Joe,
I had published it already on the list, but here goes again...The last
run we made on Thursday we advance the timing 1 more degree, (our
fastest run, 222+ MPH, was with 49 degrees BTDC). We also leaned the
mixture .002 which was a mistake. The engine detonated before Jeff
shifted out of low gear, he went on through the three mile and shut
down. We lost #4 & 6 spark plug electrodes and dusted # 6 piston near
the spark plug. Luckily, no major damage. I found that all the intake
valves had been colliding with the pistons and with deeper analysis,
found that I had degreed the cam incorrectly.
The cam card gives two numbers that got me confused. The timing spec is
.020" of valve opening, the duration is checked at .050" valve opening.
I obviously set the cam timing with .050" of valve opening, which when
rechecked at .020" read 69 degrees BTDC rather than the spec of 53
degrees. After resetting the cam timing, I had more than .100" of
clearance between the valve and piston. This was a major mistake. I had
set it up correctly when I was checking valve to piston clearance in the
beginning, I don't really have an excuse for setting the cam timing at
.050. Just brain fade I guess, but I don't think that I will do that
again. I try to learn from my mistakes.
Tom, Redding CA - #216 D/GCC
Joe Amo wrote:
>
> ""I found my
> problem when I got home"", Tom, can or do you desire to share, I am most
> interested, if you would rather not lt us in, understood, I will share some
> "learning experiences" as well, not always easy to do, depending on the find.
> Joe :) :)
>
> Thomas E. Bryant wrote:
>
> > 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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