On Engines

The internal combustion engine, when looked at basically, is a very logical and simple machine, but when one wants to tune an engine to racing pitch you need to be a highly skilled mechanical engineer as well as having a maths and science degree to succeed in this highly competitive field.

When a motor needs a major overhaul - this is the time to do the tuning

- Dave Parry

This, though, need not worry the enthusiastic motorcyclist who wants more power, with the same engine torque characteristics to cope with everyday traffic as well as charging up a motorway. He can do this, given the aptitude, at a relatively low cost, provided he does not fall for some of the so-called 'goodies' that claim to boost your power output to blow off any Honda four and the like, that is going. I suppose if we were to be taken in by all the advertising and fitted every conceivable accessory we would need a pilot's licence to go on motorways.

Now the most important part of getting more horses is to fettle to perfection what is already there. It is no use, for instance, fitting a hot camshaft and 10 to 1 pistons in the motor when you have a contact-breaker gap you could walk through.

But let's start at the beginning. When a motor needs a major overhaul - this is the time to do the tuning; it is stripped, checked for wear and parts replaced where necessary - but even when doing this, it should be done carefully, paying particular attention to cleanliness and following the maker's instructions.

I prefer to leave the bottom half of the engine standard on a road bike, unless parts are available, or can be altered, so as to increase relative reliability. For example, a lighter one-piece crankshaft.

Therefore I begin tuning from the pistons and work upwards as this is where the most good can be done. I leave cam-shafts out deliberately because:-

  1. They can be very expensive items on some makes.
  2. When fitting hot cams to some models that are not designed to run in conjunction with touring carburation and exhaust systems, you get a retarded effect on power. There are exceptions that work very well, of course, but these are relatively mild cams as compared with pukka racing parts, and therefore are not a great advantage unless used in conjunction with other tuning.

Compression ratios can be used up to approx. 10 to 1 without losing too much low rpm torque, although I prefer to keep nearer to 8.5 to 1.

The cylinder head is, I think, the most important in this type of tuning, the idea being to get as much petrol/air mixture into a combustion chamber, in as short a space of time as possible. This is done by carefully opening the inlet and exhaust ports, polishing and fitting larger valves, namely inlet if possible. This sort of work can be done professionally at quite a reasonable price. Particular attention must be paid, on assembly, to valve grinding and refitting of the guides.

I am not keen on twin carburettors on twins for reasons of synchronisation, engine wear and lack of smooth low rpm running and also that not a great deal is gained. I prefer to use one carburettor of increased choke size, to blend with the opened ports.

All that is left then is the rockers and valve gear, which can be lightened considerably to give obvious benefits in as far as reducing the reciprocating weight, which in turn eliminates valve float until further up the rpm scale.

These are the basic features that I think give the most benefit at a reasonable cost and without a lot of head scratching. It is impossible, though, in a short space, to go into the technicalities of each separate part and the correct way of assembling these, but with a little common sense and patience, a considerable improvement should be gained.

Dave Parry