monty man wrote:hello, thanks for the reply,
i thought that base ign timing was 10 before TDC or -10 and if you want to run higher octain fuel then you need more time for it to ignight as it's harder to burn therefor needing more retard e.g. 15 befor TDC or -15.
You are right in that a higher octane number will allow you to run more ignition advance before you run into detonation but this will only be of benefit to you if you are having to retard the ignition to avoid detonation in the first place.
The ignition is slightly retarded in a tuned motor because the flame speed has increased and therefore the mixture can be lit later. The other reason is to get you out of trouble because the compression ratio has been pushed up too high for the fuel available, what happens is that the cylinder pressure goes too high just after the mixture has been lit, this causes the still-to-be-burnt part of the mixture to sponatiuosly ignite. This flame front colides with the one started by the spark plug and the result is a hole in your piston. (This differs from pre-ignition where you don't have flame fronts coliding, the whole mixture in ignited by a hot spot in the combustion chamber).
This so called cure for too much compression results in a engine that is not as nice to drive as one with the correct compression for the fuel being used and the right amount of ignition advance.
Your 99 RON fuel will allow a higher compression, it will allow more ignition advance before detonation occurs but unless your mixture is still burning ATDC it won't do any good. Having said all that maybe most cars run slightly retarded from optimum as a safety feature.
Also too much advance at idle might cause the motor to hesitate when you floor it due to the vac advance kicking in.
The proper way to sort this out is to find out how much advance your motor needs at peak RPM (and possibly peak torque) then modify the dizzy so that you get this setting whilst not over advancing the ignition at idle. (Infact you would do this every 500 rpm).
This is all very expensive and time consuming dyno work and will always be a compromise unless you have a mappable system. I have not so on my Rover lump have just messed about with the bob-weights and springs, this is a far cry from what is possible!
By all means have a go using seat-of-the-pants methods, just don't push the max advance too far!
One final thing...running a motor on fuel with a very high octane rating when the motor does not require it may cause you to lose power, the fuel is too stable to burn rapidly!
Regards,
Pete
