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Wisdom on Torgue & Power PART 2

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Last month we spent a little bit of time on the fundamentals of torque and power, and proved the mathematical truths about the source of all good things – torque.  And we showed that power is generated by taking torque and “spinning” it.  We hope that you have seen that all engines make nearly the same torque per given capacity and compression ratio.  Even the latest F1 engines make more or less the same “specific torque” as the humble single cylinder 600.  Interesting hey?

written by Stefan Boshoff (Senior Instructor - Country TRAX Off-road academy, Free State)

All of that is good and well, but the one thing that we really “feel” whilst riding a bike is the rate at which the engine can reach the maximum power and torque values and their associated rotational velocities (rpm).  Each engine has its own “character” which we can put on paper by drawing curves showing torque and power against rpm.  The curve shown here is for the new BMW 1200 GS, downloaded from webbikeworld.com.

torque curve

The torque curve is determined by how well the cylinders are filled with fresh fuel / air mixture, how well it burns that mixture and how much loss there is in the process. At a certain rpm the cylinders filling ability will level off and start to decline. At the same time internal friction increases with rising rpm’s. Combine these two and you have decreasing torque output.  That is why the torque starts to decline above 6000 rpm in the curve shown.

Keep in mind that even though torque figures begin to fall off, the power output (kW) continues to rise because rpm increases faster than the torque falls off up to a point , then horsepower too will start falling away, due to friction.  The torque curve shown proves why the 1200 is such a pleasure to ride.  Above 100 Nm of torque between 3200 and 7500 rpm, and over 80 kW from 6100 rpm to the limter….  Quite impressive if you compare that with an R 80….   I have heard some people complain about the “flat spot” at around 5000 rpm.  Some deny it.  But if you look at the curve it is clearly there, although it stays above 100 Nm all the way through the “dip”, which is not too shabby….

The one factor that is not shown on a typical torque and power curve is time.  How fast the engine will get to a particular rpm value under given load conditions.  It is no good having an engine which can produce huge numbers, but takes ages to get there.  There are some “secrets” here where designers can still play and manipulate the “time” factor but still stay within the mathematical truths.  It is therefore quite logical that the “fastest” engines are the ones with the most valves, lightest valve driving mechanisms, variable ignition and valve timing and other features that help moving air through the engine’s combustion chambers faster, enabling the ability to rev quicker and faster.  Electronic knock sensors and lambda sensors also help keeping things optimized, preventing pre-ignition and keeping the mixture ideal through the rev range.  The “top end” of an R1 looks very different to that of a Harley.  The secret is to move more air through the engine; many see an engine as an air pump…. This is the reason why every engine tuner worth his keep has a flow bench to optimize the air flow through an engine.

We are not even going to discuss the “forced” ways of moving air – turbo’s, superchargers, etc.  These force “more” mixture into the combustion chambers, resulting in more torque.  But if the engine is not designed for the increased temperature, pressure and mechanical loads, there will be a dramatic decline in reliability and engine life.  The pleasure of increased performance is normally followed shortly by an untimely financial kick in the butt.  And you don’t have to be Einstein to realize that when fuel / air mixture moves through an engine at higher rates, there has to be an increase in fuel consumption.  To try and counteract this, the mixture is made leaner, resulting in even higher combustion temperatures and always ends in a catastrophe.  The thought that comes to mind here is “fools rush in where angels fear to tread”.  Think carefully before an aftermarket turbo is fitted to anything…..

Hopefully this and part 1 from last month will arm you with a little bit of ammunition to defend yourself against the sales talk you hear in some showrooms and the wisdom that is seen and heard on internet forums and around braai fires.

By the time you read this, we should be across the Northern border of Namibia, and well on our way into Angola for an exciting 2 week trip of a lifetime.  We will let you have some feedback here, watch this space!  In the mean time, don’t forget to look at our website – www.countrytrax.co.za.  All the scheduled dates for the Introduction to Off-Road Riding, Weekend Off-Road riding, Sand, Advanced and Dirt Bike Fundamentals training courses are there.  Join us at one of the great venues country wide and lift your enjoyment of the game we all love so much a few notches.  Until next month….

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