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Old 07-22-2008, 08:42 AM   #1 (permalink)
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Advance or Retard Timing for better mpg?

Just as the title states, I am not too familar with the whole timing = better mileage concept, can someone give me the DL? I have a '94 Metro, and I believe its at +4 degrees now.

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Old 07-22-2008, 09:15 AM   #2 (permalink)
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Advanced spark timing is said to be better. I've never tried to quantify it myself.

advanced timing - Google Search
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Old 07-22-2008, 11:40 AM   #3 (permalink)
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I was wondering when you'd answer. Thanks. I will set it a little higher tonight.
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Old 07-22-2008, 12:47 PM   #4 (permalink)
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I had read in a few computer tuning articles that retarded on the highway would net the most mpg. I have also heard that advanced would net the most.

I tested this on my suburban using Jet DST. I pulled four degrees out accross the board and let the computer relearn it's new tuning. I went on a trip with cruise set and mile markers for starting and stopping the test. My scangauge read 18.*mpg I then bumped the timing up eight degrees, four to return it to stock and four more beyond that. Relearn and drive the same stretch of road less than an hour after the first test. The result was 22.* mpg. All tests were with cruise on and running 55mph. I also made another run with autotap running and no knock was detected. I still have more room to go up. 89 octane was used.
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Old 07-22-2008, 12:54 PM   #5 (permalink)
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You are asking about ignition timing, right? There is an optimum ignition timing which changes according to engine speed, load, temperature, fuel type, fuel mixture ratio and probably several other factors. Most engines have the ignition timing set a little more retarded than the most efficient setting. This lowers nitrogen oxide emissions, reduces the chance of knocking, reduces the pressure on pistons, rings, and connecting rods, and reduces peak temperatures in the combustion chambers. It is easier on the engine than overly advanced timing. On the other hand, if the timing is too retarded the exhaust valves and valve guides can be damaged because of hotter exhaust gases. If you advance the timing you should do it in small steps, such as 2 degrees at a time. Listen carefully for knocking when you accelerate. Bring along a younger person if you don't hear high pitch sounds well. If you do hear knocking then retard the timing a little until it is gone. If you have high nitrogen oxide emissions during an emission test, that would be another reason to retard the timing.

Here is a nice link about ignition timing:
http://auto.howstuffworks.com/ignition-system1.htm

Last edited by Andyman; 07-22-2008 at 12:55 PM.. Reason: adding link
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Old 07-22-2008, 01:32 PM   #6 (permalink)
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Mine's currently a couple of degrees advanced above stock as well. I just earballed it.
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Old 07-22-2008, 03:47 PM   #7 (permalink)
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How would I adjust the timing in my car if I wanted to ?
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Old 07-22-2008, 05:43 PM   #8 (permalink)
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Quote:
Originally Posted by Andyman View Post
There is an optimum ignition timing which changes according to engine speed, load, temperature, fuel type, fuel mixture ratio and probably several other factors. Most engines have the ignition timing set a little more retarded than the most efficient setting...
I agree, lots of influences on what 'perfect' is for any particular engine. Pile on fuel octane and altitude to the list.

A little retarded is safe, but if you listen for ping, have a timing light and aren't afraid to experiment, the payoff is worth the trouble if you are chasing every last mpg to advance timing right to the edge, just not over the edge.

I think spark timing and fuel octane are often times misunderstood. The consensus among several top race engine builders, and what I've learn through experience, says to use the lowest octane fuel you can get away with, and advance the timing 'til it pings (then retard it 2 degrees). A higher octane number means the fuel burns a little slower (87 octane has no more energy in it than 92 octane) There is no magic timing advance across the board, only a series of compromises and 'almost perfect' settings.

I've played with old carburated VW's for a long time, this one in particular I was trying to squeak out every last mpg (circa 1981). I wound up recurving the distributor's advance curve through different springs on the flywieghts and bending the stop ears, to match my reduced accelerator pump shot volume and 'egg under my right foot' acceleration style. Through tedious seat of the pants trial and error I found best FE to be right at the edge of pinging. On hot days, I sometimes had to break out the 10mm socket and retart it a couple degrees.

Remember that initial timing and full advance are NOT the same thing. Your engine burns most of it's fuel at full advance. This is key, since not all distributors act the same, not all timing advance computers are the same, your metro might have slightly different initial timing advance needs than mine. I'm a little embarrassed to say, I have no idea what my (or your) Metro uses for an advance curve mechanism (mechanical or computer). I'm a Metro newbe, but I'll find out. Mine pings a little under heavy load when the mercury goes north of 95 deg F, I leave it where it's at since we get 50 days/year over 95F. Either way, a simple test will give full advance and initial advance numbers.

From my race experience, maybe by no coincidence, top torque readings on a normally aspirated race engine is usually a degree or two before it starts to ping (hard to hear with open headers in the dyno room, but you see it in the data, the observed torque curve falls off like a stone). My guess is top FE is at or about the same timing.
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Old 07-22-2008, 07:05 PM   #9 (permalink)
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Quote:
Originally Posted by Tourigjm View Post
How would I adjust the timing in my car if I wanted to ?
On a car with a distributor, you adjust timing by loosening a bolt or bolts that hold the distributor to the engine and turn the distributor one way or the other. If you turn it the same way that the rotor turns, you retard the timing. If you turn it the opposite way, you advance the timing. When you are satisfied with the timing, you tighten the bolt(s) so it doesn't change while you drive.

It would be a good idea to know what the recommended timing is for your engine before you adjust it. Often that can be found on a label in the engine compartment. If not, you can find it in a repair manual for your car. Usually the timing is set at idle with any vacuum advance hoses disconnected and plugged. Some cars (such as BMW's) should have the timing set at a higher speed.

To see how you are setting the timing, you need a timing light. You clamp one of its cable ends over the ignition wire that goes to the number 1 cylinder. Most timing lights also connect to the battery. You point the light at the timing marks, which are either on the flywheel (you would look through a hole in the transmission) or on the crankshaft pulley area. The flashing light shows the position of the crankshaft when the number one spark plug fires. You may need to look at a book to understand how to read the timing marks. Sometimes you may need to clean up the timing marks and mark the lines with white paint so you can see them better. Be sure the engine is off before you do that so your fingers don't get caught in the belts.

On some newer engines, there is no distributor and no timing adjustment. In that case you may need to change the computer's ROM chip to change the timing.

Many older cars have something called vacuum advance. It advances the timing when there is vacuum in the intake manifold, except during idle (on most vehicles). It is needed for maximum efficiency because the fuel mixture burns slower when the pressure in the cylinder is lower. If it doesn't work, the engine will still make normal power but its fuel economy will go down. The vacuum advance depends on a diaphragm to move some distributor parts. If it leaks, it may stop working. To test it, you can hold the throttle open a little and pull off the vacuum hose to the distributor. The engine should slow down because of the later timing. There should be vacuum in the hose which you can feel with your finger. With the throttle closed the vacuum may go away. On my Honda, there are two vacuum hoses and two diaphragms. One of them leaked and I had to clog one of the two hoses to get any vacuum advance. The engine worked better at low throttle after that. The diaphragm that leaks is only used when the engine is cold so I don't think it's important to replace it.

Older cars also have weights inside the distributor to change the timing according to engine speed. Sometimes they can get stuck if they get rusty. In my Honda they were stuck and I had to spray some oil into the bottom of the distributor and twist the distributor rotor back and forth to get it to move again. If the weights are stuck in a retarded position, the engine will lose power at higher engine speeds. If they are stuck in an advanced position, starting the engine while hot can be difficult and it may knock during acceleration. When they are free to move, the rotor can be twisted by hand in the same direction as it normally turns and it will snap back (because of a pair of springs) when released.


Last edited by Andyman; 07-22-2008 at 07:11 PM.. Reason: safety tip
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