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Compression Ratio
Air Filter
Compression Ratio
Crankcase Stuffing
Cylinder Head
Electronic Ignition
Engine Ports
Exhaust Silencer
Expansion Chamber
Magneto Ignition
Spark Plug
Transmission Roller

Tuning For Torque

THE Geometric Compression Ratio on a VeloSoleX S 3800 with a TUM M 80A CM 6.5 cylinder is:

where Cylinder Head Volume = Total Unswept Cylinder Head Volume.


On the same VeloSoleX S 3800 2-Stroke Piston-Ported engine, the exhaust port is fully closed when the piston top has travelled 9.8 mm up from BDC (Bottom Dead Centre) and reached the top of the Exhaust Port, so:


Hence the Swept, Trapped or True Compression Ratio is:


The Cranking Compression Pressure in bars is:

This Cranking Compression Pressure of 6.63 bars (663 kPa or 96.11 PSI) is only a theoretical figure and in reality will depend on whether the piston rings need to be worn in or if the engine is worn or in need of decarbonization.


The Peak Combustion Pressure in bars is typically:

This Peak Combustion Pressure of 36.44 bars (3644 kPa or 528.58 PSI) is only a typical figure and in reality will depend on many other factors.


Since VeloSoleX S 3800 owners usually use Unleaded Fuel 95 RON trying to increase the True Compression Ratio much above 7 : 1 will encourage detonation with this Octane Number of fuel. While it may be argued that increasing the Compression Ratio will lead to an increase in Torque & Acceleration at low RPM, it will also cause an increase in Pumping Losses at high RPM and therefore it is NOT RECOMMENDED to try to increase the Compression Ratio on the VeloSoleX S 3800. A higher Compression Ratio will also cause the mixture to burn faster and the engine to get hotter. Advancing the Ignition Timing will also create more heat during the combustion phase and encourage detonation.

If the Exhaust Port height has been modified by radiusing as explained in the Engine Ports section, the Compression Ratio may be slightly low. This can be corrected by replacing the standard Spark Plug (Bosch W6AC or W7AC, Champion L86C, Cyklon 43, Flashpoint FP5CR, Marchal V36A, NGK B6HS) with a Hotter, Projected-Nose type of Spark Plug (eg: Bosch W8BC, Champion L95YC, NGK BP5HS). Another benefit of a Hotter plug is that it will minimize Oil Fouling of the Spark Plug.

On an unmodified engine, using a Hotter, Projected-Nose type of Spark Plug will reduce the Cylinder Head Volume by about 0.2cc, thus:

Compression Ratio Calculator

The calculator below calculates Compression Ratio given Bore, Stroke, Exhaust Port Height (ie: height of visible exhaust port above top of piston while at BDC), Cylinder Head Volume and Spark Plug Displacement due to a Hotter, Projected-Nose Spark Plug compared to a standard plug. You may enter your own values in the  yellow  boxes below:


* 0cc for NGK B6HS spark plug but up to 0.3cc depending on Spark Plug type


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Copyright 2008 BRIAN'S VELOSOLEX. All rights reserved. This is a personal website containing information for and by VeloSoleX enthusiasts. It is not intended to replace any official information and whilst every effort is made to ensure the accuracy of all published information no guarantee is offered nor liability accepted for any loss or damage that may arise from any errors or omissions.