how to centerline a dohc without a cam card

and early intake valve closing point) then the mixture may end up being 'over-compressed.” This will lead to excessive compression losses, detonation and could even lead to head gasket or piston failure. If a high static compression ratio is used with a mild cam (i.e. As a result, performance at high RPM will suffer. However, at high RPM the intake valve will close before the maximum amount of air/fuel mixture has been drawn into the cylinder. Compression ratio is also dependent on cam duration.Ī mild cam with an early intake valve closing point will work well at low RPM. The intake valve closing point is intimately related to an engine's dynamic or 'effective' compression ratio. And in hydraulic street cam applications, it would insure that the closing ramps are not so fast that they result in noisy operation.Ī late closing intake valve will yield poor compression and will cause poor performance over most of the entire RPM range.Ī semi-late closing intake will have a good mid range and good top end but not the best.Īn early closing intake (30-35 degrees) is what we like for a heavy bike because it will give an excellent bottom end performance and a good mid-range. It wouldn't be so fast the valve bounces as it closes, allowing the charge to escape back into the intake port and disturb the next charge. It would get the valve seated quickly and not waste time in the low lift regions where airflow is minimal and there is no compression building in the cylinder. In a perfect world, the optimum intake closing point would occur just as the air stops flowing into the chamber. Of course, closing the valve too late will create significant reversion. The higher the engine's operating RPM, the later the intake closing should be to ensure all the charge possible makes it into the combustion chamber. This results in the intake charge continuing to flow into the combustion chamber against the rising far past BDC. As RPM increases, intake charge momentum increases. It also reduces exhaust emissions while enhancing fuel economy. Early intake closing is critical for low-end torque and responsiveness and provides a broad power curve. The earlier it occurs, the greater the cranking pressure.

Intake Closing: The intake closing point has more effect on engine-operating characteristics than any of the other three opening and closing points. All right, so now that we have all this behind us we can move on to the cam itself. A dresser is going to need more bottom end than a FXR due to the weight and wind resistance. Now we also have to look at the bike we ride. So here is where we are going to look for our torque.

In a STREET motor, we do not need it at the top, we do need some at the bottom, but we really need mid-range. If you have it at the top, you will not at the bottom and if you have it at the bottom, you will not at the top. There is a law of camshafts we need to keep in mind. For the purposes of these articles I'm not interested in RACING motor but rather street motors and street motors need to develop TORQUE and they need to develop TORQUE in the 2500 to 4500 RPM range, as this is the range we most often operate the engine in (freeway cruising). Now one more thing before we dive in to the mystery and that is we need to understand our objective here. The result of all the camshaft specifications is where in the RPM band the motor will make the best power. It determines how much, when and how long. Airflow is everything and the camshaft is the controller of the airflow. There is a little clue here for you sharpies and that is the word AIRFLOW. Intake: The intake must also flow enough air to support the cam and cylinder fill.Įxhaust pipes: The exhaust pipe must not only flow enough, they must be designed so that the reversion pulse is compatible with camshaft timing. There are a number of things, which affect the cam’s performance.Ĭylinder head flow rates: The cylinder heads must be able to flow enough air for the time that the valves are open.Ĭompression Ratio: Static compression ratio and cam choice should be considered as a system. #2 - is selecting a cam that is not compatible for the RPM range that we plan to operate the engine in. #1 - the most common camshaft error made by people is to OVER cam the engine.

How to centerline a dohc without a cam card