fuel pressure for replacement of vacuum lift

[markpb]

I ask for any  help on substituting an electric feed for a Stewart tank supplying a Marvel carburetor  on a 6 cyl  1925 vehicle.  At present the 'waterhead' (in fuel), from the middle of the vacuum tank to the float chamber floor,  is about 12" - this may equate to 0.4 psi apparently.

When electric pump is set to this pressure (regulator valve), there is sporadic starvation, especially on hills.  I would like to know what the safe pressure capacity of the Marvel float chamber is considered to be. I was wondering if the volatility of the fuel may counter the calculation based on height alone, as reckoned from the layout of the supply in the car.

[nickelroadster]

I think you could run a little more than .4 lbs.  Your regulator may be having a little trouble going this low and you should also note that going up hills would add to the head you need as most cars have their tanks in the back.  When your fuel comes from a vacuum tank it is very close to the carburetor but an electric fuel pump is a longer distance.  A pound or pound and a half will probably work just fine for you assuming you have no other obstructions in the fuel line.

[markpb]

Thank you very much indeed. I will try at 1.5lbs.  (Yes, fuel tank at back- of chassis - so starvation on hills as you explained makes perfect sense to me!)

[Rusty_OToole]

Before fuel pumps some cars had an air pump that pressurized the gas tank. I believe they ran at 2 PSI. Whether this would be suitable for your car I don't know but 1.5 should work.

[markpb]

Thanks for that. Yes that is interesting. Yours, and post of 'nickel roadster', convince me that increasing the pump pressure to any more than a couple of PSI is not my answer. After a  long story of faults and possible remedies, I am now reverting to getting the vacuum tank once more into dependable operation....

[Spinneyhill]

1 hour ago, Rusty_OToole said:

Before fuel pumps some cars had an air pump that pressurized the gas tank. I believe they ran at 2 PSI. Whether this would be suitable for your car I don't know but 1.5 should work.

Didn't the "mechanic" in some early races have the job of pumping up the fuel and oil pressures?

[PFitz]

Many of the early mechanical fuel pumps of the late 20's early 30's were only rated to put out 2-4 psi. It's not fuel pump pressure so much as pump output volume that you need to be concerned about. 

 

If your vacuum tank is re-buildable, it will supply plenty of volume. If it's not re-buildable and you opt to use an electric pump, chose a low pressure one and check the running float level after it's installed. Any change in fuel pressure is a change in float level and many of the later updraft carburetors are VERY sensitive to changes in  float level.

 

Paul

[markpb]

Thanks for that. Following from what you say, it may indeed be that the output is too low on the electric pump I had fitted when at 2 or so psi  -and clearly, from the advice already received, it would not be a wise step to use higher pump pressure to try to deliver more fuel to the float. I take your point about the change in float level altering the carburation - isn't it  always charming to hear an old motor at tickover mumble with successive gasps  as the vacuum chamber charges - which I suppose is a manifestation of the sensitivity you are referring to.  I am trying to set up the vacuum tank again, and abandoning my electric pump substitute, which I have not got right.

[PFitz]

45 minutes ago, markpb said:

Thanks for that. Following from what you say, it may indeed be that the output is too low on the electric pump I had fitted when at 2 or so psi  -and clearly, from the advice already received, it would not be a wise step to use higher pump pressure to try to deliver more fuel to the float. I take your point about the change in float level altering the carburation - isn't it  always charming to hear an old motor at tickover mumble with successive gasps  as the vacuum chamber charges - which I suppose is a manifestation of the sensitivity you are referring to.  I am trying to set up the vacuum tank again, and abandoning my electric pump substitute, which I have not got right.

 Your welcome,

Yup ! I love to hear a properly operating vacuum tank drop the engine rpms slightly as it cycles to pull fuel up from the tank.

 

Paul

Edited July 22, 2017 by PFitz (see edit history)

[hchris]

21 hours ago, markpb said:

Thank you very much indeed. I will try at 1.5lbs.  (Yes, fuel tank at back- of chassis - so starvation on hills as you explained makes perfect sense to me!)

 

 Not forgetting that vac tanks are reliant on vacuum, their big downfall on long or uphill gradients are the lack of vacuum with large throttle openings associated with driving uphill, some manufacturers provided larger capacity vac tanks to increase the storage capacity to diminish the problem.

 

And yes, any updraft carb designed with vac tank feed will struggle with pressures above 1.5 psi.

[markpb]

Thanks for that. I greatly appreciate your comment. 

[Larry Schramm]

18 hours ago, Spinneyhill said:

Didn't the "mechanic" in some early races have the job of pumping up the fuel and oil pressures?

 

Yup, along with changing flat tires during the race.  

 

My great uncle Ernie Olson was a riding mechanic for Jimmy Murphy in the 20's.

 

He also told me that another important part of his job was to walk the track (especially road races like LeMans France) the night before the race to gather any information on the racing surface condition to help the driver during the race.   When visiting him in the 60's he showed me pictures of the track surface at LeMans that they raced on.  Remember that most of the race at LeMans is run on city/county roads and were graded to the normal road maintenance standards at the time, not current standards.

Edited July 23, 2017 by Larry Schramm (see edit history)