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Old 12-12-2017, 03:02 PM
Tom Vaught's Avatar
Tom Vaught Tom Vaught is offline
Boost Engineer
Join Date: Dec 2001
Location: The United States of America
Posts: 30,813

Typical carbs today, which you are talking about, I believe), are in a Advertising "Dream World".
For over 50 Years 20.4" of water - 1.5" of Mercury (Hg) was the Carburetor Test Standard.
Then Carb Modifiers read that Smokey used 28" of water test pressure for flow testing because it matched numbers on HIS DYNO. Just because the numbers matched on one guy's dyno do not mean that all dyno testing and Flow bench testing has to be done at 28" of water.
Bob Mullins had World Championship Hemi engines where he flow tested the heads at 3" of water. A BUNCH OF THEM. So which is closer to reality for a large port Head, use a small block port Test Pressure number or use a 500 cid HEMI flow test number. Never could understand why people could not see that 28" is not a Magic Flow Number for the test pressure. But Smokey wrote it in HIS book so it must be Gospel. I knew Smokey personally, RIP, and he was not a Rocket Scientist, just a really hard working guy.

But back on the question. Ok your actual 750 cfm carb flows 950 cfm at 28" of water. REAL 750 cfm carbs work well when the engine wants 700 cfm worth of flow. DOH!

So the 950 CFM number you ask about is a number used for advertising games.

Holley had to play that game because Barry Grant and other Modifiers were playing that game. People would see a 750 CFM carb vs a 950 CFM carb and say GOT TO HAVE that 950 CFM carb for bragging rights. The only true 950 cfm rated Holley was the 3-Barrel unit flowed at 20.4" inches of water. But it also was a Vacuum Secondary carb so the engine and Primary Vacuum Signal actually told it how much air to let into the engine.

One final Comment, on the airflow deal.
When we ran the Booth and Aarons Pro Stock 350 cid engine on Holleys 10,000 rpm Water Brake Dyno at 3500 rpm the engine would barely idle. There was a Fuel Reversion Cloud 3 feet over the carb inlets at that rpm. At 5000 rpm the "cloud" disappeared and the Engine finally was not pumping fuel back out of the carbs/engine. At 10,000 rpm the engine was making sweet music. Carbs Individual Runner Intake and single Carb Bore were right where they wanted to be.

So why did Holley not go into production with that system? Well you blow a couple of Glass Front Ends off of Chebby Vegas going down the track and
the owners are not happy, no matter how big the carb cfm is.

So then we did the Pro Dominator Intake.
Much more like Tunnel Rams you have seen for many years now. Not like the Pontiac NASH TR intake. Nice large Plenum.

It all has to do with PHYSICS and where the carb matches what the engine wants, guys.

Tom V.

"Engineers do stuff for reasons" Tom Vaught

Despite small distractions, there are those who will go Forward, Learning, Sharing Knowledge, Doing what they can to help others move forward.

Last edited by Tom Vaught; 12-12-2017 at 03:08 PM.