Spark Timing Myths Debunked
A widely-held myth is that maximum advance always means maximum power. Here's what's wrong with this thinking:
spark plug ignites the mixture and the fire starts burning. The speed of this
flame front depends on the mixture, this means
how many air and fuel molecules are packed together in the combustion chamber.
The closer they are packed together in the same volume, the easier it is for
the fire to jump from one set of molecules to the other. The burning speed is
also dependent on the air-fuel-ratio. At about 12.5 to 13 air-fuel-ratio the
mixture burns fastest. A leaner mixture than that burns slower. A richer
mixture also burns slower. That's why the maximum power mixture is at the
fastest burn speed. It takes some time for this flame front to consume all the
fuel in the combustion chamber. As it burns, the pressure and temperature in
the cylinder increases. This pressure peaks at some point after TDC. Many
experiments have shown that the optimum position for this pressure peak is
about 15 to 20 degrees after TDC. The exact location of the optimum pressure
peak is actually independent of engine load or RPM, but dependent on engine
all the mixture is burned before about 70 deg ATDC. But because the mixture
density and AFR in the engine change all the time, the fire has to be ignited
just at the right time to get the peak pressure at the optimal point. As the
engine speed increases, you need to ignite the mixture in the combustion
chamber earlier because there is less time between spark and optimum peak
pressure angle. If the mixture density is changed due to for example boost or
higher compression ratio, the spark has to be ignited later to hit the same
If the mixture is ignited to early, the piston is still moving up towards TDC as
the pressure from the burning mixture builds. This has several effects:
- The pressure buildup before TDC tries to turn
the engine backward, costing power.
- The point where the pressure in the cylinder
peaks is much closer to TDC, with the result of less mechanical leverage
on the crankshaft (less power) and also causes MUCH higher pressure peaks
and temperatures, leading to knock.
people with aftermarket turbos don't change the spark
advance very much, believing that earlier spark creates more power. To combat
knock they make the mixture richer. All that happens really then is that the
mixture burns slower and therefore hits the peak pressure closer to the right
point. This of course reaffirms the belief that the richer mixture creates more
power. In reality the flame front speed was adjusted to get the right peak
pressure point. The same result (with more power, less emissions and less fuel
consumption) could be achieved by leaving the mixture at the leaner optimum and
retarding the ignition more instead.
charging or increasing the compression ratio changes the mixture density (more
air and fuel molecules are packed together). This increases the peak pressure
and temperature. The pressure and temperature can get so high that the
remaining unburned mixture ignites by itself at the hottest part in the
combustion chamber. This self-ignition happens explosively and is called
'knock'. All engines knock somewhat. If there is very little unburned mixture
remaining when it self-ignites, the explosion of that small amount does not
cause any problems because it can't create a large, sharp pressure peak.
Igniting the mixture later (retarding) causes the peak pressure to be much
lower and cures the knock.
The advances in power of modern engines, despite the lower
quality of gasoline today, comes partially from improvements in
combustion chamber and spark plug location. Modern engines are optimized so that the flame front has the least
distance to travel and consumes the mixture as fast as possible. An already
burned mixture can no longer explode and therefore higher compression ratios
are possible with lower octane fuel. Some race or high performance engines
actually have 2 or three spark plugs to ignite the mixture from multiple
points. This is done so that the actual burn time is faster with multiple flame
fronts. Again, this is to consume the mixture faster without giving it a chance
octane fuel is more resistant to self-ignition. It takes a higher temperature
and pressure to cause it to burn by itself. That's why race fuels are used for
engines with high compression or boost. Lead additives have been used, and are
still used to raise the self-ignition threshhold of
gasoline, but lead is toxic and therefore no longer used for pump-gas. Of
course a blown engine is toxic to your wallet.
Klaus Allmendinger is the VP of Engineering for
Innovate Motorsports, a division of Innovate!
Technology, Inc. Innovate develops
digital tools for tuning internal-combustion engines.