Weber Venturi Size, Peak Power and Choke Size, 8 Barrel Venturi Size

[Ron Earp]

Some time ago we had a post that looked at weber choke size (or EFI 8 barrel venturi size) and used the formulas from Weber that approximated where the peak power for the motor would be.

http://www.gt40s.com/forum/tech-engines-induction/17216-twm-weber-manifold-flow-2.html?highlight=weber+venturi

I'm sure these values are approximate, but it appears that some users have found them fairly accurate given some recent results. Contrary to what I thought the webers or single stack EFI can be fairly restrictive to a motor once the size of the motor and target RPM range climbs.

I took the formula and ran it through various engine sizes to produce the graph shown below. One thing to remember is a 48mm IDA or IDF does not have a 48mm opening - the carb must have a choke installed and the largest these carbs fit is a 44mm. So, if you don't have 44mm chokes, and many do not from Pierce, Inglasie, Top End, then you might have an even smaller diameter breathing hole than you thought.

Once you get up to about 342/347/351 inches of motor it becomes clear that a 48mm carb body with 44mm choke will put your peak power at 6500 RPM or lower. While the webers feed smaller engines to 7000 RPM+, not so with larger engines. I've seen some 427s with 48 IDAs too, and it is clear their peak power is going to be fairly low in the RPM range unless the carbs are modified to handle much larger chokes (I think they can go to 52mm).

Sure, its all theory and approximated as well, but seems to hold up pretty decently under experimental testing.

R

 

[Paul Thompson 'Hooligan']

Fuel Inject it

These big enough?

http://kinsler.com/Cat_31_Whole_PDF/0201234_screen.pdf

 

[Paul Thompson 'Hooligan']

These are kinda cool too!

 

[Dean Lampe]

I run the 48's on a 408w and I have the 37 chokes. The flow bench says that this carb flows around 330 cfm per barrel. Thats a lot of air! My heads will flow 300 cfm at .500 lift which is very good. These carbs pull hard to 6500 and would probably go futher but the hydraulic rollers start to float.

 

[Mike]

Hi, Dean. Are you by chance a member of the SLKCC?
Re. 330cfm capacity, what Ron's formulas take into account is that 330cfm per cylinder/choke does not add up to 8 x 330 total airflow. What it does do is limit the airflow to one cylinder to 300 cfm. On a carb set up with a big plenum/manifold, the plenum accumulates air while one cylinder closes and the next cylinder opens due to inertia and 'harmonic?" effects etc., so at any instant when a cylinder is open, it has access to considerably more than 300 cfm. Peak power in the mid 6000's seems to be the norm on most 8 stack set ups, depending on the displacement of the engine. On a 408ci motor, I think you're doing well to get peak power as high as 6500.
Re. valve float above 6500, what springs do you use? I'm running Crane 300# dual springs on a custom Comp cam, and have no problems getting to the 7000 RPM limiter.
Mike (St. Louis)