The cool thing about ignition timing is you can usually get a considerable performance increase for little or no money. But, some time and effort will be required.
Let’s start with a simple explanation of why we need spark advance and how much of it we want.
First When you hook up the timing light to the #1 spark plug
wire and it flashes a light beam onto the timing tab, it’s showing you
when the spark occurs, NOT when the ignition occurs and the explosion that starts to push the piston down the cylinder.
We want this to happen just as the pistons reach the top of the
compression stroke, but it takes time for the spark plug to ignite the
fuel mixture. This “time” is measured in crankshaft degrees and is the
difference between when the plug fires and the explosion occurs to push
the piston down the cylinder producing the power stroke.
Next How much “time” or how many degrees of advance is correct.
Well, that changes with each application but in most cases it will be
as much spark advance as we can get as soon as we can get it without
having detonation or “ping”.
Now that we know why we want spark advance and that we want all we can get without detonation How do we get it?
If the engine is already in the car and running, we will have to
hope the timing mark is correct. If you are building an engine or
having one built at a shop, make sure the timing mark is correct; use a
dial indicator on the #1 piston head to find “top dead center.” Also,
if the damper is easy to get to, put a 38° mark on it to use as a
reference on the stock timing tab. The formula we use is Dia.×3.1416÷360×38 (Diameter of the
dampener/balancer × PI ÷ 360°×38°, or with numbers:
8×3.1416(25.1328)÷360(0.0698" per degree)×38°(2.6529" which you mark on
your balancer away from your TDC mark), this distance will be 38° on whatever diameter damper you
Now we know the timing tab
is correct, and we have a 38° mark on the damper, we can now start to
work on our timing “curve.”
and street performance ignitions do not have vacuum advance and the
stock type distributors that are sent to a shop to be recurved should
have the vacuum advance removed and locked out. For this reason we will
deal only with mechanical advance. If you send your
distributor to a shop to be recurved they can get you pretty close to
the correct curve on a distributor machine. We usually install a 26°
mechanical curve that starts about 100 rpm higher than the engines idle
rpm and have all 26° in by 2,800 rpm, this is a good general purpose
timing curve when used with 12° initial timing set at engine idle.
This, however, is not perfect or optimum for any one combination.
A street/strip car that runs on pump gas may “ping” with this much
spark advance and will require less initial advance or heavier springs
to slow down the “curve.” Where as a low compression, low stall speed
converter car may respond better with more initial spark advance or
lighter springs for a quicker “curve” but watch it if you begin your
mechanical advance curve at or below the idle rpm, the car will be a
real pain in the ass to drive and tune.
For circle track applications, these motors almost always operate over
the rpm where total advance is needed so the timing “curve” is not
nearly as important as having the correct total spark advance. This is
where the 38° mark is very handy to have. We have had much success with
instant advance curves in these engines. This is where we start engines
on about 10° of spark advance and the instant it fires, the timing goes
to the total advance as long as you use high enough octane fuel this
gives a nice clean idle and very quick response.
As you can see, there’s a lot you can do with ignition timing, and
every engine will require a specific curve and total spark advance, but
if you take the time to sort out what’s best for your engine, you will
gain performance without spending a lot of money, and isn’t that what
we are all after.
Tech Tip courtesty of Jensens Engine Tech