DaiGwy profiles are based on typical results for engines ‘as standard’ from the manufacturer. It is probably fair to say that most motorcycles get modified in some way during their lifetime, some are purely cosmetic changes to enhance the appearance and some will be tuning changes to produce ‘more Power’.
Many books have been written on Engine Tuning and how to do it well. There are many excellent mechanics, tuners, performance workshops and accessory manufacturers who will advise the best and most suitable modifications to your bike. However, the list of options can grow and grow. Before you know it the estimate tops £1000 and keeps on going. Sure, everything is possible and you get what you pay for – usually, but lets take a step back and ask WHY?
Why bother? Why spend all that money? Why do you need a Big Bore Kit? Why should you throw away perfectly serviceable carburetors? Why do you need a new Air Filter? and Why have you just put a Nitrous Oxide Kit on your Christmas list?
The purpose of this section is to look at the main components that affect engine performance, to explain why they help or hinder the "power" output and to outline why many owners decide to make changes to them.
If you haven’t read the ‘Torque and Power’ page yet we recommend you do so now.
The Quest for Power
The quest for power starts with a brief overview of the three principal types of efficiency that affect the power output of your engine.
Thermal Efficiency (TE)
An engine is a machine for converting fuel energy into rotating motion. The fuel for a motorcycle is usually petrol, which is mixed with air to produce an explosive vapour. Factors such as compression ratio, spark plug location, chamber design and ignition timing all affect the TE of an engine.
Thermal Efficiency is the percentage of fuel energy that is converted into useful mechanical work.
A typical engine runs between 26% and 30%TE. Race engines may reach 35%.
Most of the thermal losses are out of the exhaust (around 40%) and most of the rest are lost through heating of cooling water, oil and other mechanical components.
Tuning for TE is really a job for the professional race teams where the gains are worth the money and effort in determining the effects of different set ups.
The power measured at the crank is as a result of the 26% Thermal Efficiency extracted from the fuel.
Mechanical Efficiency (ME)
Getting the power from the crank to the back wheel is the next challenge. Most engines are about 94% mechanically efficient so 6% is being lost turning bearings, alternators, water pumps, valves and gears. Uprated, lighter components are usually the way to go for improvements but this can get very expensive as lighter and stronger parts are often made from very expensive materials in small production quantities.
The final drive from the engine to the back wheel is usually a chain drive. These are about 98.5% efficient and the only serious choice for those on the quest for power.
A shaft drive is much less efficient and crucially it is heavier. This affects the unsprung weight of the back wheel and therefore affects the function of the suspension and handling. So for some owners of bikes like a Vmax the shaft drive is binned in favour of a chain-drive conversion. This is an expensive modification to make but one that significantly improves the available power and handling of the machine.
Volumetric Efficiency (VE)
Open your mouth wide and take a deep breath. Now breathe out quickly. Now do it again but this time with your mouth almost closed. It takes a lot longer to fill your lungs through the greater restriction caused by closing your mouth. It is exactly this effect we see in every cylinder in your engine.
Volumetric Efficiency is a measure of the actual air/fuel mixture volume sucked into the cylinder compared to the total volume of the cylinder. Typical values for normally aspirated engines vary between 80% and 100%, a figure between 80% and 90% is assumed for a motorcycle.
Many factors affect the VE from the condition of the air filter to the exhaust system used. It is not uncommon to hear the term ‘Helping the engine to breathe’ and this is the basis of most tuning modifications
When we ‘Tune’ an engine we are in fact looking to improve its ‘Volumetric Efficiency’. Higher VE leads to higher Torque and that results in a more powerful engine.
What volume are we talking about?
If the smooth flow of air/fuel mixture through the engine is critical to our quest, then just what volume are we talking about? Let’s do some maths. Don’t Panic!
Take a typical 1 litre
sportsbike for example
1000cc, 4 cylinders, 250cc per cylinder
Peak Torque occurs at 8,450 rpm
Each cylinder is filled every two revolutions of the crank.
That’s 4,225 times per minute or 70.41 times per second
That’s 17.6 litres per second passing through each cylinder at 100% efficiency.
Simply getting that volume of air through a cylinder using atmospheric pressure is very difficult. At Peak Torque the volumetric efficiency will peak at about 90% and only 15.84 litres of air are able to pass through per second. The rest could not reach the cylinder due to the restrictions to air flow in the inlet tract. Further away from Peak Torque rpm the efficiency will fall to figures around 80%. The Torque Curve for every model indicates this efficiency across the rev range.
If we can reduce the effect of these restrictions we will be able to improve the airflow and by allowing the engine to breathe more easily we can burn more fuel, the VE will rise along with the Torque and Power.
Follow the AirFlow
Let’s take a trip through an engine and look at the components that affect airflow. We can then describe the most common tuning modifications to breathe fire into your beast.
Airboxes used to be ‘somewhere to house the airfilter’ but in recent years the Airbox has been elevated to a critical first stage in producing more power on today’s Ram Air equipped sportsbikes. As the motorbike drives forward the air vents allow air to be forced into the airbox. At 70mph a bike travels 31.3 metres forward. If the inlet area is only 10 sq.cm. the swept volume is effectively 313 litres and while this does not all enter the engine it helps to force more air into the system. Unfortunately ‘Static’ Dynos cannot measure the Ram Air effect but riders of Ram Air equipped bikes will testify to the effect that results in a tremendous ‘top end rush’.
Airfilters are your engines first and only line of defence against the airborne grit and dust that will cause terminal damage if allowed to enter the cylinders. Do not neglect this vital component of your machine. Unfortunately filters get clogged over time.
The best thing you can do to your airfilter is buy a new one now. There are many good aftermarket brands available that offer higher airflow than the standard items but do get advice as higher airflow without carburettor adjustments will weaken your fuel/air mixture and may damage your engine.
The filtered air is fed straight into the ‘Carbs’ where it is mixed with fuel to form a vapour. The carburettors other function is to control engine speed by choking off the airflow as the carburettor slide descends to the closed position.
The bore size of the carbs is the important thing here as a larger bore is less restricting to the airflow. At small throttle openings the effect in minimal but from half open to full throttle the effect can be huge.
More sums –
A 37mm bore Carb has a sectional
area of 1075 sq.mm. Fitting a larger 41mm bore Carb with an area of 1320 sq.mm.
will be able to allow 23% more air through at full throttle. In reality many
of the other factors that follow do not allow for this increase to be realised
so for any given engine configuration there is a limit on how much bigger
Carbs will help. If in doubt, trust the manufacturer’s judgement and
leave the standard carbs on.
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