In order to do the necessary calculations, you need to know a few things about the engine. In some cases you can guess, although results obtained without accurate input data should be treated with extreme caution.
Even a seemingly small difference in the values used can have quite a distinct effect on the results.
Some suggestions for calculating/measuring these quantities have been provided, where possible.
Engine Size
The engine size is usually expressed in cubic centimeters (cc), and is often rounded up to a 'neat' figure. e.g. a 850 engine is actually nearer 848 cc. Check any reference data for your engine to determine the actual specified engine size. Alternatively, if you have bore/stroke information use that instead.Cylinder Bore
The engine bore diameter (in mm) can be found in reference books. The following are the 'standard' values:
| Engine CC | Bore |
|---|---|
| 850 | 62.94 mm |
| 1000 | 64.59 mm |
| 1100 | 64.59 mm |
| 1275 | 70.61 mm |
Engine Stroke
The engine stroke length (in mm) can be found in reference books. The following are the 'standard' values:
| Engine CC | Stroke |
|---|---|
| 850 | 68.25 mm |
| 1000 | 76.20 mm |
| 1100 | 83.72 mm |
| 1275 | 81.28 mm |
Piston Dish Volume
The volume of the 'dish' in a piston can either be obtained from reference books, or may be measured (approximately) by burette.Note that certain pistons (such as 998 Cooper) with a raised crown have an effective negative dish volume (in the case of the 998 around 4.5 cc).
Ring Land Volume
This is the space between the piston and the cylinder, from the top ring up to the top of the piston. This is of greater importance when and engine has been overbored and oversize rings are used, as opposed to oversized pistons, hence making this a relatively large volume.Gasket Volume
This is the volume created by the head gasket thickness. It varies from about 1.4 cc for the smallest engines up to 4.2 cc for the larger bore engines.The default figures are sufficiently accurate for most purposes
Piston to Deck Volume
On standard engines the piston at TDC usually is not level with the block deck. This leaves a volume to be accounted for. For standard bore engines this can be calculated by multiplying the piston to deck height (in mm) by the cc/mm from this table:
| Engine CC | Volume |
|---|---|
| 850 | 3.15 cc/mm |
| 1000 | 3.35 cc/mm |
| 1100 | 3.35 cc/mm |
| 1275 | 4.13 cc/mm |
N.B. For pistons with raised crown, measure from the lowest part of the piston.
Combustion Chamber Volume
The main volume is contained in the the combustion chamber. If you can identify your cylinder head, then select from the following table, otherwise select according to engine size.
| Cyl. Head ID | Chamber Vol. | Engine |
|---|---|---|
| 2A 628, 12A 1456, 2A 629 | 24.5 cc | 850, 948 |
| 12G 202 | 26.1 cc | 997, 1098 |
| 12G 206, 12G 295 | 28.3 cc | 998, 1098, 1100 |
| 12G 940, 12A 185, AFG 163 | 21.4 cc | 1275, 1300 |
Alternatively, the chamber volume can be measured as follows:
Place the cylinder head face up, with the valves and spark plug(s) installed - (This need only be in one chamber) - and ensure the head is level. Then, with a suitably accurate burette or similar device place liquid (something like glycerine, although water with a small amount of detergent to reduce surface tension, can also be used) into the chamber until the liquid is level with the deck of the head. The amount of liquid used is therefore equivalent to the chamber volume.
Measuring For Skimming
This method is particularly useful when the full chamber volume is not already known.Place the cylinder head face up, with the valves and spark plug(s) installed - (This need only be in one chamber) - and ensure the head is level. Then, with a suitably accurate burette or similar device place liquid (something like glycerine, although water with a small amount of detergent to reduce surface tension, can also be used) to the amount desired for the chamber volume, into the chamber. It is then merely necessary to measure the thickness of metal from the surface of the liquid to the deck of the head. Obviously, accurate measurement is the key here.
Calculating For Skimming
This method assumes that the full chamber volume is already know.Place a sheet of carbon paper, carbon side up, over the chamber. Alternatively, mark the edge of the chamber with graphite or similar. Place a sheet of graph paper, grid side down over the carbon paper. While holding the paper carefull in place, rub the edges of the chamber. This will transfer the outline of the chamber onto the graph paper.
Now, by counting squares, the area of the chamber can accurately be determined. To calculate how much needs to be skimmed to achieve any given chamber volume, subtract the required volume (cc) from the original volume (cc), and then divide the answer by the area of the chamber (square cm) to give the thickness (cm) to be skimmed from the head.
Useful Conversions
| Metric | Imperial |
|---|---|
| 1 cc | 0.06 cubic inch |
| 16.4 cc | 1 cubic inch |
| 1 cm | 0.39 inch |
| 2.54 cm | 1 inch |
| 1 mm | 39.4 thou |
| 0.0254 mm | 1 thou |
� 1998 [email protected]
References
Some data from "Tuning BL's A-Series Engine" by D Vizard.