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 ?

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.

Might help if you state what engine you have in mind. The 3.3 subi?

For the price of a decent carb set (and ignition) on a 6cyl I'd just look at doing a megasquirt with modified bike Tb's.

Yes 3.3 Subi.
ID be trying for 6 one per cylinder. Or maybe 4 (prefer one per cylinder)
Yes I realize a megasquirt could be more efficient , bla, bla bla.
Carbs would be much more unique and im not building this car for efficiency, lol.

AS for what carb, well thats gonna depend on what sizing i should use.

Three 40mm IDF webers on a custom mild steel tube manifold.
But a 4 barrel single would also likely work.

As for research...unless someone has done it you are on your own.

This is a 2.2 with Dellorto 36's:



It might be a little under-carbed (it's also plumbed for NO2) but there weren't any problems. The Dellorto's flow about the same as Weber 40IDF's, so that should be about the right ballpark. MegaJolt for ignition.

YAh they are about 205 - 210 CFM per choke so like 850 cfm combined which seems like a lot but on IR manifolds carbs can be so much bigger and run great.

Call PMO The Porsche carb specialists
They have reproduced the Weber triple throats with many improvements....
and in a few different sizes.

On the 3.3 the head casting is the biggest restriction. Even a mild port job can increase your flow. There is a ton of room. I would think a set of triple webers made for a 3.2 porsche should be close. Remember a 4 valve head should out flow a 2 valve head.

But realy I would think a cool looking snow machine throttle body and an injection system would give you the cool look and modern control. Or macanical injection!! Wouldn't that be a mess to set up.