Tech Dept.

Tech # 6

Ignition Timing

Ignition timing is straight forward and by the book when the engine is stock and the appropriate fuel is used. When your engine is stock you should not alter the ignition timing from what the factory specs call for. However, how many British cars running around today are stock? ANY alteration you make to your engine, intake, exhaust or fuel requires a different ignition timing. Worse yet, a different advance curve is also required.  The problem is very complex, especially when so many other factors affect what is the best ignition timing for a given engine. For example, Here are some basic factors that control what ignition timing can be used.

1. Temperature. -- High outside air temp. and/or high engine temp. restrict how far advanced timing can be.

2. Engine load -- High engine loads, especially at low RPM restrict how far advanced timing can be.

3. Octane rating of fuel -- The Octane rating of a fuel restricts how far advanced timing can be. (Low octane rating requires a more retarded ignition timing and a high Octane rating allows a more advanced timing to be used, keeping in mind that compression ratio also has control of the available Octane rating that can be used.)

4. Fuel mixture can effect what ignition timing can be used.

5. Spark plug heat range can effect what ignition timing can be used.

6. RPM effects what ignition timing can be used.

7. A mixture of driving conditions, driving habits, combustion chamber design, engine condition, intake and exhaust system design all can effect what ignition timing can be used.

8. The BIG restrictor of ignition timing is "Abnormal Combustion" (knock, detonation/preignition)

I know what you are thinking, "How on earth can I use all of that to figure what ignition timing to use?" To get the best timing through the whole range of driving conditions it would take a lot of equipment and know-how.  However, armed with a little know-how you can get in the ball park of a good ignition timing for your particular car.

First we will cover some basic facts. Low Octane fuel burns fast and not so smoothly. High Octane fuel burns slower and much smother. High compression and high combustion chamber pressure speeds up the combustion process.  Lean mixtures burn hotter than rich mixtures. Octane rating is a anti knock rating, nothing more nothing less. The combustion process should be a controlled burn not an explosion.

In general you could say that the most power you can get from any engine as far as ignition timing goes is that timing that is just short of causing "Abnormal combustion".  There lays our tool. If you are careful, it is an inexpensive tool. If you are not careful it will be a VERY expensive tool.

Lets look at how we can use this tool and even how modern engines are using it as a tool on the street and racetrack today. Abnormal combustion is basically an uncontrolled explosion in the combustion chamber caused by any one or combination of more than one of the items listed above as restrictors of ignition timing. In normal combustion the spark ignites the compressed fuel/air mixture and a smooth burn travels through the combustion chamber and building combustion chamber pressure as it goes. This flame travels through the chamber by the time the crankshaft has moved about 15 to 30 degrees after top dead center (ATDC). In Abnormal combustion the air, something in the combustion chamber or even the whole chamber is too hot for the fuel or it is compressed too high which itself causes the temp to raise too high, causes the fuel to explode. This explosion produces extremely high temperature in a localized spot and causes a pressure spike so high and quick that it is like a hammer strike. This extremely high temp/pressure spike can cause extreme damage too. The weakest point in a chamber gives way first. This hammer like strike hits the top of the piston and attempts to go down between the piston and cylinder wall and hits the top ring. which slams down on the ring land below it, braking it down against the second ring. At the same time, the extreme localized heat starts to melt the aluminum piston. On true hemi engines (spark plug in the center of the chamber) The extreme heat can first weaken the piston closer to the center and the pressure spike can knock a hole in the center of the piston. In a more wedge shaped chamber (plug on the side of the chamber) the damage usually in done opposite the plug. Abnormal combustion damage is easy to ID. The underside of the piston crown is blackened, the ring land just below the top ring on the opposite side from the plug is broken in a downward pattern and the land above the top ring will be melted and burned away opposite the plug. There is also damage to the top rod bearing as the hammer effect usually beats out the rod bearing.

The tool I spoke of is the sound it makes. It is an unmistakable rattling sound, usually occurring under acceleration from low RPM. It is misnamed "valve rattle". It is a pinging noise like marbles shaken in a tin can. It is harder to detect at high RPM. High RPM valve float can give you a similar symptom. Engine is pulling strong up to a certain RPM then abruptly drops off on it's pulling and usually has associated change in engine sound. It may or may not make a metallic sound. Valve float on a hemi is of course, results in the end of those valves and maybe that engine but a valve that is in line with the piston and flat top pistons can get away with some valve float. Not much though. With high RPM detonation the power drops off abruptly and damage can start immediately. A cast piston with thin ring lands is very susceptible to damage where a piston with wide lands and/or a forged piston has a greater chance of surviving some detonation.

If you plan to do any high RPM ignition tuning, you should consider purchasing a knock sensor system. They use a knock sensor attached to the head or block and have an LED display to tell you that you are getting Detonation. This can give you a good indication even if you have a loud exhaust. This allows you to alter your advance curve. Even with a well tuned curve you can get detonation by bad driving habits. Like puttering along in a higher gear and applying full throttle all of a sudden. Lugging an engine can cause detonation even if the advance is correct and the curve is correct. Attempting to alter an advance curve to correct poor driving habits is a dead end road. Some car manufactures use a knock sensor to control ignition timing and even fuel mixture. While another uses it to control the timing of each individual cylinder. 

You can get close without that detonation sensor just by listening. Here is how I do it. I first warm the engine to operating temp and keep it there for a time by driving somewhere. The reason for this is that I want the oil temp up also, which takes longer than coolant temp. I then connect up a timing light and loosen the distributor so it can be moved by hand. I make a quick throttle open from idle then move the dist toward advance (against the direction of rotor rotation) then I repeat the quick throttle open. I quickly arrive at the detonation and only do it one time as I have seen the damage it can do. Now I check the timing to see what it is. At this point you have to keep in mind if you have advance or retard units on the distributor. I write down what the timing is now. This is my "Too Far" point. On this engine I have a vacuum advance unit and a internal centrifugal advance. I remove and plug the vacuum line and repeat the process. If the detonation is at the same timing then the vacuum advance is not a factor at this RPM. I found on most of the older Lucas distributors you can remove the felt plug under the rotor and remove the screw and put a rubber "O" ring in under the screw, it will lock the centrifugal advance mechanism. now I can try it again to see what the timing is at detonation. This now gives me a picture of who is effecting what. This is just at low RPM but it is a starting point without any special tools. Aftermarket detonation sensor systems are not expensive and I advise anyone making any engine modifications to get one. Just knowing the facts listed above can get you in the ball park.

You can make a chart of what your ignition advance curve is by first disconnecting and plugging the vacuum line. On your front pulley or on the timing cover is a timing mark for TDC (0 deg.), 5 deg., 10 deg. etc. measure the distance between the 0 deg and the 10 deg mark. With touch-up paint or what ever you have to mark with, put in a 20 deg, 30 deg and a 40 deg mark. With a timing light connected check the timing at idle, at 1000 RPM, 2000, 3000, 4000 and 5000 (providing your engine is in a good enough condition to go up to 5 K.) If you will put these figures on a piece of graph  paper and you will now have the curve of the centrifugal advance. To get the curve of the vacuum advance unit you will need a "Mighty Vac". This test can be done at idle. First check the amount of vacuum your engine produces at idle. If your engine has a vacuum advance unit note that there is no vacuum available to the unit at idle. This is because the vacuum connected to an advance unit only is "Ported Vacuum" meaning it only supplies vacuum off idle. If you have a vacuum retard unit it does have manifold vacuum applied all the time. (some later engines used delay valves in the vacuum line for emission purposes.)  With the Mighty Vac on the unit, pump up 5 inches of vacuum and check the timing and log it. Then pump 10 inches and log the timing. (check your specs on the centrifugal advance to see at what RPM you start getting an advance. This is because as you advance the timing at idle with the Mighty Vac, the RPM will climb and you don't want to see centrifugal advance acting on your vacuum test. It is no use pumping up over 18 to 20 inches of vacuum as your engine will probably never achieve that. Remember that high vacuum is at idle and as you open the throttle it will go down to about 5 inches at full throttle. 

An example of setting timing by known facts and very little equipment, In 1980 I received a grant from the Dept of Energy to design and construct a farm size alcohol still and convert a car engine to run on straight grain alcohol. Ethanol has an assigned Octane rating of 110 Octane. (anything over 100 Octane is a mathematical guestamation). Alcohol is a slower burning fuel than gasoline so I would normally need a more advanced timing. However, I increased the compression ratio which speeded up combustion which in turn made a more retarded timing needed. The end result was that the exact same timing was correct for the alcohol engine as it was for the gasoline engine. 

Another form of abnormal combustion is "Preignition". This just means that the explosion occurs before the spark ignites the fuel. The causes can be the same as above. Timing can not always correct Detonation/Preignition. for example, a highly carboned up chamber, a piece of a steel head gasket protruding into the chamber, a sharp edge of the head or the exhaust valve can act like glow plug igniting the fuel before the plug does or a wrong heat range of plug will do the same. Oil burning is a common cause of high RPM detonation. Oil has a very low Octane rating which detonates at high RPM.

If I choose to tune the total advance on a modified engine, I always keep my test runs short and quick as damage can occur very quickly at high RPM. I've seen many modified engines destroyed in the tuning process. I always start testing with the stock ignition settings then creep very slowly toward advanced settings. Before I try to change ignition timing I make sure that I have a slightly rich mixture. A lean mixture runs chamber temp up and can itself cause detonation. Keep in mind that horse power increases with mixtures richer than stoichimetric (ideal mixture) and decreases with a lean mixture. Most engines are set up at the factory with a slightly rich mixture at idle, a slightly lean mixture at midrange and a rich mixture at full throttle. Most gasolines get peak horse power at 10% richer mixture than stoichimetric with some claims of as high as 40% richer mixtures.

Most cars had very poor performance from the factory in 1968 and 1969 because they had to try to meet emission standards and were not prepared. With a little work, these cars can be upgraded to burn cleaner and boost performance. However, you must do your homework first. Don't just start removing emission control devices, thinking you are going to get more power. You will not gain anything unless you upgrade everything to the later model engine. This way you will gain performance and cleaner emissions.

Back