Fuel pumps 101

[diverdan]

Pump pressure and flow: They basically have an inverse relationship for a given pump: The higher the pressure, the lower the flow. The greater the flow, the lower the pressure.

Series: Put two pumps in series and you will double the pressure for a given flow rate.

Parallel: Put the two pumps in parallel and you will double the flow at a given pressure.

What's hard on a pump? Pressure will reduce the life of the pump. If a pump is designed for a given number of revolutions or hours, increased pressure will reduce its life mainly due to the heat factor. (Note: the flow will be reduced which also reduces the cooling effect of the fluid. So, its a double whammy.) If you have two equal pumps in series, I don't believe that one pump would suffer more than the other. The actual pressure on each pump is equal. (They still increase the pressure by the same amount and the flow through each is also identical.)

Ratings: LPH and PSI. Please note the first paragraph. Then one studies all the pump specifications from various distributors and manufacturers. Any given pump you will have wildly varying flow and pressure ratings. For example; for the Bosch 044: One rating stated 400 LPH and 180 PSI as maximums. Our host lists specs of about half that for the same pump. Again, note paragraph one. (At 180psi, it won't flow 400 LPH. At 400 LPH it won't have 180 psi. About 200 LPH at 90 psi might be realistic.

Pipes and inlet and outlet diameters also affect pressure and flow. One kink in the line and you need more pressure to achieve a given flow.

In addition, you will note a huge difference in the amperage requirements of the various pumps. The 044 pump for example uses about twice that of a Bosh pump that flows half as much at the same pressure. (Don't worry, your 25 amp fuse will handle it easily. It would take two of those pumps working at about full capacity to blow.)

What does this mean in regards to high performance FI systems? If the system needs higher pressure to operate, the flow from your existing pump, even if it is rated for a higher pressure, will be less than when the system was operating at a lower pressure.

What if you convert your CSI to EFI? EFI systems generally operate at lower pressure. If the EFI requires less pressure to operate the injectors, then your existing pump(s) will flow a greater quantity of fuel than when the system required more pressure.

So be very careful when you see that a given pump is advertised being able to support X number of horsepower. That may not be adequate for your application! If your systems needs more pressure to operate than the one the advertiser is talking about, then that pump will not provide enough fuel to achieve that same power.

So how much will the pump flow, at the pressure needed, to achieve full power in your application?

What did I leave out folks? I think I got it right, but I'd like to learn from those of you who have been on the front lines.

Diverdan