Engine and Transmission Tech

Hi all,

I have a Rover 3.9, 10.5:1 comp, ported, headers, moderate Erson street cam with an Edelbrock 1404 carb. I run it only on the street. I have an electric fuel pump. It is a Procomp (no-name knock off of a Holley), 7 PSI, 97 GPH. (It does make a lot of clicking noise). I have a fuel regulator to restrict the fuel pressure. I have it set to 4psi at idle (It isn't the one in the photo. It is a non-bypass type.)



After a while when idling, the pump starts to "groan" intermittently. Watching my fuel pressure gauge when it does it, it jumps to 9psi for a second or two, then drops back. The groaning stops for a bit then does it again.

The other day I tried bypassing the regulator. The car wouldn't run properly. Way too much pressure for the carb. So, I'm wondering if I have too much capacity for the regulator and/or carb/engine combo. Maybe I need a bypass regulator (and some extra plumbing) or a lower volume pump?

Any thoughts on what is the best combo for this engine? What minimum volume would I need (gpm) for the engine?

Thanks.

Martin

Hey Martin,

What's happening is your fuel pump is overheating the fuel and vaporizing it.
With a deadhead style regulator the pump will see almost no fuel volume at idle.
So to regulate itself it bypasses the fuel around and around inside the pump.
This process heats the fuel very quickly and once the fuel vaporizes the pump gets noisy because it's cavitating.
The pressure spike is air going through your regulator that it can't control.
The cheapest fix for this is to supply a return line to the tank from the pump.
This lets you keep your present regulator and fuel lines intact.
You can tee off of the pressure line from the pump and return fuel back to the tank through a restriction.
You can use a brass tee and drill and tap the return port on the tee to take a Holley carb jet.
The jet size isn't critical but something small like a #56 or so will work just fine.
If it were me I would drill and tap the pressure regulator plug on the pump and use that as my return port
This lets fresh fuel into the pump no matter what the rest of the system is doing. And the pump has more than enough volume to cover for this small intentional "leak".
The best way to deal with your problem though is to add a bypass regulator as near to the carb as is practical.
And plumb in a proper return line.
And finally the High tech solution is an electronic fuel pump controller.
This unit lowers the pump speed in low demand situations, keeping the fuel cooler and extending the pump life.
A nice by product is a much quieter pump.
The ultimate system would use the bypass regulator in conjunction with a fuel cooler and the fuel pump speed controller.
But that's getting spendy and really more than you need.

Cheers
Fred

Thanks Fred.

Your explanation makes sense.
Guess I'll have to get under the car this weekend and do some plumbing. :)

Still, I wonder what kind of minimum fuel flow (GPM) is required for a 200-250hp engine.

M.

As a very loose rule of thumb you can just divide your horsepower by 10 to get max fuel flow for your engine.
This is based upon the assumption that the average NA street engine will have a BSFC of 0.5
Which means that you need 0.5 lbs of fuel per horsepower per hour.
So lets say that you have 225 hp which works out to 22.5 gallons per hour.
If you factor in loses from piping, filters, regulators etc.
Add in a 20% fudge rate and you come out at 27 gph minimum fuel supply to your engine.
Then if you have a bypass type regulator double that number.
Now we're at 54 gallons per hour.
So at 97gph for the Pro Comp pump. You are almost 100% too big.
That means that even at maximum fuel use by the engine. 75% of the fuel pumped is being recirculated and heated.

Hope that all makes some sense.

Cheers
Fred

Actually, it makes a lot of sense and explains the causes of a lot of the issues I've been having.

Thanks!

M.