argclh6670 said:
It used to run fine with NO ping before the topend. Is it possible that the new topend added enough compression to cause the detonation?
Of course. You raised the combustion chamber working pressure and the working temperature along with it.
Jetting can have an influence on knock. It's important to remember that the air/fuel ratio that produces the best power also tends to have the best chance of knocking. Running on the rich side of best power on a two-stroke tends to cool the piston crown and minimize knock.
A couple of other things to consider:
- it will be harder to jet for good throttle response with alcohol based fuel because ethanol has a fixed boiling point around 173 degrees f and a very high latent heat of vaporization, along with requiring a different A/F ratio then the rest of the fuel. In short it acts very different then pure gasoline and screws up the fuels distillation curve which can cause jetting issues.
- the octane distribution of ethanol based fuels will be very different than pure gasoline. As a result the knock sensitivity can be very erratic from load to load of fuel. It's best to tune on the safe side till you get used to running with ethanol. Two-stroke riders who switch to ethanol based fuels without other changes very often run into knock and detonation issues they never had before.
A simple list on ways to lower the octane requirement would include:
Lower the combustion chamber temperature
- Overly rich mixtures will tend to do this to a point
- Efficient cooling systems will help this
Lower the cylinder pressure
- lower mechanical (static) compression ratio (CR)
- advancing the exhaust timing will lower the dynamic (CR) and bleed off some cylinder pressure
Speed up the combustion process to outrun the temperature and pressure rise
- correct squish band design will help here
- higher density charge from increased cylinder filling or increasing trapped charge purity through proper pipe tuning
- higher rpm speeds up the process
Decrease the amount of oil in the pre-mix. Whether or not this works will depend on the oil
Decrease the amount of time available to heat the charge
- retarding the ignition timing will decrease the time available to raise the temperature
- higher rpm speeds limits the time available to overheat the charge
Your best bet might be to start with some basic investigation and tuning to determine a baseline to work from.
Here are a few things you can start with:
1) Look at the carbon pattern on the piston crown as well as the underside of the piston along with the cylinder head to get an idea how well the engine is scavenging , how well the cylinder is filling, and how hot the engine runs on average. If you can keep a higher density charge in the chamber you'll speed the combustion process, which will go a long way towards lowering the octane requirement.
2) Get an accurate squish clearance, squish angle, piston crown angle, and squish area measurement. Often times it's as simple as accounting for production tolerances, other times the factory just gets the design wrong for your type of riding.
3) Document the specific patterns that cause the engine to ping, and the specific brands of pump gas and oil that cause the most problems. Working around a detonation problem can be as simple as switching brands of gas, or buying from a different location. Not all brands of 93 octane pump fuel are the same, plus the 93 octane rating at the pump is not always going to be what comes flowing out of the ground tank. This is one of the toughest parts of the exercise. You have a random variable in the equation that is impossible to predict, so you have to err on the ultra conservative side, which leads to BAD performance. This random variable will also have a significant effect on jetting, especially on-off throttle transitions , which tends to be the major culprit in these types of cases.
4) Determine the running water temperature to verify cooling system efficiency.
5) Check silencer efficiency to minimize back pressure.
6) Measure static compression ratio from powervalve(PV) full open position,and determine PV open rate as compared to throttle position, to see if there is a correlation between part throttle pinging and PV position. Measure blowdown timing (exhaust to transfer open).
Hopefully this will provide some help.