Hi all,
I'm trying to work out the fuel system requirements prior to building my MS system in a 1970 Lotus Elan. 1600 Twincam crossflow Ford type engine. Currently has a mechanical engine driven fuel pump.
I was going to install a small surge tank in the boot, run an electric carb type fuel pump to supply the surge tank and a high pressure EFI pump to supply the throttle bodies via a plastic EFI fuel line. Then I was going to use the pre existing fuel line as a return from the regulator back into the fuel tank - not the surge tank. Hopefully to aid in cooling rather than returning hot fuel back to the surge tank.. but..
What is the potential for the primary/carb fuel pump to not be able to keep up with the EFI pump/engine/return considering the fuel from the return is going back into the main fuel tank instead of the surge tank?
I'm not really sure how to size all this up. Suggestions?
Cheers,
Neil
Surge tank/Fuel pump sizing
Moderators: jsmcortina, muythaibxr
I had to use a surge tank in my carb conversion due to no baffling or sump in the original tank. The LP pump would get air and move it to the HP pump and rails with poor effect; unlike a carb which can tolerate larger air bubbles. My system used a return off the top of the surge tank to purge the air to the main tank.
The typical factory mechanical pump flows so much fuel when at zero pressure it's not an issue - but you can do a flow test as outlined in service manuals to be confident in the capacity. Due to this large flow from the LP pump, the surge tank fuel is diluted with cool fuel quite rapidly. In my setup with a large 44 ounce surge tank and original factory 110 gph pump, IIRC it would flow enough fuel to recirculate the surge tank over 5 times every minute. 44 ounces was enough to feed 430 hp for about 45 seconds with no resupply.
Worked for my problem...
David
The typical factory mechanical pump flows so much fuel when at zero pressure it's not an issue - but you can do a flow test as outlined in service manuals to be confident in the capacity. Due to this large flow from the LP pump, the surge tank fuel is diluted with cool fuel quite rapidly. In my setup with a large 44 ounce surge tank and original factory 110 gph pump, IIRC it would flow enough fuel to recirculate the surge tank over 5 times every minute. 44 ounces was enough to feed 430 hp for about 45 seconds with no resupply.
Worked for my problem...
David
110 gph is monstrous. My engine will be lucky to see 150 bhp and the biggest electric LP pump I can find is a red top facet moving about 38 gph. I don't want to use the mechanical pump.
I guess the facet should suffice but again not sure how to size the system theoretically. The issue for me is the return direct to the fuel tank and not to the surge tank and whether or not the LP pump can keep the surge tank full enough to supply the HP pump without starvation.
So, in simple turns must I return from the regulator back to the surge or can I get the best of both worlds and return to the main fuel tank?
I guess the facet should suffice but again not sure how to size the system theoretically. The issue for me is the return direct to the fuel tank and not to the surge tank and whether or not the LP pump can keep the surge tank full enough to supply the HP pump without starvation.
So, in simple turns must I return from the regulator back to the surge or can I get the best of both worlds and return to the main fuel tank?
As Mike_Robert indicates, you may have no issues in the first place. I'd set it up like his and if you have issues, then add the surge tank inline like mine, and tee a surge return into your regulator-to-main tank return. Best of both worlds with options but without unnecessary initial hassle.
BTW - at a typical .5 pounds per HP per hour, the LP Facet should supply more than triple the demand of 150hp.
BTW - at a typical .5 pounds per HP per hour, the LP Facet should supply more than triple the demand of 150hp.
As PSIG stated/implied in his post the sizing isn't as critical, once you have two seperate fuel loops. I was restricted on surge tank size, so I used a piece of chain link fence post. It's about 1 5/8 ID and about 9 inches high. This tank alone, held enough gas to run a 200 HP engine for over 15 seconds without fuel from the low pressure pump. This is long enough for any cornering I will be doing with my unbaffled fuel tank.
The two loops:
1 - low pressure: from the in-tank pump into the surge tank and overflowing out the top of the surge tank to push any bubbles back to the fuel tank.
2 - high pressure: off the bottom of the surge tank to the High pressure fuel pump, through the injection system and back to the surge tank.
I have three connections near the top of my surge tank and only the inlet to the high pressure pump near the bottom.
Jack
The two loops:
1 - low pressure: from the in-tank pump into the surge tank and overflowing out the top of the surge tank to push any bubbles back to the fuel tank.
2 - high pressure: off the bottom of the surge tank to the High pressure fuel pump, through the injection system and back to the surge tank.
I have three connections near the top of my surge tank and only the inlet to the high pressure pump near the bottom.
Jack
Good judgment comes from experience.
And where does experience come from?
Experience comes from bad judgment.
. . . . . . . Mark Twain
And where does experience come from?
Experience comes from bad judgment.
. . . . . . . Mark Twain