Cylinder Head Performance Improvement Considerations
It is really un-nerving for me to think about all the
conversations I have with people that think that simply
installing a bigger camshaft, a smaller supercharger pulley, a
high flow exhaust or a better intake will be "THE" answer to
more performance. The over-used statement that the internal
combustion engine is a simple 'air pump' never seems to get the
point across. The Engine is really a complex number of
components that MUST work together to produce the power to turn
the crankshaft, that turns the transmission that turns the
wheels that move the car. It is simplistic to think that simply
changing one component without considering the rest will result
in the optimum performance increase.
A factory engine design team makes a major effort to make all
things work together well, while working within cost,
performance, drivability, reliability and emissions design
constraints. They establish a target for performance and each
component is chosen to optimize towards that point. If they
target to have "X" horsepower, they can then adjust each and
every component to that point. Everything from boost pressure,
throttle body port size, exhaust system, valve sizes, etc. all
are established to meet the target. Changing any one of the many
options may indeed gain some performance but by no means is
optimized.
Going back to the popular 'air pump' analogy, let's consider the
single most popular 'upgrade' done to the Mini Cooper "S";
increasing the boost pressure. There is no question that there
is HP to be gained by simply increasing the amount of air being
forced into the combustion chamber. While the factory by design
must be conservative in their choice of components and thus will
always build-in a margin for error in favor of reliability,
there are real engineering limits to what we can expect from a
single simple component change. Increasing boost pressure
consumes horsepower and generates heat which must be dissipated
by the intercooler. The intercooler on the MINI is marginal at
best, and its capacity is easily exceeded, negating the
potential gains of increased boost pressure. The real question
regarding performance enhancements, is how to know when we
exceed the point of diminishing returns, where we no longer
achieve optimum performance from an upgrade.
The MINI supercharger might be thought of a multiplier. It takes
horsepower to make it rotate and in return it forces more
air/fuel mixture into the combustion chamber, which in turn
results in more HP. The intent is to get more than you give.
But increasing the air out of the pump is only the first step,
you then have to get it into the combustion chamber and to do so
you have to pass it thought the ports of the cylinder head and
then past the valves. As we increase the blower's output, we can
quickly approach or exceed the head porting design limit
established by the factory. Getting back to a more optimum
balance of components will have many beneficial results. If we
make it easier for the supercharger to deliver the desired
amount of air it will work less and since the basics tell us the
work equals horsepower, we will consume less hp for the better
results. And if the pump is not working as hard it also can
deliver a cooler air mixture which results in a more dense
mixture, which again equals more horsepower.
So what can we do to the cylinder head to regain some of the
balance of components? Air flows into the valves by ways of
'ports'. The shape and size of these 'ports' control 'flow' and
'velocity'. These are the 2 critical components of port design.
The optimum design will yield the highest flow (volume) of air
while maintaining the highest flow (volume). If you think of the
port as a straw it is easy to understand that as the diameter is
increased the flow will increase but with the same input
pressure, the velocity will have to decrease. The result of flow
and velocity might also be thought of as low RPM verses high RPM
performance. You need the velocity at low engine speeds to
produce the HP but if the volume is not there at the high RPM,
the engine will not achieve maximum horsepower and torque. This
is why we leave porting to the professional with access to the
proper flow testing equipment. An amateur with a die grinder
will tend to make the ports way too big, drastically dropping
the intake charge velocity and ruining power (not to mention
ruining the head).
When building a full race motor. it is not uncommon to give up
low end performance in favor of total HP at high RPMs that can
be maintained on a race track. Since most of us also want to be
able to use our cars on the street or least need a wider RPM
range- then we MUST balance Flow and Velocity.
This is the real 'art' required to get the optimum performance
from a 'port job'. As we have probably already increased the
pressure from the blower (15% reduced pulley) we now must modify
the cylinder head to again re-establish that optimum balance of
flow and velocity. The 'art' typically requires not just an
understanding of how to do it but also often requires a number
of hours of very intensive hand operation with high powered die
grinders followed by hand sanding to finish the job.
Why isn't this level of attention given at the design and
manufacturing process? While the factory engineers well
understand the benefits of porting cylinder heads, it is one of
the engineering/cost compromises that must be made in modern
production engines.
The general practice In cylinder head porting is to carefully
hand reshape both the intake and exhaust ports to enlarge them
slightly while straightening out the airflow path and reducing
obstructions that result in any sort of pumping loss. As you
reduce the turbulence you increase air flow. This is also a
balance as a perfectly smooth mirror finish port wall can result
in a negative if the fuel atomization is compromised. Most
cylinder head tuners will leave the intake port 'looking'
rougher than the exhaust for just this reason. It is very common
for a good porting facility to test both their progress and
their finished product on a flow bench. This device is the best
way to quantify the gains, short of putting the head on an
engine and engine dyno to prove the results. A good port
technician can establish a baseline on a flow bench and then be
able to do many ports and heads to the exactly results that can
be verified on the flow test bench.
The next step in the process is to get the air past the valves.
There are two levels of performance that can be considered. The
first hurdle is how to get the best flow from stock valves. The
more important hurdle is how to balance increased pressure from
the supercharger by increasing the size of the valves. It has
been proven that 50% of the gain typically found in cylinder
head tuning is achieved by the simple process of doing a high
performance 'valve job'. The factory again accepts compromise
for cost and manufacturability and thus you will find single
angle valve seat in a production head. These are typically a
wide area 45-degree cut on the edge of the valve and
corresponding seat area in the head. These are easy to do and as
the valve size, etc. has been designed for x-horsepower there is
no need for the factory to spend more money.
Just as we reshaped the ports of the cylinder head to improve
flow, we can do the same with the seat area of the standard size
valves. A 'multi-angled valve job', typically 3 or 4 angles will
result in smoother transition for the air and greatly enhance
flow, while maintaining adequate valve seating area for the
necessary dissipation of combustion heat. This precision process
is considered standard practice in performance tuning shops.
Multi-angled valve seats are one improvement that does not
require the trial and error 'art' that is found in cylinder head
porting. Requesting a 3-angle valve job should be considered the
minimum that you would ask from your machine shop. Typical
3-angle valve jobs include an angle cut on either side of the
actual sealing area of the valve seat. The high accuracy
required of this technique means that the seat area is kept to a
minimum (often less than half of the original seat area). The
net result is that the gasses are encouraged, or funneled"
through the valve-to-seat opening. The flow increase is
dramatic. The intake seat width is typically about .040" while
the exhaust is kept a little wider (.050"0 to allow for better
heat dissipation. It is critical that the corresponding cuts in
the valve seats be matched and thus it is important to not
switch valves between cylinders.
15% pulley reduction increases boost by 10%, from a max pressure
of 10lbs to 15lbs. As a positive displacement pump this means
you get a higher velocity of air sooner although not much
increase in flow or volume. Porting and a good valve job will
still leave you short of the needed improvements to reach
optimum potential provided by the pulley size reduction. The
next logical step is to increase the size and lift of the
valves. Again, this must be done with the understanding the need
to balance flow and velocity. Remember that we have said the
flow is more important at high RPMs while velocity is critical
at the lower RPM. Since the blower upgrade mostly produces more
velocity, then in order to keep this balance the focus needs to
be on flow. This is best accomplished with increased valve
sizes. Another compromise that we must keep in mind is the need
to keep the correct ratio between intake and exhaust flow. In
the ideal cylinder head the exhaust should flow at about 70% of
the intake. This balance is maintained both in port design and
in valve sizes. For the MINI we have discovered that by
increasing the intake valve size by 2% and the exhaust by 6% we
not only can regain the ideal balance of intake verses exhaust
but the increased flow (combined with a top quality porting job
and valve job) allow you to get much closer to reaching the
potential benefits of reducing the pulley size.
While all this head work provides great benefit in and of
itself, a performance camshaft, with properly engineered
duration, overlap and lift, will put the finishing touches on
your re-engineered, high performance MINI cylinder head. With
other engines, the available space above the head allows for
many variations in camshaft design. Discussing these choices and
the resulting performance and drivability impacts is a subject
for another paper at another time. Suffice it to say that for
the MINI, the physical limitations of the camshaft area on top
of the head limits the performance gains and potential impact to
drivability. These limitations include taller lobes interfering
with the metal spark plug tubes, and requirement of maintaining
the stock base circle diameter of the lobes because of the
integral rocker-arm/hydraulic lifter design. The net result of
these physical limitations is that all performance camshafts for
the MINI (that we know of) all perform similarly.
Our intent with this paper is to provide you with some guidance
as to understanding the considerations pertaining to cylinder
head work. We hope we have helped you to decide whether to
undertake such a project yourself with the skills of a local
"head guru", to trade in your stock head for ported head in the
interest of expediency, or to table the whole matter and leave
the pursuit of serious power to more hard core enthusiasts.
Whatever your choice, we say to you "Happy Motoring".
Visit www.Minimania.com for further technical information.