gve

I was looking thru the Holley catalog to buy a carb, can anyone explain how they rate the CFM based on the following



Venturi Dia. Venturi Dia. Throttle Throttle
Prim. Sec. Bore Prim Bore Sec.

750DP R4779 1-3/8 1-3/8 1-11/16 1-11/16
870 street avenger 1-3/8 1-3/8 1-11/16 1-11/16
850 vac R80531 1-3/8 1-3/8 1-11/16 1-11/16
850DP R4781 1-9/16 1-9/16 1-3/4 1-3/4
Only so much air can go thru a sized opening why do the 870 and 850 use the same size as the 750. I'am I missing some thing here,they price their carbs by CFM but are we getting the CFM?

bashcraft

Different types of boosters will effect the CFM rating.

lars

Holley has a standard testing method to determine cfm flow. Flow through the carb is limited not just by throttle plate size and venturi size, but by the configuration, placement, and size of the booster venturi.

early shark

Lars,

There has been a long history of argument over the Holley 4150 vacuum secondary model carburetors. In a lot of GM literature of the time, this being 1971. GM lists this carburetor in its usage on the 1971 LS6 engine as 800 CFM, in many different ads. Holley refers to it as a 780 CFM. Could you reflect on this difference?

lars

It's all a matter of testing method to achieve the marketing numbers you want...

The flow measured through the carb is determined by the pressure differential that the test was conducted at. Holley uses a specific number for testing on all their carbs. Barry Grant, for instance, uses a different number, and GM has a separate testing method. If Holley, BG and GM all tested the same carb with their own tests, you would get 3 different numbers for that same carb. There is no universally accepted industry standard for carb cfm testing and rating, so the specific numbers for the carbs will have some variation. For example, a BG 750 carb will test at higher than 750 cfm if tested to the Holley standards.

I've seen the same thing with head airflow numbers: There is a generally accepted standard pressure differential for head airflow, but some sources use a different "delta-P." This will scew the numbers significantly... you gotta watch it to make sure you compare apples to apples...

For the GM versus Holley cfm, you can't compare the numbers directly, but you can use them as a guide for carb selection. Since GM rated the Q-Jet at 750, the 800 rating just tells you that the Holley carb probably flowed about 10 percent more than the Q-Jet. If you use the numbers comparatively rather than as absolute, you'll get an idea of performance potential and application for the carbs.
Lars

early shark

Lars,

Thank you. That explanation makes perfect sense, why there is a discrepancy in the CFM rating between Holley and GM's stated flow numbers on their Holley applications. Now when this conversation comes up between musclecar fanatics both quoting that their rating is the right one, we'll know that each entity used their own testing data in conclusion to the carburetors application rating.

gve

I have a 454BB with the 750DP I was thinking of trying a 850 cfm. From what the holley book shows maybe I should buy the 850DP so I'am sure I'am getting a bigger opening. I'am concerned about buying a 850 with the same throttle bore as my 750.

TheSkunkWorks

For anyone interested, Barry Grant rates Demon carbs with all barrels at WOT simultaneously using wet flow (with a fluid which simulates gasoline) at 1.5 in/Hg. If measured dry like everyone else evidently still does, their flow numbers would be higher. Yet another issue adding to the confusion...

427Hotrod

I've been in the Holley factory several times with some of the Guru's there. I've seen lots of carbs being wet flow tested and measured for fuel rate and airflow.


JIM

olescarb

The older classic Holleys seem to have thier flow rating (cfm) rated with different standards than the Avenger series carbs are rated at. This may be a way to allow keep the customer from buying too big (cfm) of a carb for thier needs.

I would stick with the classic holley double pumper series, if you need a 850 cfm holley double pumper use the List #4781. The Barry Grant Mighty Demon carbs are also a very good high performance carb, but they flow slightly more than thier rated cfm.

The correct (cfm) carb for maximum power will cause the engine to have about 1/2 inch of vacuum at wide open throttle @ peak/max power rpm. if the vacuum at WOT is lower the carb may be too big, if the vacuum is higher than 1 inch of vacuum at WOT the carb is too small for maximum power. but remember that a larger carb will often hurt part throttle drivabilty so select the carb that is correct for the way you normally drive you car.

You may gain more power tuning your original carb (and ignition advance curves) than you gain by switching to a bigger carb.

i hope this helps Henry @ oles carb

lars