Techno’s
“Know your car” Series # 10
Mazda MX-5
What Happens When the
Timing Is Advanced
It has long been held, and demonstrated
at the Club's Dyno Days, that advancing the timing to
13 to 14 degrees before top dead centre (BTDC) is a cheap way of achieving
lower rpm performance in the Mazda MX-5 1.6 litre cars.
So what happens when the timing is
advanced, and why is it so?
Let's start our journey of discovery with a few basic principles.
1. Power is determined by a number of factors and the compression ratio is one
of the prime factors. Higher the compression leads to more force generated when
the air/fuel mix is ignited.
2. On a stock NA model MX-5 the "timing point" is set at 10 degrees
before top dead centre (BTDC) when at idle and there is no load.
3. Top dead centre refers to the position of the piston. The piston is at its
highest point when at TDC. The crankshaft angle is 0 degrees and the
crank/connecting rod angle is 180 degrees. Imagine a fully up stretched arm
with your elbow being the crank/connecting rod pin and your fist being the
piston.
4. There are 360 degrees in one rotation of the crankshaft. When you change the
timing from 10 degrees BTDC to 14 degrees BTDC you actually move the point
where the spark is fired from 350 degrees of rotation to 346 degrees of
rotation of the crank. Imagine the end of the crank being a clock dial with 12
o'clock being 0 degrees. At 11.58 and 20 seconds the ignition process starts at
10 degrees BTDC. Change the timing to 14 degrees BTDC and the ignition process
starts at 11.57
and 20 seconds.
5. When the crank reaches the timing point an electric current is fired off to
the spark plug. The spark ignites the air and fuel mixture. This ignition
starts at the spark plug and flame fronts travel up to the top of the chamber,
out to the sides of the chamber and down to the top of the piston. When the
flame front reaches the top of the piston it pushes it down, the crank rotates
and there is movement in the drive train.
6. Whereas the engine revs can vary between 900 and 7,000 rpm; there are a
constants for the speed at which the current passes to the spark plug, the
ignition process and the burn rate of the fuel.
7. There is a point in the piston's downward movement where the flame front
impacts on piston. For this example we assume that this point is when the crank
has turned to be 15 degrees after TDC. On a 1.6 litre engine the location of
the piston is now 1.865mm below its TDC position on its downward stroke. To
check this use the formula {HT = (r + c) - (r cos (a)) - SQRT(c^2 - (r sin (a))^2)} where HT = position location ATDC, r = stroke/2, c =
con rod length, a = radians of crank angle. For a 1.6 litre the stroke is
83.6mm and the con rod is 132.9 (+/-0.05) mm
8. At 900 rpm the crank rotates one degree in 0.18519 milliseconds.
The crank turns 360 degrees in one rpm. Therefore 360 degrees X 900 rpm =
324,000 degrees per minute = 5,400 degrees per second = 1/5400 of a second per
degree = 0.00018519 seconds or 0.18519 milliseconds per degree.
9. By advancing the timing by 4 degrees you start the ignition process 0.741
(0.18519 X 4) milliseconds sooner at 900-rpm (0.111 milliseconds at 6,000 rpm).
10. At 900 rpm it takes 4.629 milliseconds for the crank to travel the 25
degrees from 10 degrees before TDC to 15 degrees after TDC. At 6,000 rpm this
movement takes just 0.694 milliseconds. Remembering that the flame front has a
constant burn rate then to have the flame front and piston converge when the
piston is just after TDC requires the combustion process to start earlier in
the cycle at higher rpm. Otherwise the piston would be lower down in its
location before the flame front reached it.
11. Therefore as the rpm increases the ECU changes the timing so that, in a
stock 1.6 litre, the advance increases through the rev range to a maximum of 36
BTDC at 5,500.
See Table 1 below for the
transition points on a normally set up Mazda MX-5 1.6 litre running standard 10
degrees BTDC timing
Table 1 Mazda MX-5 1.6 litre Timing Movements (Standard Set)
|
RPM |
500 |
1000 |
1500 |
2000 |
2500 |
3000 |
3500 |
4000 |
4500 |
5000 |
5500 |
6000 |
6500 |
7000 |
|
BTDC |
10 |
10 |
12 |
17 |
22 |
24 |
28 |
28 |
31 |
34 |
36 |
36 |
36 |
36 |
12. If you advance the timing at idle to
14 degrees BTDC then you increase the timing by 4 degrees across the rev range.
13. In our stock model we had the flame front meeting the piston when the crank
is at 15 degrees ATDC (when the piston is 1.865mm on its downward stroke).
Therefore by advancing the ignition process by 4 degrees the piston receives
the flame front 4 degrees sooner. Ie at 11 degrees ATDC.
The piston is located 1.00753 mm from TDC at 11 degrees ATDC.
14. When the combustion commences there is an expansion of the volume (remember
the formula for volume of gasses at various temperatures from our first year
physics classes). The rate of expansion is determined by the burn rate of the
fuel. In our model this is constant.
15. Combustion actually commences while the piston is on its upward journey.
The time it takes for the flame kernel at spark plug to expand to the top and
sides of the chamber permits the piston to pass top dead centre before it
receives the flame front. The expansion of the gasses during the period from
ignition point (BTDC) to flame front reception (ATDC) increases the volume in
the chamber and effectively creates higher compression.
16. Now we can see why it is that when the timing is advanced the higher
position of the piston at 11 degrees ATDC (compared to 15 degrees ATDC)
increases the effective compression and gives more power on the downward
stroke. That equals more torque. See rule number one above.
So, in summary, for those folk with NA model cars the advancing of the timing
creates a change in the position of the piston when it receives the flame
front.
Our dyno days show that there is a correlation between
advancing the timing and an increase in low-end torque. In general, each
1-degree of additional advance seems to bring the torque curve back by around
100 rpm. By advancing the timing by 4 degrees it has the effect of delivering
torque at 2,600 rpm that would normally be realised at 3,000 rpm. Of course, it
has also been noted that top end torque is adversely impacted in about the same
ratio.
The risk in advancing the timing is that the flame front (the increase in
pressure) from the burning air/fuel mix strikes the piston head before, at, or
too close to TDC to create the optimum forward rotation of the crank.
If the downward force reaches the top of the piston before the piston is able
to react with a downward motion then the force is directly to the crank, rather
than to the rotation (a bit like how landing on a stiff leg transmits the jar
to the hip).
The ignite and burn time for 98 RON fuel is longer
than 92 RON fuel. Lower RON fuels means the flame
front hits the piston top sooner than with a high-octane fuel. If the flame
front hits the piston too soon the crank angle will not be in the optimum angle
range to commence its downward stroke. The result can be pinging cause by
premature detonation. That is why higher-octane fuel can be good insurance if
the timing is advanced.
In hot weather when the air is warmer the amount of oxygen atoms in a given
volume of air will be less that in cold air. This leans out the air/fuel mix.
Once again, higher-octane fuel in the summer is good insurance. Similarly, in
lower altitudes the air is denser and the stoichiometry (ratio of fuel to air)
moves to lean so a higher octane fuel may be beneficial to avoid pinging. So if
you have advanced the timing and go for a mid summer drive down to the beach
you might like to take on a tank of 98 RON.
So if you achieve harmony between the timing advance, the air/fuel mix ratio,
the temperature of the burn, the octane rating of the fuel you will be on your
way to improving performance. (Spark plug choice (move to cooler plug) is also
a consideration)
If you do not get it right then you will end up with loss of power potential or
a fatal detonation.
Simple is not it.
Rob (Techno) Spargo
Mazda MX-5 Club of Victoria
October 2003