WARNING - Long winded explanation of spark advance follows. Delete if
This is a bigger question than can be answered on the list. Engine
timing depends on the type of engine, operating speed, fuel, shape of
combustion chamber, location and type of spark plug, fuel metering
system, number of plugs, operating temperature, etc., etc.
Here are some simple things that need to be considered. At 4000 RPM,
each piston must go up and down in 0.015 seconds. That ain't much
time! On the power stroke, the engine has just sucked in a fuel/air
mixture and is starting to compress this mixture so that it can be
ignited by the spark plug. Now the question is WHEN do you ignite. If
you fire the spark plug too soon, then the pressure builds while the
piston is still moving upward and you are resisting the desired motion
of the engine. Fire it too late and the engine doesn't have time to
extract the power from the fuel/air mixture.
When the spark jumps the gap in the plug it starts the mixture burning.
But it takes some time (albeit small) for the flame to propagate through
the combustion chamber. Obviously, you want the combustion process to
take place during the quarter cycle from just before top dead center on
the compression stroke to the point when the piston is near the bottom
of the cylinder and you have lost the mechanical advantage of the
crank. That only takes 0.0075 seconds!
The trick is to get the greatest power out of the engine at any given
RPM and engine load. For typical automobile engines, idle speed
ignition timing should be 10 to 20 degrees (crank travel) of advance.
By igniting the mixture before the piston reaches the top of the
compression stroke, it gives the flame front time to propagate
throughout the cylinder.
As the speed of the engine increases, it is necessary to increase the
advance of the spark timing to 30 or 40 degrees. Racing engines that
run at very high RPM need even more advance. Full load operation
requires less spark advance than partial load because the more fuel/air
mixture you stuff into the cylinder the faster the flame front
propagates. The vacuum line from the manifold to the distributor makes
the adjustment for load. Wide open throttle means low manifold vacuum
and big load. So the vacuum advance actually retards the ignition as
the load increases. (And now you know why turbos need less advance.)
There is lots more to this than I have just described. But these are
the basic ideas.
When the Federal government instituted air pollution controls in 1968 it
forced some changes. Normal 16 degree advance at idle resulted in more
pollution than no advance. So Datsun changed the distributor curve so
that the engine idled at TDC (top dead center) but advanced to 35
degrees at high RPM. That is 35 degrees of total advance and if you
look inside the distributor at the rotating advance plate is is stamped
with 17.5 indicating 17.5 degrees of distributor advance. (Since
distributors turn at half the speed of the engine, you have to multiply
by two to get engine advance.) Unfortunately, when idling at TDC the
engine gets hot faster.
The proper advance for our engines is the original pre-smog curve and I
recommend that you get the parts (springs, rotating advance plate and
weights) or a rebuilt distributor from one of our suppliers. You will
be impressed with the improved low end performance. And you are less
likely to overheat in traffic.
Orlando, FL - 94F but not many thunderstorms. Yea, I drove the roadster
Daniel Neuman @SMTP@Exchange wrote:
> Hello All,
> I was wondering today about advance curves. I have been told that
> setting the timing to 16 BTDC will give me more power in low rpm's compared
> to 0 BTDC. Why is this exactly?? I have also been told to make sure that
> the total advance is not more than 35 deg. Why is this?? What will happen
> if it is more than this?? It seems that we are striking a balance between
> a high initial advance (at low rpm's, which is good for some reason) and
> not too much advance at high
> rpm's(which is bad for some reason). Does someone have an idea of an
> 'ideal' advance curve for our engines and why its ideal?? Some engines,
> turbo engines for example, are actually retarded at higher rpm (I believe).
> Why is this??
> Any enlightenment would be greatly appreciated.
> Daniel Neuman
> Dept. of Radiation Oncology
> 533 Parnassus Ave. Rm. U368P
> University of California
> San Francisco, CA 94143-0226
> phone: 415-502-5177
> fax: 415-502-5175
> email: email@example.com