It’s been said many, many times – necessity is the mother of all invention, and in Ron Bohn’s racing program, that’s exactly the case. When we called to chat with him about his Trailblazer inline-six turned racing powerplant, we tore him away from the porting bench in his home shop.

“I’m a journeyman toolmaker by trade, and I have a small shop here at home,” he says. That shop includes a CNC vertical mill, a CAD system, and numerous other tools that assist him in building whatever he wants, whenever he has the time.

Bohn is a former Competition Eliminator racer who competed with a 305-cube small-block Chevy in F/Altered and E/Altered Automatic, and is putting the tricks into this engine that he learned from that experience. After struggling to keep up with what he calls the “wallet racers” in Comp, a different challenge was on the horizon. “You’d be racing, and get the car to where it really ran well, and be .58 under the index, and then the guys like Panella or Patterson would come out with a new cylinder head using a casting that I couldn’t even buy,” he says.

This current project – the aforementioned Trailblazer engine – was torn down to scratch and completely improved for strength, durability, and performance, creating one of the most unique Homegrown Horsepower bullets we’ve seen yet.

Bohn leaned on his previous experience and built an engine that isn’t widely popular, giving himself plenty of room to improve it without threat of another racer running under the index and making his work obsolete. In fact, the current engine design has set the world record three times in G/Econo Dragster, with a best elapsed time of 7.73 at 170-plus mph using a pair of carburetors and an automatic transmission.

The 242 cubic inch inline-six Trailblazer engine currently rests between the framerails of a Spitzer dragster and has a home in the J/Dragster class, made even more interesting by the fact that it’s a stick-shift class with a 5.5 pounds-per-cube rule. A Liberty air-shifted five-speed transmits the power.

Building A Custom Crankshaft

Initially, he had a local machinist put together a custom billet crankshaft for the inline six, but in the interest of saving weight and enabling high-RPM use, that project was finished with only four counterweights on the crank, meaning that the number 2 and number 5 cylinders had no counterweight at all.

“The engine didn’t vibrate with that crankshaft, but dynamically it put 12,800 pounds of force on each throw and was flexing the crankshaft, shearing the oil off, and beating the rod bearings out of it,” he says.

After searching high and low for another crankshaft and finding quotes in the neighborhood of $7,000 from some of the industry’s big names, he decided to use a different tactic and just build the crankshaft in the shop. Starting with an American steel E4340 billet from Timken that weighed 370 pounds, the billet has been whittled down to 54 pounds in its current form.

“The crankshaft was designed on the computer. An engineering friend of mine and I collaborate on a lot of projects. When we designed the crankshaft and sized the counterweights, we used the CAD system to balance it,” says Bohn. Proof of concept was taken a step further, as he designed a single-cylinder prototype to ensure that their crankshaft design would work properly, and after checking it on an engine balancer, he was within seven grams of balanced right off the bat.

Block Mods

The engine block has been modified to within an inch of its life, as has the cylinder head. During his initial research with the engine platform, he found that there was an inherent problem with the cylinder rigidity in the inline-six casting, so the fix was in before he ever built the rest of the engine.

“The block is basically just a cover for the cylinders as I installed a set of Darton sleeves. There’s a billet aluminum insert that I installed also – there’s no water in the block at all,” he explains.

Dozens of hours of machine work later, the entire block was gutted and the six-hole insert was installed with an interference fit. Each of the main studs is also long enough to extend into the cylinder insert, and Bohn has found this arrangement to be remarkably consistent. He torques the block with a deck plate, and with the garage-machined billet main caps installed prior to performing the honing process, and he says that the cylinders are nearly distortion-free – in the .0005 range.

“The head studs and the main studs actually touch in the block,” says Bohn. “Each head stud is about nine inches long.” The threads for the head studs are also located way down at the bottom of the cylinder bores – past where the piston rings travel – in an effort to enhance cylinder stability.

Other internals include a set of custom GRP connecting rods, Precision Products titanium wristpins, and a full set of Wiseco piston forgings. “I worked with Sue Nash over at Wiseco; they supplied me a set of piston blanks and I did the final dome work here in the shop,” says Bohn.

Other Notable Changes

The oiling system has also been redesigned, as the installation of the cylinder insert basically eliminated the stock oiling system.

“I wanted a cross-bolt design, similar to a Hemi. The mains and the crankshaft are fed from the pan rails on the sides, so the oil goes through the main cap up into the block to feed the mains,” notes Bohn.

The cylinder head casting is also from a Trailblazer, and it’s been modified to perform like a champ in this application. “I do all of the portwork myself. I’ve got my own flowbench, and do all of my own testing to determine how the modifications I make stack up,” he says.

“The cylinder head is really the killer. Getting the correct port velocities and that sort of thing right has been a big challenge. I’m using a pitot tube to calculate port velocity in feet per second; the current cylinder heads are too fast, and it shows on the dyno. When it hits a certain RPM it starts to nose over pretty bad.”

Fixing this issue has Bohn redeveloping the port profile to better take advantage of the discoveries he’s made on the flowbench.

The beautiful billet throttle bodies, the billet intake manifold, and other assorted parts have also been constructed by Bohn. Although the carburetor makes more power on the dyno, Bohn has found that using the fuel injection system with a smaller injector and nearly 100 psi fuel pressure, he’s had a much easier time of atomizing the fuel and making more power on the dyno.

Can’t Do It Alone

As an old “carb guy”, you might think that the challenge of fuel injection might be difficult for Bohn to grasp, but he assures us that’s not the case.

“I’m using the FAST XFI 2.0 with the individual cylinder timing controls, and a spark map where I can create a curve. I’ve found quite a bit of power in that. It’s amazingly tunable,” says Bohn. “John Meaney and Lance Ward were very helpful in letting me pick their brain when I was still racing in Comp, but I’m still learning – life is a learning process.”

Learning what it takes to put together one of these wild creations has come with help from many folks – Bohn has picked the brains of legendary engine builders like Darin Morgan and Bob Book among others, along with lots of trial and error during the development of this unique combination.

The engine is tested regularly at Darren Mayer Performance Engineering, where the inertia dyno tells the tale of each round of modifications Bohn performs on the engine.

Future plans include a move to a Honda 1.88-inch rod bearing journal dimension, which will permit a bit of stroke to be added to the crankshaft to top displacement at 252 cubic inches, as he’s currently fighting the struggle of getting the car down to weight. Minimum weight for the combination is 1,340 pounds, and he’s currently about fifty pounds overweight.

“It sounds like a pissed off bumblebee. 560 horsepower at 9,200 rpm is the peak number, and it’s still making good power at 10,600 rpm. I’m pulling the stick at 10,500 rpm,” he says.

“There’s a lot of error – sometimes it works, and sometimes it doesn’t. Sometimes you can learn more from your mistakes than you can from your successes.”

Thanks for sharing your amazing engine with us, Ron!

If you’re interested in submitting your garage-built engine to Homegrown Horsepower, send a few quality photos and brief writeup of the details to [email protected].