Airflow vs. G force;

Larry Woodfin · Post by **Larry Woodfin** » Sun Dec 18, 2005 11:00 pm

Just for mental exercise, give me your thoughts about this question. Here is the setup.

Since I am most familiar with a 355 Chevy let me use this example. Visualize a drag race car facing the tree, ready to launch. It’s a Trans brake racer using a 4200-RPM low limit chip. So, at the leave the engine jumps to 5800-RPM on the converter and begins the run.

The car weighs 3000 pounds, makes a wheels up launch, and the 60-foot timer shows 1.45 seconds. The quarter mile ET is 10.60 @ 125 MPH. [a common small block at about 500 horsepower]

Here is the question, will the G force at launch move some air/fuel mixture rearward or is airflow greater, thus overcoming G force?

Just curious.

Larry Woodfin

MadBill · Post by **MadBill** » Sun Dec 18, 2005 11:22 pm

No question, it will move fuel towards the rear, but not air. How much is hard to know unless data logging air/fuel ratio cylinder by cylinder, but Cup cars these days have carbs and manifolds tweaked to compensate for cornering gs.

Larry Woodfin · Post by **Larry Woodfin** » Sun Dec 18, 2005 11:45 pm

In that case,

square jetting is common for single four barrel, non-power valve, drag race carbs. So, would one jet smaller in the secondary be a valid experiment? Or, since G force tapers off quickly is square jetting [on the average] the best move?

Larry Woodfin

Post by **SchmidtMotorWorks** » Mon Dec 19, 2005 3:52 am

I think there is another factor that may be even bigger.
Depending on which way your carb sits on the manifold, the acceleration has an effect on the column of fuel over its horizontal path. I have been meaning to calculate it but I guess a way to look at it is how well do you think your carb works when it is tilted at 30 degress, (I guess that is about the effect of 1 G)

aedcarburetors · Post by **aedcarburetors** » Mon Dec 19, 2005 5:01 am

i think that given the air speed of an engine that size on the transbrake down through booster and with a reasonable shear/ fuel cone produced by booster...the vector of the fuel cone directed back due to acccelerating g forces would be negligable

aedcarburetors · Post by **aedcarburetors** » Mon Dec 19, 2005 5:21 am

fuel levels and angles in bowls are critical to emulsion in 2x4 sideways setups ... that is why ya have beveled floats at rear of bowls so that the mean fuel level at centre of bowl under hard acceleration determins float drop and therefore timing of all emulsion stages..... also metering blocks should have stagered emulsion circuits in the first emulsion stage giving even metering under hard acceleration to around the 200 foot mark

Larry Woodfin · Post by **Larry Woodfin** » Mon Dec 19, 2005 8:19 am

I had in mind a modified for drag racing 750 Holley, centrally mounted, bowls are standard front to rear mounting. Also, an open plenum, single plane intake.

Thanks for the intresting comments.

Larry Woodfin

Also, it does sound reasonable that engine air flow is stronger than G force on the fuel at this performance level. [assuming uniform mixture, which I think I have judging by performance]

MadBill · Post by **MadBill** » Mon Dec 19, 2005 8:50 am

However large the relative influence of mixture velocity vs. g forces, the fuel droplets are hundreds of times denser than the air and some effect is inevitable. However as touched on by SMW, for a non-crosswise carb the pressure head on the primary jets is increased by g loads sloshing the fuel higher at the rear of the bowl and similarily reduced on the secondaries, which would richen the front and lean the rear, thus compensating or even over-compensating for any droplet deflection.

Bottom line: The effect is likely far less than the static AFR maldistribution of most carburetted vehicles being drag raced, as probably not one in a hundred has been tweaked cylinder by cylinder.