Dumb Question Contest: Timing

excelguru · #1 (**permalink**) 03-01-2002, 07:30 AM

Okay guys, I'm a mechanical engineer, not a sparky as we like to call them. If it pumps liquid or blows air or holds something down, I can pretty much figure it out. But this whole "retard timing" and "vacuum advance" crap has me stumped. But I know I'm in the right place to get answers, so here goes:

I understand what advanced and retarded timing are (firing slightly before or slightly after TDC, unless I'm really clueless). I just don't understand how they physically effect the engine's performance.

And what the heck is vacuum advance for crying out loud? Does it provide more advanced timing at higher revs, based on the manifold pressure? Why?

Some people can just listen to a car idle... or accelerate... or lift throttle... and just by the sound they can tell that the timing is off and in what direction it's off. How?

Keith

CobraEd · #2 (**permalink**) 03-01-2002, 07:47 AM

Keith, Give me a call 202-268-8264. I can easily explain the whole thing to you. Too much to type though.

It all has to do with magnets, mirrors, pullys, cable, and gear oil!

Ed

excelguru · #3 (**permalink**) 03-01-2002, 08:00 AM

What? No smoke?

KeithBrown · #4 (**permalink**) 03-01-2002, 08:30 AM

Quote:

Originally posted by excelguru
What? No smoke?

You forget to mention the mirrors.

Keith II

ERA535 · #5 (**permalink**) 03-01-2002, 08:49 AM

For others who may have the same question

1. After the fuel/air mixture is lit by the spark plug it takes some time for the mixture to burn.

2. The faster the engine turns the earlier this mixture has to be lit. (think of trap shooting - the faster the clay the more lead you need) "Lead" in this case as in "you can lead a horse to water.....not lead as in lead poisoning will kill you...In fact in some circles advance is called "lead".

3. There is an optimum time to light the mixture, too early and it wants to all get burned before the piston goes into the power stroke resulting in pre-ignition (noise), too late and power is down.

4. The distributor is what determines when the plug fires.

5. Vacuum is a method used to physically control the distributor so that the point at which the mixture is lit varies - vacuum is used with centrifigul weights in the distributor to do this.

I tried to be as simple and concise as possible.

excelguru · #6 (**permalink**) 03-01-2002, 01:33 PM

In addition to the great information provided by ERA 535 above, I also got an in-depth, over-the-phone course on timing from Ed. Thanks guys. I've learned something today. Can I go home now?

Keith

Roscoe · #7 (**permalink**) 03-01-2002, 03:26 PM

http://www.priveye.com/dls/Timing.doc

Whaler · #8 (**permalink**) 03-01-2002, 07:51 PM

Taken from "how stuff works"

Timing
The timing of the spark is important, and the timing can either be advanced or retarded depending on conditions.

The time that the fuel takes to burn is roughly constant. But the speed of the pistons increases as the engine speed increases. This means that the faster the engine goes, the earlier the spark has to occur. This is called spark advance: the faster the engine speed, the more advance is required.

Other goals, like minimizing emissions, take priority when maximum power is not required. For instance, by retarding the spark timing (moving the spark closer to the top of the compression stroke), maximum cylinder pressures and temperatures can be reduced. Lowering temperatures helps reduce the formation of nitrogen oxides (NOx), which are a regulated pollutant. Retarding the timing may also eliminate knocking; some cars that have knock sensors will do this automatically.

Whaler · #9 (**permalink**) 03-01-2002, 07:54 PM

Roscoe
Good read..I printed it off for myself to read over and over before I flash up this spring

Tim

Clois Harlan · #10 (**permalink**) 03-02-2002, 11:53 AM

As usual you have provided some valuable information. I printed it off and gave to my son who has been asking some these same questions the only thing, your answer was better constructed.

Jack21 · #11 (**permalink**) 03-02-2002, 01:31 PM

Well, here goes my $.02. I'm an Environmental Engineer (aka jack of all trades, master of none). But have been working with this stuff since a "Program" was what was playing on TV.

(There will be a quiz at the end of the class, so take some notes.)

A fuel/air mixture burns when ignited. This burn occurrs over a period of time to exert a push on the top of the piston. Ignited too soon, and the mixture pushes down, while the piston is still moving up. This is not good. Ignited too late, and the piston is already on its way down before it gets its push. This is not good. There is an optimum time based on 1) RPM of the engine, and 2) load on the engine, when this burn must start. Burn start is a function of the spark plug. Timing of the burn start is a function of the distributor. With me so far?

Distributors are dumb devices with springs, weights, and diaphrams that we adjust and set based on engine power output and behavior at different RPM, and load.

There are two devices in a typical distributor that alter the timing (advance, retard, lead) of the burn start sequence. Centrifugal, and vacuum.

The centrifugal advance senses engine RPM. The more RPM, the more advance, up to a pre set limit. This is what the springs and weights do. As RPM increases, the weights move out, and move the rotor tip forward relative to its direction of rotation. This times burn start earlier or advanced.

The vacuum advance senses engine load. The little silver cannister on the side of the distributor with the hose on it does this. It pulls vacuum either from the intake manifold directly (below the throttle blades), or from the carburetor (just above the throttle blades). It has a rubber diaphram inside that moves back and forth with engine vacuum, and is connected to the distributor with a lever. It rotates a plate inside the distributor which changes (advances/retards) the timing of the burn start.

An engine (an air pump), draws air (& fuel) in through an inlet restriction (carburetor). Between the inlet restriction, and piston, (intake manifold) there exists a vacuum. An engine with more load is using more throttle opening to overcome the load, and subsequently will produce less vacuum. An engine under light load, such as while cruising at a steady speed, will use less throttle opening, and will produce more vacuum.

(As a point of confusion, diesels, have no such inlet air restriction, have no vacuum, and operate at wide open throttle (air) all the time.)

The time it takes for a fuel/air mixture to burn is a function of the ratio of fuel and air within limits. More fuel in the air will burn faster, such as under high load conditions. Less fuel in the air will burn slower, such as under low load conditions. The vacuum advance senses engine load and times the burn start earlier or later to coincide with the engine load (vacuum) demands.

Both of these systems, vacuum and centrifugal, work to optimize the burn start timing for optimum power output, and optimum fuel economy. (And have given hot rodders and performance enthusiasts trying to achieve this optimum setting grey hair for years).