Joe,
In the past, before I tried to get technical, I just set the cam by the
marks and checked it against split overlap. I check piston to valve
clearance with modeling clay. Put the clay on the piston under the valve
area and put the head on, with gasket, rockers, push rods, etc. and
valves set to the spec I run at. Turn the engine over a few times and
pull the head and check the thickness of the clay. If you try to do it
with a dial indicator, you would need to check every 10 degrees or so
through out the valve action, the clay is just easier for me.
Tom, Redding CA - #216 D/GCC
Joe Amo wrote:
>
> Tom, so bare with me here as many of us bike guys have DOHC, we degree cams
>in using
> lobe centerlines, do you not set the cam timing using lobe centerlines? It
>sounds like
> you use actual degree numbers of opening and closing is this correct?? If
>you use
> centerlines, the centerline stays consistent no matter if checking at 0.20"
>or at
> 0.050", do you then check valve to piston at say from 10 degrees before to 10
>degrees
> after TDC? It would seem that checking 10 before to 10 after would show the
>valve to
> piston discrepancy no matter the method used to set the cams. Please
>enlighten me. Joe
> :)
>
> Thomas E. Bryant wrote:
>
> > 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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