Mechanical Injection Systems

Back in the heyday of the Holden six mechanical injection systems were often fitted, mainly for speedway or drag engines. Probably the most commonly used was the Australian-developed McGee system, though other manufacturers components can be used as well. These systems can (and have) been used on the street though it's not recommended for reasons we'll cover later. They have the advantages of simplicity and ruggedness, and are not affected by G forces, unlike carbs. Performance wise, they are unlikely to show much if any advantage over a well-developed carb system, though it might be easier to use appropriately sized intake runners with injection.

They basically consist of a vane type pump driven from the engine, a barrel valve linked to the throttle butterflies and a set of injector nozzles. The pump flow is in direct proportion to the engine revs, and the barrel valve directs excess flow to the tank depending on throttle position. Full throttle mixture is set by the size of a by-pass "pill" or jet that meters return flow from the nozzle manifold block back to tank. Because the engine will usually require less fuel either side of the torque peak it is usual to use additional bypass jets for both idle/low speeds and high speeds. These are constant flow systems, with all the nozzles being plumbed back to a common manifold block.

Nozzle size should be the smallest that can be used while keeping full-throttle/high-speed fuel pressure within reasonable limits. Mixture quality at medium to high speeds should be fairly good, but at idle and lower speeds the nozzles will be dripping and dribbling rather than spraying. For this reason much of the fuel will pass through the engine unburnt at low speeds. To maintain a burnable A/F ratio then much more fuel will have to be injected than should normally be required. The engine may run cleanly enough but it's quite likely that excess fuel will be washing the cylinder walls at low speeds and this is the main reason to avoid using these systems on the street. Fuel economy is also likely to be very bad on a street engine.

For a competition engine - especially on methanol - mechanical injection can give good response and power output while being simple, rugged and virtually immune to acceleration, braking and cornering forces. For precise metering over a very wide range of speeds and loads however, EFI or a carburettor is usually a better choice. J.Zed (see links section) can supply complete mechanical systems to fit the J.Zed head or Sonic Injection can supply throttle bodies/manifolds to suit the 9 port Holden head.