I used Magnaflow high flow cat #94136 because of a posted recommendation. I haven't had any check engine lights and the car has performed very well since September of last year.

 

montoya: he has alread cut up his stock header, thats the main reason i mentioned it, and its the most popular. I dont really want to do it either. Having the cat permenant to the header is stupid, but then again, it is illegal to tamper with it anyway, myabe thats whey they did it....

Thanks for the clarification on which is ceramic and which is metallic, now, pop quiz, any one deleted the cat and run with just the pre cat? although i dont think it flows very well, but i dont know for sure would be a cheap mettalic cat at least (free is good, plus the ones left over from prior obx doings). I think our car is too small to put to small a cat on. Like i said, the dynatech 1000hp 454 cats are smaller than ours! mind you they run two, but half of one of them is still pumping out alot more than we ever will, lol

Beecher

oh, i just remembered, a friend said he would give me his old stocker that he cut the cat off of (for his obx even, haha). I will put that on, and fill in the gap. looks like i am gettinga bit of a header right now after all, im holding off on my custom one until post head and cam. I will see if it throws codes or anything. that is actuall more proof, the precat is tiny even compared to our regular cat, and no one ever talks about burning that out or anything.... this is getting interesting

Beecher

 

I had a random technologies cat on my 1989 nissan 240 sx and it was a noticeable difference in hp and tq.

 

I replaced my stock header with an OBX (high temp coated), after the pre-cat melted, using the stock cat. I was getting the dreaded SES light at least once a week until installing an O2 sim. Everything seems good now. I had purchased a Magnaflow cat before the install, thinking the stock cat was toast, but after removing the stock header and finding the melted pre-cat I re-used the stock cat as it still looked good and had the flange already in place. I will use the magnaflow cat if I ever melt the stock cat, a side effect of track days. I only wish that the OBX wasn't as loud as it is.

 

+1
Header and cat will arrive on Friday...may take a week or so to find time to install.

 

Partsman - that would be great. Also is there such a thing as too high flow?

 

This is what I am gathering - a big black whole. The driving force of buying a high flow cat is vendor recommendation and from the magnaflow website it seems driven mostly by vehcile weight which I assume some how translates into engine size approximates.

Could it be I have stumped NAM with a question that cannot be answered?

 

I don't think there is such a thing as too high flow; rather too large a change in cross section will act like a large expansion chamber and kill velocity as the exhaust pulses suddenly slam into it. That would reduce flow.

That and it might take longer to light off, setting a CEL.

 

here is some info from random technologies

" When a replacement converter is required, a high flow model is the typical choice if performance is a consideration. But many times a “high flow” converter isn’t quite what it seems. According to Clay Ingram of Random Technology, "Replacement converters aren’t subject to the same requirements as original equipment models, so most standard replacement converters offer increased air flow potential. The 'high flow' label is a result of this increased flow capacity. However, a replacement converter designed for use on a four-cylinder engine will likely not have as high a capacity as an original equipment converter (with the same size inlet and outlet pipes) designed for a V8. Although converter manufacturers certify each converter type for a maximum engine displacement and vehicle weight, some dealers have no qualms about ignoring certification criteria. If a “high flow” converter has an extremely low price, chances are it’s not really a high flow model. Additionally, if a converter is installed on a type of vehicle for which it wasn't designed, it may not be very effective at controlling emissions."

Obviously, the bricks within a converter create the major resistance to exhaust flow. Over the years, various brick densities have been used, with the most common now being 400 cells per square inch. Converters with bricks having 200 cells per square inch were once common, and might appear to offer high exhaust flow potential. However, the walls in 400 cell bricks are thinner, so flow capacity isn’t much different, given the same face area. And face area is a major player in determining the flow efficiency of a catalytic converter.

But another factor, and one that’s often overlooked, is brick length - longer bricks offer higher flow resistance. On the other hand, if a brick is too short, it won’t offer sufficient area to effectively control exhaust pollutants. Converter manufacturers use different precious metal loadings of washcoats and vary them according to brick length and density. Since all catalytic converters must meet standards established by the Environmental Protection Agency, (EPA) their efficiency in controlling exhaust pollutants is a given - provided a particular converter is installed on the type of vehicle for which it was designed. However, the super-cheap models typically don't have enough high quality materlials to allow them to be effective much longer than the EPA-required 25,000-mile emissions compliance warranty period."

for the entire write up - http://www.randomtechnology.com/technical.html

 

Called Random Tech - very nice people.

what I learned

1) smaller the cell number the larger the holes. So if you have two cats equal length lower number would flow better. However for a 100 cell to do its job, it will have to be longer than a higher cell number. So a 100 cell cat that is twice the lentgh of a 200 cell cat (exampple only) doesn't necessarily flow better than the 200 cell.

2) One key is understanding flow rate and most do not publish this.

3) for our cars he recommends their 7000 series which is about 5.5" long and 4" diameter. Flow rate is 482 cfm @ 28lbs of pressure with 2.5" ends. It is a 300 cell metallic cat and it sells for 250 bucks appr.

 

Good info in your last two posts, Bart. Thanks for doin' the diggin'.

The RT ones are even welded...

...all the better.

 

So at red-line, we're dropping about 20 PSI across the cat? That's horrible!

Matt

 

so your saying that at redline our exhaust pressure is at 48 psi?

 

no, i dont think thats what hey saying... I think what he means is that it takes 28psi to flow the 428, meansing at redline (im assuming he knows what out r cfm is at redline) we that would equate to 20 pounds backpressure. I think, but i know he will clarify for us both, i might be wrong

i read and article a few weeks ago, and it was about 1.8 vw's. I believe the highflow cat (cant remember whos) only gave them a couple horse over the same header with the stock cat. These were all done back to back. They said for the street it wasnt really worth the moeny for the cat, but for race it would be. This is of course dependeant on what their stock cats are like tho.

Beecher

 

I think our cars run ~400 cfm of air. While this is into the engine, it was a good ball-park to use.

Matt

 

So help the car imparied?

 

when gas burns, there's a lot of water and CO2 generated. The CO2 is a one for one exhange with O2, but the water, H2O, creates two molecules per O2 in. So the total amount of gas out is larger than the total amount it.

Matt

 

Matt I ordered a 300 cat from Davesport. I liked that they put on the ends and they swear by them for racing. I had them put on 3" for me so I can weld on some V-Bands, so I can swap out with a straight pipe. I am putting right after my custom 3" DP. Once I finish the rest of the 3" back system, I will be able to eaisly do any tests on the capability of the cat in a twincharge setup. Currently I am running cat free with a 2.5to3back to 2.5 dp to exhaust. I am waiting for the rest of my parts to come in and the exhaust will be done. Could have it done by the Dragon, but I still need a volunteer to drive my car & me down.....

 

You sure that's not 28" of water instead of 28psi?

 

and that makes sense, as that's the normal (or close to?) pressure for flow testing heads....

Matt

 

28" of water is the same as 2.05" of mercury or 1psi.

28psi doesn't seem to pass any test of reasonableness, since the water manometer would be 65 feet high

 

Thanks to all who are contributing info. in this thread as it's quite helpful. I know this will all come in handy after my cat has experienced one too many HPDE. I picked up a stock manifold/cat off the marketplace for when that happens, but if I want/need a higher flow version, it's good to know some brands that work.

mb

 

Its still unclear (atleast fro me) what is the right "high flow" CAT.

DocO mentions 20psi out put so how does one calculate the CMF when Iwe have figures for 28lbs?

 

is in force per area. The notion of 28 lbs just isn't a unit that makes sense. I think the 28" water is a better number..... While this is a rather arcane unit, it is pressure. This would be the pressure you have to use to push a column of water 28" up.

What I did was take 400, and scaled down the quoted number from what was listed in one of posts, and came up with 20.

To find flow. Take the displacement of the motor, times red-line, divinde by 2 (for four stroke motors), and multily by the volumetric efficiency. So 1.6 litres times 7000 RPM, diveded by 2, times about 2 for ~15 psi boost at red-line. Do the math, and you come up with ~400 CFM (you have to convert from liters to cubic feet as well). This is rough, but ball-park.

Matt

 

Could someone in the know also kindly explain the advantages/disadvantages of a ceramic cat versus an all metallic cat?

I looked up the Magnaflow referenced earlier at SummitRacing and found it's a ceramic cat...that surprised me. I thought metallic was 'better'.

Thank you!