45-Degree V-Twin Engine Named ‘The Gunfighter’ - Engine Builder Magazine
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Engine of the Week

45-Degree V-Twin Engine Named ‘The Gunfighter’

Running on 100 percent nitromethane with an estimated 375-400 hp, this V-Twin engine named ‘The Gunfighter’ built by Kirby Apathy set a blistering Pro Fuel class quarter-mile record. Find out what went into this history making engine.


Breaking racing records can be a tough task. That’s exactly why engine builders and racers alike are out there trying to break them. They want to be recognized as the best. However, some racing records can last years or even decades before being broken, and others may only last a few minutes. For Kirby Apathy, it was the latter.

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Having raced in professional, all-Harley classes for about 35 years, Apathy has spent the last 10 or 11 years with nitro bikes in the Pro Fuel class. He opened his shop, Vee Twin Racing, in 1983, and is currently located in DeLand, FL.

“When I first started with nitro we were running regular knife and fork motors, like a regular Harley motor,” Apathy says. It didn’t take long before Apathy was requiring engines far more custom and capable of setting records.

Apathy’s answer for the Pro Fuel motorcycle class was his “Gunfighter” 45-degree V-Twin engine, which after just 40 passes set a record quarter-mile time at 7.07 seconds at 189 mph, breaking a five-year-old record of 7.09 set by Johnny Vickers.


“Our motor was a collaboration between myself and Larry Janczak (nitropartz.com),” he says. “It’s a 120 cid, side-by-side rod motor. It is 5.25˝ stroke, 3.812˝ bore with aluminum GRP rods, plain bearings, Lieneweber cam, VTR heads, Kibblewhite valves, and Jims lifters and rockers.”

The engine’s 120 cid is the limit for the Pro Fuel class. “Through development we’ve had a couple of setbacks, but the motor runs really strong,” he says. “On the first pass I ever made on it with no tune we were right at 7.60 seconds at 186 mph, so we knew we were in the ballpark.

“I just wanted to get to a motor that was a lot more reliable and a lot easier to work on. My motor before this would run real strong around 7.14 seconds, but after about eight passes we needed a complete engine disassembly.”


To create the “Gunfighter,” named such because Apathy’s competition had engines called the Derringer and Magnum, Kirby enlisted the help of his long-time pal Janczak.

“This engine was a collaboration between the two of us,” Apathy says. “He did all the CNC machine work and I did all the manual machining.”


Janczak made the cases, flywheels, cylinders, cam cover on his Mazak CNC mill and lathes. The strong case and oversize bearings allow a three-piece crank to work. And a redesigned crank pin with a larger diameter keeps the crank from flexing as much. Apathy designed and built the oil system and tuned the engine for its record-setting runs. All that hard work paid off when The Gunfighter helped produce a Pro Fuel record pass at 7.07 seconds.

However, as the story goes sometimes in racing, the celebration only lasted 8 minutes until Johnny Vickers reclaimed that fastest pass with a run of 7.06 seconds.


“Between us and Vickers, we are the only bikes that have run 7.0 times. The rest of the bikes behind us run in the 7.30s or 7.40s,” he says. “Now we’re going to try to put one of these bikes in the 6-second range.”

Running on pure nitromethane with an estimated 375-400 hp and torque around 400 lb.-ft., Apathy anticipates the Gunfighter will target the 6-second quarter-mile mark in the not-so-distant future.


UPDATE: As of press time, Apathy’s Gunfighter engine was the second fastest Pro Fuel engine out there. However, over the weekend (11/18-11/19) the Gunfighter reclaimed a pass record in the quarter mile at the MAN CUP World Finals. The Gunfighter is now the quickest Pro Fuel engine in the world and only Pro Fueler to run a quarter mile in the 6s. Apathy qualified No. 1 with a 7.04 second time, and won the race with a 6.85 @ 184 mph.

The Engine of the Week eNewsletter is sponsored by Cometic Gasket.

If you have an engine you would like to highlight in this series, please email Engine Builder magazine’s managing editor, Greg Jones at [email protected]



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