Ford 351 Cleveland Engine Ignition Guide: Timing

Adjusting ignition timing must be performed with a timing light with an accurate harmonic balancer. Be sure your harmonic balancer has its timing marks properly indexed. You’d be surprised how many times you get it wrong using a worn-out balancer or one improperly indexed from the factory. Even a new harmonic balancer must be checked for proper indexing. I’ve seen them as much as 11 degrees off, which causes a lot of confusion.

 

Ford 351 Cleveland Engine Ignition Guide: Timing 1

The standard Autolite/ Motorcraft single-point distributor looks like this. If you’re going to stay with points, always use Motorcraft vented contacts with phenolic pivot and rubbing block, which wear the longest. Use plenty of distributor lube on the cam and rubbing block.


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Ford 351 Cleveland Engine Ignition Guide: Timing 2

The basic Autolite/Motorcraft ignition system consists of single breaker points and a condenser inside a dual-advance distributor. What makes it “dual advance” is a vacuum advance and a mechanical centrifugal advance. The vacuum advance is operated off throttled vacuum advancing the spark when you step on the gas. As revs increase, vacuum advance smoothly hands off to the centrifugal advance, though both work continuously.

Ford 351 Cleveland Engine Ignition Guide: Timing 3

Beginning in 1968, Ford went to a dual-advance/retard unit on some engines to retard spark under deceleration and advance spark under acceleration. This was known as “improved combustion (IMCO). The advance/retard unit isn’t in full operation until the engine reaches operating temperature. This happens through a thermal vacuum switch (valve) at the thermostat housing, which opens to allow vacuum to reach the retard side. The retard port is closest to the distributor housing. Retarding spark under deceleration reduces tailpipe emissions.

Once you have established the balancer is properly indexed, you must find true TDC at cylinder number-1, which is midway in the crank rollover point for the crank journal (12 o’clock). Install and index the distributor at cylinder number- 1. This is called static timing, and where you begin tuning.

Be ready to set ignition timing right off the bat with the engine at 2,500 to 3,500 rpm. With the vacuum advance connected at 3,500 rpm, where is your ignition timing? Igniton timing should be checked two ways at 2,500 to 3,500 rpm: vacuum advance connected and disconnected. Total ignition timing with vacuum advance connected should be no more than 36 degrees BTDC. At idle, it should be 6 to 12 degrees BTDC.

 

Ford 351 Cleveland Engine Ignition Guide: Timing 4

By 1973, distributor vacuum control became more complicated, with some applications having an advance/retard unit and some having only an advance unit. Unless you’re restoring to stock, this should never be of concern.

Ford 351 Cleveland Engine Ignition Guide: Timing 5

Ford factory vacuum advance units have a spring and series of shims designed to control rate of advance. The more shims (washers) you add, the slower the rate of advance. When you remove shims, there’s less spring pressure, which speeds up rate of advance.

Ford 351 Cleveland Engine Ignition Guide: Timing 6

Aftermarket vacuum advance units are adjusted by inserting an Allen wrench and slowing rate of advance by turning clockwise (increasing spring pressure), or increasing rate of advance (decreasing spring pressure) by turning counterclockwise.


 

 

When dialing in total timing, begin conservatively at 30 to 34 degrees BTDC and observe operation. Push it as far as 36 to 38 degrees BTDC, but no higher. Under hard acceleration or loaded on a dyno, listen for spark knock. If you have spark knock, retard ignition timing 1 degree at a time and try again.

When you’re checking ignition timing, you want to know total ignition timing (advance), and the rate of advance when the throttle is opened. When you goose the throttle, timing mark movement should roll with throttle movement. You want spark advance to roll quickly with RPM increase and throttle movement. If you don’t have a vacuum advance, spark advance should increase in linear fashion with RPM—more slowly. Ultimately, at 2,500 to 3,500 rpm, total timing should never be more than 36 degrees BTDC. Of course you can push it to 38 or even 40 degrees BTDC, but it is risky even if you don’t get spark knock.

 

Ford 351 Cleveland Engine Ignition Guide: Timing 7

Dual-point ignitions were conceived to handle high-RPM operation by increasing dwell time. This allows the ignition coil to achieve greater saturation and a stronger discharge. However, the main reason for dual-point ignition is to get stability at high RPM. Go with Motorcraft ignition points, which have the best track record based on breaker arm tension and rubbing block durability. Many aftermarket ignition points have too much tension and they wear faster.

Conditions change when it is hot and you’re roaring around a road course at wide-open throttle, which is when you can get pinging (spark knock) and not be able to hear it. That is when the damage is done. This is why you want to start out conservatively with ignition timing and change it 1 degree at a time.

 

Written by George Reid and Republished with Permission of CarTech Inc

 

 

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