by Room42
blue_vl_t Rising Star

blue_vl_t

Posted
  • Author
Posted

running no cat on the street is not the go, i realize this and will be running it, the question asked is, will the engine make more power without it? For the dyno tune and to use a decat for the track

blue_vl_t Rising Star

blue_vl_t

Posted
  • Author
Posted

Your turbos will be a bigger restriction than that cat if it's a 100 cell. Have made multiple 550hp at wheels using those cats

How do i know if its a 100 cell, the brand is Matrix Metal Cat, can i tell by looking inside it?

good to see real results, but i suppose what i'm saying is, the cars making 550hp at the tyres with a 5" cat fitted, would they make 2, 4, 10, 20??? rwkw more if removed and a straight pipe fitted, FOR RACE AND DYNO USE :laugh:

Hanaldo Grand Master

Hanaldo

Posted
Posted

I'm probably not making enough power (currently 350rwhp) yet to be of any use to you, but for what it's worth I've got a 5" body 100cell Venom metal cat; the car only made 5rwhp more after dropping the whole exhaust from the front-pipe back.

Harey Veteran

Harey

Posted
Posted

How do i know if its a 100 cell, the brand is Matrix Metal Cat, can i tell by looking inside it?

good to see real results, but i suppose what i'm saying is, the cars making 550hp at the tyres with a 5" cat fitted, would they make 2, 4, 10, 20??? rwkw more if removed and a straight pipe fitted, FOR RACE AND DYNO USE :laugh:

I would guesstimate less than 10rwkw different between a quality 100 cell metal cat 5" body and a decat pipe at 550rwhp.

Deano 1 Mentor

Deano 1

Posted
Posted

Why don't u just run It on the dyno with your street setup on. Than drop the exhaust infront of the cat and do a run, you will than no if there's a restriction, and if you running a 4inch exhaust the hole way, it will be pretty simple figure out that it's the cat resticting it, cause I doubt a 4inch exhaust would.

blue_vl_t Rising Star

blue_vl_t

Posted
  • Author
Posted

Why don't u just run It on the dyno with your street setup on. Than drop the exhaust infront of the cat and do a run, you will than no if there's a restriction, and if you running a 4inch exhaust the hole way, it will be pretty simple figure out that it's the cat resticting it, cause I doubt a 4inch exhaust would.

this is very true, will do.

when i eventually get on the dyno i will post the results for others information either way

blue_vl_t Rising Star

blue_vl_t

Posted
  • Author
Posted

Dont be soft. Xrated down here was running nos on his stock bottom end. Was good for a 10.6 from memory

fair enough....

so what do you run in the way of exhaust Rob?? Do you run a cat, or some special "high flow" version :whistling:

have you tried different setups or with the exhaust dropped on the dyno to see the difference?

GTRPSI Mentor

GTRPSI

Posted
Posted (edited)

I had a decat pipe and bolted on a FIA approved 100 cell metal substrate race cat on our R32 GTR with twin high flowed HKS2540 turbos.

Yes at top end, say 5600rpm onwards you can feel some loss of power and turbo response, the higher you revved, the more you lost.

Now heres the funny part after i spent all my money on it.......its still not street legal, it will not pass emissions testing, it does not meet the requirements, its just a race cat for classes that specify a cat must be installed.

I was better off staying with a gutted out cat with flanges and keeping the most restrictive/best scrubbing cat on the side in case i needed to go for emissions testing.

Learned the costly way.

Edited by GTRPSI
WHITER33GTS-T Veteran

WHITER33GTS-T

Posted
Posted

fair enough....

so what do you run in the way of exhaust Rob?? Do you run a cat, or some special "high flow" version :whistling:

have you tried different setups or with the exhaust dropped on the dyno to see the difference?

With my old 300rwkw setup and 3" exhaust I gained 6rwkw by removing the high flow cat.

When I ran the twin turbo setup on the RB25 I had a side pipe but didnt do any tests with the full system

The current system 4" dump, no cat, 3.5" system with a cannon at the rear was fine at 475rwkw.

Will be making a new 4" system shortly but not sure about a cat.

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