Fuel pressure drop

Fuel pressure is dropping at the track and I was only running 1/8mile. Still a fresh build, it’s a 493 with a pro form 850. Has a Holley black pump dead headed, 3/8 feed line, with a 3/8 sending unit out of the stock tank. I set the regulator to 7psi and it’s down to 3 while I’m still in second gear. Pump is ground on the frame, seems like a good ground, power out of a fuse box I installed then to a 20amp toggle switch then runs to pump. What am I missing?

Volume ?

Disconnect fuel line from carb. Run hose into 5 gallon pail. Time one minute and measure fuel volume. Compare to requirements for hp and et.

If volume is low measure voltage drop to pump on positive side and then negative side at running voltage.

If you want to get really into it. you could get an oscilloscope and current ramp the pump, acquire frequency and calculate rpm, and pattern integrity. This is how we teach fuel pump diagnoses on non-race cars.

I’ll check volume tonight, I don’t know what it’s requirements are. Being I have a stock tank, could it be not venting enough?

It doesn't take a whole lot of opening to vent , but that is certainly a possibility.
After a pass , remove the fuel cap and if you hear any pressure ( vacuum ) release , that is a problem.

Something else I just thought about, fuel slosh in the tank? Stock tank has no baffles. Anyone ever run one?

I use a stock tank w/ a sump welded on...no issues w/ fuel pressure.

1/2 line minimum......

ON STOCK tanks, i always drill a hole on the bottom towards the back, and thread in a brass bushing. (3/8 internal threads whatever external) I then solder this in. Screw in a nice AN fitting 3/8 X -8 or etc, Use that as the feed your your pump. Works very well . Your fighting a lot of gravity with the stock pick up and the faster you go, the harder it is to pull the fuel up that pickup.

What do you mean by sump?

Is this a replacement 3/8 seding unit and pick up? If so did you retail the sock that covers the pick up. If so this could be an issue. We had problems with higher volume pumps trying to suck the sock into the line.

Yes it’s a replacement, what you mean by retail? All opened it and installed it, didn’t do anything honestly

Sump.
I drill 1 " holes with the hole saw inside the perimeter of the sump , and weld it on the tank.
This acts to keep fuel at the pick up point under deceleration.......sort of a slosh baffle.

What size I.D. is your current fuel line and gas tank pickup ?
Fuel gremlins come in many forms
The smallest hole in the system is the biggest restrictor to fuel flow
You may want to test that system another way at the track, jet the carb up one size at a time to see if you have enough fuel volume to that carb to slow the MPH down If you can slow it down by jetting it richer you have enough fuel volume delivery for your current power
If not fix it

if your battery is in the trunk i would use the toggle switch to a a relay in the trunk to the fuel pump, i wouldn't run the pump off a toggle switch

Sorry it should have said did you retain the sock on the pick up. My fat thumbs don't do well on phone keyboards.

Sump pics

Now that’s just mean!

Sometimes setting the pressure high at idle results in larger drop in pressure.

It depends on how the regulator is designed and the fuel pump pressure and flow rate (gals/min) relationship.
A deadheaded pump is delivering close to zero gallons per minute at idle.

It is going to depend on the regulator pressure and the flows and pressures it will see (ie the the pump).

We took a look at a couple different regulators, and then the effect of control pressures and flow rates in this thread.
https://www.forabodiesonly.com/mopa...b-stumble.437541/page-20#post-1972625252
(read down from that post for pump output volume and cudafever's tests)

You can probably test in a similar manner, but start with the pump data from the manufacturer.
There's some basic rules of thumb for fuel consumption, and you can also look at engine dyno data for similar engines.