Special X wrote:Why fit a device that simply allows you to over advance the ignition timing at low revs then takes it back to (something aproximating) normal at high revs. Doing this makes the assumption that pre ignition at low revs is acceptable . . . . . it isn't. Set your timing at 18 BTDC on an Avanti and use static ignition and your power curve will be as dictated by the porting, and the pipe.
Or you could fit a device that slightly advances as the revs go up, in line with scientific principles, and 2 stroke manufacturers research (Yamaha, Suzuki, Kawasaki etc)
It's not that simple as it is not the case that static timing is ever 'correct' at one setting but, rather, it is a compromise of burn rate across the rev range that the bike operates at and this is a trade off that sets limitations for other aspects of the motor's specification , e.g. Compression. For no other changes, increasing a motor's secondary compression will give much improved pick-up and mid range (can limit rev on though as there is more opposing force that stops the motor from spinning so easily at higher revs) BUT we all know that increasing comp without reducing ignition advance increases engine temps and can cause piston holing
However, retarding a static ignition will then cause a motor to lose much of the low rev power that was gained by increasing the comp ratio and you are then back in the compromise situation.
Having a variable ignition timing allows you to get round the compromise above but can also be used to provide benefits in other ways too. If you've ever played with tuned barrels or rev by pipes, you normally find that you gain power at higher revs but commonly lose at least the same power at lower revs. Great fun when you are screaming along a flat road at high revs but no fun when you go 2 up with luggage or try crossing the Alps. In order to reach the higher revs that the tuned motor runs best at, and to stay in the power under most conditions, you commonly then need to fit lower gearing at run at constantly higher revs. This is not simply a case of over tuning a motor, even Innocent's own motor development saw ratios get lower (higher number) and static firing points retard as revs increased, necessary to allow the higher revving motors stay cool and reliable (e.g. 200s dropping from 4.4 to 4.8 to 5.2, with static ign reducing from 23 to 21). Had a decent auto advance/retard been available to Innocenti at the time the GP200 would have had more torque at low revs and been able to pull the longer SX or even GT low gears, right up through the box, then benefitting from the bigger carb and freer flowing later spec pipes. Nearly all modern bike manufacturers have made the switch to auto advance/retard ignitions, especially on smaller cc motors, where starting at an advanced firing point results in much increased torque/power, making a small cc motor out perform bigger motors that are limited by static ignitions.
Whilst technology has allowed us to realise variable ignitions, there are many other elements of motor design that could return real performance benefits whilst remaining reliable, which would introduce reliability issues if a static modification was made (piston holing, etc.). Variable gearing (automatics punch way above their cc on the race track), variable compression, variable exhaust port timing (YPVS , MB's variable development, etc.), methods of increasing inlet timing without the limitation of extending piston port durations (reed valves, rotary, etc.), are examples of how we can benefit from circumventing the limitations imposed by static features. A really good example of this is the variable length expansion pipe that allowed the pipe's length to be hydraulically changed from launch to flat out when sprinting, resulting in greatly reduced times and massively increased top speeds! (
http://blog.motorcycle.com/2009/03/30/m ... r-auction/)
Being able to vary engine elements in use is all about negating the limitations imposed by static settings. Many of these variable elements have been standard figment on production motorbikes and scooters for decades. In the same way that an Li125 Special runs rings around a standard Li125 or LD125, development of engines can be a very good thing
There is a lot more that could be said about the benefits of variable ignition kits, google it and you'll get far better info than I can provide, and give you an understanding of why most modern bikes have made this switch to benefit from the increased performance AND improved reliability that auto advanced/retard ignitions offer.
Adam
Ps - do note that fitting the wrong auto advance/retard kit, or setting it up incorrectly, may result in loss of performance, poor starting, kick-back, or reliability issues... But so will getting static ignitions set incorrectly!