I am very sure that the amount of ignition advance you run tells you a
very important thing about your engine, well two actually. The first and
most important thing that I have learned is that there is a direct
relationship between the amount of the ignition advance and the design
of the combustion chamber. This is a very important thing to anyone
racing. The better the chamber the more power that can be extracted from
the fuel, the engine becomes more efficient. You have pointed out a
very good example with the 50 degree timing and the shrouded plug. If
you unshrouded the plug on that engine and did nothing else other than
reset the timing you would of made more power. The reason for this is
pretty simple. When you fire the plug that early and start the fuel
burning more of the energy goes into the surrounding metal as waste heat
rather than push the piston down. The ideal time for peak pressure as I
have understood it is right around 15 to 17 degrees after TDC. With that
said the most efficient time to burn the fuel is about 15 to 17 degrees
ATC so that all the energy goes into pushing the piston down and less
goes into the cooling system. The only problem with that is it takes a
certain amount of time for the spark to occur and the flame front to
propagate. This time is compensated for by ignition timing advance
expressed in degrees as it is the easiest unit to measure. If you look
at the timing as microseconds of delay time instead of degrees a very
interesting thing happens. First thing it does is take rpm out of the
picture and explains why you have to add more timing as engine speed
goes up. It also gives you a very nice number to show the relative
efficiency of the chamber. Different cylinder heads have delay
time/advance curves that are very common to them. ex, SBC 23 degree
about 38 degrees, Cosworth DOHC 32 , Ford Yates style head 34 and so
on.. what you want to do is work on the head chamber plug combination so
that the ignition timing is the smallest number that will burn the fuel
completely. The other important point is the faster the burn time in the
chamber design the less likely you will any preignition/detonation as
there is less time for a second flame front to develop in the first
place. So when I hear that someone is running a lot of timing to get
peak power I see a bomb waiting to go off and also an engine that is in
real need of further development. The timing is a crutch for a problem
in engine design, a lot of which can be avoided by proper chamber design
and spark plug placement and shrouding.
When the tuner becomes expert and sees big timing numbers a bell should
go off in their head and tell them to pull the heads and find the real
problem they are covering up with ignition advance and not write it off
to a tuning thing. Every time you change the timing or jetting or fuel
mixture/burn time in any way there is something to be learned. the
engine is telling you all about how it is doing and how good the overall
design is, but you have to listen to it too.
Hope I did not bore anyone with this but my 2 cents worth..
Dahlgren
Marge and/or Dave Thomssen wrote:
>
> On spark advance from an old-timer.
>
> Correct spark advance is a matter of experience and experimenting. Every
> engine set up differently will need different advance. Some setups are so
> similar that they take the same advance. When I used to run my 8:1 ARDUN
> with the plugs somewhat recessed I set it at 52 degrees(!) with 10 pounds of
> boost or unblown. I worked up to that number gradually. Now that this one
> is 9:1 with better quench 42 degrees seems good blown and unblown. My 12:1
> ARDUN with unshrouded plugs likes 36 degrees. My flatheads like 28 degrees
> when there is a lot of clearance between head and piston. If the clearance
> is tight or the blower in attached they like 22 degrees. Some guys run
> flatheads at 10 degrees. The engine tuner becomes the expert on spark
> advance with experience.
>
> Dave Thomssen
> #322 XXFSTR
> The Hayseed
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