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Big
Bore Information
Most modern sportbikes have closely spaced bore centers, this
is necessary for a compact engine design but limits the amount
of material surrounding each piston. The bore center number minus
the bore diameter determines the amount of material between each
piston. The amount of material between each piston determines
more than anything else how large a bore of any engine design
will tolerate. Factors involving cylinder design such as a wet
sleeve, heat shrink sleeve, or nikasil type coated cylinders
must be taken into consideration.
Zlock Racing uses sleeves of our
own material and design in all our big bore conversions. Coated
aluminum bores in factory stock condition give excellent service.
When used in a big bore application we have seen problems with
ring wear, compression loss and smoking. The use of sleeves also
allows us to use an exclusive gasket retention system reducing
the chance of a head gasket leak. All Zlock Racing cylinders
are precision bored and three stepped honed to ensure rapid break-in
and excellent ring seal.
Compression
Ratios and Cooling
All things being equal more compression will bring more torque
and ultimately more horsepower. The limiting factors on large
bore motors are usually detonation and cooling capability. On
smaller bore engines 750cc and less the short stroke length and
large valve area usually prevents you from getting as much compression
as you would like.
Most OEM Japanese cooling systems
will handle our big bores with up to 13.1-1 cr. under all but
the most severe conditions. Special considerations may have to
be made for racing applications. We would be happy to advise
on your particular project.
All
About Cams
Horsepower gain from cam replacement is definitely possible but
less than you might expect. Typical gains will be toward the
top of the rev range often above the stock rev limit. An exception:
When a big bore is installed the engine will make more power
but at a lower rpm, the addition of a performance intake cam
will bring the power peak back up and increase total power output.
Japanese sportbike engines are usually over exhausted negating
any power increase replacing the exhaust valves.
Adding lift to an intake cam is
generally the most reliable method of gaining horsepower through
cam modification. Casting new cam blanks and grinding to a higher
lift is probably the ideal solution but not cost effective for
small numbers of cams. The more usual solution is the hard weld
regrind technique where material is welded to the nose of the
cam and the profile reground. The advantage is cams can be modified
one at a time and all stock valve train components will fit.
The disadvantage is the hardwood can be subject to separation,
inclusions and pitting, many times causing damage to the cam
follower. The cam can also warp from the heat of welding causing
excessive journal wear. The newer hollow cams seem more susceptible
to warping. Another method is to simply regrind the existing
cam to a higher lift. The advantage is no welding complications.
The disadvantage is limited lift increase and possible exceeding
valve clearance adjustments.
Zlock Racing cams are built by
regrinding a stock cam to a different lift and profile. The lift
and profile is engineered for Zlock Racing and a new master created
for the regrinding process. This process removes material from
the base of the cam lobe and may move the valve clearance out
of adjustment range. To compensate for this the cam may be provided
as part of a cam, valve and valve spring set. Normally a racing
cam will require the use of different valve springs and stronger
valves anyway so the cost remains much the same.
Cylinder
Heads
Cylinder head designs vary widely and the modifications are endless,
so to limit the scope of information I will comment only on the
most common type of head we see, the modern multi-valve motorcycle
head.
Questions usually center around
three basic issues, porting, valve jobs and larger valves. Porting
has always been seen as a mysterious business tied in some way
to horsepower gain. Most people have the idea that more flow
is always better and big cfm (cubic feet per minute) numbers
on the flow bench will equal more power at the rear wheel. This
is rarely true. Current thinking shows that more attention to
velocity and direction of flow will yield better results. Current
heads generally have excellent design so major work is unnecessary.
A small amount of work in the area from the e valve seat to the
guide gives good results and is cost effective.
A good valve job is essential
to a performance engine. Any engine with more than a season of
racing or 5000 street miles will show some power loss due to
valve and seat degradation and carbon buildup. In a factory valve
job the seat is cut by machine to a specific size with two or
three angles. The valves are installed as is and are allowed
to seat by running. This method is cost effective but results
can range from poor to excellent. Race valve jobs vary according
to the rules. Most production rules allow a maximum of three
angles and no blending. The five angle job was a method used
to avoid the rules and has been largely outlawed. Fully blended
seats are best where no rules apply.
Highly modified engines can sometimes
benefit from larger intake valves. An assessment must be made
of valve seat thickness, cylinder head support and anticipated
results based on engine capabilities and head design.
While Zlock Racing can provide
almost any cylinder head modification our most popular and cost
effective is our street and competition valve job. This involves
decarbonizing the head by walnut shell blasting, fully blending
the valve seat to the port, and cutting and lapping valves and
seats as necessary.
Transmissions
Transmissions: For street use most transmissions give good service
without modification. They vary in design but most have five
or six speeds and operate by rotating a shift drum with cam
slots that move shift forks which in turn slide the gears back
and forth on the shafts engaging and disengaging the gears.
The gears are mounted alternately on the shaft in either a
splined or freely rotating condition. Adjacent gears have engaging
dogs or slots that lock together when moved into contact with
each other. When locked together a pair of gears is then able
to transmit power from one shaft to the other at a particular
ratio.
A common failure mode for this
type of transmission is when the locking dogs fail to stay engaged.
Symptoms usually involve jumping out of gear or chattering when
power is applied. This will typically occur in only one or two
gears initially. If left uncorrected the gears, shift forks,
and shift drum may be damaged. A good transmission will have
all the engaging dogs undercut on both the upshift and downshift
sides and the gears shimmed to avoid thrust loading the shift
forks. These modifications provide for positive gear engagement
and minimal power loss through the transmission. Many stock transmissions
have no undercutting or shimming, some have one or more gears
done, almost none have all gears modified.
Zlock Racing has transmission
modification services available for selected models. The 98-99
ZX9R is a common example as it had a nasty habit of jumping out
of gear. Your particular needs are best addressed as an individual
case based on model, performance, intended use, and cost. If
you think transmission work is needed we would be happy to discuss
the specifics of your engine.
Clutches
Clutches: Kawasaki clutches are very strong as are most Japanese
sportbikes. Except for the most extreme conditions the only
modifications needed are stiffer springs and sometimes aftermarket
plates. Most of the time the stock clutch is up to the task
even in a modified engine. If clutch slippage becomes a problem
most of the time it is because the plates and/or springs have
become worn and need replacement. Remember, plates and springs
are normal wear items.
Crankshaft
and Rods
Crankshaft and rods: Stock components will usually handle performance
gains of 10 to 15 percent with acceptable reliability, provided
stock rev limits are maintained. If larger gains are anticipated
or more RPM is desired then rod replacement should be considered.
Another cost effective and worthwhile modification is to lighten
and balance the crankshaft, the engine will accelerate faster
and run smoother. Highly modified engines may run billet crankshafts
and/or long rods. Hard-weld type long stroke crankshafts are
sometimes used to increase displacement.
Ignition
Systems
Coming Soon
Carburation
and Airboxes
Coming Soon
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