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Old 11-24-2003, 09:54 PM   #1
Kevin Thomas
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Default Ignition Timing Observation

Hey all,

Before I came into work tonight, I was tooling around with my OBS with an OBDII scanner. I was checking out the ignition timing and notice something I considered strange. At idle the ignition timing is around 20 degrees. As soon as I go WOT, it immediately jumps to the high 20's and low 30's and proceeds to go all the way up to 40 degrees advance at the top of the rpm range (around 6100rpm). I tested this on 3 different occasions and it did the same thing. Even sitting at a light and revving the engine made the timing jump up.

I was checking out my 2004 Toyota Matrix XR's timing a couple of weeks ago and it seems to suck in comparison. It simply trolled at around 20 degrees and dropped to the single digits as soon as I hit full throttle. Then it proceeded to go up to the high teens and low 20's at the top of it's rpm range (around 6100rpm...go figure).

Yes, they are totally different cars but ignition timing is works in a completely different fashion. Just for note: I have 93 octane in the Subaru and 87 octane in the Matrix. I just thought that it was pretty cool that the Subaru is taking advantage of the higher octane this way. Or is this just normal? Subaru is completely stock 'cept from the 2nd cat back and a pulley.
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Old 11-24-2003, 11:54 PM   #2
Ahzlon
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Well I was doing the same thing with a Scanner also.. u would think that the higher the RPM the lower the number. I also noticed that the vacume goes in reverse too... so i came to this conclusion ... Subarus read in reverse... so when it reads 24 in/Hg it's reading the differance from outside air... so i'm guessin that when it gets close to 40 degrees of timing it's 40 degrees from max retardation ... this is only a guess from a lil observation.. unless our Subarus are more FUBAR then anyone suspected :P
i'm running a 2000 RS
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Old 11-25-2003, 01:26 AM   #3
DoorKnob
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Acutally timing should advance at higher rpm. If you think about it, a spark takes time for the spark to travel from the ignition coils down the wires and to hop across the spark plug. It takes the same amount of time regardless of engine rpm.

Here's an example... the numbers are way off, but bear with me.

Say if the engine is at 3000 rpm, from bottom dead center it takes the piston 1 second to reach top dead center. If the spark takes 1 second to reach to the plug, then the ignition coil must fire at piston bdc.

If engine speed is 6000 from bdc it'll take 0.5 seconds to reach tdc. Then the ignition must 0.5 seconds before bdc, maybe even when the piston is at the top of the exhuast stroke.

So I hope that explains why timing should advance at higher rpm.
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Old 11-25-2003, 09:39 AM   #4
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I'm sure you'll get other, more knowledable posts, but the main reason the spark must occur increasingly before TDC with increased rpm is because of the 'flame front' or 'peak' charge detonation. It takes time for the air/fuel charge to burn and, in theory, you want the peak pressur to occur near TDC, wankel engines use two spark plugs with a slight delay between them due to the weird, long shape of the 'combustion chamber' for the same reason - to create a flame front that peaks at the right monent on the cycle. The spark also has to ignite the charge BEFORE it 'diesels' (detonates,pings,etc.) to prevent damage, lose power, etc. 40 degrees (if that's what that really means) indicates a fairly extreme attempt to prevent detonation. And it probably means you could run even higher octane. It would be a fun exercise to see what the ECU backs up to if you put one or 2 gallons of toluene in a tank and ran the same test.

fun stuff

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Old 11-25-2003, 10:29 AM   #5
Legacy777
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I'll have to take a look at my select monitor, but i do believe my timing stays around 20 at idle. It does move around a bit, but I never really payed attention to which way it's going.

Kevin, you may want to look at your knock correction value as well. If you have a value that is in the + range, the ECU is adding more timing to the base curves, which would explain the higher timing.

On my car I believe the knock correction value can range from +32 to -32
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Old 11-25-2003, 11:28 AM   #6
Tim Sanderson
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Also, the ej251 is a 10:1 compression engine. If I'm not mistaken, most of the Toyota engines are high 10's-low 11's. I don't think you would want to advance the timing as much on a higher compression engine. At least not a daily driver.
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Old 11-26-2003, 01:20 AM   #7
Ahzlon
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Hey is it possiable to advance the spark some maunaly on our ej25's??? cause when i was changing the belts i did notice timing marks down near the crank shaft... or were those numbers more for puttin the block togeather???
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Old 11-26-2003, 09:35 AM   #8
Legacy777
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no, you can't manually adjust it.

To be quite honest I'm not quite sure what the hell those marks are for, considering you can't adjust timing and the ECU does it.

Like you said, probably for when you're building the motor.
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Old 11-26-2003, 10:37 AM   #9
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Quote:
in theory, you want the peak pressur to occur near TDC
Actually, peak pressure at TDC will tend to beat up your piston and bearings and stuff rather than turn the crank, as everything is in a vertical line at that point and the rod is not acting as a lever to transfer the piston stroke into crank rotation.

Check this out, facinating stuff:

http://www.avweb.com/news/columns/182084-1.htm

It's for airplane engines, which have a few different factors going on (e.g. fixed ignition timing, having to worry more about heat, having what amounts to a CVT to drive the propellor, etc.), but the principals are the same. The other columns in the series are worth a read, too. It makes me want to find a $50 car and see if I can run mad boost way lean (at the price of max power, of course).
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Old 11-26-2003, 04:29 PM   #10
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yeah you want your ignition to be before TDC, how much before will depend on engine rpm, type of fuel used, etc. Basically you want the flame front of the explosion to reach the walls of the cylinder just a hair after TDC. This should also be your peak firing pressure as well.

If it reaches the walls before TDC, you're slowing the piston down. If it reaches the walls at TDC....you can break things.....so you really want it to reach the walls and give your peak firing pressures just after TDC.
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Old 11-27-2003, 12:48 PM   #11
Charlie-III
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I agree with the "time for fuel to burn" comments above.

As to the "it takes time for the spark to get to the plug", at close to the speed of light, it doesn't make much difference in ignition timing.

The timing marks are an easy way to check if it looks reasonable without a scanner (use a timing light). Also for setting the static timing when installing new belts.
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Old 11-29-2003, 05:03 AM   #12
DoorKnob
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Hmm, you guys are right. I completely forgot about flame theory... And I'm supposed to be the engineer

Anyhow, if I remember my combustion correctly, the flame speed also depends on the air/fuel mixture. Peak flame speed should occur at a slightly rich mixture. Anything else and the mixture burns slower.

Air/fuel ratio also has an effect on emissions but that's off topic.
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Old 11-29-2003, 04:14 PM   #13
Charlie-III
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Quote:
Originally posted by DoorKnob
Hmm, you guys are right. I completely forgot about flame theory... And I'm supposed to be the engineer
Last week I couldn't spell Engineer, now I are one.

Sorry, I couldn't resist.
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