Head to Head Dyno Test: Bob Johnson Has a New Concept in Cylinder Heads

by Jim Cooper

I get this phone call. "Coop, an old racing buddy of yours has an interesting project."

It's Buzz, the Publisher. "Bob Johnson has done some unorthodox cylinder head work and says that he's got a near stock Sportster producing over 80 horsepower at the rear wheel. Are you interested in testing it?"

That's not unorthodox: that's unheard of! Conventional dyno calculations usually account for a 16-19 percent loss at the rear wheel due to friction. That would conservatively make this about 97 hp at the crankshaft.

You bet I'm interested. Sure, let's find out for ourselves and get the details. We'll ask Mr. Dyno to verify it. He won't lie.

I met Bob Johnson more than 10 years ago at the dragstrip, where he was the National Record Holder in SS/A (10.52 et. at 132 mph.) This was a restricted class allowing only the addition of an exhaust system to an 1100 Suzuki. His tuning abilities are extraordinary. Bob has devoted over 25 years to high-performance vehicles, and makes his living as a subcontractor doing R&D work and tuning for a major independent automotive client. Their cars are possibly the fastest in the world, and that's all he will allow us to divulge. Yes, he's somewhat secretive. But with good reason, as we'll find out.

Bob speaks matter-of factly in acronyms and other performance terms: BMEP, individual runner engines, normally aspirated, standard NASCAR approach, piston speed, etc. He makes my head spin with what seems to him and his associates conventional wisdom.

So it is with engineers. I just nodded often when he spoke, and privately cursed at my limited knowledge. We arranged to meet at Lightning Cycles in Norwalk, Connecticut, where owner Phil Nardi was as interested as we were to document this on his Dynojet Dynamometer, Bob thought it would be a good idea to warm the bike up on the road before any testing and offered me the controls, with a word of caution: "Shift at 6000RPM and be ready - this thing revs quick!" I took his advice and claimed a patch of uncrowded highway for a run through the gears. WOW! Can you say Midrange Response? It pulls so fast through the stock gearing you're deceived to allow it to rev more. Don't. That is the benefit of this bike's extremely early torque peak. The stock XL ignition module won't allow more RPM, and you don't need more. Road test: PASS.

Before we go on the dyno, we'll fill you in on Bob's project bike and its components. He specifically wanted to keep as many of the stock components that are available to illustrate his brash concept in cylinder head design. Bob's choice to modify the Sportster was the result of years of high-tech experience. With his depth of knowledge, he recognized that the piston speed on even a stock stroke Sportster rivaled Formula I engines, yet all conventional improvements included more RPM to gain horsepower. Flow bench tests were misleading, as they don't show velocity - only volume, He designed a port shape that had a better "Gulp Factor," as he calls it, which "captures the signal strength of the engine's acoustics better than had been designed into it." This required extensive redesigning of the cylinder head and ports. The intake manifold was reshaped accordingly, as were the stock valve shapes - which were all wrong, according to Bob, all of this was done to match the existing torque peak, which was the basic criteria for all of his math. We couldn't pry any more specifics than this out of him. But we did get these details on his 1991 XL 883 five-speed. As note above, it carries many factory stock components: complete lower end including case; crankshaft, gearbox and camshafts. No cam degreeing had been performed, and we verified stock lift at valve (.405-inches) measured with a dial indicator. Coils, pickup, and ignition module are all stock.

The cylinders were bored to 1200cc with unmodified Wiseco pistons. It now has 10.5:1 actual compression ratio. The cylinder heads are stock H-D with ports and combustion chambers significantly reshaped and modified. The valve train has stock size 1200 valves, a reshaped exhaust valve, lightened retainers, pushrods, and lighter springs The carburetor is a 1988 Sportster 40mm Keihin CV without an accelerator pump, so chosen for tuning isolation, and modified in the mouth and air tract for improved air flow. Modifications were also made in the fuel delivery ports. We ran Number 185 main jet and changed two slide needles in testing.

The air cleaner has a K&N element, Screamin' Eagle housing with some further unshrouding of the element performed. As for ignition timing it's tuned for the torque peak at less than 35 degrees. The exhaust is Harley Davidson/Kerker individual tapered megaphones with crossover pipe. Bob's oil choice was Kendall GT-1 20w50; gas was Exxon 93 octane. So here's what Mr. Dyno found: Our testing started right after my test ride and was more impressive than even I'd thought. We did two runs for every change and report the best of these. Air temperature was 65 degrees at sea level.

Tests I &2: As delivered (N7 needle) 78.3 hp at 5800RPM, 80.2 foot-pounds torque at 4200RPM.

Tests 3 & 4: We change the needle in the carb to N8 This is a needle available on some 1340cc model carbs. Bob reports that for better drivability he prefers the N7 yet the N8 makes the most power. Results: 80.3 hp at 5800RPM. 82.3 foot-pounds torque at 4200RPM.

Tests 5 & 6: This time we used an open carb, with no filter or cover. Results: 84. hp at 5700RPM, 85.4 foot pounds torque at 4200RPM. What does this mean?

This is a bike that has the capability of mid-11seconds at the drag strip. It has gone 11.7s ETs at 113 mph ridden by the owner. With those numbers and its very useful low powerband, it's a bike well suited for the masses. In fact it's an IDEAL concept that could be fairly easily duplicated on a mass production level, and fills the need for those who are resistant to purchase it because of low performance levels.

The exhaust pipes, although not original, had Harley's logo on them and I'm sure some suitable system could be manufactured. All the modifications to the carb were just simple smoothing of the air flow - this could be cast as such without modification required. Change the blueprints.

The biggest gain comes in exact matching of the engine design with it's components. Obviously we needed a fresh approach.

Bob has been down to the patent offices in Washington D.C. to protect his unique design. His focus is to sell his design to any interested parties - good for a price. His testing is continuing on Big Twin motors next. We wish him success, and you can be sure we'll be following his progress.