JIMS Box 3 1208-1353 (all models of 120, 131, or 135 Alpha, Beta and EVO Mount Engines (Including Kits) - Manual (Page 17)

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No.1208-1353
A Division of Thiessen Products, INC
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While the Screamin' Eagle air cleaner element is one of the highest flowing units commonly
available, air cleaner flow can be increased. Several larger filters may flow more air volume than
the supplied filter. However, they are also likely to interfere with riding comfort.
Combustion Chamber Pressure:
Power output and combustion chamber pressure are directly related. The higher the pres-
sure, 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 cham-
ber design, these pressures can exceed 210 psi.
An engine that operates under a variety of conditions must have its compression pressure
compromised so that it is not damaged under the worst of those conditions. For this reason,
your JIMS 120, 131, or 135 Race Kit engine does not develop the very highest possible com-
pression pressure.
Compression pressure is found by performing a common compression test as outlined in
Harley's shop manuals. In this test, the mechanic removes both spark plugs and fits a com-
pression 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 cylinder.
The common method used to raise compression pressure is to remove material 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 damage the engine. However, if
they are energetic or frequent enough, things break. A piston can be destroyed in a few en-
gine 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 chamber or engine overheating.
If you choose to alter the cranking pressure, ignition timing or carburetor settings of your
JIMS 120, 131, or 135 Race Kit engine, please be aware that any of these alterations, im-
properly done, can result in engine damage. You should only consider such modifications if
competition pressure demands them. And, make them only if you have the experience or ex-
pert 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 compression 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 afternoon, 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.