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How much air will a throttle body or carb flow?

Posted: January 11th, 2007, 6:08 pm
by Guy Croft
As a yardstick a 33mm diameter hole (broadly speaking) flows 102 cu ft/min at 10" depression.

If your throttle body (or carb choke) is 40mm diameter it will flow roughly more by the ratio of radius squared (^) ie:

33^2 = 1089 sq mm
40^2 = 1600 sq mm

1600/1089 = 1.47

1.47 x 102 = 150cfm

if the hole is smaller, invert the ratio.

A normally aspirated engine will need to cope with the volumetric flow rate of a single cylinder only, makes no difference if there is one throttle body (or choke) per cylinder or one throttle body feeding all of them. A pressure charged engine must be able to cope with the full blower or turbocharger free air delivery or manifold pressure will not build up properly.
Where a single throttle plate is concerned, it is pointless fitting a bigger one in expectation of gain if the existing throttle plate (or slide/roller) flows sufficient air. The loss of flow due to friction (viscous loss) thru a throttle body is almost negligible. Moreover, a good throttle body should flow the air demand at full throttle not part throttle, because particularly with single throttle bodies on normally aspirated engines the response on opening can be un-progressive if it's too big.

The higher the pressure difference across a throttle the more it will flow, the limit is the Mach number (supersonic velocity is Mach, or Mach 1 as some refer to it). Things get very borderline (unstable regime) around 0.5-0.6 Mach.

I'm putting together a computational table to determine Mach numbers for all types of engine setups but it's nowhere fully operational yet.