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

Inlet Port

(Unless stated otherwise, all parameters below are based on the TUM M 80A CM 6,5 Cylinder - V4 (MBK) from March 1986 and an engine speed of 2500 RPM)

THE Inlet Port TIME AREA (0.0100294 s-mm²/cc), ANGLE AREA (1.504 deg-cm²/cm³) and OPEN DURATION (115.9°) are INSUFFICIENT to allow the mixture to enter the engine efficiently at speeds above 4000 RPM.

The TIME AREA is primarily influenced by widening the Inlet Port which was actually done on the VeloSoleX S 3800 engine from its launch in 1966. The ANGLE AREA is altered by changing the height of the port OR as RECOMMENDED here by filing part of the Piston Skirt. This must not be overdone on Piston-Ported engines otherwise mixture will escape back into the carburettor as the piston travels upwards before the Inlet Port has a chance to close and stop it. This is not quite as much as you would expect since the mixture gas does have INERTIAL ENERGY encouraging it to flow in the same direction (BUT enlargening the Carburettor Venturi Diameter will increase this type of loss). Changing the width of an Inlet Port is usually preferred to changing its height.

If your engine has the Long Piston (49.45 mm long [measured at the edge] for cylinders made before March 1986 such as the TUM 80 [V1], TUM 80A [V2] or TUM 80A CM 6,5 [V3]), see What is the effect of shortening or lengthening the Piston? in the FAQs section.

Transfer Ports

THE Transfer Ports  are satisfactory for most practical engine speeds and therefore it is NOT RECOMMENDED to modify them.

Exhaust Port

THE Exhaust Port is satisfactory when used at engine speeds BELOW 4000 RPM.

At engine speeds ABOVE 4000 RPM, the Exhaust Blowdown (the time needed to clear exhaust gases from the cylinder before fresh mixture enters and measured from when the Exhaust Port opens to when the Transfer Ports open to allow fresh mixture in) TIME AREA (0.0033635 s-mm²/cc), ANGLE AREA (0.505 deg-cm²/cm³) and BLOWDOWN (18.6°) are INSUFFICIENT.

Changing the width of an Exhaust Port is usually preferred to changing its height. On the VeloSoleX S 3800 both the width and height of the Exhaust Port should be radiused thus increasing both the TIME AREA and ANGLE AREA. It is important though that the width of the Exhaust Port is not too wide otherwise Piston-Ring wear and seizing are more likely to occur.

Simply putting a 1 mm radius on the upper sides and top edge of the Exhaust Port (but NOT the lower edge) is recommended which also allows the piston rings to expand and contract more easily as they traverse the port.

This gives a new Exhaust Blowdown TIME AREA of 0.0039798 s-mm²/cc, ANGLE AREA of 0.597 deg-cm²/cm³ and BLOWDOWN of 22.0°.

This also changes the Exhaust Port TIME AREA (0.0235901 s-mm²/cc), ANGLE AREA (3.539 deg-cm²/cm³) and OPEN DURATION (130.7°) to a TIME AREA of 0.0248227 s-mm²/cc, ANGLE AREA of 3.723 deg-cm²/cm³ and OPEN DURATION of 137.5°.

Some engineers align the open edges of the piston rings away from the ports but without stop-pins in the piston grooves there is no real guarantee that the rings will not turn back towards the ports after the engine has run for a while.

Changing the height of the Exhaust Port reduces the Swept Volume (Effective Swept Volume) from 37.01 cc to 35.78 cc. This also causes the Swept, Trapped or True Compression Ratio of 6.44 : 1 to change to 6.26 : 1 but this can be corrected by using a Hotter, Projected-Nose Spark Plug.

As a result of radiusing the Exhaust Port, better Exhaust Blowdown occurs with better high-RPM power (and slightly less low-RPM torque).


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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.