Ignition Timing (Part 1)


Ignition Timing (Part 1)



This article will deal with the basics of ignition timing on classic bowtie V8's.  Following articles will explore more details of this "taken for granted" but "often misunderstood" subject and explain how to tune your V8 for  more power and better mileage.

Ignition timing is basically the amount of distance measured in degrees, out of 360, (one complete crankshaft revolution) that the spark plug in each cylinder fires before the piston reaches Top Dead Center (TDC), very top of its travel in the cylinder, on the compression stroke.  The plug must fire before the piston reaches TDC, because, the gas and air mixture "explosion" is actually a "burn" that takes a short time to complete.

The amount of timing advance must be optimized to cause the maximum amount of downward push on the piston after it passes TDC.  The advance varies to suit different running conditions such as rpm, engine load (vacuum), etc.  Many factors affect optimum timing advance such as; design, modifications, vehicle weight, gearing, operating conditions and gasoline octane levels.

Too early a spark can cause detonation or "pinging", loss of power and severe engine damage if left unchecked.  This is when the mixture burn is trying to push the piston down while it's still on the upward push.

Too late a spark is inefficient and can cause loss of power, poor running, reduced fuel economy and overheating.  It's best to err on the side of too late or not enough advance. Timing is controlled in older engines (pre-mid 1970's) by three systems; initial, mechanical and vacuum advance.

Initial is the amount of advance set with a timing light by lining up marks on the harmonic balancer with marks on a stationary tab mounted to the engine.  This is adjusted by loosening a clamp and rotating the distributor.  Clockwise rotation reduces advance and counter-clockwise increases advance.  It is the commonly seen timing "spec" in Before Top Dead Center (BTDC) degrees.

Mechanical (centrifugal) advance is built into the distributor, however, it can be modified, and is controlled by weights and springs (rate) and a pin and bushing in a slot (amount).

Vacuum advance is also built into the distributor and is controlled via a mechanism connected to the engine manifold vacuum by a hose to the carburetor or intake manifold (pre-emmissions era).  Vacuum advance mainly advances the timing at idle and light load cruising conditions to help fuel economy, idle smoothness and reduce the tendency for the engine to overheat.  Initial and total timing is normally set with the vacuum advance disconnected and plugged.

Total timing normally consists of initial advance plus mechanical advance.  Most Chevrolet service manuals call for initial plus mechanical advance in th 30-36 degree range.  However, most Chevrolet V8's run best in the 35-40 degree range.  Finding the right range requires some experimentation.

To set total timing you must first determine how much mechanical advance is built into the distributor, which can be done with a timing light and measuring tape or  a dial back timing light.  I'll explain how in one of my next articles.  

To be continued...