JIMS Box 3 1208-1353 (all models of 120, 131, or 135 Alpha, Beta and EVO Mount Engines (Including Kits) - Manual (Page 16)

555 Dawson Drive, Camarillo, CA 93012 Phone 805-482-6913 • Fax 805-482-7422
15
Rev I
9-12
No.1208-1353
A Division of Thiessen Products, INC
IIn ns st tr ru uc ct tiio on n S Sh he ee et t F Fo or r B Bo ox x 3 3 1 12 20 0” ”,, 1 13 31 1” ”,, o or r 1 13 35 5” ”
E En ng giin ne e A As ss se em mb bl liie es s O Or r E En ng giin ne e R Ra ac ce e K Kiit ts s
Tuning Factors:
The Air:
As with any other normally aspirated engine, the JIMS 120, 131, or 135 Race Kit engine is af-
fected by atmospheric conditions. Changes in air pressure, temperature and humidity all have
their effects on air/fuel mixtures, power output, engine temperature and ridability. At a con-
stant altitude, most such changes are small, not very important in day-to-day operation and
can generally be ignored. Altitude changes have a more dramatic effect. As a rule-of-thumb,
you can expect a pressure loss of three percent for each 1,000 feet of altitude gain. This means
that at 10,000 feet an engine is going to lose almost 30-percent of its power, at best.
The loss is normally even greater because air/fuel mixtures also richen. Carburetor settings can
become too rich for either smooth operation or maximum power production. If you are going
to operate your JIMS 120, 131, or 135 above about 3,000 feet, you may find it necessary to
alter the jetting. In Denver, at one mile above sea level, you may need to install the next
leaner needle, next leaner slow jet and go a couple of sizes down on the main jet. A highly
experienced engine tuner should make such changes.Atmospheric conditions are much more
important when racing where small differences can determine whether you go on to the next
round or go onto the trailer. Races are often won or lost on a tuner's ability to get the jetting
(among other things) "right" as the atmospheric conditions change during the day. If you do
not understand jetting, if you cannot 'read' the air, you are in for some hard lessons at the
track. Ask questions. The great majority of race tuners are more than willing to share much
of what they know. Do not expect to get all the details - it is racing after all. But expect to get
more information than you can probably use right away. Drag racers are a congenial group
and they want you to be there, racing with them, and enjoying it.
More Power Air Flow Volume:
Ultimately, the amount of power your JIMS 120, 131, or 135 Race Kit can produce is going to
be determined by two factors: How much air it can process in a given time, and, how efficiently
it uses that air to supply useful power to its crankshaft. The JIMS 120 Race Kit delivers about
121, 131 at about 130 horsepower, and the 135 is about 136 horsepower. It is very possible
to increase that output by 20% or more. It is also possible to reduce it by a similar percent-
age. It is much easier to loose power than to gain it. Each and every part of this engine and
its accessories are in balance. Each has been designed and developed or selected to work to-
gether for maximum airflow, power output and drivability. If, for instance, you fit a larger air
cleaner or carburetor that flows more air, the result may not match your expectations. A big-
ger carburetor does little to improve power if the total airflow is limited in some other part of
the engine system. Substantial power gains require substantial modifications.
Increased airflow through the cylinder heads of your JIMS 120, 131, or 135 Race Kit engine
requires that the manifold and both intake and exhaust ports be carefully and skillfully mod-
ified. Cylinder head porting continues to be an art form and should not be attempted with-
out guidance from a practiced porting expert. Better yet, contract such an expert to do the
work for you.