measureing or calculating FI CFM capacity

[Chris S.]

Re: measureing or calculating FI CFM capacity

CFM Would change with velocity, take the square inches devide by 144 and multiple by fpm. that would give you cfm. I do it all the time in the hvac business. is this what is being asked? i would say full cfm would have to be measured at full throttle. but than agian i am not sure if this is what was asked for a question?

[Duke W.]

Re: measureing or calculating FI CFM capacity

The isentropic flow tables are based on ratios of absolute pressure and density - downstream divided by stagnation.

For stagnation conditions I'll select standard sea level conditions, and use a 1.5" downstream pressure drop (same as is used industry wide to test four-barrel carburetor flow), so the pressure ratio is 28.42/29.92 = .950

At this pressure ratio the density ratio is .965 and the Mach number is 0.27.

From a practical standpoint, if Mach number is less than 0.3, the flow can be considered to be incompressible to reasonable approximation, but in this case the density ratio is 0.965, so we do have a 3.5 percent reduction in density.

Given sea level sonic velocity of 1100 ft/sec the velocity is 297 ft/sec, throttle bore area based on an outlet diameter of 3.105" is .0529 square feet (not taking an allowance for the throttle shaft). Isentropic flow rate is velocity times area, 297 x .0529 x 60 = 937 CFM.

If measured flow is 600 CFM, then the flow coefficient is 600/937 = 0.64.

Applying the same calculation to the contemporaneous Holley four-barrel, which has four 1 9/16" throttle bores, total throttle bore area is .0532 square feet, so isentropic flow at 1.5" Hg depression is 949 CFM, and at the rated 585 CFM, the flow coefficient is 0.62.

So both the Holley and FI air meter represent about the same restriction and have nearly the same flow coefficient, but the FI system has more efficient manifold runners and much more equal fuel distribution under all operating conditions, which is why, given sufficient head flow (a big IF) the FI system should make more power.

With OE machined heads, the FI system only has a slight advantage because the head flow is the major restriction. But once the heads are "massaged" the manifold becomes the major restriction, so the FI system should make significantly more power than the Holley and manifold, all other things equal.

Duke

[Clem Z.]

Re: measureing or calculating FI CFM capacity

this may be of interest to people reading this thread. the dyno operator who signed the dyno sheet was ron mccaferty a friend of mine who move to calif when gulf research labs closed.http://www.racingicons.com/gs/004/gulf.htm

[George J.]

Re: measureing or calculating FI CFM capacity

Fascinating. Thank you!

George

[Jerry G.]

Re: measureing or calculating FI CFM capacity

NOW were getting technical.This is what I like to see on a technical board. Good stuff. So Duke, in your example you compare the Holley and the Rochester. What would be the street rating of the Holley your comparing to. IE is it a "750" or an "850" or ? For the Rochester unit what effect will the the cone and annular opening have on the flow of the throttle bore. Will it be proportionately reduced. If the annular opening and cone area are 70% of the throttle bore area would the CFM capacity be reduced by 70% or would velocity increase through this area to compensate for throttle bore capacity. What is a reasonable flow coefficient to strive for 60%, 70% 90%? To find the flow coefficient do I need to put the air meter on a flow bench. I'm happy to do this to get the data. My dyno measured CFM requirement for the race motor is 700CFM at peak reequirement to the engine. Can this air meter do this?

[Duke W.]

Re: measureing or calculating FI CFM capacity

Theoretically a "perfect" (isentropic) verturi will have no effect on flow until the Mach number at the throat is one, which is called "critcal flow" and the pressure ratio is always 0.528 at critical flow.

At this point flow rate will not increase no matter how low the pressure ratio is made. When Mach number equals one the flow is "choked".

There's no way to calculate the flow restiction of venturis and various obstructions in the flow path. It's a matter of testing the device, computing the isentropic flow and then calculating the flow coefficient. The higher the flow coefficient, the better the design.

Some of the "tricks" used to improve real flow, and, therefore the flow coefficient, is to completely remove the choke valve/shaft, replacing the conventional butterfly throttle valves with slide valves, and smoothing down any casting flash. Also the approach to the air horn is very important, which is why completely removing the air cleaner assembly typically reduces flow, which can be seen as less power in a dyno test (more about this in the next Corvette Restorer due in your mailbox this month).

Holley 750-800 CFM carbs have 1 11/16" throttle bores, and I believe the difference in flow rates is due to different venturi sizes. Holley 850s have 1 3/4" throttle bores, but a flow coefficent of 0.6 to 0.65 is probably typical for all.

So anything you can do to the carb or air meter to reduce obstruction will improve the flow coefficient for less density loss. You really can't mess with the venturi without upsetting metering, but the FI air meter's annular venturi means not all the incoming air goes through the venturi, which helps the flow coefficient.

I'm not sure about the details of all the "tricks" used to improve the air meter's flow, but, for sure, removing the choke valve and shaft should help along with grinding down any casting flash in the flow path. My understanding is that a properly massaged air meter can flow about 750 CFM at 1.5" Hg versus about 600 as built.

As far as sizing is concerned a high performance road engine should have a four-barrel carb or venturi type air meter that provides the engine's peak air demand at no more than 1.5" Hg depression in order to maintain good part throttle metering and response without excess restriction at peak flow demand. Using 85 percent volumetric efficiency at 7000 (which is the ballpark figure with massaged heads and the LT-1 cam) equates to 562 CFM, so OE carbs/FI air meter are well sized.

For a racing engine, where low-end metering and throttle response are not issues, the carb/air meter sizing should provide peak air flow at no more than 1" Hg, and since flow rate varies with the square root of depression they should provide sqrt (1.5/1)562 = 688 CFM, so a 700 CFM rated carb/air meter should be in the ballpark.

However, if we assume that your engine makes 0.85 VE at 7500, then the engine's maximum demand is 603 CFM, and 738 CFM at 1.5" Hg depression will provide the requisite 600 CFM at no more than 1" Hg pressure drop. So, rules permitting, a 750 CFM carb or air meter is best.

The least restrictive induction systems are slide throttle individual runner fuel injection systems, which is what you see on purpose-built naturally aspirated racing engines. Slide throttles means you have essentially a straight pipe with no restrictions, and the flow coefficient can be on the order of 0.9.

Duke

[John D.]

Re: measureing or calculating FI CFM capacity

Why is it that when the CFM's is drastically increased in a mid year FI unit the spill valve has to be altered to compensate for it?

Gulf Research played around with the dual air meter 63 FI units. At the NCRS national at Seven Springs ski resort in 1994 there was one of those units on display in Festival Hall next to Rich Masons Grady Davis car that I worked on as a kid.
The spill valve in this unit is a modified thumbtack style one. The spill valve has an insert pressed into it to make the thumbtack seat area smaller. The thumbtack itself is smaller also. The nail is the same.
Trying to say they reduced the size of the interior of the bore of the body.
Jerry do you have to do this with your FI unit? Do you need a special spill valve?
Ask Chris Wickersham this question and get back to us please. Maybe Duke knows the answer.
Somewhere I have one of the Gulf Research spill valves. Got it from John Eyestone at the NCRS national at Flint/Warren years back.

[Duke W.]

Re: measureing or calculating FI CFM capacity

Two air meters means half the flow goes though each, so the venturi signal at each air meter is only one-quarter of what it is with a single air meter, so there has to be some way to amplifiy this signal.

I've never known the details of how is was done, but reworking the spill valve linkage to create four times the leverage would be a logical way. Fuel metering at low load and revs would probably not be as accurate as with a single air meter, but on a racing engine, it's not a big issue.

Duke

[Jerry G.]

Re: measureing or calculating FI CFM capacity

I'll take the air meter in next week and put it on a flow bench to correlate the data Duke calculated. With a single air meter that has been modified to flow more CFMs I'm not sure if the fuel meter/spill valve needs to be modified. I've done the calculations on the nozzle orifices and they will flow enough to support the horsepower the engine can develop. Race engines run in a fairly narrow operating range so we'll just have to put it back on the engine dyno and see what we've got.

[Duke W.]

Re: measureing or calculating FI CFM capacity

I think the only adjustment required with a modified air meter is to make sure the power stop is properly adjusted to provide and A/F ratio of 13.5-12.5:1 in the upper third of the useable rev range.

Duke

[Jerry G.]

Re: measureing or calculating FI CFM capacity

I'm a little fuzzy on this. Let's say I set the power stop at 6500 RPM is set to be at correct A/F ratio. The amount of fuel being bypassed at that signal to the control diaphragm is correct for that point. As the RPMs drop off the signal should weaken and the amount of bypass fuel should increase putting less fuel into the cylinders. So far so good. The slope of the curve is what concerns me. I don't know if the slope of the curve is predetermined by the balance arm geometry or if there will be a compensation for the larger volume of air being passed in lower RPMS by my modified air meter.

[Jim L.]

Re: measureing or calculating FI CFM capacity

I don't know if the slope of the curve is predetermined by the balance arm geometry or if there will be a compensation for the larger volume of air being passed in lower RPMS by my modified air meter.

Jerry, as I remember, the big change you made was to make the opening for the venturi cone choke plate completely circular. I also recall you made other changes that were more subtle.... knife edging the throttle plate, for example.

In total, I tend to believe your changes might cause the venturi signal to drop off slightly more rapidly as engine RPM drops than it would if you had a stock venturi cone.

What would this mean to driveability? If there is any noticeable effect at all, you might experience a slight lean "hole" in the transition from the idle and low speed manifold vacuum circuit to the main venturi vacuum circuit.

Jim

[Jerry G.]

Re: measureing or calculating FI CFM capacity

Hi Jim. I suspect you are correct. Whether it is significant I can't tell yet. I'm not aware of anyway to "trim" the slope of the fuel spill curve. Is anybody else??

[Jerry G.]

Re: measureing or calculating FI CFM capacity

Why is it that when the CFM's is drastically increased in a mid year FI unit the spill valve has to be altered to compensate for it?

Gulf Research played around with the dual air meter 63 FI units. At the NCRS national at Seven Springs ski resort in 1994 there was one of those units on display in Festival Hall next to Rich Masons Grady Davis car that I worked on as a kid.
The spill valve in this unit is a modified thumbtack style one. The spill valve has an insert pressed into it to make the thumbtack seat area smaller. The thumbtack itself is smaller also. The nail is the same.
Trying to say they reduced the size of the interior of the bore of the body.
Jerry do you have to do this with your FI unit? Do you need a special spill valve?
Ask Chris Wickersham this question and get back to us please. Maybe Duke knows the answer.
Somewhere I have one of the Gulf Research spill valves. Got it from John Eyestone at the NCRS national at Flint/Warren years back.

John, would reducing the thumbtack seat area amplify the signal? I may need to do this. Do you have a photo? Jerry