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Camshafts are arguably the most important part of the
engine. They determine how the car performs, idle, and
character of the car as a whole. Understanding the function,
design, and limitations of the camshaft profile is
essential. This information lets the builder choose which
camshaft(s) will maximize the intended performance of the
engine / turbo configuration.
Function
There are a total of four steps of the combustion process:
intake, compression, power, and exhaust. The crankshaft
position, stroke, and rod length determine the position of
the piston in the cylinder. The camshaft is what determines
the position of the intake and/or exhaust valves during the
stroking process. The camshafts control the valve timing of
the engine. This is what makes or breaks the total
performance of your car. The performance characteristics of
the engine are determined by when the valves open, how high
(lift) and how long they are open for (duration).
Power Band
Where does the engine make power? Where does the
engine begin to drop power? When the power does come on, is
it flat and consistent or is it jumpy and aggressive? These
questions can all be answered with the description and
understanding of the engine’s power band. A narrow power
band would only range from 6000 to 8000 RPM. A wider one
would range from 3000 to 7000 RPM. More so than any other
internal components of the engine, the camshaft and the
complimentary valve train components will establish the
power band.
Ideal Cam
What is the perfect cam? The cam with huge low-end
torque, a redline set at 10,000 RPM, glass smooth idle, and
a power band from idle to redline does not exist. Never has,
never will. But an aftermarket performance cam that
optimizes the rest of your performance engine does exist.
Dollar for dollar, there is a good chance that someone
somewhere does make a camshaft that’s the best investment
you can make.
Camshaft Shaping
From a performance standpoint, the faster a valve
opens and reaches full lift, the better. Horsepower relates
directly to how much air and fuel can actually be pushed
into each cylinder, and this can not happen if the valves
are not open. Cams that open the valves quickly are said to
have an aggressive lobe profile. Unfortunately, the laws of
physics govern the maximum amount of possible valve
acceleration. If the camshaft tries to open the valve too
fast, excessive wear or valve train problems can occur. When
returning a valve to its seat, a camshaft once again cannot
do this too fast or the valve slams on the valve seat. Most
of today’s camshafts are designed to optimize valve
acceleration rates by designing camshafts with asymmetrical
lobes. This is done to lift the valve faster than it lowers,
making the best of your engine’s performance while prolong
the life of your valve train.
Lift and Duration
The basic description of a camshaft’s purpose is to
open and close the engine’s valves. Regardless of the number
of cams in a motor, they all do the same job(s). The most
well-known camshaft specifications are lift and duration.
Most of the current manufacturers of performance camshafts
list the lift of the valve and not the lift of the camshaft.
This may be different for applications other than ones with
rocker assemblies.
The distance from when the valve is closed relative to the
maximum open position is known as lift. Assuming all the
other measurements are the same, choosing a camshaft with
more lift will increase the flow of air and fuel into an
engine and the flow of exhaust out of an engine. Most of the
time, cams that have increased lifts over stock
specifications and near stock duration will offer increased
performance without making large sacrifices in drivability.
Everything has a limit and cylinder heads reach a point
where airflow no longer increases with an increase in valve
lift. Before you jump to ordering a camshaft with a huge
lift and duration, remember that when valve lift is
dramatically increased so is the possibility of
valve-to-piston contact, and coil bind. Valvetrain
interference is also increased.
How long a valve is open is the next influential part of
your performance engine. The duration specification of a
camshaft is measured in crankshaft degrees of which there
are 720 in one complete four-stroke cycle. Pay close
attention when studying camshaft duration figures. Different
manufacturers may measure this duration at different valve
lifts. This may cause confusion. Lucky for us there are
industry standards of either 1mm lift or a lift of .050”.
When you compare durations between two camshafts, only
compare the figures if the measurements have been taken at
the same minimum lift.
To
avoid bent valves, broken retainers, and empty wallets, make
sure you use the necessary complimentary valve train
components and ensure your valve to piston clearances are
within specifications suggested by the manufacturer.
Duration and Power
The more lift you have the more repetitive power and
torque across the power band in most cases. Generally
speaking, increased duration will shift the torque and
horsepower peak to a higher RPM. All other specifications
being the same, increasing duration yields more top-end and
mid-range power while sacrificing low-end torque. By
shifting the power band higher, a longer duration camshaft
accompanied with the proper valve train components, will
raise the engine’s overall redline provided the block can
handle the extra rpm. Rule of thumb is for every 10 degrees
you increase the duration, the torque and peak horsepower
will shift 500 RPM higher in the powerband.
The Ending Words
A camshaft may look simple, but completes no easy
task by any means. The understanding of function, design,
and limitations of aftermarket cams will allow you to make
educated decisions about getting the right cam(s) for your
car. While it's great to have an understanding of cams,
leave the designing and measuring up to the professionals.
Always remember that the camshaft(s) are just one element of
the performance combination. The entire parts combination
needs to work together to produce the maximum in power and
reliability. Camshafts will only do their job effectively
when complimented with the correct valve train components.
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