Detonation is a bad-sounding word, and it
does bad things to your Harley engine. In a nutshell, detonation is
incomplete combustion—it robs power and decreases engine life.
This article describes what detonation
is, why it’s a problem for Harley motors in particular, and how it’s
A lot of people think they know what
detonation is and when it’s present, but for most detonation is
undetectable. You can’t feel it, you can’t see it, and most of the
time you can’t hear it. Here’s some more bad news: If you are riding
a Harley on today’s pump gas, there is a very high possibility it could
be going on right now inside your engine.
Detonation is the premature burning of
the fuel-air mixture in your combustion chamber. Usually it’s caused
when the spark from the spark plug creates an incomplete “flame”
from the fuel-air mixture during the combustion cycle.
The cure for detonation involves some
hot-rod jargon: atomization, the microscopic process of gasoline
changing from liquid to vapor, in combination with air; flame travel,
the penetration of the “fire” throughout your combustion chamber as
the spark plug explodes the fuel-air vapor; and complete burn,
the extent to which the fuel-air vapor is consumed inside the combustion
chamber before being expelled through the exhaust port.
Oh yes, and there’s one more term: volumetric
efficiency, the ratio between actual and theoretical flow of air
through your engine’s combustion chambers. That’s what determines
how well your engine “breathes,” and in some cases, whether it lives
The grim truth about today’s gas
In 1985 the EPA passed a law that
drastically changed the quality of pump gasoline. Prior to this law,
premium gas was rated at 98 octane with four grams of lead per gallon.
In case you didn’t know, lead cools your valves and generally makes
for a longer-lasting engine.
Today the best pump gas has 92-rated
octane and no lead. As a result, your cylinder heads have become the
most vulnerable part of your Harley’s engine. If they don’t atomize
your fuel perfectly, your engine will run hot and be down on power. Even
worse things can happen.
Detonation and high combustion chamber
temperatures, which were not a problem when lead was present in
high-octane pump gas, are now causing expensive breakdowns in
older-generation Harley engines running on today’s gas: warpage and
cracks, loosening of valve seats and guides, burnt valves and seats,
bronze valve guide seizing, gauling and excessive wear of valve stems,
loss of spring tension in rings, and in extreme cases, holes burned
right through pistons.
The bottom line is, your beautiful older
Harley’s cylinder heads are vulnerable to the evils of detonation.
Hey, they weren’t designed for today’s lousy gas!
But we said there was a cure. . . .
The solution—with a catch!
Cylinder head work, or just plain “head
work” as it’s known in the trade, can make your vintage Harley run
faster and longer than the original heads on old-fashioned gas. Head
work consists of porting and dual plugging, two separate
but complementary processes.
We cover porting in a separate article,
Harley Porting Secrets. The rest of this article explains dual plugging.
Your H-D motor has a single spark plug
per cylinder, located on the left side of the combustion chamber. This
chamber is deeply concave in shape and partially filled with the piston
dome at the point of ignition. Because of the location of this single
spark plug and the piston dome, the flame travel is partially blocked
and does not cover the piston evenly. The result is lost power and
detonation, or “pinging.”
An inexperience tuner may recommend
high-compression pistons to boost the horsepower, but aftermarket
high-compression pistons only increase detonation.
Complete burn is the answer
The secret of greater power from your
Harley is not necessarily greater compression; it’s more complete
In 1970 Warner Riley set a new land speed
record at Bonneville of 202 mph with a Harley-Davidson. In 1972 he went faster
with lower-compression pistons. It was later discovered that the flame
front was partially blocked by his high-compression piston domes. Flame
travel notches were used in the later 1970s by many knowledgeable engine
So, as it turns out, the cure for
detonation also happens to provide greater performance: “complete
burn” of the air-fuel mixture in your Harley’s combustion chambers.
The dual plugging solution
How can complete burn best be achieved in
your street engine?
Your Harley’s engine design goes back
to the early part of this century, a time when aircraft manufacturers
were having the same problems with detonation and power loss.
In 1917, Liberty Aircraft discovered that
simply adding another spark plug to the opposite side of the combustion
chamber and increasing the voltage at the plugs would cure detonation
and increase power.
The same dual plugging solution was
incorporated by Continental and Lycoming in the 1920s and 30s. Primarily
used as a safety factor, it also reduced spark plug fouling and
detonation and increased RPM..
For exactly the same reasons, your Harley
motor can benefit from dual plugging. Dual plugging will “undo” the
problems with its combustion chamber design and help your Harley run
happily on unleaded gas.
Dual plugging alone of a stock motor will
eliminate most detonation, stop plug fouling and hard starting, increase
horsepower by five percent or more, increase gas mileage, double spark
plug life, and lower emissions. Some tuners have reported six to eight
percent increase in gas mileage.
The combination of dual plugging and
cylinder head porting, when carried out by experienced professionals,
can transform your Harley into a faster, smoother-running, and cooler
For tuning advice about your
Harley-Davidson, call the FLO Headworks customer service hotline,