CFM discrepancy

[Randy S.]

Re 66 300HP 327

My Holley 4160 is rated at 585 CFM. The Wix air filter for it is (42095) rated 250 CFM. What am I not understanding here?

Randy

[Michael W.]

Re: CFM discrepancy

Without knowing how Wix tested their filters, the number is meaningless. Most likely the numbers and the differential are apples and oranges.

[Bill M.]

Re: CFM discrepancy

I found this:

Air Filter Flow Test
14" x 3" Round Flow # @ 5" H2O
Wix 42095 14x3 Paper 721 CFM

So I figure the drop across the filter at 585 CFM would be about 4" H2O

[Jerry G.]

Re: CFM discrepancy

Whats the pressure drop across the carb at rated flow?

[Duke W.]

Re: CFM discrepancy

Basic fluid mechanics: flow increases with the SQUARE ROOT of depression, which is the pressure drop across the flow passage. In other words to double the flow the depression must quadruple!

Also, as previously stated, flow data without the associated test depression is absolutely MEANINGLESS. One can increase/decrease the flow rating of a carburetor or air filter merely by increasing/decreasing the test depression, so unless you know the test depression the data is garbage.

At least with the case of carburetors the defacto industry standard test depression is 1.5" Hg for four-barrels and 3" Hg to two barrels. That means that the 1300-odd CFM rating for the 3 2-bbl. carbs on big blocks is not much more than the L-72 4-bbl. because you have to divide the total 2-bbl. flow by the square root of two to estimate the flow at 1.5" Hg rather than 3".

A good number to remember if you're interested in this subject is 1" Hg = 13.6" H20, so the 4-bbl. depression standard is 20.4" H20.

In the case of this air filter, if the rated flow is 721 CFM @ 5" H2O then the depression at 585 CFM would be 5(585/721)^2 = 3.3", which is very little loss.

An engine is basically an air pump, and a ballpark estimate of maximum flow volume at the redline is 80 percent VE for unmassaged heads and about 90 percent for massaged heads. An OE 327/350 at 6000 is pumping about .80(327/1728)(6000/2) = 454 CFM, so the 585 CFM Holley is not restrictive, and the pressure loss is only about 0.9" Hg, which is about 12.3" H20.

The dyno test in the Fall 2010 Corvette Restorer of a '67 Special 300 HP configuration showed that with the air cleaner assembly removed the engine lost about one percent across the range compared to the air cleaner/filter assembly installed, and the reason is explained in the article.

A basic rule of thumb for carburetor sizing on a road engine is that it should provide the engine's maximum air demand at no more than 1.5" Hg loss. For a racing engine it should be no more than 1" Hg.

Similarly an air filter should be able to supply a road engine's air demand with no more than 6" H20 loss, so the 14" x 3" air filter elements used on many Corvette engines is very generous for small blocks and reasonable for big blocks.

Duke

[Duke W.]

Re: CFM discrepancy

Here a proportioning relationship you can use to determine flow rate at another depression or depression at another flow rate.

(CFM1/CFM2)^2 = d1/d2, where CFM1 and CFM2 are the respective flow rates at depressions d1 and d2.

Or you can take the square root of both sides of the equation and work with it in this form:

CFM1/CFM2 = SQRT(d1/d2)

Say you have a carburetor flow rate of 585 CFM @ 1.5" Hg. You want to determine flow rate at another depression or depression at another flow rate. Now you have three numbers you can plug in and solve for the remaining variable.

Duke