Re: 4 degree timing mod from www.sa750.com
silvershadow55 said:
Hey folks,
I know I've asked a similar question before but I really want to be sure I have my head wrapped around the concept before I do this mod. Since I am not an engine mechanic but can do simple bolt-on mods...
So... when I do this mod I am advancing the timing 4 degrees. This means that the spark fires earlier? Assuming I am correct, this would mean less compression, right? (since the piston had not traveled all the way to the top of the cylinder) and would not less compression equal less power? Obviously there is a flaw in my understanding somewhere, else this would not be something anyone would want to do.
thanks,
Scott
Let me try to put this in a clear way, since there is a lot of people that
have similar misconceptions.
As stated before, the compression doesn't change.
As the piston goes up in the cylinder on the "compression" stroke,
the fuel/air mixture is compressed.
When the piston is at 0 degrees that is perfectly straight up on the crank.
Any backwards for forwards movement from 0 degrees will cause
the piston to fall.
Advancing the ignition timing is causing the plug to fire earlier in the up stroke of the piston. Retarding the timing, is just the opposite.
As the piston is on the up stroke, the plug will actually fire long before the piston reaches 0 degrees (top dead center).
This gives the fuel/air mixture time to ignite. As the fuel ignites,
the gases expand. The expansion rate is a given and is conservatively
setup so that as the piston goes over top dead center and starts back
down in the hole, the expanding gases push the piston down in the hole.
Advancing the timing, gives the gases a little more time to expand.
If the piston is trying to compress the gases as they are trying to expand,
then suddenly the piston goes over center, energy is released to the top
of the piston and transmitted to the crank.
If you wait, and fire the plug later in the stroke and the gases don't
have time to expand while in a compressed state, they expand in to open
space and don't exert as much energy on the top of the piston... hence,
not as much power transmitted to the crank rotation.
Advancing the timing a little allows the gases to get a head start on
expanding, a little more before the piston hits TDC (top dead center).
If you go too far advanced, the plug will ignite the gas and super heat
the cylinder. This will cause the lower half of the cylinder gases to
detonate (ignite from heat/pressure, instead of spark).
When this happens, the two flame fronts collide and cause what you hear
as pinging. This collision releases a tremendous amount of energy in
the form of heat and can do severe damage.
I have done the timing advance on my VLX and it really woke the bottom
end up. Mid-range and high RPM are about the same.
And as stated above, be prepared to possibly have to change to mid-grade gasoline.
Mine runs file on '87 in the winter, when the ambient air is 70 degrees or under.
In the summer, I have to go up to 89 octane or it'll ping. Runs great on 89.