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Discussion Starter · #1 ·
Top Dead Center is determined when?

(a) At begining of INTAKE Stroke
(b) At begining of COMBUSTION Stroke

I've found different web sites that conflict with each other.
 
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Discussion Starter · #2 ·
When the piston is as high as it gets, that's TDC. Actually, both of those answers are correct, since the "stroke" begins when the piston changes direction, regardless of whether or not the valves are open (or closed) yet. The piston is at TDC when the intake stroke begins, AND when the combustion stroke begins, since the piston is changing from an upward stroke to a downward stroke in both scenarios.
 
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Discussion Starter · #3 ·
Technically, that's true. The pistion is at TDC when it is at it's highest point on either stroke. But when changing a timing gear or a distributor which one do you use as the reference point?
 
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Discussion Starter · #4 ·
quote: "But when changing a timing gear or a distributor which one do you use as the reference point?"
The answer is:
b) At the beginning of the combustion stroke. Both valves will be completely closed (lifters on the base circle of the cam)
 
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Discussion Starter · #5 ·
If your heads and valvetrain are in place, you can tell you're coming up on the compression stroke by sticking your finger over the sparkplug hole. As the piston comes up to TDC on compression, it will push air out the sparkplug hole.
 
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Discussion Starter · #6 ·
John,
When messing with distributor, or timing gear cam and crank, you don't have to know or worry if the #1 piston is at top dead center. The 2 sprocket dots will line up any time the #1 piston is at top dead center. (2 crank revolutions per 1 cam revolution).
I have a feeling you are wondering if you put your timing gear on correctly. I had the same worry John, when I did my first motor rebuild, (at it wouldn't start). But it really is impossible to mess the phasing up with the gears by having the dots line up. You'd think it could be 180* out...but it doesnt' matter. It cannot go together any other way.
but the opti can get put on wrong and even experts can mess that up.
Hope she get's to runnin for you soon.
Did you already check that the opti was installed OK? (mounting ears flush with the front of motor? ie. the opti can go on but the pin might not be in the slot...and so the mounting ears will show about 1/4" of air space but you can still bolt her in if you didn't know).
Karl
 
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Discussion Starter · #7 ·
Orca,

You're not alone dude if you were confused. Everybody is familiar with TDC on the compression stroke for ignition timing purposes.

But as I discovered when installing my cam; when the gear dots are aligned, the #1 piston is at TDC at the end of the exhaust stroke. This initially screwed me up because I assumed it would be TDC on the compression stroke.

Doug
 
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Discussion Starter · #8 ·
Yep, this is what has happened. Got timing marks lined up ok (cam pin at 3 o'clock, by default). Opti was flushly mounted. But did not turn cam pin to 9 o'clock (the other TDC) before putting it on. My mistake was thinking the Optispark was smarter than it really is. GM had the chance to make this "completely" bonehead-proof but failed. Oh well, dead-on or 180* off is better than the many ways an older distributor could have been installed.

Car starts right up now. But I am getting a P0300 code (misfire) and the car is surging at idle speeds. I think I need to re-adjust the rockers. I probably have one too tight or something.

Thanks for all your help.
 
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Discussion Starter · #9 ·
Doug, it makes sense that the TDC you want to allign the cam and crank gears is at the end of the exhaust stroke as this is when piston # 1 has completed a cycle and is ready to start the next, it has just completed the 4th of 4 strokes in its cycle and is about to move onto the 1st of the next 4.
 
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Discussion Starter · #10 ·
<BLOCKQUOTE>quote:</font><HR>Originally posted by SS ORCA:


Car starts right up now. But I am getting a P0300 code (misfire) and the car is surging at idle speeds. I think I need to re-adjust the rockers. I probably have one too tight or something.

Thanks for all your help.
<HR></BLOCKQUOTE>

John,

Unless you have had your PCM reprogrammed and the idle bumped up to something like 800+ RPM the
P0300 code is to be expected from a '96. Remember our cars have the crankshaft positioning sensor as part of
the OBD-II "enhancements". When this sensor sees the crankshaft speed varying, such as during the lope
caused by a more aggressive than stock cam, it will throw this code.

Good luck battling the little green guys!
 
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Discussion Starter · #11 ·
So when the dots on both gears are aligned, does that mean its about to start the intake stroke on #1? Both valves completely closed?

Or is it starting the combustion stroke? (seems like both answers are given above...)

In either case, are both valves completely closed at TDC?

Also, how is it possible to install the opti incorrectly? Aren't there 3 holes, but only 1 hole that the pin will fit in?
 
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Discussion Starter · #12 ·
On the opti question, there are three holes and only one will accept the cam pin...unless you really, really want the opti to go on no matter what!
 
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