what is it and what does it do to your engine (internally)when you advance it, retard it?

 

Timing refers to when the spark plug fires in relation to where the piston is in it's travel. For example...

If the timing is set to 0* top dead center (TDC), that means the spark plug will fire when the piston is at the very top of it's travel (Top Dead Center). If the timing is retarded, you're setting it after top dead center. So, the spark plug will fire while the piston is on it's way back down. If you advance the timing, you're setting it before top dead center, which means you're firing the sparkplug while the piston is on it's way back up.

What does this do to the engine? Well...

Firing the sparkplug after top dead center will make you lose a bit of power, since the piston is already on it's way back down, the combustion of the air/fuel won't be able to push as hard as it could. However, since it's on it's way back down, it also means there's less pressure in the chambers, making it easier to start the car. SOme cars retard the timing a bit when starting to make it easier on the car.

Advancing the timing will get you a little more power out of the engine, so long as you don't advance the timing too much. IF you get it just right, the air/fuel mix will "push" as hard as it can when the piston is at or near the top of it's travel, where it can make the most power. This isn't to say that setting your timing to top dead center will make the most power.. This is because the combustion won't be complete right away, it takes a little while to burn. So generally before top dead center gives you more power.

However.. Advancing it also puts a lot more pressure in the chambers. Because ignition is taking place while the piston is on it's way up, part of the explosion will try to force the piston back down. This makes it very hard for a car to start. Also, if you advance your timing too much, detonation will occur, and you'll get engine knock, or pinging.. It's very detramental to the life of your engine.

FOrtunately, if your car is computer controlled and has a knock sensor, if it detecks knock it'll retard your timing so you don't kill the engine. Unfortunately, that generally means you'll lose a lot of power.

Timing will be advanced the higher your rpm's are. The sparkplugs need to be fired sooner, because the pistons are approaching TDC more quickly than they would be at lower RPM's.

The computer will take over timing, but you can set where the computer starts from by changing the base timing. Briefly, to do this you bypass the computer and set the timing. When you plug the computer back in, it'll advance and retard the timing based on where you set it.

That help?

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89 iroc-z 305 tbi
k&n filtercharger, open element air filter. nuffin' else

 

Thank you very much.

 

Oh, yes... And proper timing will also maximize your fuel economy.

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89 iroc-z 305 tbi
k&n filtercharger, open element air filter. nuffin' else

 

Other than explaining how base and total timing are affected and how vaccum and mechanical advance works that was a very good explanation on timing. I congradulate you on the explanation.

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Follow my racing progress on Stephen's racing page
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87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
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Best results before the 383 blew up
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87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

so, how exactly is the timing adjusted.. say on a '91 Z? Thanks, doug

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'91 Z28 - 50,000 miles. All power options leather. 350 TPI:
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S Cobra Radar Detector; Z28 SS Floormats

 

Again, as quoted from... me.


Setting the timing on the car is very easy to do. First buy a timing light and an offset timing wrench. (The bolt on the distributor is placed inconveniently if you don't have one. heh) That'll run you probably no more than 40 bucks.
To bypass the computer, look for a tan wire with a black stripe on it on the passenger side of the car. There should be a bundle of wires helt together with some black plastic tubing, and sticking out of that will be the wire. It's got a plug on the wire, just disconnect that. (Do this with your car off.)

To hook up the timing light...

Put the inductive pickup on the #1 spark plug wire. It's the wire closest to the front of the car on the driver's side of the distributor. Then hook the positive cable to the positive terminal, then the ground to any good ground you can find. Turn the car on, and after a few seconds the check engine light should come on. (Also be careful not to let the wires of the timing light get anywhere near the belts or the fan.) With the car warmed up and at idle, point the timing light down to the marks. As you've seen, the marks are made up of a bunch of triangle shaped wedges. The widest "valley," towards the drivers side, is TDC. Then, the peaks, heading towards passenger's side, are 4*BTDC and then, I believe, 8*BTDC.

Loosen up the bolt on the distributor, and slowly turn it either way to adjust the timing: Counterclockwise to advance, clockwise to retard. Don't jerk the distributor as it doesn't take much to adjust the timing. Also, when adjusting the timing the engine's RPM's will vary, so don't let that throw you off. After you get the timing set, tighten down the distributor and check the timing again to make sure it didn't move. Then turn the car off, disconnect the timing light and plug back in the EST.


It wouldn't be a bad idea to go ahead and check/reset the IAC and TPS at this point either, since timing throws off the idle a bit. There's tech articles for setting that.

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89 iroc-z 305 tbi
k&n filtercharger, open element air filter. nuffin' else

 

i think you can do it more accurately an easier with a scanner instead of the timing light.

 

Hmm.. Never heard of setting the timing with a scanner. Maybe I'm old-fashioned.

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89 iroc-z 305 tbi
k&n filtercharger, open element air filter. nuffin' else

 

You need a light to set the timing. The ECM is programmed with the stock initial timing setting listed on your emissions tag. The timing figures reported by a scan tool are based on that programmed initial setting and do not reflect any changes you make in initial advance. The ECM has no idea where the distributor is positioned.

 

ok, but a scanner can be used to see if there is an knock being sensed and then you can back off the timing..