Mechanical Efficiency; Necessary Mechanical Compensations - Royal Enfield Bullet RES 500 Service Manual

CHAPTER 12
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PERFORMANCE M O D I F I C A T I O N S
of the fuel being burnt
-
the "better" (and more expensive) the fuel, the more power its burning will produce.
Compression Ratio and Fuel Quality are highly inter-related
-
higher compression ratios are necessary t o extract the
extra performance from better fuels, and better fuels are necessary t o allow detonation-free running with higher
compressions.
Compression Ratio
:
Fuel Required
6: 1 - 7.5: 1
S
Gasoline of Some
Sort
1
8: 1 - 10.5: 1
i
Better to Best Premium
10.5
-
12.5:l
"pecial Racing Fuels/Aviation Blends
i
12.5- 14:l i MethanoVRacing Fuel
i
15: 1 plus
'
Compression-Ignition (Diesel)
A rough table of compression ratios and fuel requirements. A certain amount of "fudging" can be done by retarding
ignition timing in cases of not being able t o find fuel of a high enough quality, but the resultant net decrease in efficiency
offsets any advantage of the higher compression. Short story: never build a motor you can't get (or afford) fuel for!
m
RAISING COMPRESSION RATIO: is done by changing t o a higher-compression piston. There are a number of
after-market pistons available and/or adaptable
-
the main parameters are the pin diameter, the pin-crown distance,
skirt length, and of course the actual bore size. A minimal increase may be had by eliminating the head gasket,
instead lapping the cylinder head onto the cylinder barrel protuberance. Providing the appropriate fuel is available,
increases in compression ratio are usually the best "bang for the buck" performance improvement. For daily
street riding, with the best pump gas, compressions of higher than
9: 1
will not usually be satisfactory.
Mechanical Efficiency
Lastly mechanical efficiency
-
any decrease in relative friction in the engine will of course show itself in greater power
output. Changes here would involve choice of lubricants, flywheel assy balancing, (important at higher rpms), replac-
ing the small end bushing with needle bearings,special piston ring arrangements (thinner rings, fewer rings, different
shaped rings for better sealing with fewer rings (Dykes), and so on). Some changes would result in better longevity,
some, such as the rings options mentioned, would result in greater efficiency at the cost of lower longevity.
Necessary Mechanical Compensations
Increasing power output increases stress on all parts of an engine. It must deal with higher rpm, higher pressures,
and higher stresses on all moving parts. Consequently, it must be built stronger if it is to last. First, the metallurgy
should be checked
-
castings should be magnafluxed, X-rayed, etc., and you may find you have t o test a number
of them t o find the best ones. Ditto for moving parts, such as bearing parts, con rod, valves, etc. High-output oil
pumps are available, and of course this modification should be made. Valve assemblies should be assembled t o
minimum tolerances, since they will be operated at higher rpms if the available power of the hi-perf engine is t o
be exploited. Balancing is of primary importance
-
an engine operated at a nominal
2
to
3000
rpm all its life could
have balancing problems you'd never notice, but t o run this engine at
4
t o
7000
for hours at a time could well
cause it t o shake itself apart.
162
The Enfield BULLET MANUAL by Pete Snidal
O
2002