I'm not sure what you mean when you say that load is holding the crank rotation back.

When the transmission is in neutral and the only load on the engine is the internal friction of the engine and accessories attached to it, revving the engine will require a much lower airflow than if the car is accelerating on the road. I think there is some confusion between volume airflow and mass airflow. Let me show you what I mean with the following example.

We can agree that at say 3000 RPM the cylinders are displacing (or pulling in) 195.3 cfm. For the sake of argument, let's assume that the volumetric efficiency of the engine is 100% at all times. Let's further assume that with no load and the engine running at 3000 RPM, the manifold vacuum of the engine is 15 Hg (7.33 psi absolute) while the manifold vacuum of our 4bbl carburetor-equipped engine at 3000 RPM is 1.5" Hg (13.96 psi absolute). Let us further assume that air temperature is 70°F in both cases to simplify our calculations.

Using the ideal gas law, the density of air at 15" Hg & 70°F is 0.0374 lb/ft³ while the density of air at 1.5" Hg & 70°F is 0.0712 lb/ft³.

Mass flow @ 15" Hg is 195.3 ft³/min x 0.0374 lb/ft³ is 7.3 lb/min.
Mass flow @ 1.5" Hg is 195.3 ft³/min x 0.0712 lb/ft³ is 13.9 lb/min.

Therefore, the engine is using almost twice as much air accelerating on the road at 3000 RPM as it is with no load. Remember, the venturis are above the throttle valves so the flow just above them is always at around atmospheric pressure. Density doesn't affect the airflow very much until the air stream is past the throttle valves.

Assuming an air temperature of 70°F and a pressure of 14.7 psia, the density of the air would be 0.075 lb/ft³. Using the above mass flows, the volumetric flow to the carburetor and what the venturis would actually see would be more like:

No load -- 7.3 lb/min / 0.075 lb/ft³ = 97.4 ft³/min or 97.4 CFM.
High load -- 13.9 lb/min / 0.075 lb/ft³ = 185.5 ft³/min or 185.5 CFM.

The volumetric airflow is still about twice as much for the loaded condition as for the unloaded condition. The point of all this is that when I cracked the throttle open in my garage and saw fuel flowing through the primary venturis, I was sure that there was sufficient air flowing to get fuel flowing through the main metering system at all times.

An additional point I hope you can also see is that as you reduce the manifold vacuum (related to carburetor restriction) at wide open throttle, you increase the mass of air getting into the engine, thereby increasing its power. With a more restrictive carburetor (like the factory 1bbls) the engine produces less power because the density (and pressure) of the air finally reaching the cylinders is lower. Also, lower absolute pressure (or higher manifold vacuum) in the manifold helps an engine be less sensitive to lack of heat in the intake manifold.

I think you are absolutely right about the accelerator pump shooters. However, in my case, I had tried to hold the RPMs for more than just an instant as I was doing the thing with one foot on the brake and the other on the gas. Any fuel that the accelerator pump had shot in had plenty of time to be consumed and I noticed that I lost power as I started to slowly open the throttle. I did this to determine whether the primary metering system was causing the bog or the accelerator pump system. Because I could pretty much hold the loss of power indefinitely, the problem seemed to be coming from the main metering system which is why I started to richening the power step.

You might be disputing my claim that adding heat to the intake manifold will allow you to run a larger carburetor. Any carbureted engine is equipped with manifold heat from the factory to improve fuel distribution. Only all-out racing engines can really benefit from having a cold intake manifold. I have not suddenly discovered a new miracle cure for using larger carburetors because I am saying the same exact thing as Clifford Performance when they recommend water heated intake manifolds for the street, except they've been around a lot longer than me.

Frank

I'm not disputing any of your claims... I will get back with you after I've read your book...

D/W

_________________

If it ain't broke, fix it!

I have created html tables from my excel spreadsheet that show the flow areas of the various combinations of metering rods and jets.
My first posting on this topic helps to understand how to use it and I've updated into my web page:
Carter AFB / Edelbrock Performer Jetting Setup


I hope it helps.

Frank Raso
Raso Enterprises

What's the difference? Just the p/n's and sizes offered. This is very helpful to people dialing in an AFB, by the way, myself included, thanks. I have calculated the area for a narrow number of different sizes for my own purposes so an extensive chart like this is very helpful. I just think it should be made clear that the jets and rods are interchangable no matter which name the carb has on it.

D/W

_________________

If it ain't broke, fix it!

Yes, you're right, and thanks for pointing that out to me. I've updated the page with that note. The differences in the tables are purely with the part numbers. If you need a 2-step rod in your carb, the metering rods and jets will interchange. I included the AFB table because that was the one I was originally working with and figured that others might still be using their old AFB strip kits.

Frank