[prostreet64]

Made a few runs in the Bird on Saturday; now have a gauge in the cockpit to monitor pressure during the run. At idle it is around 6.5psi and under w.o.t. it drops to 4psi. No indication of any problems with fuel delivery but wondering what others have experienced in a car running similar #'s and with a fuel system that uses 1/2" line and a Holley HP125 pump or equivalent. Also, the stock factory tank is sumped.

[Motor Daddy]

Not running a return style regulator, eh?

[prostreet64]

The HP125 pump is factory set at 7psi and does not use a regulator. Our gauge sending unit is plumbed right at the carb so I think the 1/2psi drop is simply attrition from the distance fuel must travel to reach the carb.

[RobbMc]

ANY fuel pump (mechanical or electric) that does not use a regulator will experience some pressure drop at full throttle. Doesn't matter how big the pump is.

Electric: The pump is at the back of the car. At idle, there is little or no flow so there is no pressure drop in the line. At full throttle, the pump is still producing full pressure (assuming the pump is big enough) but the pressure at the front of the car is 3 or 4 psi lower than at the rear because there is a pressure drop due to line friction and g-forces.
If you run a regulator, the pressure drop is still there but because the pump is putting out about 14 psi, there is still about 10 psi at the front of the car which is still higher than the set pressure of the regulator, so the pressure coming out of the regulator changes very little.

Mechanical: At idle, the pressure will be about 7 psi because the spring is fully compressed as there is no flow. At full throttle, the average diaphragm level drops down to increase flow which reduces the force exerted by the spring which reduces the pressure. Running a high pressure pump and regulator fixes the problem for the same reason it fixes the problem with an electric.

Having said all this, a pressure drop from 7 to 4 psi probably won't hurt anything (although it does lean the jetting about the same as one or two jet sizes because the float level falls slightly). I think Holley recommends a minimum pressure of 4 psi for their carbs.

[Dragncar]

So how much noise does your HP pump make.I am thinking about the HP 150,Holley says its good to 900HP.

[prostreet64]

Dragncar, the HP125 is almost silent. We can barely here it running even when the motor is shut down. I imagine the HP150 is the same.

[Motor Daddy]

I use the HP150. It's a good pump, but it is as loud as the Holley Blue and Mallory 140. I've had 'em all. LOL

I think most of my noise is from the pump having to "suck" through a 3/8" pickup. I use a 1/2" supply and return line and a return style regulator. I have an in-dash electric fuel pressure gauge and it read rock solid even at WOT. I also have the ability to log fuel pressure. Here is an example of a WOT run in third gear:

[prostreet64]

md- I am surprised that the HP150 is that noisy. I didn't think anything was as loud as the Holley blue pump! Anyway, maybe it is because of your particular setup?

[dda72]

A Holley 150 rated to 900hp? Is that correct?

Dwight