Scooterotica

Please bear with me on this.
The spark need to be timed to ignite the fuel BTDC x degrees.
This can be done at whatever setting you decide and whatever book you are following, but we will use 18 degrees BTDC as an example. So thats STATIC timing (pick your point and its that all the time)
So we have various systems (black boxes) that will automatically adjust this point as the revs get higher. As the revs get higher is it advancing or retarding ? Is it getting closer to TDC or the other way Further away from TDC.
Its my understanding that once the fuel is ignited it takes a time to burn. So we light it up before TDC. as the revs increase do we want the timing to be nearer TDC eg 16DBTDC is this retarding or advancing coz Im confused.
I've just read something ont tinternet. So it must be true....that says the burn takes the same time but because revs increase the spark should be further away from TDC to give it time to burn completely.
Help

The way I see it with advance/ retard ignitions, is that it sits at BTDC at say 26 degrees with the tickover, then retards back anti clockwise to something like 19 degrees or whatever when it's revved. An adult will be along shortly with a better explanation.

Let me offer something to confuse everyone even more under the pretence that I know what I'm talking about!

Timing measured in degrees refers to the movement in degrees of the crank web before top dead centre when a spark is generated to ignite the fuel mix.So if the figure is higher, ie 23deg then the spark is generated earlier than when to figure is say 16deg.In this instance the ignition is more advanced than when it is set at 16deg and consequently the fuel mixture is burning longer before it is allowed to leave the cylinder via the exhaust port.In this situation the piston crown gets hotter and the fuel mixture has less of a cooling effect,this might be fine when the engine is igniting 4000 times a minute at 4000 rpm but if the rpm is raised to 7000 then there is more heat generated and more chance of your piston melting into small bits until it holes and is completely fcuk ed.This is why ignition retard boxes are used so that the motor can run to a higher rpm and still run at a safe ignition setting because the box regards the timing at a predetermined rate.Hope this helps,if not an expert will be along soon.lol


Cheers Jeff

Lets say your engine has been happy at 18 degrees static for years and you now decide to fit a retard unit. The biggest advantage is being able to run more advance ie firing earlier ,to raise the power and engine acceleration , then retard back to your original static setting once the revs rise. I guess fuel consumption goes down as the engine is more efficient at burning it which explains why the idle, needle and atomiser often need weakening to compensate.

....best thing I did was to get rid of my Augusto.....

The advancing it away from TDC is confusing as is Retarding it But I understand the basics. It was the fuel burn this article implied that the burn took the same time, so if its reving higher you must start the burn earlier. It was a car forum but it didn't make sense.
I was thinking if the timing was set at 23 with no blackbox at what point do you get ignition before it on its way back down again.
Im running an M Tech and an Augusto on my other bike but have holed out at full chat more than once. Its why I went down the EGT and box of tricks route in the 1st place.
Both bikes are great to start and very smooth through the power. It has taken some of the kick in the pants from my Imola set up and smoothed out the power band from the JL

PS I know no one will use 23BTDC on unleaded petrol, just a for instance.

how does a ign retarder affect combustion as revs rise?

Muppet wrote:how does a ign retarder affect combustion as revs rise?

It doesn't affect the combustion, the fuel mixture will take the same time to burn. Retarding the ignition means that more of the combustion takes place when the piston has gone past tdc and is on its way back down. This moves the heat away from the combustion chamber and towards the exhaust. The effect can be seen on EGT gauge and is one instance where a rise in the exhaust gas temperature is a good thing as it means a cooler piston......I suspect your's was a rhetorical question though Muppet

my understanding is that the theory is different between 2 stroke and 4 strokes..

the basic theory is it takes time for the spark to ignite the fuel, so in order to allow time for the ignition to get going it makes sense to fire the spark before tdc so that the burn reaches its peak just at, or immediately after TDC, giving maximum power at the right spot, kind of if you're pedalling a bike, the best place to shove on the pedal is just after the top and not before or halfway down.

in your old car, as the revs rise the distributor moves the spark so it happens further before TDC (advancing) the theory being that as the engine gets faster you need to start the fuel burn earlier so that it still reaches its peak at TDC. However in modern cars the whole thing is computerised so the ECU works out where the spark should be based on a whole lot of engine and atmospheric data and fires it at the point its calculates as best.

In two strokes the design of the combustion chamber is completely different, no valves, no long compression stroke, a squish band.. this means cryptically as the engine gets faster the way the ports feed fuel, the way the expansion works and the effect the squish has on the mix of fuel and air means it burns a hell of a lot quicker, thus you need to ignite it closer to TDC or it will reach its peak burn too early causing strain on the engine, overheating and lost power. thus in a two stroke the spark has to happen later as the engine gets faster (retarding)..

this means that on a tuned scooter using a static timing method the advance is reduced so that it is working best at peak power, which is when the engine is most stressed, this has the disadvantage that starting and poodling around the engine is less efficient and doesn't run as smooth, but its a compromise between this and blowing the engine at full power.

the various ignition boxes out there seek to electronically alter the timing of the spark to bridge the compromise.. thus an agusto is a simple device that just retards the ignition as the engine gets faster, so you set the ignition at 23 degrees or whatever to start, and once the engine gets to a predetermined speed starts to retard back gradually until say 15 degrees when the engine is going full beans. the more sophisticated boxes do the same thing, but on a more complicated pattern than a simple straight line to match the characteristics of your motor.. A little bit like the ECU in the car above, but again much simpler as they cant measure atmospheric pressures, knock, inlet temperature, exhaust lambda, etc etc..

I hope that helps explain, experts feel free to correct me and hope I'm not coming across as a smart arse

Set my Agusto at 21.5 degrees btdc and at 4000 rpm it retards to 17 degrees and at 6000 rpm it retards to 15 degrees. Beyond tthat it goes to about 14 degrees.
Did I notice any difference ?.....not really but have peace of mind