Excellent dissertation on timing. However, it seems that there is
disagreement as to when the burn needs to be completed, I had read 23
degrees ATDC, in an other publication 20 degrees ATDC, now you give a
different number, which I don't dispute. It is just that as a layman,
the way I find what works is to keep advancing the timing carefully
until it doesn't improve performance anymore and then back it up a
couple and lock it down.
Several years ago I went to a Champion Spark Plug Seminar where they ran
an engine equipped with thermal coupler spark plugs on a dyno. To show
the effects of advancing timing beyond specs, they ran the engine
measuring HP and cylinder temp at increasingly advanced timing setting.
It was interesting how HP increased up until 4 degrees over advance
while temp increase was minimal At 6 degrees the HP increased a bit but
temp climb substantially, past this there was a rapid climb in temp with
little or no HP increase. This was a stock Chevrolet engine, but I am
sure that the physics would be similar with the race motor.
I am aware that cylinder heads have much to do with needed timing. That
is why I was concerned about the timing I was putting into my motor at
Speedweek. I had to do this years ago, but the heads I am running now
are suppose to be superior to those I ran in the yesteryear. I found my
problem when I got home, but I suppose a wiser tuner would have
investigated further at the Salt. I was just having too much fun!
Tom, Redding CA - #216 D/GCC
dahlgren wrote:
>
> 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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