Ignition timing question

[Tom63050]

On SPFI/MPFI cars, does it really do any good to advance the distributor timing? Or is timing affected by vacuum advance and the fuel management system? And do they override manually advancing the disty, so that it's pointless to do so?

 

On my 86 GL. I stuck a Weber on it and advanced the disty timing for more power (and used 89 octane gas to prevent detonation). But on that car, there was no computer telling the carb what to do. IIRC there was a vacuum line going to the disty. What function does that vac line have?

[BobBrumby]

Sorry i cant help you on the first question but on the second one, the vac advances the the timing. The carbi distributers use a combination of vac advance and mechanical advance to achieve their total ammount of advance. Hope that helps

[Tom63050]

Sorry i cant help you on the first question but on the second one, the vac advances the the timing. The carbi distributers use a combination of vac advance and mechanical advance to achieve their total ammount of advance. Hope that helps

Thanks. OK, so it sounds like it is useful for carbed cars. How about FI cars? Same deal, or a whole different animal?

[Reveeen]

I'm not quite sure what you are asking.

 

Visualize your piston traveling up the cylinder, if ignition (or power) is applied BEFORE it reaches the top not only do you get "pinging", you get an engine trying to run in reverse. Once at the top is where you want ignition to happen, to drive that piston back down to make power.

Advance (in degrees) is the time that ignition happens before the piston reaches the top so that power is applied at the right time.

Many factors determine this "proper" amount of advance. Remember you are "lighting" a fire, the speed at which this fire travels (or "power is made") is due to engine design, and fuel type and mixture, so every situation becomes different. The "recommended" timing for an engine is an average value determined by the engine manufacturer to work best overall under average conditions. You can optimize this setting by trial and error, but you must remember that power will not be made if it is applied to the top of the piston too soon.

 

It matters not on the type of fuel delivery (other than as a rule fuel injected cars are more precise in fuel delivery than a carburetor).

[daeron]

The spark advance is completely computer controlled on the SPFI system. The distributor is supposed to be set at an initial timing of 20 degrees BTDC.

 

Apparently, some folks *do* get good results advancing? the initial timing a little more than spec.... but this is a subject I have not looked into enough to be able to advise you. I have been wondering myself, but have been too lazy to search old threads about it and too embarassed of my own laziness to start my own thread about it.

 

Note: from here on out the terms "ping, pinging" and "preignite, preignition" are used interchangeably, but technically preignition is one cause of pinging; they are not literal synonyms.

 

However, if you arent pinging now, and you can run low octane fuel, then you probably won't gain very much. If my understanding is correct, in a "big picture," general engine talk sense, you actually can obtain more power by giving your engine LESS spark advance and the cost of that is increased chance of preignition. "Preignition" is the primary cause of pinging, and it happens when the air/fuel mixture being compressed in the cylinder ignites due to pressure, BEFORE the spark has a chance to hit it and make it blow up in a controlled fashion. Just like the word is built, premature ignition. Making the spark happen sooner is the obvious way to avoid letting this happen. In a very vague sense, "Ideally" an engine would spark precisely AT top dead center, when the mixture is at its maximum compression; however in the practical world this is never or rarely possible.

 

In other words, as a rule of thumb, you should run as little initial timing advance as possible without pinging; but you should always run enough initial timing to avoid preignition like the plague. If you run on high octane, ping resistant fuel, (race gas) you can run less spark advance without pinging and get more power. Your SPFI EA82 subaru was built to run with 87 octane fuel, and 20 degrees initial timing (set with the green connectors plugged in, of course. those connectors tell the computer to run zero spark advance; it is the equivalent of unplugging the vacuum advance line on a carbed car)

 

If you want to depart from the subaru spec, do so at your own risk.. but I have heard tell of mild success. I am still learning, though.. that is why I was able to give you a breakdown of it all. These are questions I have been learning the answers to myself, in the last year or so.

 

Edited to reflect correction made to my original post below :-p

[azsubaru]

One way of setting the advance on these cars is to advance the timing until you get pinging, then back it off a little until it stops. Some people claim their cars run best at up to 28 degrees BTDC.

[robm]

The 20 deg is the point around which the timing is varied by the ECU. Just like an old fashioned car, changing the initial advance will change the advance throughout the range.

 

In other words, if you set the timing to 22 degrees, instead of 20, there will be 2 deg. more advance at idle, at cruise, and at WOT.

 

The ECU can't tell where it is set, it just adds or subtracts a certain amount of timing depending on the conditions of RPM and load.

 

New cars are quite different, as they take their timing directly from the crankshaft, and fooling them into giving you more or less advance is much more difficult.

 

And let's get some terms right:

 

There is "preignition" and "detonation". No "predetonation."

 

"Preignition" is when a hot spot (carbon chunk, e.g.) lights off the charge in the cylinder before the spark plug fires.

 

"Detonation" is when the spark plug fires, but the charge burns faster than desired, so the pressure rises faster than desired. This is also known as "knock" or "ping". Too much advance will give the same effect, as the charge burns and hits peak pressure before it should.

 

Ideally, ignition should occur so that peak pressure occurs just AFTER TDC. But since it take some time to burn and hit peak pressure, it is necessary to ignite the mixture BEFORE TDC. Changing the timing is designed to make the peak pressure point always occur at the same point in the rotation of the engine. At high engine speeds, more advance is needed as there is less time for the combustion to occur. This is the reason for mechanical advance on the old distributors. But the speed of combustion is also a variable, and it increases with more charge (WOT) and decreases with less charge (cruising on nearly closed throttle). That is why more advance is needed when cruising, thus the vacuum advance on old distributors. There are lots of other variables.

 

I have had good success with 22 deg. Although this summer, I haven't managed to break 14 km/l (33 MPG US) yet on the highway.

 

Rob.

[daeron]

The 20 deg is the point around which the timing is varied by the ECU. Just like an old fashioned car, changing the initial advance will change the advance throughout the range.

 

In other words, if you set the timing to 22 degrees, instead of 20, there will be 2 deg. more advance at idle, at cruise, and at WOT.

 

The ECU can't tell where it is set, it just adds or subtracts a certain amount of timing depending on the conditions of RPM and load.

 

New cars are quite different, as they take their timing directly from the crankshaft, and fooling them into giving you more or less advance is much more difficult.

 

And let's get some terms right:

 

There is "preignition" and "detonation". No "predetonation."

 

"Preignition" is when a hot spot (carbon chunk, e.g.) lights off the charge in the cylinder before the spark plug fires.

 

"Detonation" is when the spark plug fires, but the charge burns faster than desired, so the pressure rises faster than desired. This is also known as "knock" or "ping". Too much advance will give the same effect, as the charge burns and hits peak pressure before it should.

 

 

I have had good success with 22 deg. Although this summer, I haven't managed to break 14 km/l yet on the highway.

 

Rob.

 

 

Rob,

 

Many thanks for the correction; I made that post shortly before I finally admitted to myself that I was giving that whole "sleep" thing a miss for this diurnal cycle. My points were well conveyed, and the disclaimer regarding the terms, I felt was adequate apology for the way I simplified things. Will edit original post.

[Tom63050]

Excellent info, thanks all. Much clearer now.

[dave valiant]

I did a search and I dont have a repair manual. what rpm should the engine be at when setting the timing on an spfi.

[2.5GL]

I did a search and I dont have a repair manual. what rpm should the engine be at when setting the timing on an spfi.

 

700-800 RPM or so, with the two green connectors plugged in to freeze the timing. (located between spare tire and wiper motor/fuel filter area.)

 

Lewis