While the carb was on the bench, MacNeil recurved the distributor. What is recurving? To put it simply, it's allowing either the vacuum or centrifugal advance to come in sooner or later, thus changing the moment when the spark plug fires. Why? At slow operating speeds, such as at idle, the spark plug can fire later in the compression cycle because the piston is moving more slowly. However, at higher engine speeds the spark plug must fire earlier to ensure complete combustion. While it may only take 15 degrees of crankshaft rotation at idle to complete the combustion process, it may take 35 or 40 degrees of crankshaft rotation at higher engine speeds. The baseball analogy is that you must start swinging sooner to hit a ball traveling 100 mph than you do for one traveling 30 mph. Therefore, the spark plug must fire earlier in the compression cycle at a higher rpm. To accomplish this, the position of the rotor is advanced on the distributor shaft by vacuum and mechanical means to advance the sparks. The advance curve, as set by the factory, is a compromise that is intended to work in a variety of situations. Therefore, for optimum performance it's best to have your distributor tailored for your particular application.
Round Two
The recurved distributor and re-jetted carb were bolted back in place, the fuel filter was replaced, and the initial timing was changed from 13 degrees btdc to 12. Since the timing would now advance sooner, less initial timing was needed for upper rpm performance. The truck was hooked back to the dyno for the final flogging. As the charts show, the results are quite impressive. Our average horsepower increased 11 percent from 122 to 136 and we picked up at least 12 hp everywhere. Our average torque also increased 11 percent from 194 to 216 with the peak dropping 500 rpm. What's more, we saw an increase of 32-lb-ft at the new torque peak of 2,300 rpm!
The Lowdown
So how does it work? It's like a completely different vehicle. The truck now fires right up and idles dead-smooth. Acceleration is very strong across the entire rpm range where before there was hesitation below 2,500 rpm. Accelerating up hill seems effortless, and freeway passing power is far superior than before. The difference in the amount of throttle needed to hold the truck at 70 mph on the freeway is so drastic that it almost feels unnatural to have the right foot that far from the floor. However, the best part is that repeated mileage checks on a 250-mile circuit comprising both highway and city driving went from an average of 12.5 mpg to 14.6 mpg! That's almost a 17 percent improvement in fuel efficiency. The only downside is that we had a slight detonation problem due to the carbon buildup, which required a switch to 92-octane fuel. This was to be expected given this engine's mileage and approximate 9:1 compression ratio, but the extra fuel economy offsets the price of switching from 89- to 92-octane fuel. In fact, even with the added price of buying premium, we're still saving money, but we've got extra power as well!
What Time?
Timing advance is generally controlled by vacuum and centrifugal means. A vacuum advance unit is connected to a source of engine vacuum. As vacuum increases, an arm advances the timing by rotating the entire internal distributor assembly with respect to the cam. (A) MacNeil set our vacuum advance to 24 degrees. In low vacuum situations, such as wide-open throttle, there is no vacuum advance.