[steve25]

GM did not make the Q jet Rochester mainly did and then Carter for a while.

The 4 bbl test pressure has always been 1.5 hg which converts to 20.4 inches of water, not 24 inches of water.

When David Vizards residence was CA he had a super flow 300 bench and around 2016 David had a Flowlab 520, when he moved to the east coast at some point he got a SF600.

Note the none of these 3 flow benches have the capacity to test a good size carb directly at 20.4 inches or 1.5 hg, which ever you prefer.

Testing with these benches would have called for testing at a lower pressure drop and then making the math conversion up to 20.4 inches, or the carb could have been flow tested by means of 1 or 2 barrels at a time and then added up, which can be misleading as compared to all 4 barrels drawing in at the same time around the air horn of the carb.

Here are the specs for a SF 300 and 600, from what I was lucky enough to still dig up on line about the Flow Lab 520 it had a capacity of about 450 cfm@ 28”.

A SF 600 could come very close to testing out a 600 cfm rated carb @ 1.5 hg with the primary’s and the secondary’s wide open.