can someone explain to me exactly what the timing of an engine is? exactly how do you change the timing of an engine? ive always heard people say that they advance or retard the timing by x degrees, what does that do? thanks for the info, just trying to learn!

 

Ok, since nobody's jumped up and volunteered, I'll give it a try. Keep in mind that on todays engines the timing cannot be adjusted externally, the computer controls it.
If you imagine yourself looking at the front of the crankshaft (or the main pulley) as it spins, the path is a circle, and a circle can be divided into 360 degrees. Now, start at the top of the circle and call this 0 degrees or Top Dead Center (TDC). If the pulley is spinning COUNTER-CLOCKWISE any measurements to the left of zero (TDC) are known as degrees After Top Dead Center (ATDC), those to the right are degrees Before Top Dead Center (BTDC); for the most part you're only concerned with about 20 degrees on either side of TDC.
To simplify things we're only thinking about the #1 cylinder, when the pulley is at TDC the piston is at the top of it's travel, both valves closed and maximum compression. Since the fuel in the cylinder takes time to burn, ideally you would want to start the explosion BEFORE the piston gets to the top, so by the time it starts traveling back down the explosion is almost over and you can get the maximum amount of energy out of it to push the piston down. By changing the "timing" you're changing the "moment" when the spark plug fires. This moment is measured in degrees BTDC or ATDC. As I said, ideally you would want the plug to fire a few degrees BTDC or "advanced", but if you advance the timing too much the explosion will finish before the piston gets to the top and fight the normal travel of the piston, this is one of the causes of pinging or engine knock; some engines are actually tuned to run a few degrees ATDC or "retarded".
This adjustment could be made by rotating the distributor in one direction or another to advance or retard the timing. Because the the explosion takes a fixed amount of time and cannot be speeded up or slowed down, the spark has to be set off earlier and earlier as the engine attains higher speeds. To compensate for this the distributor was equiped with either a mechanical or vacuum operated advance that would change the timing as the RPMs increased or decreased.

I don't know how much you know, so I tried to keep it as basic as possible. I'm sure I missed something so someone will chime in with corrections. Hope this helps.

 

thenks a lot, i get it now. i knew it had to do with when the plugs fire, but never really knew the technical aspects of it. thanks for clearing that up!

 

one more question, is there a way to measure just how much youre advancing or retarding the timing? lets say you want to advance your timing 5 degrees, is there a tool to measure so you dont accidentally get 6 or 4 degrees?

 

If you are setting the timing on an engine which has a distributor, then you will find two things around and about the crank pulley. First, if you look carefully on the crank pulley itself (or maybe on the harmonic balancer) you will see a series of lines etched on the pulley. One will be marked with a 0 which is top dead center.... usually short lines indicate one degree, longer lines indicate a five degree mark.

Next, somewhere very close to the crank pulley, you will find a tab jutting out so the it covers the timing marks. Generally, one side is straight (the side you use for timing), and one side is not.

To actually do the timing, you need a timing light. This is a device that is attached to the number one spark plug wire, and it only fires when the number one plug fires.

As IndyFan noted, timing is measured using the TDC of the number one piston. Thus, when you point the timing light at the crank pulley, the strobe effect caused by the firing of the number one piston "freezes" the crank pulley and it is easy to read the actual timing setting (Hint: prior to setting timing, use a grease pencil or piece of chalk to highlight the timing line you want.

 

thanks a lot guys, you told me exactly what i wanted to know. thanks for the info!

 

A way to make setting and checking timing easy is to use the "dial" type timing light. This has a dial that shows degrees of advance.

You can use a single reference line for your timing mark, and turn the dial to the desired timing. Then turn your distributor to match. Either way works, but the dial back type lights make it a little easier.

On my truck, just getting the light at an angle to hit the marks is a PITA, unless you want to stick your hand in the fan!

 

One more thing I just remembered. If and when you decide to set the timing, remove the vacuum hose that leads to the distributor (if there is one). The timing should be set with the vacuum advance disabled.

 

This is for a mustang, but should help:

http://www.sn95.com/techarticles/techtip8.htm

YOu need a timing light and a haynes or chilton type of manual. it will give you step-by-step there

 

thanks for all the info guys. im not planning on trying it, i was just curious as to how it was done and all. thanks again!

 

Donate,

Why not? Changing the timing will often help with the low end power and mileage as well. My truck responds very well to timing changes.

It's easy. Also easy to get back to factory settings. I have a reference mark on my distributor base. I can change it back to within about 1 degree by eye.

To be honest, my truck is probably harder than most. The way the TB inlets block the dist there is no easy way to grab it near the bottom. As I have a high output ignition I stay the he** away from the top!

Even on mine it's not bad. I simply use a long screwdriver to move it.

 

Not to ruffle any feathers...but all engines need a reference timing to work off of ...even the ones equiped with a ECM...

The engine has to be set to this reference and then adjustments are made occording to input from various sensors...

If setting the reference on computer controlled engines the computer has to be set in a state where it no longer can control the timing..then the reference is set then the computer control is turned back on...

Although this sounds complicated it can be performed quite easily, usually with a spout connector...

This is how its done on the later model vehicles...