There was some debate a while back about the effects of fuel inlet presure on timing. I had negative experiences with a gas EFI pump, and others were running pumps greater than 7 PSI, but less than the estimated 50 PSI that the gasser pump was pushing. Somewhere in between... I said, must be a point at which the presure becomes a factor.
Soooo... I asked my pump wizard about this and he said he didn't know, because normally our pumps are calibrated at about 4 or 5 pounds inlet presure, and he never fooled with varying it. So I said next time I got a pump in your shop, try it and let me know. So, Todd C is getting a Baby Moose. It rebuilt up real nice, and before taking it off the test stand, Mr. Moose Pump ran a battery of tests. What he found was suprising to him, but not to me.
The pump was calibrated and tested between 4 and 5 PSI. The default timing curve was obtained, and the pump certified as being correct. Inlet presure was then slowly increased 1 PSI at a time, and the pump run through the full RPM range. By 9 PSI, the pump had gained a full 1 degree of advance at idle. By 18 PSI, the pump was fully saturated with what was esentially a static unchanging timing of about 12 degrees. There was a somewhat linear curve along the way, with idle producing a greater amount of advance than at 3300 RPM. So the net effect is that an increasing pump presure results in a shorter and shorter timing curve, until it is a flat line. This translates into a Holley Blue or Facet Duralift, taking 1 to 1 and a half degrees of timing variance away from your IP. Not a significant amount to be sure, but at least now you have the facts. Because of this discovery, I will now be asking pump customers what IP inlet presure they would like their pump calibrated to. We may be able to compensate slightly for the difference, but will not really be able to do anything about the decreased range internally available in the pump. At idle, inlet presure seems to have the most effect, as any change in presure upwards from 5 PSI caused some small shift even if it was only a quarter of a degree at a time.
Soooo... I asked my pump wizard about this and he said he didn't know, because normally our pumps are calibrated at about 4 or 5 pounds inlet presure, and he never fooled with varying it. So I said next time I got a pump in your shop, try it and let me know. So, Todd C is getting a Baby Moose. It rebuilt up real nice, and before taking it off the test stand, Mr. Moose Pump ran a battery of tests. What he found was suprising to him, but not to me.
The pump was calibrated and tested between 4 and 5 PSI. The default timing curve was obtained, and the pump certified as being correct. Inlet presure was then slowly increased 1 PSI at a time, and the pump run through the full RPM range. By 9 PSI, the pump had gained a full 1 degree of advance at idle. By 18 PSI, the pump was fully saturated with what was esentially a static unchanging timing of about 12 degrees. There was a somewhat linear curve along the way, with idle producing a greater amount of advance than at 3300 RPM. So the net effect is that an increasing pump presure results in a shorter and shorter timing curve, until it is a flat line. This translates into a Holley Blue or Facet Duralift, taking 1 to 1 and a half degrees of timing variance away from your IP. Not a significant amount to be sure, but at least now you have the facts. Because of this discovery, I will now be asking pump customers what IP inlet presure they would like their pump calibrated to. We may be able to compensate slightly for the difference, but will not really be able to do anything about the decreased range internally available in the pump. At idle, inlet presure seems to have the most effect, as any change in presure upwards from 5 PSI caused some small shift even if it was only a quarter of a degree at a time.
