Skye,
BTDC stands for Before Top Dead Center. Therefore 10 degrees BTDC is 5
degrees more advanced than 5 degrees BTDC (further away so to speak). The
term advance here is with respect to TDC 10 degrees before is more advanced
that 5 degrees. I think your confusion has to do with why in the heck are we
firing the plug Before the piston gets to the top?
OK, I was hoping to save myself the typing, but here goes.
To get the maximum power out of an engine, the maximum pressure in a cylinder
must occur at somewhere between 5 and 10 degrees of crankshaft rotation AFTER
TDC. But wait you say, AFTER the plug fires before. Yes, grasshopper, you
are correct. The reason for that is that the combustion of the air fuel
mixture is not, I repeat not, instantaneous it takes about 3 milliseconds to
combust the mixture in the cylinder. At 1000 RPM three milliseconds is about
15 degrees of crankshaft rotation. So if we want Max pressure in the cylinder
to push the piston down as hard as possible, we need to light the mixture 15
degrees before the 5 to 10 degrees after that I mentioned earlier. 15 degrees
before 5 degrees AFTER is 10 degrees BEFORE. If you were to retard the timing
10 degrees to TDC then Max pressure in the cylinder would occur at 15 degrees
After Top Dead Center. The Max pressure would be lower, the power produced
would be less, more heat would be transferred in to coolant, and the car would
run like a dog.
So what happens if you open the throttle? I glad you asked. At 2000 RPM you
still need 3 Ms of burn time. But now 3 ms is 30 degrees of crank rotation
instead of 15 because the crank is going twice as fast. Therefore there needs
to be something that advances the timing beyond the base. There are two used
on MG's Centrifugal and vacuum advance. Just for giggles take you timing
light and check the timing at 2000 and 3000 RPM you will see that it is much
further advanced.
<WHEW> a complete thermodynamics lesson in one e-mail <G>
I hope this helps
Rick Ewald
In a message dated 1/2/99 11:21:38 PM Pacific Standard Time, skye@ffwd.bc.ca
writes:
> > 2. Wait a minute here, 5 degrees is retarded from 10 degrees. The full
> > statement is 10 degrees before top dead center (BTDC). Top dead center
is
> > when #1 is at the absolute top of it stroke. The ten degrees refers to
> how
> > many crankshaft degrees (before or sometimes after) TDC the spark fires.
> So 5
> > degrees BTDC would be less advanced than 10 degrees BTDC.
>
> But aren't the degrees for BTDC measured BACK from TDC, hence a spark
firing
> at 10 degrees BTDC would occurr *before* a spark at 5 degrees BTDC,
> therefore
> 5 deg is more advanced (along the stroke) than 10 deg BTDC.
>
> > If after you have adjusted the dwell correctly if the timing is out too
> far
> > then the distributor needs to be loosened and turned until all the marks
> line
> > up.
>
> OK. I'll try and turn it more, but it sure seems like its at a 'limit'.
>
> > I suspect that your dist. has a lot of play in the shaft and when you set
> the
> > dwell with a meter instead of a feeler gauge it moves past the amount
that
> you
> > can adjust with the "dizzy"
>
> I was worried about a worn distributor shaft as well, but the timing marks
> are rock solid with the strobe, as is the dwell angle.
>
> Thanks
> Skye
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