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Primary Bore size with Cat theory ?
Agriv8 - 8/10/08 at 11:33 AM

More a theory tread - but hey we like a discusion on here so ere we go.

V8 7 running in the 750 MC racing, all cars are going to have to be fitted with a Cat ( or 2 in the Case of the V8 ) as per MSA IIRC.

So obviously a pair of large racing cats have been ordered.

Now I am a beliver in the keeping primaries to a sensible bore size to keep the speed of the gas flow up.

But when you introduce a Cat What would an ideal bore and size be ? has anyone come across any reading / calculators to try and keep as much Power as possible while having to force the gasses through a CAT.

Thanks

Agriv8

Mr Whippy - 8/10/08 at 02:18 PM

why do you have to force the gas through the exhaust? don't think they're all that restrictive

NS Dev - 8/10/08 at 04:34 PM

ahhh the wonderful racing CAT that does absolutely nothing other than make life difficult!!

At full throttle the cat doesn't have time to actually convert anything at all

MikeRJ - 8/10/08 at 05:08 PM

quote:
Originally posted by Mr Whippy
why do you have to force the gas through the exhaust? don't think they're all that restrictive


Cats are restrictive by the very fact that they introduce a large increase in cross-section right at the point where the gasses are moving very quickly. You therefore have fast pulses hitting a slow moving region of gas.

britishtrident - 8/10/08 at 07:48 PM

Cats also cause restriction because viscous friction of the exhaust gases boundry layers against the walls of the very fine passages of the cat's honeycomb matrix.

I have noticed since tintops were fitted with 3 way cat the dia of the primary pipes has increased quite a bit.

[Edited on 8/10/08 by britishtrident]

Agriv8 - 9/10/08 at 06:25 AM

quote:
Originally posted by britishtrident
Cats also cause restriction because viscous friction of the exhaust gases boundry layers against the walls of the very fine passages of the cat's honeycomb matrix.

I have noticed since tintops were fitted with 3 way cat the dia of the primary pipes has increased quite a bit.

[Edited on 8/10/08 by britishtrident]


I could not have put it better myself, but I can not seem to find any readding / calculations on the subject.

I thinks that the pulses are still the same as the engine is not going to change. We have now got to get that pulse through a restriction ( the CAT ) which is going to slow it down Thus cause presure buildup. So it makes sense that that presure will be less on a larger Bore than a smaller On.

There must be an optimum though ( suppose looking at the primaries of a modern v8 AUDI may release some clues ).

regards

Agriv8