Crate Motors And Bolt-On EFIs Make Things Easy
While looking through the GM Performance Parts catalogue for our '69 GMC pickup, we started thinking that it would be a great place to drop in a 502ci Crate engine. After looking at the different crate 502s we decided on the 502 HO. We could have gone conservatively with the 502 HT, but we could not resist the 550 lb-ft of torque of the HO. But we wanted some modern performance mixed in with our good old big block. We were looking for more torque, fuel economy and uninterrupted performance when climbing hills, so naturally a fuel-injection kit was looked at.
Our plan was to run the rat motor on Don West's Stuska Dyno. We would run the engine with two different GMPP Holley carburetors calibrated for the 502ci. Once the engine was broken in on the dyno, we would make a few baseline pulls with the carburetors. After that, we would install the Holley TBI and Commander 950 Fuel Management computer system. We wanted to see how well the Holley TBI would perform against the 502 Specific Holley carbs.
In our first test we installed a 770 CFM Vacuum Secondary Carb. Initial numbers were strong and the EGT's and air/fuel ratio were very safe. One observation was that manifold vacuum was over 2 inches. This indicated to Don that the carb might be a little restrictive. In the truck, a smaller carb might work better, but hey, we are testing on the dyno! We then installed a GMPP 850 Mechanical Secondary Holley Carb. We wanted to stay with a vacuum secondary 870 CFM carb but we were concerned about the secondary opening with the load on the dyno. With the added air flow we saw an increase in horsepower and torque across the entire power band. As the numbers got closer to peak power it was only about a 10-hp increase. Our manifold vacuum at WOT was down to about 1 inch. This is more in line with what Don observes on most well matched engines. Our air/fuel ratio was a point richer than with the 770 Carb. We probably could have taken a couple of jets sizes out of the 850 and picked up a little bit of power. We decided to forgo any tuning as we wanted to get the Holley TBI installed.
Once the TBI system had been installed, we had Don make a couple of sweeps with the dyno while monitoring the A/F on the Commander 950 software. Initial pulls were lean at loads greater than 50 percent. After increasing the numbers in the fuel map by about 6, another pull showed that while still lean we were headed in the right direction. We then added another six numbers again. The next pull we showed the fuel flow moving closer to our previous fuel flow with the 850 Holley. A look at the power numbers indicated that we were right-on the power numbers with the 850 carb. Our A/F was still a little on the lean side but not enough to worry about.