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
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
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
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
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.
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