A Brief Primer on Fuel Pressure
All mechanical fuel pumps ultimate pressure is a product of:
- How large the pump is
- What size and how many nozzles
- What size the by pass pill is
- How fast the pump is turning
So the pressure is infinitely variable. Generally, the idle pressures with an average small pump like a 7gpm pump will be from 4-8 lbs. The pressure at 3000 rpm under no load might be in the range of 15-20. The pressure at 3000 rpm however, under a load (in gear) will or could be double the amount of when the pump is free wheeling. You will typically see pressures between 55-110 lbs under a load, at WOT, in the 6000-7000 rpm range. Pressure must be measured before the metering valve to get true pressure. If you want nozzle pressure, then you will measure it after the metering spool – this pressure will be 20% to 25% less. Large pumps, 11gpm to 20 gpm will often exhibit pressures at idle of 10-20 lbs and 100-200 at WOT running condition under a load. The introduction of a high speed (High RPM) valve will usually be necessary to curve the fuel pressure down to keep the pump from overcoming the engine’s ability to cope with the high fuel pressure load.
Mike or Toni can tailor a high speed lean out valve for each engines set of conditions. The high speed valve is only needed on engines that see rpm’s above 6500 sustained for over 2 seconds – All sprint cars must use a high speed valve, drag racing engines that live in that rpm range and all supercharged engines need the high speed valve to prevent hydra locking.
Even the smallest mechanical pump (except worn out ones) can produce very high pressures when chocked down with a small by pass pill. Do not be concerned with idle fuel pressure, it is almost meaningless.