Most engines (except diesels) use an electric spark to ignite a fuel/air mixture. The fuel burns and powers the car. This electric spark has to happen at exactly the right time or the engine will not run properly. This is called "Ignition Timing". When a mechanic "Sets your timing" during a tuneup he is adjusting your ignition system to fire the plugs at the proper time.
Your ignition timing changes as you drive. The number of degrees BEFORE TOP DEAD CENTER (BTDC) is called the ADVANCE. This is done mechanically (centrifugal advance), or by vacuum (vacuum advance), or by computer, or by a combination of these. Whatever the setup, advance increases with engine RPM and decreases as the engine goes under a load.
SYMPTOMS OF INCORRECT IGNITION TIMING
Symptoms of incorrect ignition timing are poor fuel economy, sluggish acceleration, hard starting, backfiring, or "pinging" or "spark knock". Too little spark advance will cause low power, bad gas mileage, backfiring, and poor performance. Too much advance will cause hard starting and pre-ignition.
PINGING, SPARK KNOCK, OR PRE-IGNITION
Pinging or spark knock is properly called "pre-ignition" and is often incorrectly called "valve noise" because it sounds similar to loose valves that need adjusting. The difference is that loose valves will make noise all the time, whereas pre-ignition happens on acceleration or under a load. You might hear it when going up a hill on the interstate: a rattling sound that goes away when you downshift or get off the gas.
Pre-ignition can be caused by too much spark advance or too low octane gas. Many newer cars have "knock sensors" that retard the spark when pre-ignition occurs.
If your car does this from time to time, you may not need to do anything. You might use a higher grade of gas, but that costs a lot of money! I tell people to just not let their car do it: either get on the gas or get off the gas and the "ping" will go away. Official word from the manufacturers is that a"little" ping is normal, although they don't define "a little".
Basic Ignition System Components
Your ignition system supplies that spark to a spark plug via a set of spark plug wires, or ignition cables. The thousands of volts used to fire the plug comes from one or more ignition coils. Over the years manufacturers have used several different ways to operate these coils:
BREAKER POINT IGNITION
Cars used to have BREAKER POINTS to fire the ignition, and as they wore the timing would change and need to be adjusted. Whenever you replaced the points during a tuneup (about every year) you had to reset the timing. This was when you needed a tuneup every year or so.
Since the 1980's most every car has some type of electronic ignition. Since there are no points to wear, the timing should never need to be adjusted unless you take the distributor out of the car. The only timing change comes from the timing chain
loosening up as it wears. This is often the case on higher mileage engines. If you have to advance the ignition timing a lot to make an engine run, it probably has a bad timing chain and bad VALVE timing
On a lot of newer vehicles you can't adjust ignition timing (or idle speed, or nearly anything else) because it's all controlled by the computer. WATCH OUT! A lot of people have a rough run problem with their car, take it to 3 different shops for a tuneup, and end up just getting 3 new sets of plugs without fixing the problem.
AVOID THIS! Don't go in just asking for a tuneup: give a specific complaint: like, "It's hard to start in the morning", or "It's getting bad gas mileage".
Newer vehicles have their ignition and fuel controlled partly or entirely by the vehicle computer. Many of these have no distributor: the computer fires multiple coils directly. You cannot adjust the timing on a vehicle that has no distributor, unless you reprogram the computer.
To get the most power out of a motor the spark has to happen a bit before it gets to the top of the cylinder. As the engine goes faster the spark needs to happen earlier and earlier. This is called "Spark Advance". Older cars had vacuum advances, centrifugal advances, or a combination of both. On newer cars the computer controls the spark advance.
TYPES OF ADVANCE MECHANISMS
A vacuum advance is on the side of the distributor, usually a cone shaped metal unit with a vacuum hose connected to it. As an engine comes under a load it cannot tolerate as much spark advance. Also, when an engine comes under a load, intake manifold vacuum decreases. So the vacuum advance works out well by retarding the spark (less advance) when the engine comes under a load.
As a motor goes faster it needs more spark advance. Picture the old flyball governors on the old steam engines (and Frankensein movies) Centrifugal advance mechanisms are inside the distributor. They have weights that oppose springs, and engage a pivot mechanism that advances the spark as the distributor spins fasrter.
COMPUTER CONTROLLED ADVANCE
Newer cars use computer controlled electronic ignition to advance the spark. They use a number of sensor inputs to calculate ideal ignition timing. On vehicles like this you can sometimes check the ignition timing, but you can't adjust it.
Timing is measured in degrees of crankshaft rotation. When a piston is at the top of it's cylinder, it is said to be at "TOP DEAD CENTER, or TDC. A mark is placed somewhere on the crankshaft to indicate this position and labeled "ZERO DEGREES OR TDC. This mark is used to set the ignition timing.
BEFORE TOP DEAD CENTER (BTDC)
The spark advance varies as to engine speed, but it is set to an initial base setting, called BASE TIMING. This is measured in "DEGREES BEFORE TOP DEAD CENTER". In a manual, or on a sticker under the hood, you'll see something like this: IGNITION TIMING: 12 DEG. BTDC. Rarely a car will set BASE TIMING to a few degrees AFTER top dead center (Some VW Rabbits: 3 degrees ATDC), but these cars have VERY agressive advance mechanisms that put the timing into the BEFORE top dead center range really quickly.
TIMING MARK LOCATION
Somewhere on the motor's crankshaft there is a timing mark (usually on the front pulley, or Harmonic Balancer.) Sometimes it's just a tiny pointer which you align with notches on the front pulley. Some cars have a piece of tin about an inch long by the pulley with dregrees embossed on it. Some have degrees engraved on the harmonic balancer, with a pointer to align them with. Some vehicles have the timing marks at the back of the engine on the flywheel: you view them through an acess hole in the bell housing.
A CUTE TRICK
WITH THE ENGINE OFF !!! clean off your timing marks. Very often they are hard to see because of dirt and grease. With some chalk or paint go over the timing mark so it will be easy to see.
The typing correction fluid "White-Out" works really well for this.
ADJUSTING IGNITION TIMING: METHODS OF "SETTING YOUR TIMING"
On newer vehicles you can't adjust the timing without reprogramming the computer. You CAN buy performance computer chips, but that doesn't really count. Basically, if you don't have a distributor, you can't adjust your timing.
WARNING!!! BEFORE YOU BEGIN: DISABLE THE ADVANCE
Many cars run a lot of advance at idle. If you don't disable this advance and set the timing to the mark the spark will in reality be very retarded and the car will run poorly.
DISABLING THE SPARK ADVANCE
Vacuum advance distributors
Disconnect and plug the vacuum line(s) going to the vacuum advance on the side of the distributor
Centrifugal advance distributors
Set timing with engine RPM as low as possible: under 1000 RPM
Some folks with centrifugal distributors static time them: (see below)
Computer controlled distributors
On many cars you disconnect a wire, some you jump a wire between 2 terminals on a connector, others you put into diagnostic mode. Check out the manual for your car: the manufacturers have too many ways to list here!
A timing light is the most common way to check ignition timing. It is a strobe which is activated by whichever cylinder the engine "times" off of: #1 cylinder on most every vehicle, except some International Trucks, which for some resaon time off the back cylinders (6 or 8), and have timing marks on the flywheel . Some other motors, especially front wheel drive motors, DO also have timing marks on the flywheel, visible through a small hole in the bell housing. Some motors have timing marks both places.
The strobe "stops motion" and shows the timing mark and pointer. You can also check the advance operation: rev the engine and you'll see the timing mark move.
ELECTRONIC TIMING PROBE
Mostly on GM, a magnetic sensor fits into a tube by the front pulley (harmonic balancer). You view the timing degrees on an LED digital readout. All cars set up for this have traditional timing marks next to the hole for the timing sensor probe..
When you put a motor together you HAVE to static time it or you'll never get it started. Align the timing pointer on the harmonic balancer (front pulley) with whatever degree setting the motor uses for its base timing. Loosen the distributor and move it back and forth. Then use one of the following methods:
THE SPARK METHOD: WATCH OUT! IT BITES!
Turn the ignition on, remove the coil wire from the distributor, leaving it hooked to the coil. Put the loose end of the coil wire near a good ground AWAY FROM THE BATTERY: IT WILL EXPLODE!!!
As you rock the distributor the coil will spark. After you "play with it" a bit you can see where the spark is happening as you rotate it. Find that spot and bolt the distributor down.
THE VISUAL METHOD
When you build an engine the coil isn't working yet, but you can still static time it. Remove the cap. On the older point ignitions set the distributor where the points are just about to open, but are not open yet.
Electronic ignition distributors use 2 different sensors to fire the coil: the RELUCTOR and the HALL EFFECT SENSOR.
Reluctors are star shaped and located just below the distributor rotor. The reluctor is magnetic, and it induces a current in a PICKUP COIL (WHICH IS REALLY A SENSOR) which fires the IGNITION COIL. Align one of the points on the reluctor even with the center of the pickup coil and you are static timed.
HALL EFFECT SENSORS
Hall effect sensors are also under the rotor, but they have a series of windows cut in a piece of tin. These windows pass between a pickup assembly, where they interrupt a magnetic field in a HALL EFFECT SENSOR.
These fire when the window just clears the center of the sensor.
In all cases static timing should be done like this: your "finishing turn" should be in the direction OPPOSITE the rotor rotation direction. This way you are moving in the direction the engine rotates and you get an accurate setting.
RACE TUNING, OR TUNING BY EAR
To get the most power out of an engine you really need to give it as much advance as it will tolerate without preignition, or pinging, also more accurately called "spark knock" This setting is varies according to how good your gas is, outside temperature, humidity, and altitude above sea level, to name a few. A lot of racers (and mechanics) I know set their timing by specs, but then leave their distributor slightly loose. They'll advance the spark until it pings, then back off it a bit.
For a rough timing setting you can turn the distributor one way until the engine starts to die. Turn it the other way until it starts to die. The correct timing is approximately halfway between these 2 points.
A "CUTE" SETUP: MID 60'S TO MID 70'S TOYOTA COROLLAS
These Corollas had a small white knob on the side with "R" and "A" and a couple of arrows. This knob when turned advanced or retarded the ignition timing a bit. The reason was to allow drivers to compensate for various octanes of gas. Get a cheap tank of gas and your engine pings a bit? Turn the knob a bit. Find some good gas? Advance it a bit for better gas mileage. This in effect gave the driver the ability to easily "race tune" his motor without tools or major hassle.
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Gainesville has been my home since 1974, and I've loved Gvl and the Gators since I came here in the fall of 1974 to attend the University of Florida. I loved it so much I stayed and opened my car repair business. Originally it was out of the back of a 1963 Chevrolet wagon, but in 1977 a fellow mechanic and I opened an auto repair shop with actual walls, etc. I stayed in the same location for 26 years, and recently moved my operation to property I bought 15 miles east of Gainesville. I am doing most all the repairs myself now, having reduced my overhead from $1500 per month to practically nothing. I do work by appointment only. I mostly work only on my established customers cars, but I will occasionally take on new clients. E-mail me and I will either make arrangements to look at your car, or I will recommend you to someone who will.
George G. Scott, Jr.