Anyone with a knowlege of physics

[Shvegel]

Really this is a 113 question, I promise.

 So if you double the pressure of a fluid through a fixed orifice how much would the volume increase.
 I am trying an experiment on my 113 that involves doubling the pressure in the fuel rail(This was done in many later Bosch fuel injection systems) and am just wondering what the net pressure increase will do to the volume of fuel put out at the cold start injector.

[George Davis]

An engineer buddy's guesstimate is approximately 1.4 times the flow.  But, the system is designed to operate at about 15 psi as I recall, whereas many of them actually operate at lower pressure, like down to 8 psi in one case.  I personally would not increase the pressure over 15 psi, seems a bit risky to me, might increase leakage at the injection pump seals (your other post) and at the CSV or other places, and might seriously shorten fuel pump life, too.  Also, we prefer that you not immolate your car or yourself!

George Davis
'69 280 SL Euro manual

[dwilli3038]

The scientific answer is that  the pressure drop is proportional to the square of the flow, which makes the flow propotional to the square root of the presure drop, so if you double the pressure, you will increase the flow by he square root of 2.

A rather long way to say that George's engineer buddy is correct.

Daryl
'64 230 SL

[dwilli3038]

the question I have is why do you wish to do this? are you trying to fix another problem by making these adjustments? If so you may be better off if you diagnose and rectify the acual problem

Daryl
'64 230 SL

[Shvegel]

Hi,
 I am just playing with a few things more or less for fun(I am also thinking of adding a modern 5-speed trans to my car).
  I have a hot start problem that I have traced to vapor lock due to  leaking ball check valves at the FI pump outlet and I do not want to pull my pump and send it out in the middle of the summer(I have not yet found or made the tool to do my own check valves).
  Since Bosch stepped up the rest pressure of their systems(rest pressure defined as key off residual pressure)In the later CIS and Electronic systems I figured they must have decided that 15 PSI was not enough pressure to stave off vapor lock.
  What I have done is added a fuel pressure regulator/accumulator immediatly downstream of the return of the FI pump which I feel does several things. It provides a higher pressure in the fuel lines and rail to prevent vapor lock. It will maintain pressure long enough for the engine to cool to a point that vapor lock is no longer a threat. It also makes the ball check valves almost superflous(except for some minor reversion on the pump piston down stroke).
  As for fuel pump wear I am running a Bosch pump designed for a CIS system in the car which is capable of much higher pressures. The fuel should not leak into the oil because the same seals are pressurized to 150PSI when the pump is in operation.
  As for self-immolation, the first thing I bought for the car was a fire extinguisher.
 Thanks,
 Pat

[Albert-230SL]

Hi Pat,

One year ago, I had vapor lock problems in my 230 SL (it stalled with hot weather and traffic conditions). I checked the fuel return line with my mechanic, and we found very few fuel returning from the injection pump: the 230 SL has a valve united to the injection pump to adjust the fuel pressure in the system. This valve was nearly closed, and it sent too much pressure to the system (1,3 - 1,4 kg instead of 0,8 - 1,1 kg). Once adjusted to the correct pressure, this valve allowed fuel to return to the tank (cooling the fuel system) in around the correct values (3 liters per minute). With the clogged return line and during low consumption moments, I guess that fuel pump was beatting all time the same fuel until it was transformed to vapor.
No more vapor lock problems since then.

Have you checked the fuel return line of your car, before and after your changes in the system? If you have no enough fuel returning to the tank, you could have similar vapor lock problems

Regards,


Albert de la Torre Chavalera
Barcelona (Catalunya/Spain)
Feb.'64 230 SL Euro 113042-10-002432

[Cees Klumper]

I can definitely support Pat's suggestion to always carry a fire extinguisher with you. From my personal experience (fortunately NOT on my 280 SL) I can testify that having one of these on board can make the difference between incurring only slight and absolutely massive damage to your car ...

[George Davis]

Regarding vapor lock, Albert is right that having a clear fuel return line is critical.  The fuel system in the W113s recirculates a lot of fuel, and clears vapors out by pumping the vapors back to the fuel tank.  There is, however, a little more to it than that.

The earlier cars, originally fitted with the "large" fuel pump, have a hole in the fuel overflow valve in the injection pump.  This hole allows vapors to pass through, into the return line and back to the tank.

The later cars, after about mid-1969 with the later style "small" fuel pump, have a vapor bypass fitting and line from the fuel pump to the return line, and no hole in the injection pump overflow valve.

In between, some cars were fitted with the small fuel pump, but the early vapor relief system.  These have the bypass fitting on the fuel pump blocked, and have the hole in the injection pump overflow valve.

If a fuel pump with no bypass is fitted to a later car that orignally had a bypass, you end up with no provision for evacuating vapors.  In this case, the overflow valve with the hole in it should be installed.

Raising the fuel pressure may help reduce vapor lock, but if the fuel return line is clear and the car has one of the correct vapor relief systems, vapor lock shouldn't be a problem.

George Davis
'69 280 SL Euro manual