Measuring the RPM of a Vire 7

or any single-cylinder 2-stroke engine with magneto ignition

RPM is an abbreviation for Revolutions Per Minute. For any engine, the RPM can be measured either Directly or Indirectly.

Direct Measurement of the RPM

The name is self explanatory. By measuring the rate at which a key part of the engine revolves, the RPM can be calculated directly. The easiest item to observe is the flywheel, which rotates once for every revolution of the engine. (This applies to all internal combustion engines).

Direct measurement can be done mechanically, although the Vire 7 is not set up to use such systems on a permanent basis. Jean-Francois describes his simple tachometer:- "I have this small counter (fits in the hand) that increments by one every turn it goes, and by simply pressing it's rubber end right in the middle of the flywheel and holding it there for one minute and then checking the value on the counter gives you the precise quantity of turns it went, thus giving you the RPM.

More permanent solutions use either a contact or an optical system. These require a specialised tachometer, such as those supplied by SenDEC Corporation. However, this is not necessarily the cheapest alternative. Read on...

Indirect Measurement of the RPM

This involves measuring some parameter that is directly related to the RPM, although the parameter measured may need to be multiplied or divided by a conversion factor to calculate the RPM. The principle is very suited to gasoline / petrol engines, where the ignition cycle can be measured easily, with relatively inexpensive equipment.

For the Vire 7, at least three methods are applicable:-

  1. Using a ready-made customised electronic tachometer, designed for use with a single-cylinder 2-stroke engine. One such is a SenDEC Engine Maintenance Meter which has four functions including Tachometer up to 6,000 RPMs, Hour Meter, Job Timer, and Service Alarm. It works on the principle of electrical induction, using a wire wrapped around the spark plug cable. Tiny Tach provide a choice of very similar meters. In Europe, the RevCo MTL1-P was another suitable meter, but may no longer be available. (March 2006)

  2. If you want to build your own tachometer from a kit, Velleman of Belgium do a suitable digital rev counter in kit form. To make it you just need to have a reasonable amount of experience at soldering and be able to follow 'pictogram' instuctions. The kit number is K2625, and you can look at its specification on the Velleman website. It costs £25 (40 Euro / $40US - Price at August 2002) from Maplin in the UK (Order Code VE01B), and they do mail order delivery. It runs from a 10-15 volt supply, and draws just 200mA. It can be configured for 2 or 4 stroke engines of 1-12 cylinders, reads to 100 RPM accuracy, and connects across the contact breaker points.

  3. Using a hand-held Tachometer / Dwell Tester. These are available for as little as 25 Euro ($22US) for an unsophisticated version (Price at September 2001). A digital display is always going to be accurate, which can't necessarily be said of analogue-display meters. Most models are designed for use on 4-stroke engines with four or more cylinders, but if your computational abilities extend to being able to multiply by two, then this does not present a problem (see below).

How to use a Tachometer / Dwell Tester to measure RPM

The meter comes with a cable and two crocodile clips:-

  1. Attach one of the crocodile clips to the terminal at one end of the stop cable - this is the black cable that leads to the engine stop switch from the four-terminal connection rail on top of the flywheel housing
  2. Attach the other crocodile clip to earth
  3. If the meter has a setting for a single-cylinder 2-stroke (or a 2-cylinder 4-stroke) engine, select that. The meter will then give a true reading for the RPM of your Vire 7.
  4. If the meter doesn't have a suitable setting, select the setting for a 4-cylinder 4-stroke engine - the meter reading will then be half of the true RPM, e.g. 500 RPM on the meter indicates that the engine is revolving at 1000 RPM. Click here for the arithmetic underlying this conversion.

Happy Engine Tuning !

Top of Page Vire 7 Home Page Operator's Manual - Contents Site Map