I was researching the Ferrari 250 GT Lusso and realized that it has an electrical pump at the fuel tank and it also has a mechanical fuel pump driven by the engine. The electrical pump feeds the mechanical pump which feeds the Weber carburettors. Why did they decide to use both an electrical and a mechanical pump at the same time? Wouldnt only an electric pump be able to fullfill the job without needing a mechanical pump? This video at the timestamp for example https://youtu.be/AVuSf6tkfpQ?t=68 he talks about how electric fuel pump works together with the mechanical pump in the 250 gt lusso. Or in this website author says you can install a modern pump instead of using the old electrical and mechanical pump combination http://tomyang.net/blog/2010/09/15/mechanical-fuel-pump-i/ In here they talk about how the electrical pump only feeds into the mechanical pump and doesnt influence the fuel delivery from mechanical pump to the carburettor. https://www.gtoengineering.com/post/2019/03/21/fastidious-about-ferrari. Also one last question, was it like this on every car? or were there some cars that only used a mechanical pump without any help from an electrical pump?

  • Not all cars are Ferraris...
    – Solar Mike
    May 23 '21 at 5:09

I can't tell you specifically for the application you're talking about, but most of the time when manufacturers use two pumps as you describe, one is a lift pump to get the fuel from the fuel tank to supply the mechanical fuel pump which then pushes the fuel into the carb.

You can still see this used today, but with direct port fuel injection. On GM vehicles, there's a separate lobe on the camshaft (for overhead valve engines). The lift pump (or low pressure pump) gets the fuel to the high pressure pump which is actuated off of the cam lobe. From the electric pump, the fuel is pressurized below 100 psi (I don't remember the exact pressure off hand), while the high pressure pump increases the pressure to around 2100psi. The high pressure pump isn't made to pull the fuel from the tank. Anyways, just an example of how what you're describing is still used today. Diesel engines do pretty much the same thing.

As far as cars only using a mechanical fuel pump, the Ferrari you describe is an odd ball for the era. Most carbureted engines only need a mechanical fuel pump. GM's vehicles only pushed ~14psi to the carb. If more than that was used, it would push past the needle seats and flood the engine. I believe most other manufacturers ran pretty much the same. The mechanical fuel pump was more than capable of pulling fuel from the tank as well as pushing it up to the carb. It would sit fairly low on the engine, fairly close to the level of the fuel tank. Once the fuel was to the pump, there wasn't really a lot of work going on to get it from the tank to the pump.

  • ♦ Thanks a lot, i also did some more research to see if there were other cars like those Ferraris and i have found another one. W198 Mercedes 300SL Gullwing 300sl.org/technik-300-sl-w198-technology-300-sl-w198. From what i understand: you need to crank the engine for a bit for the mechanical pump to prime the carburettors right? What i just now thought is that maybe by using an electric one coupled to a mechanical one they eliminate the need to crank the engine so you can turn on the e-pump and prime the carbs and then start right away. May 22 '21 at 19:22
  • The old carburetor US cars needed very little pressure , basically just enough to push gasoline to the carb. I drove one car several hundred miles with no fuel pump, just the vapor pressure of the gas ( '72 Pontiac full size 400 / 5.9L ? fuel tank was upright in a rear fender). The pump arm that rode the camshaft was broken off so it did nothing. Wyoming to Chicago , I did not measure the miles . May 22 '21 at 21:10
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    The mechanical fuel pump can have good suction performance - one model of car was known to have its fuel tank compressed flat due to the reduced pressure in the tank... A good laugh as the customer complained they could only get 2 gallons into it before it was full.
    – Solar Mike
    May 23 '21 at 7:56

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