While automotive trends come and go, true collectors develop a personal taste that grows more interesting and eclectic as time passes. Sometimes it comes down to owning things truly unique and one-of-a-kind. This GMC inline-6 is all of that and more.
The 302 cid inliner is one of the most sought after inline-6 engines, and this particular engine may be at the top of the desirable list! Guy Henson, owner of Damn Good Motors in Minnesota built this engine by immediately tearing down a core engine and researching its inefficiencies. It just so happened that the first core he was working with had been previously built by Warren Johnson and could never be built to street specs.
By starting with a stock engine, Henson was able to pinpoint the greatest faults and develop a game plan to correct them, which increased power, reliability and, most importantly, drivability. He could also take cues from the Johnson engine by spotting modifications and deducing why they were made.
This particular engine has an enormous shared center intake runner/plenum that greatly affects the airflow velocity from the intake manifold into each intake valve pocket. By dumping the air/fuel mixture from a high velocity manifold into a large center chamber, the velocity drops and fuel will fall out of suspension. Liquid fuel puddling in the intake runners may be common in this engine, but it is certainly not a design goal.
How can it be fixed? The head can’t effectively be recast with a smaller chamber (or 6 intake ports). Machining the head or adding an insert would be relatively ineffective. Adding fuel injectors into the head, keeping the intake runners dry, is virtually impossible by design. The logical solution is to push the air through that large chamber via a supercharger.
Henson started with a fully race-prepped block, indexed and precision ground crank, and a set of stock rods that were beam polished, shot peened, resized, rebushed small ends (with oil fed pin bushings), straightened by mill boring the bushings, and length corrected in the same process. Then he added custom forged pistons with special designed domes and Teflon coating to reduce the additional friction and heat developed by forced induction. He then added a custom Hellfire ni-chrome plasma top ring and designed a vacuum reversion between the compression rings.
With all the basics in hand and an engine assembled, the fun really began! The trick behind this engine was hiding the Rotrex planetary supercharger. It was designed for inline-6 Jeep engines, but could be easily adapted to many applications with a bit of imagination. By using planetary gears, the Rotrex unit revs to 20,000 rpm quickly and provides boost from idle until its efficiency is exceeded by the engine. In this case, it provides 6-8 psi boost depending on the engine efficiency at specific rpms.
Now, how do you hide a supercharger? It’s belt driven, so Henson mounted the compressor in a position of low visibility, but it has to push air into the cylinder head and fuel has to be added at some point. The car owner insisted on maintaining the look of a carbureted engine. The real trick was to design a riser for the valve cover that could add function. Rocker arm tower trusses were integrated into the spacer design to stabilize and help correct valve train geometry, so form follows function.
The exterior design was machined to mimic the block, and the tappet cover was also machined to mimic the same look. Viton rope seals are used top and bottom to keep oil where it belongs. Once it’s painted the engine color, it all appears original to the untrained eye.
Then, an aluminum tube designed to deliver the boosted air was hand milled, CNC machined, precision welded, straightened, shaped and ported for air cleaners before being sent out for anodizing. This is really the most “trick” piece in the system. It is camouflaged in black and easily overlooked. It feeds compressed air around the front of the engine into the custom billet air cleaner housings, then down into what looks like Stromberg 97s. What are they really?
The “Strombergs” are actually throttle bodies with two injectors installed inside each float bowl. They spray directly into a set of three custom GMC badged intake manifolds that, again, are the only ones in the world. The air cleaners? Well, they actually never have any air passing through them. Hidden behind the filter is a horseshoe shaped tube pushing air directly from the black intake tube into the throttle body. Trick!
Last but not least, the cam is a custom billet grind designed to optimize efficiency and take full advantage of the boost. Later, intake valve opening times and lower duration correct the cylinder pressures and allow the engine to run smoothly on pump gas. Managed by an aftermarket fuel injection system, it burns fuel utilizing a heavily modified HEI distributor and custom crank trigger.
The Dyno Session
The Jimmy 302 was broken in and tuned on R&R Performance’s engine dyno with the help of the shop’s owner Ron Quarnstrom, John Garner from the Horsepower Ranch in Seattle, WA, Jeff Schlemmer from Advanced Distributors in Shakopee, MN, and Guy Henson the engine designer extraordinaire from Minnetonka, MN. The goal was to safely break in the cam, tune the FAST fuel injection system to safe air/fuel levels, check the operating temperatures, verify oil pressure while running, optimize the timing, and possibly get baseline torque and horsepower numbers if time allowed.
Initially, the ECU requires you to set up some basic parameters including number of cylinders, firing order, and other design parameters such as crank trigger, type of ignition trigger, MAP sensor range, IAC, and cam sensor triggers. From there the engine can be started, roughly at that. Immediately the baseline AFR (air fuel ratio) is established and the 12×12 grid-design tuning chart gets filled in with more accurate figures in each of the 144 squares.
Ron did a great job of running the engine at steady state in 500 rpm increments to establish the air fuel mixture under load. After more than an hour of setting up the fuel mixture and trying a few timing settings, the FAST computer was set up well enough to make a full horsepower and torque pull to 4,000 rpms, the design limits of where the car will be driven. This is all documented in a YouTube video (Ed. note: Watch the Jimmy 302 in action at http://tinyurl.com/cpp8uqy).
Then, in typical stress-induced dyno day fashion, the entire program disappeared from the FAST computer and everyone had to start over tuning! That’s when a very important trick was learned when tuning EFI systems: save your files while tuning to both the EFI program AND a separate folder in your laptop! Even with a constant power supply and perfect wiring, glitches like this can occur and will absorb an enormous amount of extra time.
Other lessons learned from the dyno session included finding the proper oil level in the Rotrex supercharger cooling tank (easy to overfill). The team ran into some computer interface problems with the aftermarket fuel injection computer. Even though the correct computer system with all necessary (included) sensors was used, there were a few issues with sensors not fitting the wiring harness. And despite having correct fuel pressure, air velocity, great spark, and proper timing, the system initially failed to read the rpm signal from the HEI module. However, swapping to a 7-pin HEI module cleared up the issue.
No amount of preplanning can help you prepare for a 100% successful dyno session, but bringing a well versed group of specialists and not overcrowding the dyno cell will certainly promote the best possible results. With all the mechanical, fuel, and ignition systems specialists in house, this session ended in success with great results and a very happy customer!
Jeff Schlemmer owns Advanced Distributors in Shakopee, MN. He said the final numbers for this low rpm run came in at 400 lb.ft. of torque and 250 hp at 3,000 rpm but with further tuning could be more.