Anyone out there have access to a flow bench?.....

Mark Plaia

True Classic
I am curious about the difference in flow between:
1) Three bolt catalytic converter as used on carbureted cars
2) Four bolt catalytic converter as used on fuel injection cars
3) 1 3/4" cat "test pipe"

The early 3 bolt converters used the so-called fluidic bed approach utilizing a cage full of loose beads to provide exposure of the exhaust gasses to the catalyst material. The later 4 bolt converters use a honeycomb matrix to expose the exhaust gas. Since you can actually see through the 4 bolt type, I wonder if it does, in fact, provide a lot of exhaust restriction.

A 1 3/4 inch diameter pipe has a flow area of 2.41 square inch. I don't have a cat converter handy, but I remember them being about 4(?) inch X 7(?) inch. So, that would give a nominal flow area of about 28 square inches. Now if the honeycomb matrix is sitting in there (that you can see through) blocking say 50% of the flow area, that leaves a remaining 14 square inches or 580% more flow area than the straight pipe. Even if the matrix blocks 75% of the available flow area, that still leaves a 270% greater flow area.

Now, this is simplistic admittedly, because we are dealing with a dynamic situation. Bernoulli must be served, so there is a cost when the flow goes from a smaller area to a larger area and back down to a smaller area due to pressure changes. But, my fluid mechanics tell me that the reasonably long (and non-abrupt) taper at each end of the cat converter helps to mitigate the effect of the pressure changes.

The situation for the early style catalytic converter is different because of the approach. The idea with this style is like the old vacuum cleaner demonstrations where the outflow from the vac levitates a bunch of balls in a tube. The beads in the cat converter are liable for breakage, crumbling, clumping or otherwise getting in the way. And, never is a straight through flow path presented.

So, I speculate that the catalytic converter on a fuel injected X (NOT carbureted) presents very little restriction to flow, and it may not be worth changing out to a test pipe on a fuel injected car.

Prove me right or prove me wrong.

Ciao,
 
I own one :)

I bought it at a garage sale. Really. As of right now its stored at a friends shop in Alabama.

I am curious about the difference in flow between:
1) Three bolt catalytic converter as used on carbureted cars
2) Four bolt catalytic converter as used on fuel injection cars
3) 1 3/4" cat "test pipe"
,

That's easy. And I don't need a flow bench to test it. The fluidic bead cats flow like crap. That's why most manufacturers went to the honeycomb matrix. The matix type flows way better than the beads. This is well known by the Showroom Stock SCCA racers that have to use the OE converters. Also, new converters flow better than used converters.

As for comparison with a test pipe. I haven't tested an FI X with/without a cat but I can tell you that a "test pipe" will flow better than the cat. Even if the matrix has a significantly larger area, even when considering bernoulli's priniciple, the exhaust pulses don't pass through the matrix as effectively. The converter acts much like a pulse dampener. As a result, the exhaust gases loose their velocity passing through the chamber and matrix. The loss of velocity means the loss of the scavenging effect.

Larger converters, while having more area for flow, cause a greater loss of velocity. So there is no free lunch.
 
Thanks for the information...

I knew someone had been through the thought process.

Still, it would be cool to have dyno results from a four way test: Stock 4-1 manifold & head pipe versus header, with and without catalytic converter.

A flow bench would not show the effect pulsatile flow of the exhaust stream, so that would not be as good a measure.

The stock 4-1 manifold with head pipe would probably have the pulses a bit more homogenized than a header setup, and perhaps show a bit smaller difference between cat and straight pipe than would a header.

Ciao,
 
Back
Top