Vire 7 - Hints and Tips

Adding an Alternator

Webmaster's Note: Can it be done? Will it work? Does it put too much load on the engine? In 1994, Chris Hall fitted an alternator to his Vire 6, and in 2004 he switched it to his new Vire 7, and he says "Yes, Yes and No", respectively, to the above questions. Here, in his own words, is everything that you ever needed to know about the subject (and doesn't the final installation look neat): -

Why fit an alternator?

  1. Dynastarts fitted to Vire Engines give a low output, if any at all, and a long run is needed to replace the power taken from the battery for starting.
  2. An alternator will take less power from the engine. An alternator will automatically reduce the charge to zero when the battery is charged. In contrast, the old Dynastart will continue to charge (at a low level) all the time that the engine is running.
  3. The battery will last longer - it will spend less time discharged and will not be overcharged, neither of which is good for a battery.

What kind of alternator to fit?

The alternator that I used was designed for a Rover car of about 1980, but any general car alternator from the last 30 years should do. Mine was new (surplus), but a secondhand one from a car breakers will do and should be cheap.

How do I fit it?

Three things are needed: -

  1. A mounting for the alternator
  2. A drive to the alternator
  3. Wiring to the alternator

The Mounting

I made a simple bracket that bolted through the port engine bearer, but do not fit this until you have sorted out No. 2.

The Drive

This is the only bit that may cost some money. Remove the pulley from the Dynastart and buy a double pulley of a similar diameter. Mine was a "Picador" brand (65mm). At the same time buy a single pulley of twice the diameter (mine is 125mm).

These pulleys need to be machined to fit the Dynastart and the alternator. If you have not got a lathe, a friend with a lathe, or an employer with a machine shop, then you may have to get a local engineering firm to do it for you. I have found that most small companies are keen to help and it is a simple piece of work. The double pulley is bored to the diameter of the Dynastart shaft, the outer end counterbored to allow the nut to go on and a slot is filed for the key. The larger pulley is bored to fit the alternator. Both the shaft on the Dynastart and the alternator are parallel, with no tricky tapers!

Fit the pulleys. Re-fit the starting belt and a new belt from the Dynastart to the alternator so that you can line up the alternator mounting brackets and then fit them. A tensioner strap (if you got your alternator from a car breaker the car one will probably do) like the one on the Dynastart is needed. Do not make the belt too tight; especially if the engine is on flexible mounts and the alternator is not. As the belt has a good "wrap" around each pulley it does not need to be very tight to transmit the power.

That completes the installation. Turn the engine over by hand to check that everything is free to move. DO NOT start the engine; this could wreck the alternator!


Only three connections are needed: -

  1. The earth return - do make sure that your alternator body is connected to the earth.
  2. Output - this is the large terminal on the back of the alternator. This must go to the battery with a heavy wire (mine goes through an ammeter but this is not needed). This wire must not be disconnected at any time the engine is running as it will almost certainly wreck the alternator.
  3. The field/energiser wire - this is the small terminal on the back of the alternator. This is connected via a small light bulb to the battery (my light bulb is 12v 2w). When the alternator is stopped a current will flow and this light will glow just like the ignition light in your car (it is the same thing). The small current provided by this source energises the field coils in the alternator and allows it to start charging when it rotates.

This connection needs to be controlled by a switch or it will slowly discharge the battery when the engine is not in use - I use a switch next to the stop switch for the engine.

That is all you need. You may disconnect all wires on the Dynastart except the heavy cable that goes to the large terminal on the side of the Dynastart - that is the starter cable!

In use, I start my engine and almost immediately turn on the switch for the alternator which immediately starts charging (and places a load on the engine, I may open the throttle slightly). By the time I am ready to cast off, the battery is fully charged and the alternator is no longer loading the engine. When I stop the engine the lamp in the field/energiser circuit glows and reminds me to switch off. The system has been in use for 10 years on my old Vire 6 installation and now 6 months on my Vire 7.

Chris Hall, May 2004

Webmaster's Note: In response to questions from another Vire owner, Chris later supplied the following extra information: -

The reason that the pulley on the alternator is larger than the pulley on the dynastart is to keep the load from the alternator within reasonable limits.

Please note that the speed of the alternator is controlled by the ratio of the sizes of the crankshaft pulley to the alternator pulley. The size of the pulley on the dynastart has no effect on the speed; it is just an idler i.e. the in and out sizes are the same. Feel free to experiment with the size of the alternator pulley but ...

  1. If you use the original pulley you will almost certainly exceed the maximum speed limit of the alternator (usually about 9000 rpm).The rated maximum output of the alternator I use is 70 amps.
  2. This probably equates to a 2 hp load on the engine - Do you hate your Vire that much?
  3. It is expensive:- a new pulley and a new belt for each experiment.
  4. The size I have given works well and has proven reliable.

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