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13th August 2004

Opinion: Modern Engines - All They're Cracked Up To Be?

What's so great about modern motorcycle engines? Jim Rendell ponders, and discovers that there are more questions than answers...

Why do modern engines last as long as they do? It seems that around the era of the original Mini the life expectancy of an engine could be as low as 60k miles. Today there are numerous examples of engines exceeding 250k miles and decokes seem to be a thing of the past. The factors which have changed are metallurgy, oils, fuels and manufacturing techniques and process control. The pistons still whang up and down, the crankshaft wobbles around and the valves still go clackety-clack, and that much has remained unchanged since the concept of the reciprocating engine (with due deference to the Wankel).

Mind your flares don't get sucked in by carb...


Are the materials from which the modern engine is constructed vastly better than those of their predecessors? It seems, to a non-engineer, that cast iron and aluminium alloys still feature prominently in engine construction and the humble, simple, white metal bearing still serves its purpose. Bits of titanium and carbonfibre seem mainly to be used for pseudo exotic trims. Really -- a titanium gear knob does do a lot for a car as does a Kevlar ashtray.


My constant complaint about early two-strokes was that the silencers choked up, the plugs fouled and there was the inevitable trail of blue smoke. The oil was mixed with the fuel at around 20-25 to one and most of it exited the exhaust in an unburned state. It was not uncommon to follow vehicles which gave off the unmistakeable stench of a clapped out engine, a view usually reinforced by the clouds of smoke from the exhaust.

A RealClassic motor?

Today modern synthetics will allow ratios of around 40-50 to one with very little visible smoke and almost a complete absence of fouling. To my untrained mind a good oil is one that keeps moving surfaces from metal to metal contact, maintains viscosity and withstands the vagaries of temperature and pressure.

The modern oil filter must play its part; it wasn't so long ago that a filter was a wire basket in the sump which caught the big bits. The development of the micronic, screw-on filter must surely be a major factor. Such is the progress in lubrication technology that gearboxes and differentials no longer have drain plugs and the oil is expected to survive the life of the vehicle. A modern diesel has service intervals of up to 36k miles although after a few hundred miles the oil looks to be sorely in need of a change.


There are doubts concerning the stability of unleaded petrol and we are frequently given warnings of the need to drain fuel systems if a vehicle is to be laid up.

The pool petrol of the war and immediate post-war years must have been some pretty poor stuff and Redex seemed to be the universal panacea for all ills ranging from sticking valves to an improbable decarbonisation. It offered the lead content sought to keep the valves from burying themselves in their seats, yet even so valve regrinding seemed to occur frequently.

Gasping for a shot of 100 octane fuel?

Gradually things improved and seemed to peak with 100 octane fuels like Cleveland Discol. Then along came the anti-pollution era, the disappearance of leaded fuel and with it the question of valve seat regression on older engines. Is regression a serious consideration, are additives necessary?

Manufacturing design and techniques

Who remembers the 'Friday' vehicle? The one that had everything wrong with it. The one that would fall apart in the blink of an eye and would spend more time in the garage than on the road. We are told that there is now no such thing as a bad 'un, such is the sophistication of modern, automated machine tools and CAD/CAM systems. Tolerances are tighter, tool wear is automatically monitored and controlled and the rogue components are eliminated.

Farewell, poor pushrods...There seem to be no firm conclusions about engine configuration. Parallels and Vees do not seem to naturally balance, while boxers do. Engines can have as many cylinders as you like: twos, fours, sixes and eights and even threes, fives and twelves. It seems to be a basic design failure when an engine requires the added complexity of balance shafts. Ah now! A small gas turbine motorcycle (don't get too close behind) - that would be so smooth.

The sidevalves are almost no more, there aren't too many pushrod overheads left and belt-driven cams are commonplace. The long strokers have gone, so now the pistons rather vibrate than stroke. Anyone fancy setting up the valve clearances on a desmo? Shouldn't take more than half an hour! I just hope some fool doesn't introduce rear wheel steering for bikes as they did for cars.

Process control

It is in this area that perhaps the greatest advances have been made. The humble carburettor was a simple enough device but lacked real efficiency. Under load, with the throttle wide open, much of the fuel could not be utilised by the engine and was ejected to atmosphere doing nothing for performance, washing oil from the bores and increasing running costs. The advent of computer controlled fuel injection ensures that the engine receives an optimum fuel/air mix related to the immediate needs of the engine including the cold starts formerly effected by the old fashioned choke. Variable valve timing? Now there is a concept to ponder.

Modern ignition systems can produce energy of lethal proportions and plug gaps can be around 40+ thou contributing significantly to greater burn efficiency.

Conclusions. Are there any?

They don't make 'em like that anymore...This is just a brief, simplistic contemplation of how I see things but, more importantly, how do all these improvements translate to the maintenance and running of elderly motorcycles? There is not too much we can do about the original designs and very little on the metallurgy front although better bearings are available. We have a fait accompli in the fuel we use but can probably take advantage of modern oils and filters. There are electronic ignition systems for many of the more popular old machines.

I was never sure of the durability of old engines. How long will they run reliably before major surgery is needed? I know there are many variables that influence engine life, but, by and large, with modern oils and unleaded fuel, do we still have to decoke? Can we expect extended life of components? Time will tell.

The simplicity of earlier machines offers us the chance of some self-spannering, something apparently denied to owners of modern stuff. No 'user serviceable' parts inside, what a challenging phrase.

The pistons still whang up and down. The crankshaft still wobbles around and the valves still go clackety-clack Oh, never mind...

Agree? Disagree?

No pistons whanging up and down here...

More Info

By a strange twist of fate, there's more information about modern petrol, additives and finding 100-octane fuel in issue five of RealClassic magazine. We look at oil in more depth in issue six. Anyone would we think we planned this... instead of it being cheerful happenstance. Mustn't grumble, eh?

Not a subscriber to the magazine? No problem. We can fix that with only a teeny eeny application of cash...


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