Road Race Carbs: Why 2 PV's?

[StanJ]

Curt, check the size of the orifice in those spring-loaded needle & seat assemblies before heading to the track. We’ve never used them so I don’t claim to be up to date on everything thats available now, but the ones I’m familiar with (Holley parts) have a .097” diameter orifice, which might be dangerously small (or not) for your engine. In general numbers, here’s how it breaks down…

The standard “performance” needle & seat assemblies that Holley installed for decades in their double-pumpers have an orifice size of .110” diameter. At .400” float drop (any more than that and the fuel delivery curve starts going to hell anyway) the net flow area (total orifice area less that taken up by the portion of the needle still remaining within the orifice) is .000854 sq. in. At a consistent supply pressure of 8 PSI, a pair of these will flow a bit over 46 gallons of gasoline/hour…enough to feed a 680-700 horsepower engine assuming a BSFC of .44

Using the same parameters for one of the previously described Holley .097” “off-road” spring-loaded needle & seat assemblies, the effective area drops to .00643” sq. in., reducing the capacity of a pair of them to safely feeding only around 520 horsepower. Keep in mind that these numbers are for steady-state demand; on a tight road course you’d probably have a safety factor of 10%.

A current 355” non-restrictor plate Nextel Cup engine with “front row” potential produces around 840 horsepower now, and in those applications we use a .130” needle & seat assembly with our own unique needle design (completely clears the orifice)…but even this one is pushed to its limits on some 2brl applications…500+ horsepower and only one needle & seat assembly to get the fuel through. When needed and when/where rules allow, we utilize the PCI dual needle & seat Superbowls. These are a practical necessity for high HP alcohol-fueled engines, and even some gasoline-fueled single carb drag racing and SCTA “Bonneville” LSR big-blocks.