Two-Stroke FRV Timing Calculator

Bore and Stroke

These are mandatory values. You can enter them as either metric or imperial units by selecting the unit type Radio Button at the top of the page. Uniform units of millimetres (mm) or inches are assumed for all measurements provided in the form. Note that we want the actual stroke, not the crankshaft throw (which will be half the stroke).

Piston Height

Mandatory parameter if performing exhaust or transfer timing calculation. This is the height of the piston from the lower edge to the perimeter of the crown. This means that if the piston crown is domed or conical, or has a deflector, you must omit this part from the measurement. This is because it is the edge of the crown (ie, the perimeter) that will uncover the cylinder port(s) and govern the timing.

Conrod Length

Mandatory parameter if performing exhaust or transfer timing calculation. This is the distance between the centers of the crankpin (big end) and wrist-pin (little end) holes.

Wrist Pin Offset

Mandatory parameter if performing exhaust or transfer timing calculation. The vertical distance from the perimeter edge of the piston crown to the center of the wrist pin hole (or gudgeon pin if you prefer).

Piston Deflector Step Height

This parameter is applicable only to Transfer timing calculations on engines which have a step cut into the top side of the piston, forming a deflector to the transfer charge. The value required is the maximum vertical distance from the perimeter of the piston crown to the bottom of the deflector step. Omit the parameter, or set it to zero for pistons with no step—including pistons fitted with a deflector baffle on the piston crown.

Top of Exhaust Port Location

Entering a value for this parameter will produce Exhaust timing figures. It requires values for stroke, piston height, conrod length, and wrist pin offset. The measurement required is the vertical distance from the crankshaft axis to the top edge of the exhaust port. We do not need to know the height to the port.

Top of Transfer Port Location

Entering a value for this parameter will produce Transfer timing figures. It requires values for stroke, piston height, conrod length, wrist pin offset, and optionally, deflector step height. The measurement required is the vertical distance from the crankshaft axis to the top edge of the transfer port. We do not need to know the height to the port. Transfer is sometimes called "Bypass" as this port enables the fresh fuel/air mix to pass by the piston and enter the upper cylinder.

Intake Width

This parameter is required when calculating the inlet timing. Values are also required for inlet port width, crankshaft diameter, and inlet port angle. The value is the width of the venturi inlet at the intersection with the crankshaft. In most cases, this will just be the diameter of the venturi hole. Note that it is assumed that the inlet is located on the engine centerline. This program does not (yet) account for offset inlets (which effectively increase the inlet duration).

Inlet Port Width

This parameter is required when calculating the inlet timing. Values are also required for intake width, crankshaft diameter, and inlet port angle. This is the effective width of the port cut in the crankshaft. In simple cases, this will just be the diameter of a hole drilled in the crankshaft. Some engines increase this width by milling a flat across the shaft. In this case, the value would be the length of the chord formed by this flat.

Crankshaft Diameter

This parameter is required when calculating the inlet timing. Values are also required for inlet port width, and inlet port angle. The value required is the shaft diameter measured at the inlet port location.

Angle of the Inlet Port relative to the Crankpin

This parameter is required when calculating the inlet timing. Values are also required for intake width, inlet port width, and crankshaft diameter. The value required is the angle between the crankpin and the axis of the crankshaft inlet port. Note that the the calculations performed assume a "down-draft" inlet configuration. For engines with an updraft inlet, you can adjust the angle by subtracting it from 180 and using the result—effectively transforming the engine into a down-draft inlet.

Exhaust port height

This parameter is optional. It is not strictly required in order to calculate the timing events, but does allow the program to check for Sub-piston Induction (SPI). This is a design feature that opens the crankcase to the atmosphere at TDC through the exhaust in an attempt take in more air.

Engine Name

Enter any string you like, or leave the field blank. This value is used for the header on the timing diagram page and is intended to assist identifying the timing later, should you print the page.