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1993 GMC 350 Pickup - Project Jake, Part II

More Bolt-On Tbi Power And Other Practical Mods

Photography by Jeff "Smith,"

Last month, we left our Caltrans cast-off '93 350 pickup, Jake, with a handful of simple mods that were worth decent power for not much coin. To quickly review, the 204,000-mile pickup's catalytic converter was plugged, so we bolted in a new replacement 3-inch cat and an aftercat exhaust system from DynoMax along with a tune-up and a new air filter from Rush.

This month, we decided to build on our success, so we added a set of DynoMax 151/48-inch 50-state-legal headers to our throttle-body injected (TBI) hero. DynoMax doesn't offer a performance Y-pipe to connect the headers to the cat, but we discovered that JBA in San Diego, California, offers a separate mandrel-bent Y-pipe that would work with our headers. Of course, JBA also makes a header for these trucks-which is the set employed on Jake's Blues brother Elwood that you can find in the July issue of our sister book Chevy High Performance. To visually enhance our rather drab engine compartment, High Performance Coatings (HPC) ceramic-coated the headers.

Dyno-flogger and man-about-town Ed Taylor performed the installation that required some tube "massaging" on the passenger side in order to clear the truck's upper control arm. Once installed, the truck seemed to run about the same. At this point, it is important to mention that our truck had been blessed with a 350ci engine swap earlier in its life, and we suspected that the stock 305 chip was still in place. This was reinforced during our first wide-open throttle (WOT) test of the headers when our Innovate Engineering air-fuel ratio meter warned us that the engine was running at 13.9:1 air-fuel ratio at WOT. While the engine still ran fine, had we continued with this or used the truck to pull a trailer at or near WOT, it would quickly have scorched a piston.

The first thing we tested was fuel pressure to ensure that the pump was up to the task. TBI trucks require between 11 and 13 psi for fuel pressure, and Jake delivered. This led us to believe that a chip change might solve our air-fuel ratio problem. We put in a call to Hypertech, and soon we had a chip and a Power Charger air filter adapter waiting to be installed. A few days later, we made the trek to Westech Performance where Tom Habrzyk bolted our orange beast once more up on the rollers. We soon discovered (to no one's surprise) that adding headers without additional fuel is a great way to lose power.

So the next step was to add the Hypertech chip. This turned out to be a very easy task since the Hypertech unit slips in over the existing chip. Once all the new electronic commands were in place, we ran the test again and instantly picked up 8 hp at the peak. What really interested us was the torque. On this side, we gained a massive 23 lb-ft, which is exactly what we were hoping for. It's important to note that the major power hit with the chip was the combination of the much better air-fuel ratio and the headers, not just the chip itself. This is one of those cases where the final piece in our power puzzle makes the whole package. Another note here is that while we got away with using 89-octane fuel in this case because of cool test conditions, Hypertech warns that you must run this combination on 93-octane fuel.

We could have quit at this point with 260 lb-ft of torque and 176 hp-up from the stock combo's 140 hp and 200 lb-ft of torque. But Hypertech also sent us this little air filter adapter that looked like a stainless steel mixing bowl with its bottom missing. We instantly dubbed it the "soup bowl" and laughed that this could actually accomplish a power gain. Our laughter turned to amazement when the soup bowl pumped the power up across the entire power band, adding 8 lb-ft of torque at the peak and 8 hp!

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