Moderator, if there is a better location for this, please move it there.

I was sorting through some old paperwork, and I found my fuel system testing from 25 years ago. My car is a LG4 305 carbureted 84 TransAm. After I turned my 305 into a 334, I was making 1/4 mile runs on the highway and the better I tuned the engine, the more the car would fall flat on it's face at about 100 mph after just shifting into Drive (700R4). This car comes with a 3/8" supply line, 5/16" return line, and a 1/4" line for the charcoal canister.
I tried different and bigger mechanical fuel pumps and needles/seats without much difference. I also inspected every single inch of the fuel lines for kinks - NONE. The carb's bowl was being drained faster than the fuel system could replenish it.

So I then fabricated my own fuel system tester similar to those used for testing for adequate fuel flow when using nitrous oxide. I can connect a 3/8" fuel line to one end and it has a threaded port for a gauge, but the most important part is at the other end. I can screw in my choice for a Holley fuel jet.

We know for each horsepower our gas engine creates we will need .50 lbs of fuel per hour. Expressed as a formula it looks like this:

Given Horsepower x .50 lb fuel/hp = lbs of fuel for given horsepower

To calculate Fuel Consumption for a 400 hp Gas engine with a BSFC of .50 lbs/hp/hr:

400 hp x .50 = 200 lbs/hr

To convert this lbs/hr to a more meaningful gallons per hour we use the conversion rate from lbs to gallons which is 6.2 lbs to 1 gallon of gas

200 lbs/hr / 6.2 lbs/gal = 32.25 gph

And that's it! Now you have an approximate peak gph fuel requirement for your engine that can guide you in choosing your fuel system components.

If you will be doing this calculation often, you can make it even easier by converting the BSFC you prefer for your fuel from the normal lbs/hp/hr unit of measure to a gal/hp/hr measure. This way you will only need to multiply your horsepower by that figure to arrive at your GPH rate, without the need for converting from lbs to gallons after.

BSFC .50 / 6.2 = .0806 gallons / horsepower / hour.

Again we calculate Fuel Consumption for a 400 hp Gas engine.

400 hp x .0806 = 32.24 gph

So I need to supply at least 32 gph of fuel for 400 HP. But I took my testing one step further. The largest needle/seat available for my E4ME carb at the time was .145" (and QJet carbs only use ONE). So that is the size of the orifice I screwed into the end of my tester.

The flow numbers were achieved by measuring how much gasoline was pumped and how much time it took to do so. For example, it took 15 minutes to pump 6 gallons of gas, which equals 24 gph.

Let's get right to the results: (note that the pressures showing on the tester's gauge during testing showed less than 1 psi, and the tester was connected at the output of the fuel pump).

My stock block-mounted mechanical fuel pump flowed 24 gph when testing the entire system. Removing the gas cap increased it to 27 gph. Bypassing the pickup screen and and tank pickup increased it to 30.5 gph. And - drum roll please - bypassing the 3/8" supply line from tank to FP increased it to 43 gph! That line alone is a 12.5 gph restriction. And this year does not have a fuel filter in this line under the car. All combined adds up to 19 gph of restriction.

So now I understand why bolting on a 120 gph FP to the block didn't make much difference - it's being fed by essentially a small vacuum line. My band-aid cure at the time was to do the same thing the factory did trying to overcome the "vapor lock" problem on the L69 cars - I installed a small electric FP near the tank to give the fuel a push up to the block-mounted mechanical FP.