Ignition Timing: How It Works With Nitrous
Here at the office we frequently get calls about timing recommendations for various vehicles and configurations. Timing can be very confusing and frustrating for a lot of gearheads, so we wanted to go through and explain what timing is, how it affects performance, and why we need to adjust timing. This is the scientific definition of ignition timing:
Ignition Timing is the process of setting the angle relative to piston position and crankshaft angular velocity that a spark will occur in the combustion chamber near the end of the compression stroke.
To break it down in simpler terms, timing is setting where you want the spark to occur relative to piston position and speed. Timing is referred to in degrees of angle before top dead center ( BTDC). To get the most power and efficiency out of an engine, the timing will need to be advanced. This is because from the moment that the spark occurs, it takes time for the fuel to ignite completely and begin expansion. When the timing is advanced, the spark will actually take place during the compression stroke before the combustion chamber reaches its smallest size, giving the fuel and oxygen time to begin expansion and push the piston down with greater force. If the spark occurs too early in the compressions stroke, it will expand against the piston that is still moving towards the end of the compression stroke, causing detonation (also known as pinging, or knocking). If the spark is too delayed (retarded), it will cause the maximum pressure of the cylinder to peak after the piston has traveled too far down the cylinder. If the spark occurs after top dead center (ATDC), you will lose significant power due to the gases not having enough time to expand and force the piston down.
Pictured above is the result of timing that is too advanced. The piston on the left failed due to detonation occurring over a period of time. With detonation, engine failure may not occur right away, rather taking numerous passes for the failure to occur. The spark plug pictured above shows signs of detonation occurring in that cylinder. We can tell by the small specks on the porcelain part of the plug. This is commonly called ‘peppering’, and is actually small pieces of aluminum from the piston or head.
WHY RETARD TIMING WITH NITROUS?
One of the key points of tuning nitrous is setting the timing. When using nitrous or other power adders you have to ‘pull’ or retard the timing because peak cylinder pressure will occur earlier in the ignition cycle. This is because of added oxygen and fuel, which causes the mixture to burn at a much higher rate than a naturally aspirated engine; and peak cylinder pressure will occur much earlier in the cycle. When pulling timing we are trying to compensate for the accelerated burn of the mixture and we are trying to make the peak cylinder pressure occur at the same point it did while running naturally aspirated. If peak cylinder pressure occurs too early, it will cause detonation and parts failure. If peak cylinder pressure occurs too late in the cycle, it will cause lost power due to peak cylinder pressure occurring after the piston has already started going back down the cylinder.
Retarding your ignition timing with nitrous not only will help keep your engine safe from detonation and parts failure due to heat, but it will also help you get as much power as you can out of your nitrous system. Timing can be retarded multiple ways on almost all vehicles- through a tune or timing retard box, or by changing your ignition settings on your ignition control box. The ‘general rule of thumb’ for retarding your ignition for nitrous is pulling (or retarding) 2* for every 50 horsepower worth of nitrous you intend on spraying. However, that is not always correct as many engines have different timing requirements due to a variety of factors. The best way to find out if your timing is where it needs to be, or if it needs to be adjusted is to read the spark from a single pass.