The "Porting and Polishing" Myth
It is
popularly held that enlarging the ports to the maximum possible size and applying a mirror finish is what porting is. However
that is not so. Some ports may be enlarged to their maximum possible size (in keeping with the highest level of aerodynamic
efficiency) but those engines are highly developed very high speed units where the actual size of the ports has become a restriction.
Often the size of the port is reduced to increase power. A mirror finish of the port does not provide the increase that intuition
would suggest. In fact, within intake systems, the surface is usually deliberately textured to a degree of uniform roughness
to encourage fuel deposited on the port walls to evaporate quickly. A rough surface on selected areas of the port may also
alter flow by energizing the boundary layer, which can alter the flow path noticeably, possibly increasing flow. Flow bench
testing shows that the difference between a mirror finished port and a rough textured port is typically less than 1%. The
difference between a smooth to the touch port and an optically mirrored surface is not measurable by ordinary means. Exhaust
ports may be smooth finished because of the dry gas flow but an optical finish is wasted effort and money.The reason that
polished ports are not advantageous from a flow standpoint is that at the interface between the metal wall and the air, the
air speed is ZERO. This is due to the wetting action of the air and indeed all fluids. The first layer of molecules adheres
to the wall and does not move significantly. The rest of the flow field must shear past which develops a velocity profile
(or gradient) across the duct. In order for surface roughness to impact flow appreciably, the high spots must be high enough
to protrude into the faster moving air toward the center. Only a very rough surface does this.
What does the cylinder head exchange program consist of:
I will send you
a set of my heads and cylinders all done and ready to bolt onto your bike. When the job is done put the old parts back in
the boxes and return to me and I will return the core charge. Core charge ?? I get $350.00 deposit (core charge) for the heads
and cylinders. When I receive your parts I will refund $300.00. This not an exchange where you can send me your
dirty, stained, dented or defective parts and expect to get the full credit return. The heads/cyl must be clean and free
of road dirt, oil and staining from hard riding. I am not talking about the trace amounts of oil from disassembly as that
is normal and acceptable. When you receive the parts from me you will notice that I have taken the time to wrap and pack them
very carefully so there is no movement in the box during shipping. When you send back the parts wrap them in the plastic and
pack so there is no movement of parts in the box. If there is movement there is a good chance they will be damaged in shipping.
DO NOT use Styrofoam peanuts for packing, they are the worst possible packing material, use bubble wrap or the material I
used to ship to you. Before you repack the parts look at them closely and ask your self, “would I be happy if a vendor
sent me a set of heads that looked like this” ?? I take the time to clean and send you nice looking, functional parts.
I expect the same from you. If there are questions about your returns take some pictures and send to me and we can discuss
the condition.
Thanks for your time.
Dan Vance Racing
641 Brook Rd. Hampton Ct 06247
Danvanceracing.com “E” n2ofaster@charter.net
Q. What's the big deal about exhaust , long, short, big or small. What is the best for my bike
???
A. Without careful thought about these variables,
an exhaust system can yield very disappointing results. On the other hand, a properly designed and tuned exhaust system that
is well-matched to the engine can provide outstanding power gains.
The distinction
between "maximum power" and "maximum performance" is significant beyond general discussion. Realistically,
one exhaust system may not produce both maximum power and maximum performance. For a motorcycle to cover "X" distance
as quickly as possible, it is not the highest peak power generated by the engine that is most critical. It is the highest
average power generated across the distance that typically produces the quickest time. When comparing two horsepower curves
on a dynamometer chart (assuming other factors remain constant), the curve containing the greatest average power is the one
that will typically cover the distance in the least time and that curve may, or may not, contain the highest possible peak
power.
In the strictest technical sense, an exhaust system cannot produce more
power on its own. The potential power of an engine is determined by the proper amount of fuel available for combustion. However,
the efficiency of combustion and engine pumping processes is profoundly influenced by the exhaust system. A properly designed
exhaust system can reduce engine pumping losses. Therefore, the design objective for a high performance exhaust is (or should
be) to reduce engine-pumping losses, and by so doing, increase volumetric efficiency. The net result of reduced pumping losses
is more power available to move the motorcycle. As volumetric efficiency increases, potential fuel mileage also increases
because less throttle opening is required to move the motorcycle at the same velocity.
Much
controversy (and apparent confusion) surrounds the issue of exhaust "back-pressure". Many performance-minded people
who are otherwise knowledgeable still cling tenaciously to the old school concept.... "You need more back-pressure for
better performance."
For virtually all high performance purposes, backpressure
in an exhaust system increases engine-pumping losses and decreases available engine power. It is true that some engines are
mechanically tuned to "X" amount of backpressure and can show a loss of low-end torque when that backpressure is
reduced. It is also true that the same engine that lost low-end torque with reduced back-pressure can be mechanically re-tuned
to show an increase of low-end torque with the same reduction of back-pressure. More importantly, maximum mid-to-high RPM
power will be achieved with the lowest possible backpressure. Period!
The objective
of most engine modifications is to maximize the proper air and fuel flow into, and exhaust flow out of the engine. The inflow
of an air/fuel mixture is a separate issue, but it is directly influenced by exhaust flow, particularly during valve overlap
(when both valves are open for "X" degrees of crankshaft rotation). Gasoline requires oxygen to burn. By volume,
dry, ambient air at sea level contains about 21% oxygen, 78% Nitrogen and trace amounts of Argon, CO2 and other gases. Since
oxygen is only about 1/5 of air’s volume, an engine must intake 5 times more air than oxygen to get the oxygen it needs
to support the combustion of fuel. If we introduce an oxygen-bearing additive such as nitrous oxide, or use an oxygen-bearing
fuel such as nitromethane, we can make much more power from the same displacement because both additives bring more oxygen
to the combustion chamber to support the combustion of more fuel. If we add a supercharger or turbocharger, we get more power
for the same reason…. more oxygen is forced into the combustion chamber. Theoretically, in a normally aspirated state
of tune without fuel or oxygen-rich additives, an engine’s maximum power potential is directly proportional with the
volume of air it flows. This means that an engine of 80 cubic inches has the same maximum power potential as an engine of
100 cubic inches, if they both flow the same volume of air. In this example, the powerband characteristics of the two engines
will be quite different but the peak attainable power is essentially the same.
Flow
Volume & Flow Velocity One of the biggest issues with exhaust systems, is the relationship between gas flow volume and
gas flow velocity (which also applies to the intake track). An engine needs the highest flow velocity possible for quick throttle
response and torque throughout the low-to-mid range portion of the power band. The same engine also needs the highest flow
volume possible throughout the mid-to-high range portion of the powerband for maximum performance. This is where a fundamental
conflict arises. For "X" amount of exhaust pressure at an exhaust valve, a smaller diameter exhaust pipe will provide
higher flow velocity than a larger diameter pipe. Unfortunately, the laws of physics will not allow that same small diameter
pipe to flow sufficient volume to realize maximum possible power at higher RPM. If we install a larger diameter pipe, we will
have enough flow volume for maximum power at mid-to-high RPM, but the flow velocity will decrease and low-to-mid range throttle
response and torque will suffer. This is the primary paradox of exhaust flow dynamics and the solution is usually a design
compromise that produces an acceptable amount of throttle response, torque and horsepower across the entire powerband.
A very common mistake made by some performance people is the selection of an exhaust system with
pipes that are too large in diameter for their engine's state of tune. Bigger is not necessarily better and is often worse.
Equal Length Exhaust :
The effectiveness of equal length
exhaust is widely debated. Assuming that an exhaust system is otherwise properly designed, equal length pipes offer some benefits
that are not present with unequal length pipes. These benefits are smoother engine operation, tuning simplicity and increased
low-to-mid range torque.
If the pipes are not equal length, both inertial scavenging
and wave scavenging will vary among engine cylinders, often dramatically. This, in turn, causes different tuning requirements
for different cylinders. These variations affect air/fuel mixtures and timing requirements, and can make it very difficult
to achieve optimal tuning. Equal length pipes eliminate these exhaust-induced difficulties. "Tuning", in the context
used here, does not mean installing new sparkplugs and an air filter. It means configuring a combination of mechanical components
to maximum efficiency for a specific purpose and it can not be overemphasized that such tuning is the path to superior performance
with a combination of parts that must work together in a complimentary manner.
In
an exhaust system that is properly designed for it’s application, equal length pipes are generally more efficient. The
lengths of both the primary and main section of pipes strongly influence the location of the torque peak(s) within the powerband.
In street and track performance engines with longer pipes typically produce more low-to-mid range torque than shorter pipes
and it is torque that moves a motorcycle. The question is... Where in the powerband do you want to maximize the torque? *
Longer pipes tend to increase power below the engine’s torque peak and shorter pipes tend to increase power above the
torque peak. * Large diameter pipes tend to limit low-range power and increase high range power. * Small diameter pipes tend
to increase low-range power and to some degree limit high-range power. * "Balance" or "equalizer" chambers
between the exhaust pipes tend to flatten the torque peak(s) and widen the powerband.
Among
the more astute and responsible exhaust builders, it is more-or-less understood that pipe length variations should not exceed
1" to be considered equal. Even this standard can result in a 2" difference if one pipe is an inch short and another
pipe is an inch long.
No exhaust system is ideal for all applications. Depending
on their design and purpose, all exhaust systems compromise something to achieve something else. Before performing exhaust
changes or modifications to increase performance, it is critical to determine what kind of performance you want. * Do you
want the best possible low-end and mid-range power or maximum top-end power? * Will you be using an aftermarket cam with different
lift, duration, timing and overlap? * Have you investigated the relationship between torque (force) and horsepower (amount
of work within time)? * Do you want a cosmetic exhaust system or a performance exhaust system?
