Precision Matters: Twin Plug Ignition

The 356/912 combustion chamber is far from ideal, its geometry compromised by both air cooling and pushrod valve operation, so combustion remains incomplete at most engine speeds. This means that some portion of the mixture in the cylinder is not burned, resulting in roughness, wasted fuel and lost horsepower. Starting the fire in two locations, however, seemed a more promising direction and an initial look showed the entire package workable within our 'non-invasive' approach. Hence, we have designed, prototyped and tested the entire system; twin distributor and drive, coil mounts, plug wire management and cylinder head modifications. And it turned out to be very effective indeed. Fitting the twin-plug components and increasing compression ratio to 10.0:1 increased horsepower by 15, from 91 to 106 horsepower on the dynamometer. This on pump gas, our 36mm Zenith carburetors, stock air filters and muffler. In-car performance has confirmed the dyno results, with a smooth idle, strong and clean acceleration from nearly any RPM. Most hot-rod engines remind you of their presence at all times; this one does only when you press the accelerator. Neither gas mileage nor engine temperatures have shown any negative effects.

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