What size carb?

[Herald5.7]

Anyone know of a formula (if it exists) to get correct size Carb when fitting a supercharger? I'm currently running a SBC with a 650cfm double pumper, but have just bought a Weiand 142 supercharger. Will the supercharger need more air/fuel, e.g a bigger Carb say a 780cf0, or is the difference able to be covered by re-jetting??

[cliff gould]

hi
the 142 means that blower will pump 142cfm.
so you can work out the blower od ,if any.
then work out what the engine can pump at peak rpm/hp.
then that will give you an idea of what it will need.
its only math fella
cliff

[Herald5.7]

Thanks cliff, simple when you know how - innit!!

[GreenV8S]

cliff gould said:

hi
the 142 means that blower will pump 142cfm.
so you can work out the blower od ,if any.
then work out what the engine can pump at peak rpm/hp.
then that will give you an idea of what it will need.
its only math fella
cliff

Just to clarify - that's 142 cubic inches per revolution, not 142 cubic feet per minute.

You need to work out what RPM the blower will be running at (engine rpm times blower gearing ratio) and multiply by 142 to work out the total flow rate in cubic inches per minute, and divide by 12*12*12 to get to cubic feet per minute.

[cliff gould]

ok ,if its ipr. then that makes it a 410cfm blower?at 1 to 1 overdrive at 5000rpm.
just slightly smaller than a bds 8/71 street blower(440cfm).which sounds about right.
my problem with working it out like that is if you go too the next weiand blower up the scale, the 256 .then at 5000rpm @ 1 to 1 od.
then that makes it 740cfm.
when a competition 14/71 is rated at 523cfm.
now if this figure is peak cfm .i am not sure ,but would have thought so ,as after an rpm has been
reached the blower will not pump any more due to various factors.
why cant they all work too the same scale.
not an attack greenv8s.
just an observation!
cliff
anyway screw blowers rule .
(hoping my math is right now)

Edited by cliff gould on Sunday 2nd December 18:18

[anonymous-user]

The formula for carb selection in a blown application is I believe:

Max CFM required = Engine CID x Max engine RPM / 356 x [max boost / 14.7 +1]

For a 350ci SBC, that is effectively 700cfm, so a Holley 750 would work.

HTH

[Herald5.7]

Thanks guys - all these calculations are much appreciated.
Tony