Want Opinions on exhaust flow

[DareBee]

I need to install a Cat on my 90 RX7.
Existing exhaust I believe is 2 7/16.
I am undecided between 2.5 or 3 Cat.
A Cat is somewhat restrictive so would I cone it up to 3" and back down to existing pipe to help flow?
Maybe the Cat will not function properly with reduced velocity?
Maybe I am a Dumb Ass?
I do have to passs e-test.
Should I save myself the extra work and initial cost and just install the 2.5 unit?

What do you guys thin on this subject?

[ADAM]

what car do you have?
what is its displacement?
what does it rev too?
what is the cars intended application? (full race or mostly street..or mixed)

[DareBee]

Adam Adam Adam ( I am shaking my head Here)(the one on my neck)

First line reads "'90 RX7" on the end.
That would dictate a 1300cc rotary with limiter at 9000RPM
The fact that I am putting a Cat on it would dictate that it needs to be licensed.
The only grey area I left is that it is primarily race and the Cat MAY come off imediately after passing the test.

Im not trying to be a prick, but you need to get some more sleep man!

Oh.....I hang around with Finboy sometimes.:D

[haniforama]

A new cat in either size should clean it up more than plenty to pass an e-test.

Get the cheaper one if it's a temporary measure, get the larger one if it's a keeper.

Rev it up!

Hanif

[ADAM]

hmmm don't know about rotary's sorry can't help...

[DareBee]

I also have another question

Do you guys figure I should hook the air pump back into the new Cat or go straight thru.

[haniforama]

Definitely hook it up if you want to pass emissions.

The rotary is set up to run rich with a hint of burning oil. If you don't dilute it and cause the secondary combustion in the cat, you'll blow HUGH Hydrocarbon's in the e-test.

Hanif

[DareBee]

I guess I knew that.
Just like a Baby I need plenty of reassurance.

Thanks Bud

[Cheers!]

becareful of what cat replacement you buy. Rotary engines burn HOT! i mean hot.

It is noticeable when you put your hand behind the tail pipe. Normal cars you just feel a breeze. On a rotary your hand will start to burn if you don't move.

Rotary engines are known to melt catalyic converters. I would buy a hiflow main cat, 2.5 inches. Run the air pump with the cat for the emissions test.

Then after replace it with a peice of 2.5 inch stainless steel pipe and save the cat for 2 years down the road whenu have to retest again.

[DareBee]

Jason

I am aware of the excessive heat/exhaust issues. Glasspack life expectancy is given in minutes for the rotary.
I think that I will put my presilencer back in, Stewart did a beautiful job building it and my car is too loud already.

[MiG-29 Foxbat]

I think I'll go with Cheers! on this one.

The extra cross-sectional area of the catalyst matrix, viewed perpendicular to the stream of exhaust gas, versus the cross-sectional area of the inlet pipe, for a well-engineered converter, probably compensates for the reduction in available area which the catalyst matrix provides. :o

The cat already steps up and steps down the maximum cross-sectional area of the gas conduit, but perhaps not very neatly. (Some cats may be directional.)

"Coning" the pipe up and down would make for a very heavy, complicated and expensive system that may not work very well because all the pressure-wave and flow-momentum tuning inherent in the near-stock system may be adversely affected. :rolleyes:

[DareBee]

I do believe you want to view the cross sectional area PARALLEL to the flow of waste gasses.

But good call though.
I am sure comapnies like Hedman and Flowmaster are always fighting for the best flowing and functional product and strive to size the catalyst matrix to accept the volume as it flows from the inlet.

[JoeT]

Derek,


Like many people have said here. Rotaries burn really hot, and most aftermarket ceramic core cats are not worth their salt, and will end up in a ball of burnt mush.

Go with a metal core cat, they are more efficient, witstand greater temperatures, and the air passages are about 10 times larger than the ceramic type. Of course, they are also more expensive than the ceramic type.

If you want more information on where you can pick these up, and get a custom exhaust installed at the same time. Let me know... Click the "PM" button below this post.

[finboy]

hey db..

i'm not sure if you knew this or not...

but rotaries run really hot!!

i had a dream once of a friend who had melted and blew his cat, totally empty (ended up in the muffler)

the guy ran with a hollow cat and, picked up a new cat for the
e tests.



"da fin"