Interesting story Dave.... Lets look at My Engine which you have some degree
of familiarity with....
We were running 38 degrees and building Big EGT's and adding Tons of
Heat.... Would we have cooled it down a Tick if we had taken some of the
timing out.... and reset it at Say 36 or even 35? ..... I guess what I am
asking is my combustion chamber more effiecent then I thought? Would the Hp
Loss been compensated for with the abilty to turn a Higher RPM.... ( I was
driving the EGT's for a bit there )
Keith ( NO this doesn't mean I am Falling deeper in love with this JUNK set
of Heads.. I understand there are much better out of the box stuff out
there )
----- Original Message -----
From: "dahlgren" <dahlgren@uconect.net>
To: "Marge and/or Dave Thomssen" <mdthom@radiks.net>
Cc: "Land-speed Racers" <land-speed@autox.team.net>
Sent: Saturday, August 26, 2000 6:51 AM
Subject: Re: Timing advance
> 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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