2.6
General Information
Compression Ratio Example
Bore = 6.5cm
Stroke = 6.0 cm
IHV = 17.1cc
Displacement = 3.1416 (PI) x 42.25 (Bore squared) x 6.0
(stroke) / 4 = 199.098cc
Displacement = 199.098cc
R= 17.1 + 199.098 / 17.1 = 12.643 Full Stroke Compression
Ratio
To calculate the Effective Compression Ratio, substitute the
exhaust port height for the stroke in the formulas above:
Exhaust = 2.95cm (exhaust port height is 29.5mm)
Effective Displacement = 3.1416 (PI) x 42.25 (bore squared) x
2.95 (exhaust port height cm) / 4 = 97.89cc
Effective Displacement = 97.89cc
Effective Compression Ratio = 17.1 (IHV cc) + 97.89 (Effective
Displacement cc) / 17.1 (IHV cc) = 6.725
Effective Compression Ratio = 6.725
In order to increase the Full Stroke Compression Ratio to 13.6,
how much material do you need to remove from the cylinder
head?
You know that: R = IHV + Displacement / IHV, and you want
to find out IHV.
Displacement = 199.098cc, and we want R to = 13.6, so then
IHV (desired) = Displacement (199.098cc) / R (13.6) - 1 = (12.6)
IHV (desired) = 15.801cc desired IHV to have 13.6:1 Full
Compression Ratio
Head cc Removal Example
The total number of cc’s to remove from the head = Old IHV
(17.1) - Desired IHV (15.801) = 1.299cc’s
Removed cc’s = 1.299cc’s
I = Stroke (6.0cm) / (Displacement (199.098) x 2.54
I= 0.01186 inches
To find out how much to machine off, multiply the number of
cc’s you need to remove by the number of inches to remove per
cc.
Thickness to remove = I (0.01186) x 1.299cc = 0.015”
Port Opening Duration
Port open = 81.5° This indicates the degrees after TDC that the
exhaust port opens, and also the degrees before TDC that the
port closes.
Duration closed = 2 x 81.5 (port open)
Duration closed = 163
°
Total Duration = 360°
Duration Open = Total Duration (360) - Duration Closed (163)
360-163=197
Duration Open = 197
°
Percent Open = Duration Open (197) / Total Duration (360) x
100
197/360 = 0.54722 x 100 = 54.722
Percent Open = 54.722
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