Feb 3, 2002
Does anyone see were there may be a problem paring up a set of Walbro pumps-one a 307 and the other a 340 on a double intank hanger and using the 340 as the primary with the 307 coming one at 10-12 #'s of boost?

Not a good idea IMO, the flow rate on the 307 really starts to fall off at 60+ psi, so do you really want to rely on a pump that falls on it's face at full boogie?

Doug C.
I would do it the other way around, 307 all the time, 340 when needed. The 307 will return less fuel to then tank when idling, putting around, etc. which will heat the fuel less. At WOT it won't be the same as 2 340's, but you'll have more than a single 340.
Its also very simple to modify the bypass on the 307 to make it virtually equal to the 340.
Originally posted by Scott Atk
Its also very simple to modify the bypass on the 307 to make it virtually equal to the 340.

340 motor is diff then the 307 and pulls less current the 340 is more effcient produceing less heat.
Humberto is right, the motors are different. And dconnor has a point, too. If the boost is high enough, the 307 will start to lose flow just when you need it, leaving more of the burden on the 340. There will still be more flow than if the 307 was on its own, and at much higher pressures, the 307 might not contribute anything. Sully runs the combo, and it works, at least up to pretty good boost levels. Of course, the 307 is still putting out 30 gallons/hr at 80 psi. How much higher will the pressure go? (The two pumps together will put out about 67 gallons/hr at that pressure! Lots of horsepower!) Of course, the pump curves are taken at the pump discharge, and don't allow for pressure drop through the fuel lines. Still, that seems to be a lot of pump.
How much higher will the pressure go?

In my tests, flow rate dropped to zero around 95 psi for the unmodified 307. For the 340 it was well over 100 psi, my gauge only went to 100 so I couldn't record the exact number.
My question, "how much higher will the pressure go", was more directed toward boost + intial regulator setting. If the pump or pumps put out 80 psi, that allow for 45 psi initial setting, plus 30 pounds boost, plus 5 pounds for "line loss", how many folks need more than that? I kind of remember somebody measuring pressure at the tank, and at the fuel rail, under full throttle, but don't remember the numbers. The pressures, as I remember, were measured in deciding when/whether to go with bigger fuel lines, which seem like they might be needed if you need dual pumps.
When I set my system up I figured I would need about 44 gallons/hour at 72 PSI(45+27). At the time I thought I only wanted to go high 10's. I was able to do that with a single XP Plus pump(340) which should support about 50 gallons/hour @ 70 PSI when voltboosted. I went 10.95@123 with at Bristol.
supply the additional flow I needed to maintain pressure.

If you use this chart http://www.gnttype.org/techarea/fuelsystem/walbroflow.html
and you have (1) 340 and (1) 307, you should be able to supply ~ 80 gallons/hour @ 70 PSI or maybe a bit less since you have 2 pumps feeding 1 line. According to this chart http://www.gnttype.org/techarea/misc/lubrant.html that should take you into the 8's which is well outside of my target et and mph.

The danger you run into with a double pumper is what happens if the 2nd pump doesn't cut on. That is the sole reason I use the 340 as primary. It supplies more fuel at pressure in case 2nd pump doesn't cut on, you may have more of a chance of not roasting the whole thing.

Now I do not think any of this is exact, but looking at Red's charts for his XP double pumper (which is like 2 307s) it looks like I am in the ballpark at least as far as flow goes. He has 75 Gallons/hour for an XP DP and comparing the XP and the 307 they seem pretty darn close. http://www.gnttype.org/techarea/images/doubleXP.gif

Now everything I have started here is at 12 volts and since I have everything volt boosted at WOT my flow numbers should be even higher (safety net).

Now that my target speed and ET have changed for like the 20th time, I figure I need about 55-60 gallons/hour at the same 70 PSI (42+28). The requirements for flow may be a little high but I like to be on the safer side. Although you may be able to just get by with a single pump, I figured just about any 2nd pump would