Ever wonder why one engine makes noticeably more power than a seemingly identical engine? The difference is often in the head. Not your head, in the engine head. A motorcycle motor (or most any reciprocal combustion motor) is basically an air pump. The more air you can move through the motor, the more fuel you can put with that air, and the more power you can make. Manufacturers know this so they work to optimize air flow through their engines. Yet production motorcycles are limited by the financial realities of mass production manufacturing techniques. Manufacturers can't hand polish and port each head on the production line, so they do the best they can and leave the rest to master engine builders like Tommy.
There are several techniques used by engine builders to increase performance of a particular engine – increase RPM's, increase cylinder size, increase stroke – but one of the most important and effective is increasing the flow of air through the engine head. Air must pass valves at a high rate of speed, with just the right atomization of the fuel for maximum energy burn. Experienced engine builders know that just “hogging out” the intake and exhaust ports won't necessarily lead to more horsepower – in fact it can reduce horsepower in many instances. Today's sport bike motorcycle heads come from the manufacturer with very high flow numbers and with machining optimized for performance. A skilled and experienced engine builder with a flow bench can usually increase the flow from a factory head, but it takes years of experience to know just how to work the head for maximum airflow.
Valve Seat Dynamics
One area of critical importance is the valve seats. Valves slam against the valve seat thousands of times per minute at high revs. The air flowing past that valve seat is turbulent and frenetic. The sooner you can start the airflow over the seat, the more air you can get in and out of the engine. And the smoother that air flow is over the seat and valve, the more air can be pumped into the engine to make more horsepower.
From the factory most engines come with a three angle valve job done on CNC machines. As shown in this picture, the seat is not located directly on the edge of the valve, but instead a few millimeters back into the valve seat. The actual valve seat angle is generally 45 degrees. On the combustion-chamber side of the 45-degree seat is a top angle that is usually around 30 degrees. This top angle serves two purposes. First, it acts as a radius to transition air between the 45-degree seat and the combustion chamber. Second, the top cut is used to reduce the width of the 45-degree seat from the top.
The third angle is usually called the throat angle and it also has two functions. Its primary task is to transition air between the 45-degree seat angle and the port. The throat angle also reduces the width of the 45-degree seat angle from the bottom. A five angle valve job i
s often employed in competition settings to further smooth the airflow over the valve face and seat. More angles mean less transitions for the air to traverse, improving flow and performance.
Valve Seat Position
But a competition valve job is more than just three or five angles. The position and width of the 45-degree seat angle is crucial. For the intake valve, most builders like to position the seat as high (toward the combustion chamber) on the valve as possible. For exhaust valves, builders like to position the valve seat more towards the middle of the of the valve. The reason? Exhaust valves run much hotter than intake valves and hence moving the valve seat down increases durability of the ultra-hot exhaust valves.
Vavle Seat Width
The seat width is also important to airflow. There are many theories on seat width. Narrower seats generally improve flow but are less durable. Narrow seats are typically used for racing applications, where the engine is freshened often. Harder valve seat material allows a builder to run narrower seat widths while still maintaining durability. And since exhaust valves operate at extreme temperatures, they require a wider seat to conduct heat away from the valve through the seat.
The valves themselves are just as important as the valves seats. Titanium valves used in most modern sport bikes are light weight yet strong, allowing higher RPM's than steel valves. In most street applications we use stock valves, since they are light weight, well designed and ultra-reliable. Competition race applications call for the expense of specialized valves. You can spend big bucks on high end valves. There is a lot of science in valve design. The shape and position of the valve seat is again crucial to maximum performance. And since intake and exhaust valves flow in different directions, the radius and shape of the valve angles are different. On the backside of the valve there are other tricks that can be done, such as back cutting the inside face, reduced diameter valve stems, and mating of the valves to the valve seat.
Tombo Racing Valve Job
A Tombo Racing competition valve job uses five angles plus hand polishing of the valve radius. Depending on the engine, the valve seat is typically moved closer to the edge of the valve so that air flow begins sooner when the valve begins to open and with smaller angled curves on the valve seat and face, the air is less turbulent and can flow faster and smoother. Our head specialist has been porting and polishing heads and performing competition valve jobs since 1968. All head work is done to Tombo Racing specs, which vary per engine and per the planned use of the motorcycle.
While these very minor details may seem insignificant in the overall scheme of your motorcycle, they can have a major impact on performance and specifically racing competition, where winners and losers are differentiated by a tenths of a second. When combined with other minor details of engine and bike building, they are the difference between race championships and also-rans.