Meriden Triumphs 101

- by Pete Snidal (C)2004

CHAPTER 1 - FOUR STROKE ENGINE BASICS

TheTriumph Twin engines use the four stroke design. Each full up or down movement of a piston within its cylinder has a specific function, and is referred to as a "stroke." The piston is connected to the crankshaft flywheels by the connecting rod, and up-and-down movement is translated into rotary movement of the central crankpins by the eccentric mounting of the rod journal at the extremity of the crankshaft.

Only one of two piston/rod assemblies is shown.

The Four Strokes Of The Four-Stroke Cycle

The Power Stroke is the reason for the whole thing. The full four strokes are as follows:

Each piston goes through its strokes oblivious of the other. The Triumph Twin engines, typical British Vertical Twins that they are, have both pistons rising or falling at the same time, but the strokes are arranged alternately; that is, when one cylinder is on the intake downstroke, the other is on its power stroke. Thus there is a power stroke for every 360 degrees, or one turn, of crankshaft rotation. The essence of motorcycle tuning involves ensuring that these operations occur with the correct timing, and that the mechanical considerations - mechanical, thermal, and volumetric efficiency, are met.

HORSEPOWER AND TORQUE

Different engine designs result in different power outputs, power being defined as the ability to do work in a given time. Work is defined as force-distance product. So the faster an object is moved, the greater the distance, or the greater the force, the more power is required. The power potential of the Triumph twin engine is matched to the needs of the average rider in average use. The engine is designed to produce the required range of power over a comparitively wide range of rpm - engine speed, or revolutions per minute. The idle, or "tick-over" speed of the engine is about 900-1400 rpm. The maximum rpm is about 6000 - the engine can be "revved" higher than that, but at a cost in power development and reliability.

Torque is raw twisting abilty, and is measured in foot-pounds, or kilogram-meters. If a foot-long lever mounted radially on a rotatable shaft exerts a tangential force on its outer end of 1 pound, we say it is developing a foot-pound (ft-lb) of torque. If the shaft is continually rotating, as in the case of an engine, the force must be measured with some sort of brake, which applies counter-torque to the shaft and measures the force required. This is the principle of the Prony Brake, a basic torque/horsepower measuring device, which gives HP readings as Brake Horsepower.The torque readings of the Prony Brake, combined with readings of rpm, to introduce the time factor, result in this BHP reading. A Horsepower is defined as 550 ft-lb/sec.

What all this means to the rider of a motorcycle is that the engine may be expected to produce various forces with various throttle readings at various rpms. There will be an ideal rpm for maximum power development - at full throttle, and a much wider range of rpm for less power at lower throttle readings. This is best demonstrated with a power graph. 

        A TYPICAL HP/TORQUE CHART

        40 |                                            h h h h h h
           |                                         h              h
        35 |                                       h                   h
           |                                  t  t  t  t  t  t t t
        30 |                               t    h                  t
           |                            t    h                        t
        25 |                        t      h
           |                     t       h
        20 |                  t        h
   HP(h)   |               t         h
 TORQUE 15 |             t        h
  (t)      |           t      h
        10 |         t     h
           |       t   h
        05 |    t h
           |  h
         0 |t_________________________________________________________
              5   10   15   20   25   30   35   40   45   50   55   60

                                rpm X 100
This is not meant to represent any specific engine, but is an off- the-cuff typical chart. Note the sudden drop of of HP at the end of its range, the relatively wider range of peak torque over peak HP, and the lower occurance of the torque peak than that of HP. The torque/hp reading shown would all have been read at the same throttle opening - generally full open when an engine is tested on a brake or dynamometer - the throttle is opened fully, and measured counter-torque is applied to hold the engine at given rpms in steps and the readings recorded to make the chart. Of course, in actual general use, the motorcyclist seldom, if ever, uses full throttle, but the shape, placement, and duration of the curves would be similar.

Performance Boosting

Engine modifications can be made to boost performance change the power and torque curves. A highly-tuned racing engine would have a higher HP peak, over a shorter range, higher up the rpm band. Torque would likely suffer, and the torque band would be shorter and also higher up the rpm range. Thus, the price for "more power" is measured not only in dollars, but also in tractability and ease of operation - the engine must be operated within a narrower rpm "power band," and at higher rpm in all cases, resulting in lower reliability. Performance modifications are always a trade-off, in cost, noise, reliability, and ease of riding.

So the first thing the new Triumph rider should consider is that his mount has been designed to provide the best all-around power and reliability for most riders, and that seeking more power will result in less tractability, and less reliability. For those who contemplate trading off these two characteristics for more power, however, speed tuning is dealt with briefly at the end of this manual.

Hit your browser's "back" button to return to the index.

---