Evaluation Day
After dragging the used 5.3L truck motor out of the boneyard, dyno guru Ed Taylor stripped and cleaned the long-block and then set it up with a GM Performance Parts single-plane carbureted intake manifold along with a new set of Fel-Pro intake gaskets to seal the carb'd manifold. On top we opted for one of Ed's dyno mule 750-cfm mechanical-secondary Holley carburetors. If you read last month's story on the MSD ignition box, you know how easy the ignition was to set up. We plugged in the sensors and the coil packs and we were ready to go. We bolted on a set of Hooker 13/4-inch coated headers that are actually designed as engine-swap headers for a Gen III engine in an early Camaro.
Our first shot was a simple baseline of the stock 5.3 with the carb and headers, making a credible 337 hp and 351 lb-ft of torque. Next, it was a simple task to strip the accessory drive and timing-gear cover and swap the stock hydraulic roller cam for a new Comp Thumpr cam. We also added a set of Comp beehive valvesprings to the stock heads to ensure adequate valve control at what we figured would be a higher peak horsepower engine speed. That's exactly what happened. Just bolting in this new Thumpr cam (it didn't even have a part number) pushed the peak horsepower from 5,600 to a nose-bleeding 6,800 rpm. But more torque at a higher engine speed is the formula for horsepower, which is what happened when the peak power jumped from 337 to 433 with stock heads. That's a 96hp bump with a cam swap!
After the baseline, Ed yanked the stock cam and replaced it with this Comp Thumpr cam. The specs are not all that aggressive, but with only a 325ci engine, it pushed peak horsepower up to 6,800 rpm. | 
Swapping heads was easy between the different castings, requiring a couple of sets of these Fel-Pro intake gaskets. |
We also used a set of coated Hooker 1 3/4-inch engine-swap headers that are designed to bolt a Gen III engine into a '67-'69 Camaro or '68-'74 Nova. Behind the headers is a set of Flowmaster 1/2-inch Super 44 mufflers. | |
Of course, this just made us salivate to bolt on the WCRCH Stage 1 ported production castings. That was also a quick swap, since the Gen III engines only use four head bolts per cylinder, which we replaced with a set of ARP head bolts. With the ported Stage 1 heads in place, the power jumped again, this time up to 449 hp, an improvement of 16 while the torque gained 12 lb-ft with the peak occurring at 5,400 rpm. Our final test was with the CNC-ported castings, where again the little 5.3 spun up to 6,800 rpm cranking out 460 hp with 396 lb-ft of torque. Taken as a whole, this simple head and cam swap on an otherwise stock 5.3L short-block was worth a total of 123 hp and 45 lb-ft of torque.