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What is the difference?

Discussion in 'Modifications and Projects' at netrider.net.au started by thestig, Jun 26, 2009.

  1. So I have somewhat of a technical background but it always confuses me to look at two same capacity engines and then have different amounts of power. Obviously a shorter stroke will give more power less torque but sometimes its like they both increase. Is this all to do with ECU/Carby differences or porting or what? What are the basic things that make a performance engine vs a tuned down one?

  2. Higher compression ratio (increases power everywhere)
    More aggressive ignition advance can raise power throughout the rev range too
    Mechanical Boost (turbocharger, supercharger) or chemical boost (nitrous oxide ;) )

    It also comes down to where the engine has been designed to achieve peak volumetric efficiency (more or less where peak torque occurs).

    To move the peak volumetric efficiency around, we can change:
    * length and diameter of the intake runners (and the rest of the intake system)
    * length and diameter of the exhaust headers (and the rest of the exhaust system)
    * Camshaft profile (how aggressively it opens/closes valves, and how far)
    * Camshaft duration (how long it holds the valves open for)
    * Camshaft timing (when the intake and exhaust valves open/shut during each cycle)
    * Bore and stroke of the pistons probably moves the volumetric efficiency curve around too.

    But what you'll tend to notice is that most naturally aspirated petrol engines of a similar displacement will all make a very similar amount of peak torque, somewhere around ~100Nm/L.

    By making the engine breathe most efficiently at high rpm (ie: peak torque occurs at higher rpm), the engine will generate more power.
    Power being equal to Force * Velocity, or in this case Torque * Rpm.
    Energy generated per second.

    By making the engine breathe most efficiently at low rpm, (ie: peak torque occurs at lower rpm), the engine will tend to feel more tractable. It'll have more power to use in the lower end of the rev range.

    The sacrifice is that when we tune an engine to make power at very high rpm, the flow through the engine at low rpm will be highly inefficient, resulting in an engine which is gutless at low rpm, fuel-inefficient and polluting at low rpm and in extreme cases won't even idle properly.

    At low rpm, a highly tuned engine won't have enough gas velocity through the intake and exhaust to keep the gas's momentum up, and both intake and exhaust valves will be open at the same time, which at low rpm allows plenty of fresh air+fuel mixture to go straight through the cylinder into the exhaust (and exhaust gasses will try to go back into the intake too!).

    The sacrifice for detuning an engine to achieve peak torque at lower rpm is that high rpm operation becomes inefficient and sacrifices peak power. Gas flow will be fast enough to maintain momentum at low rpm, but at high rpm the gas flow will be so fast that the boundary layer expands and chokes the flow off.

    Note again that the tuned and detuned engines probably still make the same peak torque figure; it just occurs at different points in the rev range and so changes where the engine best makes its power.
  3. Not true. You can get more torque on longer stroke engines, but there are limits and there are lots of other governing factors.

    Spots is right, but the single biggest factor as to where a motor produces it's peak power in the rev range is camshaft profile. keep the valves open longer and an engine will produce more power, but higher in the rev range.

    All the other things Spots has mentioned can help spread that power around, but the camshaft is the ultimate tuning factor after capacity/configuration.
  4. Wow. Thanks for that I learnt heaps.
  5. Dito, great info, thanks :grin: