JIMS Twin Cam 120, 131, 135 - Manual (Page 5)

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provements, it’s difficult to list all replacements. So, if you’re unable to receive
the service or parts you need to maintain your JIMS race engine, please call
JIMS Parts department at 805-482-6913 or E-mail at www.jimsusa.com
If you do find it necessary to contact JIMS, please have the following informa-
tion written down and handy; failure to do so will delay service.
Before starting or riding your new JIMS engine, please read through your H-D
owner’s manual as it is your (the riders) responsibility to know how the engine
and motorcycle function correctly. These engines are first and foremost a Twin-
Cam, designed & manufactured with its size and power in mind. If for any rea-
son this engine starts to make LOUD internal noises, safely stop the engine and
fill out the information sheet and submit to JIMS.
Caution:
These engines make almost double the HP & Torque as your old stock engine.
120” Engine both Alpha & Beta
Cylinder................................................................................................2
Type of engine.................4-cycle, 45 Degree V-twin, Oil & Air Cooled
Standard Compression Ratio ................................................10:00 to 1
Bore .............................................................................................4.125”
Stroke ..........................................................................................4.500”
Displacement .................................................................................120”
Recommended Gas ...............................................................91 Octane
131” Engine both Alpha & Beta
Cylinder ...............................................................................................2
Type of engine..................4-cycle, 45 Degree V-twin, Oil & Air Cooled
Standard Compression Ratio ................................................10:50 to 1
Bore.............................................................................................4.313”
Stroke..........................................................................................4.500”
Displacement .................................................................................131”
Recommended Gas ...............................................................91 Octane
135” Engine Alpha
Cylinder ...............................................................................................2
Type of engine..................4-cycle, 45 Degree V-twin, Oil & Air Cooled
Standard Compression Ratio ................................................10:67 to 1
Bore.............................................................................................4.313”
Stroke..........................................................................................4.625”
Displacement .................................................................................135”
Recommended Gas ...............................................................91 Octane
Camshaft Specifications
Lift: .658”/.658”
Duration: 262*/266*
Intake: 24* BTDC / 58* ABDC
Exhaust: 69* BBDC / 17* ATDC
TDC Lift: .208”/.178”
Combustion Chamber Pressure:
Power output and combustion chamber pressure are directly related. The
higher the pressure, the more efficient and more powerful the engine becomes,
within limits. That is, until the onset of detonation. A racing engine, one that
is carefully and constantly looked after by an expert, can run with very high
compression pressures. In the case of the Twin Cam chamber design, these
pressures can exceed 210 psi. An engine that operates under a variety of con-
ditions must have its compression pressure compromised so that it is not dam-
aged under the worst of those conditions. For this reason, your JIMS 120, 131,
& 135 Race engine does not develop the very highest possible compression
pressure. Compression pressure is found by performing a common compres-
sion test as outlined in Harley's shop manuals. In this test, the mechanic re-
moves both spark plugs and fits a compression tester to one of the plug holes.
He then opens the throttle fully, lifts the CV slide, and turns the engine through
five to seven compression cycles or until the pressure reading on the tester
stops rising. The measured pressure is the compression pressure for that cylin-
der. The common method used to raise compression pressure is to remove ma-
terial from the heads on a milling machine. This decreases combustion chamber
volume, raises the compression ratio and, in turn, raises the cranking pressure.
Detonation:
Detonation must be avoided. A detonation event is literally an explosion. Most
of the time these explosions are so small that they do not immediately dam-
age the engine. However, if they are energetic or frequent enough, things
break. A piston can be destroyed in a few engine revolutions from the effects
of severe detonation. Detonation can be brought on by a variety of conditions
including, incorrect ignition timing, poor quality gasoline, overly lean air/fuel
mixtures, and too much cranking pressure for the particular combustion cham-
ber or engine overheating. If you choose to alter the cranking pressure, igni-
tion timing or carburetor settings of your JIMS 120, 131, & 135 Race Kit engine,
please be aware that any of these alterations, improperly done, can result in en-
gine damage. You should only consider such modifications if competition pres-
sure demands them. And, make them only if you have the experience or expert
assistance to deal with the possibility of detonation. Performance Tuning: As
you can see, performance tuning is complex. One change often creates the
need for other compensating or complimentary alterations. If raising the com-
pression pressure makes the chamber more turbulent, a likely thing, it may be
necessary to back-off ignition timing to compensate for the increased flame-
front speed across the chamber. If moist air rolls across the track late in the af-
ternoon, a leaner main jet may be needed to compensate for the reduced
oxygen in the more moist air. If that air is also cooler and more dense, no
change may be needed at all. The possibilities are almost endless. However,
exploring those possibilities can be great fun. If drag racing and performance
tuning in general were "Cut-and-Dried", they would be boring. The stands