Catalytic converter sizes: is bigger better?

[Wlee2]

Yes Wayne, stop randomly blabbering!

A thought came to me as I read all this. What happens to the air flow when it goes through to a 2.5 then 3 and then back to 2.5? Wouldn't you be getting some back pressure? I guess this would depend on shape and seeing most cats are curved, the air would just smoothely flow through the cat and just be bottled necked by the 2.5 again.

Seems like I have answered my own question!

wtf you on about random blabbering

thats what i was saying it wouldent make a difference with a 2.5" exhaust system with a 3" inlet outlet cat.

the core of the cat Body is still the same at 4" 300 Cell Metalic cat.

[DLO01]

I have always adopted the rule "keep piping size the same throughout the exhaust". Sections of smaller or larger piping will create turbulance and therefore resistance and restriction.

For me I would opt more for a good quality metal cat of the same diameter as opposed to going for a larger cat.

[tinkerbell]

So, the question now becomes, will a cat with oversized flange ends (3" cat, 2.5" exhaust) with with the same internals be better than a cat with the same size flange (2.5" cat, 2.5" exhaust) as the rest of the setup?

yes, you are right wayne, sorry.

mugsee - stop asking un-useful questions.

[Wlee2]

yes, you are right wayne, sorry.

mugsee - stop asking un-useful questions.

Just so that people understand it wasent mugsee fault really..

the question was useful

untill we relised that quite alot of aftermarket Cats IE MEtalcat was one in particular

their entire range uses the same body but different sized flanges..

all of them used a 4" 200 cell metalic cat.. but with a range for 1.75" to 4" flanges..

same goes with alot of the stuff on ebay as well that try to immitate the 4" 200 cell metalcat..

not sure if theres actually any other in different size. but thats what we ended up looking at.

[mugsee]

Tinkerbell - it was/is a legit question, as I've seen other memebers (ie. Sangsational) with an oversized cat (3") on a smaller exhaust and was wondering if there was any particular reason behind this.

[tinkerbell]

i guess what i am not understanding is how the response to post #1 does not cover this off?

post #1 was excellently worded, and required a one word reply. you guys have both just reworded it and gone in circles.

i still dont understand how else you are going to use a oversized cat without having it flared to 2.5in...

maybe it is your use of the word "cat" that has me confused...

for example - the MetalCat "cat" is actually around 4in not 3in as you keep referring to...

[BF]

do you have proof of diff b/ween cat and non-cat?

No I don't but I vaguely remember a dyno of a dc2r with a proper high flow cat and no cat a couple of years ago, and the difference was minimal - my memory could be fabricating dyno plots though.

The question I have is how many kw atw are you going to make by flaring a 2.5" exhaust to fit a cat with 3" flange? e.g. 5kw, 10kw or 15kw? I don't think so. You make that kind of power with a complete aftermarket full exhaust, I wouldn't expect you'd see that by increasing the cat by .5 of an inch.

So is it really worth all the extra effort and money?

[DC2R]

So is it really worth all the extra effort and money?

Inshort. Probably not.

[tinkerbell]

So is it really worth all the extra effort and money?

so you too seem have been confused by the posts between post #2 and now...

the original question was:

Would attaching a larger sized catalytic converter onto an exhaust provide better gains than a respectively smaller sized one?

the answer is:

yes.

however, it is obvious that it is more expensive to buy an off the shelf oversized cat with 2.5in flanges compared to an off the shelf smaller sized one. so it is up to you wheterh you sepdn "all the extra money"...

but it is going to reduce the exhaust restriction, and therefore increase power *potential*

it MIGHT not even add any KW - if you have a restriction further up the air flow path, eg air filter, head ports, valve seats etc etc...

the reason you goto the "effort" is to ensure that it is not a *possible* restriction...

oh, here is the graph you may have been looking for:



http://www.team-integra.net/forum/di...0&TopicID=3110

[tinkerbell]

my memory could be fabricating dyno plots though.

otehr people could also be fabricating dyno results too

ask any tuner - e.g. Toda Racing Australia - they will say:

no cat > cat

[freakie]

I gained 5 kws from changing my 3 inch to a 4 inch cat, my car made 374kws at the wheels before the 4 inch cat and after, it went to 379kws.

I guess its only better if its actually a restriction in the first place.

[00dc2]

1.3%

[BF]

This is getting a bit silly really. Perhaps I took the original question a bit far by extending mugsee's question of whether there was a gain to whether such a gain is worthwhile. I do think, however, my extension was worthwhile.

Of course not having a cat is better than having a cat, but as everyone knows it is illegal to do this - In fact, a thread discussing the issue was closed in the past. So the comparison of no cat to having a cat is a bit of a moot point.

I might be missing the point entirely, but it seems like that dyno plot and freakie's post support my argument - it is generally not worthwhile upgrading your cat by .5 of an inch and flanging a 2.5" exhaust so it fits. Especially in the case of a mildly modified NA Honda.

Now I've lost 5 minutes of my life I can never get back!

[tinkerbell]

so under freakys example, a dyno proven 1.3% increase,

on a DC2R that is probly over 1.3kW ATW more...

some intakes make less than that...

i guess it comes down to your perception of 'worthwhile', doesnt it?

to me - i extensivly modify my cars, maybe you don't, but i know that every % is worth it, as it all adds up, and to me - it is all worthwhile...

also for teh OP's - here is an interesting page:

http://www.surefloexhaust.com.au/cc.htm

[tinkerbell]

Of course not having a cat is better than having a cat, but as everyone knows it is illegal to do this

sorry, i should have clarified that Toda Racings recommendation regarding no cats is for is for off road only.

my bad.