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What does the timing on your cars engine do, and how does it work?

posted by  archangle

Cam timing is what determines when the valves open and close with respect to the position of the pistons in their bores. It is set when the engine is built- by placing the camshaft and crankshaft in the correct relationship.
Advancing the timing will...

Decrease exhaust temperature
Increase cylinder temperatures/pressures
Increase fuel economy (yes, economy is better)
Increase your output of NOx (a pollutant)
Decrease your output of Hydrocarbons (a pollutant)


Raising the cylinder temperatures/pressures after already having increased the fueling pushes the engine that much closer to its design limits, but lowering the exhaust temperature helps balance against the increased heat load from the increased fueling.

At idle, your engine is turning relatively slowly, let’s say 1000 rpms. The throttle is closed, so very little fuel and air are being drawn in to the cylinders. This small amount of combustible mixture burns very quickly, so for maximum efficiency, the spark needs to start when the piston is very near top dead center. If the spark comes too early (too advanced), the pressure from the ignited mixture will hit the piston while it is still coming up the cylinder and be wasted trying to shove the piston down before it reaches the end of it’s travel. If you try to start an engine whose ignition timing is too advanced, the starter will try to turn the crank one way, and the combustion process will try to turn it the other way, and it will seem as if the starter hasn’t enough oomph to start it. Contrariwise, if the timing is set too late (too retarded), the pressure from the ignited mixture (and the power derived therefrom) will dissipate as the flame front chases the piston down the cylinder bore in the rapidly diminishing pressure of the combustion chamber. In other words, the piston is already on it’s way towards the bottom of it’s stroke, reducing the effectiveness of combustion. The is very fuel inefficient, since a larger throttle opening at idle (set by the idle speed screw) is needed allow extra fuel in to keep the engine idling. In practical terms, the position of the distributor which yields the highest idle speed is within a very few degrees of where it should be set. (If you retard the timing about 5 degrees from this point, you will be awfully close to spot-on.) Of course, this assumes your carburetor (or F.I.)is working well and that the idle mixture is correct.

When driving on the highway, your engine’s timing requirement is different. At higher engine speeds, larger throttle openings and greater loads than idle, you need ignition advance. There are two reasons for this. First, you are burning more fuel so complete combustion takes longer. Second, the combustion time, as a percentage of the time the piston is at or near top dead center is much longer because of the piston speed. What this means is that you have to ignite the charge earlier, while the piston is still coming up, in order to get the full benefit of the pressure against the piston at the right time. Too early or too late timing will have a similar effect at speed as at idle, but greatly magnified and with far more destructive potential. Too retarded timing will give low power, lousy emissions and excessive bore wear. Timing too advanced will cause pinging (a rattling noise usually heard on acceleration), overheating cylinder heads and other problems too scary to contemplate.

Distributors have little weights inside that swing away from the shaft as it turns faster. As they move out, they rotate the upper part of the shaft which passes through the plate that the points are bolted to so that the rubbing block which opens the points meets the lobes which hit it (and thus open the points) a little earlier. At what engine speed this advance begins, at what rate it advances and at what engine speed it stops advancing is determined by the shape and mass of the advance weights and the strength of their return springs; at what degree of advance it stops is determined by a limiter on the plate to which the weights are bolted . None of this is meant to be adjusted: the manufacturer sets it up for each engine family it builds, based on compression, cam profile, octane requirements and availability, among other things. Timing the ignition, then, is a matter of getting the timing correct at one end (idle or full advance) and letting the rest of the range look after itself.

I think thats what you want right? :thumbs:

posted by  Bubba

thanks for awnsering my questain

posted by  archangle

Your welcome! :thumbs:

posted by  Bubba

but let me make sure I understand,the timing tells the spark plugs and other thing when to ignite and react.

posted by  archangle

Pretty much yeah.

It is in control of all of the equipment that require timing

posted by  Bubba

thanks man,you rock :clap: :clap: :thumbs:

posted by  archangle

Your welcome again! :laughing: :thumbs:

posted by  Bubba

Good explanation Bubba - I might add the ignition timing (i.e. spark advance, etc) on most cars of recent vintage is controlled by the ECM based on vehicle speed, engine temperature, engine speed, #1 cylinder reference position, and on some engines, octane knock (knock sensor)

posted by  tbaxleyjr

Oh yes... I forgot about that part. :doh:

posted by  Bubba

I might just add/cement some food for thought:

vacuum advance is higher at low load because the flame front moves slower; due to less volatility and density of the mixture ;

at higher loads vacuum advance is retarded because, combined with mechanical advance it may result in detonation, pinking or pinging and try sending the piston back from whence it came playing jingle bells as it does so. This is particularly true on force draft engines where sometimes the vacuum actually retards the spark.

you want maximum combustion pressure at about 15° ATDC

posted by  Wally

Which cars actually use a vacume mechanism in the distributor to retard the spark?

posted by  tbaxleyjr

Some non-computer assisted cars

posted by  Bubba

Most of your older cars prior to the computer age had a vacuum advance mechanism as part of the distributor. Ford/Gm and Chrylser, including Gm's HEI (high energy ignition) in the mid 70's.

posted by  cmeseadoin

I thought the vacume advance on the cars adjusted the advance with respect to the engine vacume while a deaccelaration valve dumped vacume during de-acceleration to retard spark. As the rpms reached a ceertain point the centrifugal advance mechanism took over

posted by  tbaxleyjr

I don't know if you are fair dinkum with that query, but I will explain, rhetorical query or not :mrgreen:

Most cars with dizzys and carbys use vacuum advance mechanisms. The Rx series had a dual operating advance diaphragm that advanced the timing on manifold vacuum and actually retarded past static advance when boost came on, relying only on centrif advance.

For a standard dizzy under light load the manifold vacuum increases, thus the vacuum advance. On two port dizzys, the first stage is brought on by the primary throttle plate cracking open and revealing a port that becomes down stream relative to the leading edge of the plate and allowing the manifold negative pressure to act on it. The second stage comes on as the venturi starts working and the manifold vacuum overcomes the pressure drop of the throttle plate revealing negative pressure to a port upstream of the plate.

You can have both centrif advance and vacuum advance working simultaneously. An engine doing reasonable revs (eg 1600 or more) will have its throw out weights fully extended, but if the throttle plate is not fully open (eg cruising, decellerating) the manifold and carby barrel pressures will be in the vacuum region. Under sudden deceleration you will find the vacuum extending up to the carby horn momentarily.

posted by  Wally

What I am "hearing" is the few cars which use vacume to retard the spark have a two ported advance mechanism and the diaphram which controls the advance mecanism is actuated by the ported or throttle body source on one side of the diaphram and manifold vacume on the other. What happens here is during cruising, the throttle body or carb vacume are almost equal. During acceleration, the carb vacume is higher than the manifold vacume allowing more advance. During decelleration, the manifold vacume is higher than the carb vacume thus timing is retarded.

Some of these cars had a decelleration valve to dump carb or throttle body vacume and force maximum retard if manifold vacume reached a certain set point.

The spark retard @ idle and fast response on decelleration is to ensure complete combustion of the fuel reducing hydrocarbon emmissions

I cant answer for the mazda Rx series (although my daughter used to have a 626), Ford used these distrobutors in the seventies / early 80's before the coming of the engine managfement systems)

posted by  tbaxleyjr

Most of the Jap cars here with carbs have a fuel cut (to the emulsion tube) if the manifold pressure gets into the mid twenties.

How can a carby barrel have a higher vacuum (closer to absolute 0) than the manifold? I know venturis are good, but I must admit I haven't seen an event where the static pressure is less than the difference between total pressure and velocity pressure. The manifold is the source of the vacuum after the valves, thus the tendancy for air to travel in that direction.


Seems odd to retard the spark for emmission control and to advance when accelerating as the torque will suffer significantly. An advanced timing results in a more complete burn although the combustion heat transfer is greater to the water jacket and the EGT goes down a bit as a consequence and there is little ooportunity for the exhaust valves to get much cooling from the charge especially underload when reversion is high back through the throttle.

I take it your domestic market has a high turnover in exhaust valves, rings and spark plugs?

We are talking the same language here? When I refer to retard I'm saying the spark occurs close to TDC while full advance occurs around 30° to 40° BTDC.

You are not one of those people who think ramcharging has an effect are you?

posted by  Wally

I don't have a good scanner @ home or I would show a picture of the dual ported advance mechanism and its internals for everyone reading the post to have a better idea what we are talking about

This system at least as used by Ford was used in the 1970s primarily. At that time in history, American manufacturers were coping with a double whammy of pollution control regulations and fuel economy regulations and this was one of their early attempts to cope before the computer controlled systems began showing up in the late 1970s- and throughout the 1980s. You are right about engine performance since those engines built in that time frame really stunk performance wise compared to similar engines built in the 1960s and built today

As far as ring wear and general engine wear, engines built when I was a teen in the 1970s would be lucky to make 100K miles between rebuilds. Now, one can push over 150-200K or more. Plug wear is a similar story - Due to advances in combustion control, plug change intervals extended from he norm of 12000 miles to 60000 miles plus today.

posted by  tbaxleyjr

100K and then a rebuild? Every car I've had in my hands to maintain has lastest a great deal longer. The only engine I had to rebuild infact had 400K on it. Ran great, just had low oil pressure. My moms crown vic is running upwards to 200 now and I just recently put on a timing chain. Before that I rebuilt the heads. An engine life is all in the oil.

posted by  DodgeRida67

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