S&S Cycle Replacement Cylinder Heads for 1984-1999 Harley-Davidson Evolution Engines and 1986-2003 Sportster Motorcycles - Manual (Page 9)

Carburetion
●
All S&S ® test engines are run using S&S carburetors. S&S
Super E and G carburetors are recommended for most
applications with the Super G being used more often on
larger displacement, freer breathing engines with higher
compression ratios. Typically, engines equipped with S&S
heads require the same or slightly leaner jetting than those
engines fitted with stock heads. Consult the carburetor
jetting instructions for specific jetting recommendations.
● If another type carburetor is used, it must be made to run
rich enough to operate properly and to prevent engine
damage. If you have a problem with another carburetor,
S&S cannot help you and recommends you call the carb
manufacturer with any questions you may have.
● If the motorcycle is used exclusively on a drag strip where
engine temperatures vary, slightly richer jets may be
necessary for best performance. Larger jets/richer mixtures
will enable one to run a colder engine which is sometimes
desirable. This is best determined by experimentation.
● Carburetor jetting and spark plug color - While spark plug
color may be used to help determine carburetor jetting,
S&S recommends that our instructions be used as primary
jetting guide and that plug color indications be used only
as secondary aid. We have found that different brands of
gasoline, gasoline additives, engine heat (due to ignition
timing), and brands of plugs and heat range used distort
plug color drastically making plug reading difficult for the
average tuner. Also, new plugs usually require a road test
of 10 miles or more to properly develop the color which
means that quarter mile tests may not be long enough and
hence, not always a good indication of carb jetting. It is
best to use proven spark plug combinations and to consult
the spark plug manufacturer if you have questions.
● Cams and exhaust systems can make some engines difficult
to carburate. We have found that certain cams and
exhaust systems will cause poor performance at a specific
rpm, and attempts to correct jetting for that specific level
usually destroys carburetion over the balance of the range.
A combination of cam overlap, reversion, and back pressure,
or even lack of back pressure can cause mixture dilution at
certain engine rpms. This dilution will cause engine
roughness or misfiring when engine is held in this range.
Exhaust Systems
●
Drag pipes - While drag pipes can be used with good results
to establish performance guidelines on the 1984 and earlier
engines, they are generally not recommended for 1984-
1999 big twin engines in street applications. These big twin
engines are easier to carburate with muffled systems.
● Muffler systems: Most stock and many aftermarket exhaust
systems are too restrictive and made exclusively for looks
with little consideration given to performance. A very good,
economical street system for big twin engines consists of the
stock header pipes with the crossover tube and a set of low
restriction mufflers. S&S offers a series of mufflers that can
be used with stock header pipes that work very well. This
combination will typically produce more horsepower and
torque than drag pipes in the midrange. Since the midrange
is where the vast majority of normal driving occurs, it makes
this system ideal for the street.
Gearing
●
Gearing depends on the total weight of the machine and
rider/s, the size of the engine, cam(s), exhaust system and
type of riding to be done. Most high performance
engines, and particularly those with larger displacements,
are capable of pulling more gear. We suggest you break
the engine in with stock gearing to minimize the load on
the engine. After the engine is broken in you will have a
better feel of its potential and can change gearing
accordingly.
● For those who wish to determine their final drive gear
ratio the formula is as follows:
Engine Revolutions Per One Revolution of Rear Wheel =
(Clutch Sprocket*) x (Rear Wheel Sprocket*)
(Motor Sprocket*) x (Transmission Sprocket*)
*Number of teeth on each sprocket
Compression
●
Generally speaking, big twin engines with the proper
camshaft selection can operate using higher compression
ratios with fewer problems than their earlier counterparts.
Keep in mind, though, that while engines with higher
compression ratios make more horsepower and perform
better, they also tend to lose that performance edge faster,
require more maintenance and start harder. As a rule, we
prefer to limit the compression ratio to no greater than 11 to
1 for engine combinations used in normal street operation.
A word of caution is in order. Before building an engine you
may regret later on, carefully consider your riding needs,
riding style and overall performance objectives.
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