Fuel pressure

[Deleted member 19315]

Agree with both these points.

 

[Michael Oritt]

Ray

I hope so as that is what my installation is based upon.

 

[John Turney]

John--

I have one as well--rebuilt/converted a few years' back by Dave DuBois--and have it wired so that I may select BETWEEN one motor or the other via an "On-Off-On" switch mounted through the bulkhead behind the driver's seat. I assume you have wired your pump so that one of the "On" positions connects both sides of the pump to a hot lead.

Please correct me if I am wrong but--assuming proper motor operation and unimpeded flow--why would running both motors supply more fuel and/or more fuel pressure? I have always assumed that the limitation of flow and pressure is a function of the pump's design and operation and not a lack of power to it.

I have two on-off switches (and an inertia fuel cutoff switch) mounted on the heelboard below the rear "seats". Each switch feeds each end of the pump.

The pump flow is rated by SU as twice that of a single-ended pump, 30 gph vs. 15 gph. With more power, the pump can operate faster.

 

[Deleted member 19315]

Agreed you will have more volume with both pumps on...I checked last night on the 911 and 2 facets equals 3.5 psi and x2 the volume for a given time. So a high fuel demand beyond the delivery VOLUME of the single pump can be managed if the motor demands it whilst maintaining correct delivery pressure.
My 2.8 litre 911 on carburettors does about 10 mpg driven briskly and manages on one pump with no starvation issues. I supose a very large capacity motor might need more fuel but have no experience to speak of around very large car engines.

 

[Randy Forbes]

I've never designed a fuel injection setup, but I'm reasonably familiar with a couple of the BMW M Rdstr I-6 versions. On the <2000 models, the fuel rail has a supply line at one end (rearmost) and a return line back to the tank at the other. The FPR (fuel pressure regulator) is mounted to the underneath the floor alongside a unibody "frame-rail") and senses manifold vacuum to compensates accordingly. Dynamic line pressure runs from about 48 psi at idle to 52 psi @ max acceleration. Fifty (50) psi being the factory "spec" for diagnostic purposes. FPRs that don't fluctuate thru that little bit of range are suspect.

The >2001 version is similar, except that the return line is from immediately after the regulator (under the floor as earlier cars, approximately under the drivers left hip).

If I was going to put a return line on my Healey, I'd come off the rearmost (3rd Weber ) carb__the supply being attached to the 1st carb and dual-Banjo fittings feeding #s 2 & 3 (as opposed to the "fuel block" with a line to each carb). If I did so, it would be as red57 says, to minimize the potential for vapor lock, as my Facet interupter ("cylindrical" in their new parlance) doesn't seem to be a limiting factor to either pressure or volume.

 

[Deleted member 19315]

If you did need to put a return line in on your low pressure system feeding those weber's a top tip is to restrict it . Commonly folks use a carburettor main jet as a restrict or. Running effectively open will lead to a pressure loss which as we have established is a bad thing.
I like the thinking over where you would fit the returnext if you did so.
I kind of wish I'd welded an extra return to tank fitting to my new allow tank when I fitted in the Healey but so far I've had no fuel problems that would demand me use it...Just nice to have the insurance if you can.

 

[RAC68]

Hi All,

As I see it, Hassad has stated the thought many of us are thinking. Its nice to discuss how we would approach solving a fuel starvation issue through the implementation of a continuously pumping fuel pump and return line but, since no one seems to have the issue, at this time the discussion is only theoretical.

Or, other then fuel starvation and vapor lock, is there another benefit that can be realized from this approach?

Just my thoughts,
Ray(64BJ8P1)

 

[Deleted member 19315]

No other benefits.

 

[vette]

.
My 2.8 litre 911 on carburettors does about 10 mpg driven briskly and manages on one pump with no starvation issues. I supose a very large capacity motor might need more fuel but have no experience to speak of around very large car engines.

I'm running a stroked, 383 cu. in, small block, dyno'd at 467 hp and 490 lb ft of torque in my '70 Vette. It is supplied fuel by an original, mechanical, engine block mounted fuel pump with a return line. This fuel line set up is original to the '70 Vette. The supply line from the tank to pump to carb is 3/8th inch, and the return line from the pump to the tank is 1/4 inch. Fuel pressure is a nominal 4 lbs. This engine experiences no starvation issues thru out it's Rpm range up to and including 6400 rpms.

 

[Novamonte]

I used to have a -66 Mercedes W111 250 SE Convertible. It had a Bosch Kugelfischer mechanical injection pump (same as in some Porsches from the era) and the standard set-up was a Bosch continuous flow electric pump (virtually bullet-proof, my original pump ran perfectly after 50 years) with a return line to the tank. There was a pressure regulator on the return line that kept the pressure high enough to feed the injection pump. This kept the fuel cold, never any vapour locks. That car was nowhere nearly as attractive as the Austin-Healey, but I must admit that the engineering and parts quality were far superior. That is actually one reason why I sold it, I like to tinker with my cars now and then but the Merc was so problem free that I ran out of things to do once I had sorted the interior and some other issues. I don't seem to get to that point with the Healey, always a little something to fix.

 

[RAC68]

Hi All,

As I conclude by the comments of many on this thread, installing a return line on a Healey is likely a non-started project with little benefit over our present single-line installations. Although the elimination of vapor lock could be a consideration for the few experiencing this problem, other approaches are available to address this issue within the context of our present fuel feed architecture that will not involve a somewhat difficut implementation.

Again, my conclusion,
Ray(64BJ8P1)

 

[steveg]

I used to have a -66 Mercedes W111 250 SE Convertible. It had a Bosch Kugelfischer mechanical injection pump (same as in some Porsches from the era) and the standard set-up was a Bosch continuous flow electric pump (virtually bullet-proof, my original pump ran perfectly after 50 years) with a return line to the tank. There was a pressure regulator on the return line that kept the pressure high enough to feed the injection pump. This kept the fuel cold, never any vapour locks. That car was nowhere nearly as attractive as the Austin-Healey, but I must admit that the engineering and parts quality were far superior. That is actually one reason why I sold it, I like to tinker with my cars now and then but the Merc was so problem free that I ran out of things to do once I had sorted the interior and some other issues. I don't seem to get to that point with the Healey, always a little something to fix.

I agree re the tinkering. Neighbors go: "you're always working on your car." I tell them "it's my electric train set: this week I'm taking out the mountain range and putting in a village."

 

[John Turney]

I agree re the tinkering. Neighbors go: "you're always working on your car." I tell them "it's my electric train set: this week I'm taking out the mountain range and putting in a village."

I tell them "It keeps me off the streets."

 

[RAC68]

Hi John,

Something seems wrong when you own a Healey that keeps you off the Street!

Merry Christmas … Buy a Sleigh and Reindeer,
Ray(64BJ8P1)

 

[aero3113]

I got around to hooking up a test gauge to the line going to the carb after the regulator to test the pressure. With the test gauge it’s showing just under 5 psi. With the new gauge 0 psi. So my new gauge is no good. The new gauge does read with air pressure, but must not be reading low pressure. I contacted the seller and he’s sending me a new one.

 

[steveg]

My fuel pressure gauge has a pressure release button on the side. I think most do.

 

[aero3113]

I received and installed the new gauge today. The gauge now reads pressure with the key on and the fuel pump running!