Rear Wheel Horsepower
Superflow Chassis Dyno
Tested At Westech
On the secondary side, we...
On the secondary side, we replaced the original 407 jets with a pair of 392s. On the primary side, the factory 413 jets were replaced with leaner 404s. With the jetting change, power went up to 325 at the wheel, while the average air/fuel ratio leaned out to 12.21:1. More power and less fuel-we like that.
Satisfied with the WOT mixture,...
Satisfied with the WOT mixture, we turned our attention to the part-throttle and cruise response. With the leaner jets, we now developed a lean condition at part-throttle.
To fatten up the cruise mixture,...
To fatten up the cruise mixture, while keeping the WOT setting unchanged, we needed a metering rod with the same power step diameter at the tip, while having a thinner part-throttle step higher up the rod. We settled on a 6547 rod, which measures .065 inch on the cruise step and .047 inch on the power step. The stock rods were 6847s, which measure .068 and .047 inch on the cruise and power steps, respectively.
The rod change brought the...
The rod change brought the cruise mixture to where it needed to be, showing a 13.5:1 air/fuel ratio. While it cruised cleanly, there was a lean hole when applying throttle from cruise to power, or tip-in.
We figured that getting the...
We figured that getting the rods to switch from the cruise to power position sooner would allow the mixture to fatten up a little sooner as tip-in throttle is applied. Changing the metering-rod springs will affect how early the rods will move from cruise to the power position. Metering-rod springs are rated by the vacuum level at which they will move the metering rods to the power step. Stiffer springs will lift the metering rods to the power position sooner. We used the silver springs, which move the rods to power mode at 8 inches of vacuum.
With the power and part-throttle...
With the power and part-throttle tuning done, the icing on the cake was setting the idle. With some careful adjustments using a vacuum gauge, we finally ended up with 11 inches of vacuum at a 750 rpm idle-not bad for a cam with 239/243-degrees duration at .050 and .537/.549-inch lift.