Calculate Carb Cfm From Cylinder Head Flow Specs

[Larry Jones]

I’m finishing up my Gen 1, 23 degree 350 engine for my ‘59 Corvette roadster. My roller cam redlines at 7000 rpm and the following flowchart shows the individual cylinder flow from the head manufacturer.
is there a method for calculating optimum carb CFM for a double pump carb using the flowchart and heading upward of 7000 rpm?
Thanks for any help.

 

[Grumpy]

keep in mind that each cylinder draws from the common plenum only every other revolution, and
max port flow is only occurring during at about 90-160 degrees of the 720 degree repetitive cycle, each cylinder max draw is spaced 90 degrees apart, remember the crank makes two full revolutions for each full revolution the camshaft makes

I've rarely seen any benefit from a carb larger than a 750-780 cfm on any 350 cubic inch engine
and most ports rarely flow to the max rated flow,
and in many cases both lack of valve lift and a restrictive exhaust help restrict intake flow rates.
Id also point out that a carb rated at lets say 700 cfm is rated at a vacuum of 1.5" of mercury,
if the vacuum increases so does the flow through that carb.
yeah your heads might be rated to flow at 258cfm,
but its very unlikely your heads are the restrictive component,
its more likely the intake manifold, exhaust system or cam timing is restricting air flow,
and that potential max head flow would be drawing at that rate for only about 40 degrees on average,
out of the plenum per cylinder as its intake valve lift reaches near its cylinder max lift over the 720 degree cycle

https://www.inchcalculator.com/carburetor-size-calculator/

http://www.wallaceracing.com/calcafhp.php

http://www.wallaceracing.com/carb-cfm-vacuum.php

http://www.wallaceracing.com/calcdchg.php

http://www.wallaceracing.com/chokepoint-rpm.php

http://www.wallaceracing.com/chokepoint.php

USE THE CALCULATORS
http://www.rbracing-rsr.com/runnertorquecalc.html
http://www.wallaceracing.com/chokepoint.php
http://www.wallaceracing.com/header_length.php
http://www.superchevy.com/how-to/en...-0902-chevy-engine-port-variations-measuring/
http://www.hotrod.com/articles/choosing-the-right-camshaft/
http://garage.grumpysperformance.com/index.php?threads/bits-of-383-info.38/

https://www.badasscars.com/index.cf...ct_id=440/category_id=60/mode=prod/prd440.htm

http://garage.grumpysperformance.com/index.php?threads/port-speeds-and-area.333/#post-72826

http://garage.grumpysperformance.co...orsepower-from-head-airflow.14939/#post-84278

http://garage.grumpysperformance.co...gine-to-match-the-cam-specs.11764/#post-55571

 

[Larry Jones]

Incredibly knowledgeable answer. I was looking towards a Holley or Demon 750 CFM. So I’m a happy camper. Thank you for taking your time to answer my question. you are appreciated.
Larry

 

[Grumpy]

always glad to help