220B pressure questions

[dillonkbase]

a few pressure questions about my 1965 220b

what should i be getting pressure wise from my fuel pump and oil pumps. short sweet and to the point.


the long version is im looking into installing an electrinic fuel pump with an oil pressure safety switch hence an good idea of the pressures im looking for would help

[t walgamuth]

most of the benzes i have had say minimum pressure 1/2 bar. i don't know what that is in psi though.

on the fuel pressure only about 3 # is required. a simple diaphram type pump will work fine i imagine.

the goal is admirable but the oil pumps fail so seldom it seems kindof un necessary though. is there a problem with yours?

tom w

[Mike D]

One bar is 1 Barometric pressure or about 15 p.s.i. at sea level.

[Mark DiSilvestro]

I believe the goal here is to refit the car with an electric fuel pump, and add an oil pressure safety switch to cut power to the pump in case the engine should stop running, such as after an accident.

Is a replacement mechanical fuel pump no longer availible?

My Mercedes factory repair manual calls for .15 to .20 atm delivery pressue, with engine running, for the carburetor fuel pump.
.15 - .20 atm is approx 2.25 - 3 psi., and .5 atm oil pressure would be 7.5 psi. (Though at idle-speed on a hot engine I like to see at least 15 psi.)

If you can't find an electric fuel pump that operates with low enough pressure to prevent overpowering the float-valves and flooding the carburetors, you can fit an inline pressure regulator between the pump and the carbs.

If you fit an oil pressure switch to operate the fuel pump, you may also want to consider adding a pushbutton switch, or relay off the starting circuit to ensure the electric pump supplies ample fuel before or during cranking, without having to resort to extended or excessive operation of the starter motor, especially in case the car has sat for awhile and the fuel level has dropped in the carbs.

Happy Motoring, Mark

[Mike D]

1 Bar (barometric pressure) equals 1 ATM (atmospheric pressure). Just a simplification for people who don't deal with bars and atms. Although, usually when I go to a bar I need to hit the ATM first!

[dillonkbase]

you hit the nail on the head Mark,

can you do a little more detail on the saftey switch buypass

[Mark DiSilvestro]

Well first, will you be able to rig a tee to tie into the oil pressure guage line fitting at the oil filter, so you can install an oil pressure switch?

Did you check with Phil at PartsShop on this Forum for a mechanical pump before going through all this trouble? if the early style pump isn't availible, it may still be possible to retofit the newer style.
Ask him by phone. He's helped me out with stuff that's not listed in the online catalog.

As for adding a safety circuit for an electric pump, since the vast majority of oil pressure switches ground the circuit to a warning light when there's no oil pressure, you'll probably need to wire it to a relay anyway to operate the electric fuel pump. You'll need a relay with contacts that are normally closed until the relay coil is grounded by the oil pressure switch. And so the relay coil isn't on while it's grounded with the ignition off, of course the hot side of the relay coil would also be switched through the ignition circuit.
You can add a pushbutton switch to the circuit to activate the relay, if the oil pressure is insufficient at cranking speed, or to pre-fill the carbs if the car has sat for awhile. Or you could run part of the fuelpump circuit to the starter switch so the fuel pump would get power immediately when the engine cranks, instead of waiting for oil pressure to build. But this all may be unneccessary if the oil pressure during cranking is enough to quickly activate the fuel pump circuit. You'll have to rig it up and see.

For an alternative safety circuit, instead of using an oil pressure switch, you could wire a fuel pump relay to the charging lamp circuit. However if the carbs are empty, you would still need to have an initial fuel pump activation before or during cranking, by wiring to either a pushbutton, or the starter switch circuit.

Happy Motoring, Mark

[dillonkbase]

thanks mark, still dont know what the real issue in my car is its going to the shop in a few weeks, just checking out all my options and weighing the costs and benefits.