measureing or calculating FI CFM capacity

[Jerry G.]

Heres one for you techkies.
I have a modified FI unit on my 64 racecar. The inner air inlet ( where the choke would sit) has been machined open. I would like to know if anyone has ideas about cross sectional area of the opening vs CFM capacity. Jerry

[Jerry G.]

Re: measureing or calculating FI CFM capacity

How about any correlation between carb or air meter opening area and CFM? I can measure the area of the opening I now have if anybody can tie that to CFM. Or is there any instrument or bench test that can be done to determine CFM?

[Duke W.]

Re: measureing or calculating FI CFM capacity

One could calculate the isentropic flow at a given depression based on the size of the throttle bore opening, but actual airflow will be less due to real world phenomenon such as heat transfer and friction, and the ratio of actual measured airflow to isentropic flow is the flow coefficient.

In a 1957 SAE paper by Duntov et. al on the FI system air flow was quoted as "24 pounds per minute", but no depression was given, which means the data is useless. I expect the omission was intentional.

Based on what I've heard, the airflow at 1.5" Hg. depression - same depression that is industry standard for measuring 4-bbl. airflow - is about 600 CFM, but the Chris Wickersham mods get is up to about 750.

If you measure the throttle bore diameter, I can do the calculation. I think it's about 3", but have never personally made the measurement.

Duke

[Jack H.]

Re: measureing or calculating FI CFM capacity

One race dyno facility I once used in Golden had a flow meter as part of their fixture. When they'd run engine dyno tests, one the parameters measured & reported was air intake performance. Basically, the fixture was a tube that mounted to the carb air horn that had a light weight propeller inside which turned in lock-step to air flow into the engine.

I didn't see what all adaptors they had for different carbs or what they did when the engine was FI based (no carb air horn), but I presume the shop had facilities to measure those setups too...

[Jerry G.]

Re: measureing or calculating FI CFM capacity

OK Duke if your game here are the area measurements. The inner cone area is 3.97 in sq. The outer sencing ring area is .67 in sq. So the total inlet area is the sum of the two or 4.64 in sq. This is not a stock set of dimensions. I have machined out the center cone. Jerry

[Paul Y.]

Re: measureing or calculating FI CFM capacity

Jerry, F&M Auto @ I70 and Wadsworth has a flow bench. Good bunch of guys too.

[Jerry G.]

Re: measureing or calculating FI CFM capacity

Jerry, F&M Auto @ I70 and Wadsworth has a flow bench. Good bunch of guys too.

Thank you Paul. I don't know if a flow bench can be adapted to measure flow through an FI intake system. I'll check it out. Jerry

[Duke W.]

Re: measureing or calculating FI CFM capacity

OK Duke if your game here are the area measurements. The inner cone area is 3.97 in sq. The outer sencing ring area is .67 in sq. So the total inlet area is the sum of the two or 4.64 in sq. This is not a stock set of dimensions. I have machined out the center cone. Jerry

What I need is the diameter of the throttle bore. You will have to remove the air meter to measure it.

Duke

[Jerry G.]

Re: measureing or calculating FI CFM capacity

Duke I'm happy to do that but I don't think throttle bore is the limitation. The inlet cone and annular opening for the sensing ring are the smaller of the two areas and feed the throttle bore. Am I missing something? Jerry

[Tom P.]

Re: measureing or calculating FI CFM capacity

Duke I'm happy to do that but I don't think throttle bore is the limitation. The inlet cone and annular opening for the sensing ring are the smaller of the two areas and feed the throttle bore. Am I missing something? Jerry

Jerry,
I'm glad you asked this question.
It WOULD SEEM TO ME that the air meter bore just before the plenum opening (throttle bore) would be the limiting factor in actual air flow. BUT, with a cone ahead of the bore, as well as the throttle blade/shaft itself, it would make sense that those parts right in the middle of the airstream (or air flow) would have an impact on the actual flow characteristics as the air passes through the air meter at wide open throttle.

And last, would there be ANY difference at all between an air meter attached to a flow bench device with air being sucked through it vs any possible ram effect when the car is traveling at speed, as air is being pressurized at the air inlet in the grill area?
It would seem that there MUST be some difference in air flow through a stationary air meter (on a test stand) and pressurized air buildup at 100-150+mph.

[Jerry G.]

Re: measureing or calculating FI CFM capacity

I agree. You should be able to move more oxegen molecules through the air meter when it is pressurized by the static pressure that builds up at the air intake up in the front of the car where the air cleaner inlet line starts. I think the calculation that Duke is doing will give a base number and this should improve with speed. We will never get to sonic velocity , which is the pysical limit at local atomspheric pressure. Jerry

[Clem Z.]

Re: measureing or calculating FI CFM capacity

take the air meter assy off and take it to a shop with a flow bench. they should be able to give you a number just like they flow carbs. take your air cleaner and flow the whole ball of wax.

[Duke W.]

Re: measureing or calculating FI CFM capacity

Duke I'm happy to do that but I don't think throttle bore is the limitation. The inlet cone and annular opening for the sensing ring are the smaller of the two areas and feed the throttle bore. Am I missing something? Jerry

The isentropic flow calculation is based on a "perfect" duct equal in diameter to the throttle bore.

A carburetor and the FI air meter has obstructions because of the venturis, throttle and choke valve shafts, etc., so the flow coefficient will be lower than a straight through throttle bore that would be used for a speed-densityi EFI system, which only has the throttle bore shaft as a "restriction".

All the other paraphenalia in the air stream just represents more restriction and friction, which will result in a lower flow coefficient than a straight through throttle body of the same diameter for a speed-density EFI system.

The same philosoply applies to cylinder heads, which you will be reading about this month in The Corvette Restorer. The flow coefficient, which is based on the valve diameter is a measure of how good the design is relative to the perfect, frictionless duct of equal cross section area to the valve head diameter.

Duke

[Jerry G.]

Re: measureing or calculating FI CFM capacity

OK now that the fire crisis is over, I live in Boulder Colorado, I have the throttle bore diameter. it's 3.105". do you need anything else to do the calculation? jerry

[Duke W.]

Re: measureing or calculating FI CFM capacity

No, that's all I need, but I'm just taking a break from chopping wood, so I'll do the calculation tonight or tomorrow.

Duke