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Maybe you've just built up a brand new engine, or upgraded to new heads and a cam, perhaps you're simply trying to dial-in an existing combination. In either scenario, one area of tuning that is highly overlooked and greatly misunderstood is timing. All too often we see people dropping in their distributor, making a quick adjustment with their timing light, and setting off to make another pass.

Timing is everything, and without a proper timing curve, every thing else goes out the window. Jetting changes, fuel pressure adjustments, are all useless if first the timing is not set correctly.

So what is timing? In a nutshell, timing or 'ignition timing' relates to when the sparkplug is fired in relation to piston position. At idle, when engine speeds are the lowest, the plug fires just before the piston reaches the top of its stroke. As engine speeds increase, the time between piston strokes is less, and therefore the plug must fire sooner. In all cases the plug is fired in advance of the piston reaching top dead center. There is a small window of time in which the combustion need to take place in order to produce peak power. Too late and power is lost, too soon and detonation occurs, which can lead to melted pistons.

In reality, ignition timing, is a complex physical process, dealing with multiple variable, including compression ratio, volumetric efficiency, combustion chamber shape, cylinder temperature, etc. Very interesting stuff indeed, but we wont get into it here.

In this article we're going to focus primarily on carburated, non-computer controlled, engines which have fully adjustable distributors. The EEC-IV computer controlled Fords allow for setting initial timing, but the rest is adjusted by the computer. The newer modular engine Fords have distributor-less ignitions which offer no adjustability from the factory, although companies like Steeda have recently developed timing adjusters for these engines. Some Fords, particularly in the 70's and early 80's, had distributors where timing was fixed due to emissions reasons.

When it comes to timing the most common myth is that adjusting the timing simply means moving the distributor clockwise or counterclockwise. While this does affect the timing, it is not the correct way to adjust the timing curve. To explain why, we first we need to define some terms.

Advancing and retarding timing refers to increasing or decreasing the 'time' at which spark is delivered to the cylinders. This 'time' is measured in crankshaft degrees, signified by marks on the harmonic balancer, and a reference pointer on the block or timing chain cover. When the piston is at Top Dead Center (TDC), this is synonymous with zero degrees on the balancer. Ten degrees before that point would mean the piston is ten degrees of rotation from being at TDC.

So how does the crank position relate to the distributor?
The distributor shaft on Ford engines is driven by the camshaft gear, which is turned at half-crank speed by the timing chain connected to the crankshaft. Thus there is a direct correlation between the position of the crank and the position of the distributor. Remember, the distributor is a switch. Regardless of the type of distributor you have, there is a fundamental design common to all of them; the shaft is in a fixed position, spinning in direct relation to the crankshaft. On the shaft sits the trigger which activates the switch. On electronic distributors the trigger may be a magnetic sleeve with eight openings, or in the case of points, its simply an arm that open and closes the points. The distributor housing does not spin and it contains the actual switch, such as the Pertronix box, which is mounted on a breaker plate. By rotating the housing you in effect move the position of the switch, changing when it triggers a spark. When you rotate the distributor to "adjust the timing" you are moving the switch on the housing side in relation to the trigger on the shaft.

Rotating the distributor housing clockwise on a Ford advances the timing (i.e. spark is being fired a greater number of degrees before the piston reaches TDC), and counterclockwise decreases the timing.

When referring to timing, there are really four terms that must be considered; initial timing, mechanical (or centrifugal) timing, total timing, and vacuum advance. There is also cam timing which is more appropriately termed valve timing, since it deals with when the valves open and close in relation to crank position. We won't talk about this since it has no dynamic bearing on ignition timing.

Initial: This is the most common adjustment that people associate with timing. At idle, with the vacuum advance hose disconnected and plugged, this is the timing that you would see if you flashed timing light on the timing marks. On typical stock engines you'd see as low as 0 to as high as 15 degrees. Most Ford shop manuals specify around 6-8 degrees initial timing advance for the 289-351 motors.

Mechanical/Centrifugal: Most V8 distributors contain an internal advance mechanism consisting of two each of weights, springs, and slotted 'reluctor' arms. There is also a stop tab for the arms. On Fords this assembly can only be seen by removing the cap, rotor, and breaker plate; we'll get to removal a bit later. As the distributor shaft spins with increasing rpms, the centrifugal force acts on the weights, which begin to force outwards against the springs. This movement rotates the shaft and thus advances the timing. The slotted arm controls how much the weights can move the assembly, and the springs control how fast the assembly reaches that limit. The reluctor arm on a Ford has two slotted sides, only one side contributes to the timing, the arm can be flipped around if more advance is needed (see pictures.) On Fords each side is stamped with a number, usually 10L and 13L; or some have 15L and 18L. These numbers refer to 1/2 of the total degrees of timing that will be obtained when using that arm. So for example a 15L arm would contribute 15 x 2= 30 degrees of timing when full against the stop.

Total Advance: So far we have looked at initial advance and mechanical advance. Both of these combined gives total advance. Say for example initial was found to be 6 degrees, and we visually verified that the reluctor arm was on the 15L side. Total timing, theoretically, is then the initial + mechanical. In this case 6 + (15 x 2) = 36 degrees. If we shined a timing light on the marks (with vacuum hose disconnected and plugged), at idle we'd see 6 degrees, then as we increased the engine speed, we'd see more and more advance, until at some point the total centrifugal advance would be reached, and we would see 36 degrees. When exactly the total advance occurs is of great importance when it comes to performance, and we discuss this in the section below on "curving."

Vacuum Advance: Most Ford distributors include a vacuum advance mechanism. This consists of a diaphragm vacuum canister, an arm from the canister to the breaker plate, and a hose connected to an engine vacuum source. The purpose of this mechanism is to provide spark advance when the engine is not spinning fast enough to create the centrifugal advance talked about earlier. In other words this is an engine-load dependent advance. This would be a typical situation when climbing a steep hill, or driving at low rpms, light throttle, conditions. In these conditions there is high engine vacuum, so the vacuum signal applied to the diaphragm in the canister, via the hose, will cause a 'pull' effect on the arm, which moves the breaker plate and results in a timing advance. During full throttle conditions there is very little engine vacuum, and thus the vacuum advance does not contribute to total advance.

Vacuum advance is tricky to tune because there is no direct measurement like total. In fact, the reason you must measure initial and total timing with the vacuum hose disconnected is because when the engine is in neutral there no load, thus the vacuum is high, and if the hose were connected you'd see as high as 60 degrees advance and think something is really wrong! The only way to tune vacuum advance is on the road, by feel, and AFTER the initial and total are adjusted.

In short, vacuum advance was developed to optimize fuel economy and reduce emissions. It is not a bad thing to have on a car which sees a lot of street driving, and in such conditions the engine will perform better with it properly adjusted. However many factory and aftermarket performance distributors do not even come with a vacuum advance. The reason is simply because race cars do not spend much time at part throttle.

Curving for Performance
A timing curve is simply a plot of how much ignition advance takes place over the rpm range. In other words, when the timing advances is just as critical as how much it advances.

When it comes to performance there are many different engine combinations, buildups, components, and uses….Each requiring slightly different timing curves. On the other hand if you have a stock motor, and do not care for every extra horsepower, you really do not need to do more than follow the shop manual procedures. However even a stock or mild daily driver motor can be made to accelerate faster with a five minute timing curve adjustment.

The rule of thumb is that the higher the compression ratio, the less total timing it can handle before detonation, and also the higher octane rating it needs to control detonation. Low octane fuels ignite faster, thus require less timing advance. Conversely high octane fuel can handle slightly more advance. Dyno testing has shown that most small block Fords with 9:1 to 9.5:1 compression make peak HP with 38-42 degrees total advance. Engines with 9.5:1 - 10.5:1 run best with 35-38 degrees total, and above 11:1, should not go higher than 35 deg. total. When using power adders such as nitrous, super or turbo chargers, the timing should be advanced accordingly.

The first step in curving a distributor is to set you initial and total advance. As detailed above and in the picture captions, the total is determined by the reluctor arm setting plus the initial advance. Ideally you should keep the initial between 10 and 20 degrees, and the total in the ranges listed above for your compression ratio. For example, if you are shooting for 40 degrees total, and your reluctor arm is on the 15L slot, you would have 30 degrees mechanical advance, requiring the initial to be set at 10 degrees.

The second step is to dial-in how fast the distributor achieves the total advance. This is controlled by the springs which hold back the weights, under the breaker plate. A stock distributor usually has one light and one heavy spring, and brings the timing in really slow, such that the distributor may only reach the total timing at very high engine speeds, 4500+ for example. For performance driving, the best acceleration comes when total advance is achieved before 2500 rpm. To adjust this rate, you can replace the stock springs with lighter tension springs. You can also bend the tabs on which the springs connect to change their tension.

Once you've set the initial and mechanical timing, and adjusted the curve, you should be very very close, if not right at, the optimum timing curve for wide-open throttle performance. You should use timing light at this point to confirm that the initial timing is where you set it, and steady, and then check the timing from idle to 3500 in 500rpm increments. The curve should increase a few degrees at every checkpoint until 2500, where it hits the maximum. After 2500 it should not go beyond the total advance.


Final Thoughts
Hopefully we taken some of the mystery out of properly curving a distributor. Keep in mind these are ballpark ranges, and every engine responds differently. Aluminum heads, large overlap cams, differences in cylinder pressures, all affect timing. Optimum timing can really only be determined on a dyno, or under very controlled and repeatable track conditions. When we dynoed Project 11.99 recently on a chassis dyno, we saw first hand a difference of 30 rear wheel horsepower from timing at 30 degrees total and 42 degrees total! We've also seen gains of up to eight tenths due to improper timing. It is a cheap and relatively quick modification that can be worth significant power. F/M


Typical Ford distributors. Shown are a stock with Pertronix conversion (left), Mallory Unilite (right), and Ford Duraspark (below). The stock distributors have a vacuum advance canister, while this Unilite does not.
 

Notice how all distributors use the same arrangement; the switch is mounted on the breaker plate, while the trigger is on the shaft. Changing when these two make contact results in advancing or retarding the spark mechanism. The Duraspark distributor uses this vaned piece to trigger the switch.
 

The Unilite uses a notched plate which passes through a light beam switch.
 

The vacuum advance mechanism is a canister which houses a rubber diaphragm. A hose connects the canister to an engine vacuum source. As vacuum increases the diaphragm constricts, pulling on the arm, which in turn moves the breaker plate. Because the switch or points sit on this plate, the movement results in the triggering the spark sooner, i.e. advancing the timing.
 

Underneath the breaker plate is a the centrifugal advance mechanism. A pair of weights force against the slotted reluctor arms, which are held back with spring pressure. Each side of the arm is stamped with a number which corresponds to the half the amount of timing that arm will yield. Only one of the slots is actually functional. In this picture the arm at the bottom (red pointer) reads "10L", which means a total of 20 degrees of timing (10 x 2). You can also see the stop tab in the slot of the 10L arm. Flipping this reluctor pair to the other side would yield 30 degrees, since it is a "15L" arm.
Changing the springs to lighter ones will bring the timing in faster.
 

Crane sells a "re-curve" kit, (part no. CRN-99606-1). This kit comes with light, medium, and heavy tension springs, which can be swapped out with the stock springs to tailor advance speed. It also comes with an adjustable vacuum canister.

Typically it is best to replace the stock heavy spring with a lighter spring, or run the two lightest springs in the kit. Reassemble the distributor and check the curve with timing light. If the lightest spring combination doesn't get full advance in by 2500, you can also bend the spring tabs inward through the access holes in the breaker plate, to get the perfect advance curve
 

Vacuum advance canisters are usually adjustable with a 3/32-in. allen wrench, as shown here. The small screw inside the housing adjusts the tension on the diaphragm spring. If you detect knocking and loss of power, back the screw out (counterclockwise) to decrease advance. If the engine pops and surges, tighten up the screw to increase advance.

Note:When checking initial and total advance, always disconnect the vacuum advance hose. Otherwise you will get very high timing readings.
 
Tuning Vacuum Advance
The last step, after the total advance curve is set, is to dial in the vacuum advance if you have one. There should be a vacuum line connected from the carb, or the manifold, to the vacuum canister. There are two types of vacuum sources that you should be aware of. One type is known as "full" vacuum or "manifold" vacuum. This is a direct connection to the manifold, and if the hose is connected to this port you will get vacuum in the line at idle. The other port is a "timed" port, which only yields a vacuum above a certain rpm. At idle the line will have no vacuum. Most carburetors have both ports. On Holley's the timed is above the throttle blades, and the "full" is below, near the base. On Carter/Edelbrock carbs, the timed port is on the passenger side and the full is on the driver's side. The easiest way to confirm what port you have is to hook up a vacuum a gauge and check for vacuum at idle. The preferred vacuum source is the timed source. This way there is no effect on the initial timing setting.

Remember vacuum advance is load dependent, so you cannot check it with a timing light with the car in neutral. The best way to set vacuum advance is by feel, under real driving conditions. Connect the vacuum line and drive the car up a steep, long grade, with the car in high gear and at a low speed, 30-40mph. Occasionally push the accelerator to the floor, and listen and feel for knocking and/or loss of power. If you detect this, immediately back of. This means the canister is advancing too much and you should adjust the canister so the diaphragm is 'tighter', by turning the screw counterclockwise.