Text
by Jim Cooper Photos by Butch Lassiter
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.
|