Hi David, it's pobably worth 40-50 more HP. The 312PB & T's ran Slide Valve Injection (also P4's) for up to about approx. 550 HP.
Two main reasons the injection can increase HP = Flow & Flow {
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With nothing in the Bodies/Stacks/Runners to interfere, air flow and velocity is dramatically increased, while at the same time fuel flow can be increased to accomodate the increased air flow, the result is considerably more power. A side benifit of electronics over mechanical is metering ability to a wider range of use conditions and throttle settings.
Motec conversions are expensive but worth it imo.
Regards, JRV