Anyone understand IR manifolds, And how they affect carb sizing ? Options

[got914?]

Im looking at putting carburetors on a motor and trying to figure out how to select the carb size ?

Is it all based on head flow or is that only part of it ?
There isnt much on the internet other than proven engine combos.

Now a 250 Chevy straight 6 can run triple webers that flow 250 cfm per choke for a total of 1500 CFM, this is where i know IR manifolds work with higher CFM because obviously a single 1500 carb on a common plenum manifold would be way over carb'd.

Now its a 3.3 Subaru motor H6 boxer engine. 202 CI
HEad flow data found here

http://speedtalk.com/forum/viewtopic.php?p=189677

States it maxs out at about 165 cfm SO I would assume I can use that as part of the calculation. Obviously you cant go higher than that or your over carbing.

What are your thoughts ?

[Mueller]

As copied from Jenvey:

[b]What is the best throttle body diameter ?

Factors influencing size are; Power output, RPM, cylinder head design, cylinder capacity, position of the throttle body in the inlet tract and position of the injector.

Choice of bore size is a balanced compromise resulting from the following;
1) A larger bore leads to lower flow resistance, but obeying the laws of diminishing returns.
2) A smaller bore leads to better throttle control and response (never underestimate) and improved fuel mixing.
3) The system should be considered in total - from (at least) trumpet flange to cylinder and proportioned accordingly.

Basic references for BHP per cylinder, assuming ca 120mm from butterfly to valve head and a max of 9,000 rpm are;
Up to 30 BHP - 30mm, up to 33 - 32mm, up to 39 - 35mm, up to 46 - 38mm, up to 51 - 40mm, up to 56 - 42mm
Up to 65 - 45mm, up to 74 - 48mm, up to 80 - 50mm, up to 87 - 52mm, up to 93 - 54mm.
These power figures may be increased by up to 10% in a purpose designed and well proportioned system.
As butterfly to valve distance increases, butterfly size will need to increase in proportion to system taper and vice versa.
Lower revving engines and those with injectors placed before the butterfly will generally accept a larger body.[/b]


Please don't say you are not doing a single carb on flat motor!

google carb or itb calculator for a start.