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D E T O N A T I O N

Knock Knock -- Who's There?

Do you know when your engine is calling for 'HELP'?

Of all the worlds big problems, the pinging sounds that are coming from your engine when it's under serious load, are probably not the foremost in your mind. World starvation, are the Chinese smart enough to control the bomb, is Iraq's "Saddam Insane" for real, are all more important.

If your engine requires a steady diet of high octane fuel, but none is available, there are some possible solutions to this pressing problem.

First, there are two reasons for the sound you are hearing, pre-ignition and detonation. Pre-ignition means what the name implies, ignition starts too soon, even before the spark plug fires. Normally this is caused by a piece of carbon or a sharp edge object in the combustion chamber that ignites like a diesel engine glow plug. Detonation occurs after the spark plug fires. While the flame front travels across the combustion chamber, if the pressure and temperature suddenly spike up, the scene is set for a possible spontaneous combustion from the chamber edges. This ignites a second front back towards the first. It can be related to hitting the piston with a hammer instead of the normal smooth burning and pushing down on the piston. Either one makes the similar sound of pinging or knocking that you hear under high load conditions.

Since I am writing this for 2-cycle people, I'll try not to relate to four cycle possibilities, although they do overlap on these two types of engines.

For stock engines, start off by setting carb and timing adjustments to stock factory figures. If you bought the vehicle used its very possible these adjustments were modified by the previous owner. Setting them back to stock could solve all your pinging problem easily.

For modified engines, I suggest you check these adjustments carefully and make note of the numbers for future reference. The first thing to do is make sure you are running the required octane gas to match the compression you have in your engine, assuming the engine is in good mechanical condition. If this is right, then I suggest you retard the ignition timing, provided it can be adjusted. By retarding the timing you lower the combustion pressure and temperature. Only retard the timing until the knocking stops, as too much retard will kill the torque, and hp will drop. If you are lucky, this will work with only a little change.

Now lets say you tried the above fixes on your modified engine but it still sounds like marbles on a tin roof. The head(s) will have to come off to measure the trapped volume, which determines the compression ratio. A possibility that you may find a burr (hot spot) or high carbon deposits is real, and should be addressed first before modifying anything. Of course not finding this easy fix and you determine that the ratio is too high, you have some choices to consider in how to lower the compression ratio. A good way to determine this is with my T.S.R. 'SQUISH' velocity program and my 'COMPRESS' program. Two very informative tools that can help you to set up the compression ratio numbers and the distance from piston to the cylinder head. You normally will have to remove metal from the cylinder head(s) and quite possibly set the squish band to a different angle and depth. The many T.S.R. Cylinder Head Design software programs can make your life much easier doing this.

Luckily for you most stock 2-strokes only need a good tune-up or decarbonized by a qualified mechanic. If you need to work over the head(s) you should check out my T.S.R. Software. Once this is fixed you can get on with fixing something else on your hot-rod engine. Yeah, do that, its more fun anyway.

This material is for personal use only. Republication and redissemination, including posting to newsgroups, is expressly prohibited without the prior written consent of TSR Software.

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