Performance That's Right for You
Evaluating Dyno Charts
Porting: The Black Art
Power Transfer to the Rear
Corrected Compression Ratio
PORTING: The Black Art
by John Hildreth
Mysterious…misunderstood…misused. At times, the black art of porting seems to be able to achieve the impossible. When done by a capable craftsman, porting can subtly change shapes and curves to drastically increase the flow of fuel and air through a motor. However, bigger is not always better. Having too much of one thing can hurt more than not having enough. This is especially true when discussing ports.
Ports are the orifices that bring the fuel/air mixture to the cylinder and allow the spent gasses to leave the cylinder. Ports are required to move a certain amount of cubic feet per minute (CFM) of air depending on the size of the motor and the peak RPMs the motor is capable of turning. If the ports aren’t capable of meeting the CFM requirements, overall performance of the motor will suffer. Likewise, if the port’s CFMs far exceed the engine’s requirements, overall performance will be hindered.
Inadequate CFM delivery adversely affects the engine. Low RPM performance may be good, but mid-range can be short-lived, and there will be no increase in the top-end. On the other hand, significantly more CFM flow results in a motor that has little or no power at low RPMs and the mid-range suffers, while top-end performance will be healthy.
One of the limiting factors as to how many RPMs an engine is capable of safely turning is the length of the stroke (how far the piston travels top to bottom). The longer the stroke, the fewer RPMs you can turn and still have the engine stay together. The formula for determining maximum, unsustained RPM is: 24800/length of stroke = maximum unsustained RPM (‘unsustained’ means hitting the top of an RPM range and quickly shifting.) This would only be for motors constructed of quality components and that are in good condition. Even with perfect conditions, you should not hold the motor at the top of RPM range for long.
Porting these orifices means removing obstructions and sharp edges, and smoothing the radii and blending transitions to reduce resistance to air flow. An increase in flow, both on the intake and exhaust will allow the engine to continue producing horsepower and torque at increasing RPMs. However, when increasing the CFMs of flow through a head, care must be taken not to significantly reduce port velocity. A drastic reduction of port velocity results in the loss of low RPM performance. In addition, it allows the air and fuel atomization to be lost, causing lower combustion efficiency, and can ultimately lead to detonation.
If the CFM requirements for your engine package can be achieved with stock-size valves, going to larger ones will only result in a loss of low-end performance and, in some cases, it can also hurt your mid-range as well. To achieve a wide powerband, the best results come from staying with the smallest valve and port combination. By using the smallest valves capable of the CFM requirements, and keeping the port volume as small a possible, you can have a power band that will pull hard from 2,500 to 6,000 (EVO Big Twin) or 2,500 6,500 (Twin Cam)
It is true that a hand-ported head is more time-consuming to build. A head with an efficient small port using a stock valve that can deliver the required CFMs is much better than hogging out ports and installing large valves. Hand porting also allows you to achieve higher port velocity over after-market, big valve castings. A flow bench is used to measure the CFMs flowing through an orifice. A competent porting service will have access to a flow bench, and will be able to provide you with both “before” and “after” numbers for the work they do.
The level of performance that people desire varies widely from high mileage requirements to high horsepower needs. Some riders are happy with a bone stock bike, some want additional get-up and go, and then there are those who want every ounce of horsepower available for a given motor. With hand porting you can get heads made to meet your personal requirements and desires. Tailor-made components generally produce better results than off-the-shelf products provided the engine builder knows what he’s doing. Be honest with yourself in determining how much performance you want. Talk to a reputable engine builder who can also produce what he claims. Purchase only what is necessary to achieve your goals. It is important to have a balanced engine package. Decide what your power goals are and choose products that match or compliment your flow requirements.
If you would like to know more about increasing the performance of your Harley, stop by Rocket Harley-Davidson in Huntsville, AL, and see me or drop me an email at firstname.lastname@example.org.