Project Jake, Part 1 - Tech - 1993 GMC 3/4-Ton Pickup

Tom observed that Jake was far from consistent, and that the hotter he baked the catalytic converter, the better the truck ran. This led us to the conclusion that the cat was at least partially plugged. After all, it looked like the original unit, and Jake had 204,000 ticks on its clock. After returning home, we fashioned a backpressure gauge, drilled a hole in the Y-pipe ahead of the cat, and went for a testdrive. Even with the factory fuel shut-off at 4,600 rpm, we saw an amazing 11 psi on the gauge where 4 psi was more appropriate. It was time for a new cat and new exhaust.

This was a perfect opportunity to install a new 3-inch DynoMax catalytic converter and after-cat single exhaust system. Jake ended up on the hoist at Moore Automotive, and in a matter of minutes the old cat and exhaust were in the trash. Since the factory catalytic converter was welded to the Y-pipe, we had to do the same. The rest of the system quickly connected to the back of the cat, and within a couple of hours we were back on the road. Jake felt a little friskier and the backpressure gauge now read a more reasonable 4 psi at the top of Second gear. We also performed a minor tune up to complement the exhaust work, adding a set of ACCEL U-Groove spark plugs, MSD plug wires, a new MSD cap and rotor, a new Bosch oxygen sensor, along with a high-flow Rush air filter that fit in the stock air cleaner housing. Back on the dyno, the improvements paid off when Jake spun up an impressive 168 hp at 4,100 rpm and 229 lb-ft of torque. Now we were cookin'!

ConclusionWith just a hotter spark and removing the restrictions in the exhaust, we picked up a stout 20 percent in horsepower-and there's more to dig up, but we'll save that for next month. We've got more than just power plans for Jake, including some image-enhancing items like Eibach springs to drop the ride height as well as a Goodmark steel cowl hood and a Summit rear roll pan. We'll save those for next month when we subject Jake to an aerobic workout that includes a set of headers and a fuel curve boost from a Hypertech chip, but here's a hint-they work very well together. We're also searching out the best place to transform our Cal Trans-orange pumpkin into something a little more aesthetically pleasing. So come back next month when we inject Jake with a little more power and class.

Note that we picked up 28 hp (20 percent) by uncorking the exhaust system with a more efficient cat and after-cat system. But don't overlook the almost 30 lb-ft of torque we gained at a very useable 3,800 rpm. That's roughly a 15 percent improvement in mid-range pulling power.

By Jeff Smith

Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!