Cell count is just one thing and most people don't seem to understand that. Comparing cell count without knowing the diameter of the catalyst is totally useless and tells you nothing. A 100 cell 130mm catalyst will flow much more that the same 100 cell 100mm catalyst. It's all about the cross sectional area. The larger the area, the more voids you have, and the more flow you will have as a result. How much more? The cross sectional area of a 100 cell 100mm cat is 78.5cm. The cross sectional area of the same matrix 100cell 130mm cat is 132.7cm. This means that the 130mm 100 cell cat will flow 69% more air than the 100mm version. Again, not all 100 cell cats are the same. That's why I say you need to ask your exhaust manufacturer what diameter cats they are using. Hardly anyone advertises that because the cost delta between various diameter 100cell cats can be quite large. I know Europipe uses 130mm HJS/Emitech 100 cell cats on their 996TT exhausts and even bigger 144mm HJS Emitech 100 cell cats on their 997TT exhausts. Unobtainium Welding uses 130mm HJS/Emitech cats. You have to look past the smoke and mirrors...

 

AGREE FULLY.
I have been posting that over the years with my exhaust testing on the dyno and reasrch.
That's why my Milltek works so good with the Large130 diameter 100 cell HJS cats...Its not just pipe diameter, it is the combination of the two.
I found that an average 200 cell cat is only good up to around 500 rear wheel HP. .Also at my 625 rear wheel HP that my large 100 HJS cats did not cost me any peak HP/TQ over catless, just a little bit spooling at low to mid .

 

powder and John,

I disagree and do not believe what you are stating to be true. One of our catalytic converter manufactures has repeatedly stated that a large cat that necks down to 3" outlet is worse for flow than a narrower cat of proper size for a 3" outlet and in fact causes turbulence that a narrower cat will not. Think about that a bit and it makes sense. The reason for a larger diameter cat is to get more "scrubbing" action of the gasses, but not for flow reasons. With 100 cell cats, 4" is sufficient for a 3" outlet to provide unrestricted flow with minimal turbulence. 5" is not necessary and does not provide for any better flow. I have been advised it is actually worse for turbulence/flow than a straighter cat and my supplier has done flow testing to support their claims. This is from a very trustworthy source, no smoke and mirrors.

 

WSMy information comes straight from HJS, the premier source of German HJS/Emitec catalysts for OEM and Motorsport applications worldwide. I would deem them a trustworthy source also. I spoke with them at length when deciding on a catalyst for my application. Maybe they are dead wrong and have no idea what they are talking about, but according to HJS, the larger diameter catalysts do flow better, the greater the diameter, the more horsepower they can handle without undue restriction. Their largest diameter catalyst flows the most horsepower. Invariably the larger diameter catalysts have more scrubbing action and will generally maintain their function longer than a smaller one. We shall agree to disagree on this one.

Which catalyst manufacturer / supplier provided you the data regarding turbulence of the larger cats?

 

We use a 4 catalytic converter suppliers depending on the application, including HJS for many of our products and I have had many technical conversations with HJS myself. As a nature of our business, we are in contact with catalytic converter manufacturers weekly and this subject has been discussed many times. That said, I am not going to drag cat manufacturers into this debate. I do agree that a larger cat will support higher horsepower. By support, I mean (and I am pretty sure HJS meant when you spoke to them) that it will process the gases of a high HP vehicle at an acceptable emission level in accordance with anticipated requirements. For instance, a 4 cyl motor may not require a 5" cat to comply to specific levels and support that motor, whereas a V8 with all the extra gases will, etc. Also, some vehicles need the extra scrubbing action to prevent a CEL and others are not as sensitive. As for flow, the exhaust requires enough volume in the cat to process the gases without undue back-up. Larger than that and you create a large chamber, which in turn creates additional turbulence, by promoting uneven flow of gasses at different rates of speed for no reason. There is a point where too large is not necessarily better for flow. So yes, I guess we will just agree to disagree on this...

 

I don't have a horse in this race, but as an outsider reading the posts, both of you have rational hypotheses...and that's what they will remain until a statistically valid data set can be obtained to shed light on the subject. Not anecdotal data from CAT suppliers or data of the correlative kind. I mean an actual designed factorial experiment to expose both factor and interaction effects. Now back to your regular programming.

 

How is turbulence being created when what's being flowed through isn't an empty shell but is instead a chamber fitted with what amounts to a flow screen? It's like a set of parallel tubes is it not? Is any of this flow data published? You would think it would be in the best interest of the manufacturer to do so...

 

Here are a couple of graphs taken out of a white paper published by Emitec dealing with Catalyst efficiencies including layout and effect of catalyst diameters on HC conversions and flow. In the case of flow, this is listed as pressure drop in the paper. Pressure drop is the difference between pre-cat and post cat pressures for a given amount of flow. A decrease in the pressure drop number equals a decrease in back pressure which equals an increase in flow. To cut through a lot of technical jargon, I have posted a couple of graphs taken directly out of the paper which clearly show that a larger diameter catalyst will provide lower pressure drop numbers. Unless I'm mistaken, you will not find a discussion of "turbulence" anywhere in this technical paper which leads me to believe it is of no relevance. Here is a link to the paper:

http://www.emitec.com/fileadmin/user...01_01_0929.pdf


The graph below clearly shows that for a given constant air mass flow (in this case 176kg/h) and a given catalyst matrix (cell count), an increase in catalyst diameter will equal a corresponding reduction of pressure loss. For example, a 400cpi catalyst with a diameter of 80mm will have a pressure loss of 110 mbar while the same 400cpi catalyst with a diameter of 120mm will have a pressure loss of 25mbar. In other words, a 50% increase in catalyst diameter will have a 77% reduction of back pressure (or 77% increase in flow).

[url=https://flic.kr/p/rNvqM8]


This second graph further reinforces the diameter vs. flow comparison. Here, Emitec has a larger diameter 118mm but more restrictive 600cpi catalyst compared with a smaller diameter 100mm but less restrictive 500cpi catalyst. Again, for a constant flow of air, the MORE restrictive 600cpi larger diameter catalyst is proven to have less pressure drop (103mbar vs. 108mbar) than the smaller diameter less restrictive 500cpi catalyst. In other words, a 600cpi cat flows better than a 500cpi cat. Remember, lower pressure drop = lower back pressure = greater flow. Again, no mention of turbulence. The one advantage of a smaller diameter catalyst is the fact that due to the increased back pressure which results in higher gas velocity though the matrix substrate, you will reach catalyst efficiency sooner as the catalyst will heat up quicker. Hope that helps..

[url=https://flic.kr/p/s7Gjdb]

 

There is no doubt that a larger diameter cat will flow more air more efficiently. The problem is when you introduce 2.5 or 3" of airflow into a 5" cat, then try to jam that 5" worth of flow back down to 2.5" or 3" with a sharp cone decreasing the flow, as we do on the 996TT. Now you no longer have the simple larger diameter flow example and you introduce other flow dynamics, including obstruction and turbulence.

 

Please include supporting data showing the effect of turbulence reducing flow by flaring from a 3.375" flange into a 5" catalyst and then flaring back down to 3" piping. Without data, it's all conjecture. Not arguing, just curious. I come from an aviation background and just because something doesn't look like its optimal for flow, doesn't necessarily make it so. Thank you John...

 

Our 997 TT cats are actually size 153mm/6 inch. Also note that we don’t use a sharp cone after the cat but an equal sized 6 inch plenum.
We don’t use HJS cats though. Besides HJS is not a cat manufacturer.

 

I was doing some calcs earlier for something unrelated, and it turns out that the pressure drop for an 85mm pipe that necks down to 52mm in 7cm (14 degree taper angle) is on the order of 0.03bar, or 0.43psi for the metrically challenged. This was for hot exhaust gas at a flow rate of 33lb/min, so I don't think a 6" section that necks down to 3" is any cause for concern whatsoever...

 

Correct, HJS is a supplier of Emitec cats if I'm not mistaken. The 153mm are Emitec cores Stef? Why do you use such large cores instead of maybe 100mm versions?

 

The bigger the total surface area of any cat ,to a given point which we couldn't even run one that big, is going to improve flow over a smaller one no matter if its our outlet size..
Isnt that why a 100 cell cat flows more then a 200 ,300 cell cat because it lets more volume of air thru it and they both have the same size pipe outlet ,, same principle basically.

 

Aren't most modern catalytic converters a honeycomb flow design which in itself causes turbulence so it would make sense to increase the surface area to be exposed to the exhaust gases and really doing the proper flow dynamics of the exit chambers to off set the increase in exhaust flow. I would think the bends in the exhaust systems would do more to flow restrictions than the larger catalyst exiting the unit.