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Old 03-09-2010, 05:17 PM   #1
Batman1980 Batman1980 is offline
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Default 4 degrees before top dead center?

What does this expression mean, I have no idea?
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Old 03-09-2010, 07:22 PM   #2
LeAnn LeAnn is offline
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Here is the wiki answer for a chevy:

Put a timing light on the number one spark plug wire. Start the engine and remove the vacuum advance hose from the distributor and plug the hose with something such as a screw. Loosen the hold down bolt on the distributor just enough where you can turn the distributor. Shine the timing light on the harmonic balancer and notice the mark in reference to the index mounted to the timing chain cover. It shows degrees before and after a center line which is Zero or top dead center. Rotate the distributor slightly in each direction, and notice how the timing mark moves on the index. Turning the distributor counter clock wise advances the timing. Clockwise retards it. When the line on the harmonic balancer is lined up with four marks advance on the index, tighten the distributor and you're done. Reconnect the vacuum hose and remove the timing light, and go.
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Old 03-09-2010, 07:23 PM   #3
LeAnn LeAnn is offline
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And from wikipedia on what is top dead center:
In a reciprocating engine, the dead centre is the position of a piston in which it is farthest from, or nearest to, the crankshaft. The former is known as top dead centre (TDC) while the latter is known as bottom dead centre (BDC).
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Old 03-16-2010, 07:34 PM   #4
phlydude phlydude is offline
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it is basically when you want the spark plug to fire - you want the plug to fire 4 on the circle before the circle hits 360
the crankshaft pulley generally has the mark - if the car was spec'd to 4 before TDC (top dead center), then when you shine a timing light on the pulley, when adjusted properly, the light should flash on plug wire #1 exactly when the two reference marks line up

the piston is on a crankshaft that spins in a circle causing the piston to go up and down - TDC is when the mark is @ 12:00 on the circle, BDC (bottom dead center) is when the mark is on 6:00 on the circle - you want the spark plug to fire just before the circle hits 12:00 and the 4 mark allows that to happen

the piston uses 4 cycles:
1 - from TDC the piston draws down the air/fuel mixture into the cylinder
2 - from BDC the piston compresses the air/fuel mixture

4 from TDC the spark plug ignites

3 - the piston from TDC is forced down from the controlled explosion
4 - the piston from BDC pushes the exhaust out of the cylinder
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Old 03-02-2011, 06:06 PM   #5
U4K61 U4K61 is offline
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Originally Posted by Darth Anakin View Post
What does this expression mean, I have no idea?
Top Dead Center. This is when piston number one is all the way up on its compresson stroke; when it's furthest from the crankshaft. BDC is just the opposite. It's when the piston at its lowest point or nearest to the crankshaft. It would seem logical to ignite the air/fuel mixture at TDC, but combustion is not instantaneous. It takes about 0.003 sec and should finish up at about 10 ATDC. At 625 RPM, the crankshaft rotates 11 in 0.003 sec, so the timing of the spark should be at 1 BTDC. This is called basic timing, which is for a specified engine speed. At 1,000 RPM it's 8 BTDC. At 2,000 RPM, it's 16 BTDC. Normal ignition by the spark plug is about 10 to 40 before TDC depending on engine speed and load. This allows us to time the combustion to develop peak pressure and extract the max amount of work from the expanding gases.

If you have a timing light clamped to the number one wire and can see the timing marks which are often found on the crankshaft pulley, you can view the concept in action. First take a look when the motor is running at idle. This is about 750rpm for my Jeep. The timing marks should line up. At higher RPM, the marks will start to move in the opposite direction the crankshaft is turning. My old 1969 Dodge Dart, the first car I owned w/slant 6, used mechanical distributor weights and springs to do detect engine RPM and vacuum advance using manifold vacuum for engine load. Today's computerized systems use spark advance curves or maps.

On some cars, the marks may be on the flywheel and viewed via an access port.

Spark advance map.

The 4-Stroke Engine
The movement from one end of the cylinder is called a stroke. In the 4-stroke engine, four strokes complete the operating cycle: The crank must turn twice or 720 (360 x 2). The timing light will flash on every other revolution.
Intake - 180
Compression - 360
Power - 540
Exhaust - 720

Crank Position Sensor CPS/CKP
Determines the position and RPM of the crank. The input is used by the Engine Control Unit (ECU) to control ignition and fuel injection. On my Jeep, it's located on the passenger side of the transmission bell housing.
Engine cranks but does not start.
Fuel gauge and voltage gauges not working properly.
May or may not throw a Check Engine Light (CEL) trouble code.
No spark at the spark plugs.
Code checker not working. Can't pick up any codes.
Unplug CPS. If fuel and voltage guages now work, replace CPS.
Resistance between A and B leads should be 125 - 275 ohms.


TDC - Top Dead Center.
BTDC - Before Top Dead Center.
ATDC - After Top Dead Center.

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Last edited by U4K61; 08-22-2011 at 08:56 PM.
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