George Lendich wrote:

Paul,

I'm doing a comparison on inlet sizes of the 12A to the 13B.

Area sizes!
Did you know that the 12A Carb butterfly is 18% bigger than the inlet ports. Allowing for the usual losses I guess!  - Nothing unusual here !

Did you know the 13B primary is only 19% bigger than the 12A - while the secondary is 89% bigger !!

Did you know that the total (one rotor) 13B port area is 69% bigger than the 12A.

So I figure taking the tube size you recommend of the 13B and finding 31% of that I will be right, anyhow it's not a bad cross check - not so!!

(50mm Al tube with a 3mm wall gives an ID of 44mm x 31% =  14mm)

I figure I need at least as much as the primary + secondary in the
12A  Carby i.e. 21mm Radius= 42mm ( 1.65") Dianeter ID.

So The 13 B must be + 69% of that ( 42mm+ 69% =71mm or 2.8").
Even without the 20 % extra, it's 2" ID.

My conclusion is either the 12 A is well undersized or the 13 B is well oversized OR both.

I then looked at in engine size 1: 1.1413 or 14.13% difference - so figure if 44mm ID is acceptable for the 13B then than 38mm ( 1.5") should be OK for the 12A. However if 1.5" is OK for the 13B
then 33mm ( 1.3") for the 12 A OR somewhere in between.

I don't know about you, but I'm confused!!
The sooner we work out the ideal port sizes the better !!
Please feel free to disregard all content, if it's too confusing.

George (down under)

You are not being too clear here George. Try a table.

Paul Lamar

 
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