Scanguage IGN guage, what is good? 40 either is really bad or really good

[rmay635703]

I am messing around with the fuel in the buick and would like to determine what the IGN guage is actually showing. It is supposed to be timing but I doubt 40 advanced or retarded is possible (thats where I seem to be at highway) premium it goes to about 38 which is really terrible or really good.

When I start the car on a cold day and shift into gear it moves to -3 while the engine is running slow.

So what is a good IGN value? As low as possible? As high as possible?
Is it REALLY showing an Advance of 40 and is that a good thing.

Just not sure how to guage what is ideal for that number.

Cheers
Ryan

[gone-ot]

...gasoline combustion takes time, and more time means "advanced" timing.

...the more advance, typically, the better the burning rate (not detonating or pinging).

...timing is commonly "advanced" for both acceleration and higher engine RPMs.

[tasdrouille]

The higher the ING number without pinging the better.

[jfitzpat]

Originally Posted by tasdrouille

The higher the ING number without pinging the better.

I must be missing something, that seems somewhat misleading and wrong to me. The basic reading is degrees before top dead center.

For simplicity sake, lets just say 'when the piston is at the top of its stroke'. Typically, the piston is attached to others, so it will go down even if we don't ignite the fuel at all (a misfile). So what we want to do is ignite the fuel so that the resulting heat and expanding gasses will put the most downward force on the piston, not heat the cyl or go out the exhaust as heat.

The reaction takes time, so we generally ignite the fuel early. But the speed of the flame front is not fixed, it can vary with different factors, especially the air fuel ratio. At the risk of oversimplifying, lean mixes burn faster, rich mixtures burn slower. So, under significant load, a well tuned engine will generally be around 16 degrees before TDC. This isn't a hard number, since it is the 'effective' timing that matters. Different spark systems, even cyl shapes, will move this. When the timing gets yanked significantly from this, it is usually in response to something like knock detection. Basically, sacrificing 'work' for lower peak pressures.

When there are very light loads, a lot of modern engines will use very small amounts of fuel and have relatively slow piston travel rates. So, to get the most oomph, timing is often very close to TDC.

I'm sure you know all this (I'm reiterating it more for the lurkers), and you are probably thinking of some of the other factors, like exhaust pressure, that I haven't even mentioned. I'm just struggling with the 'higher number without knocking is good' as a blanket statement. Often, the higher number is the ECU reacting to knocking, and this isn't generally considered a benefit in combustion efficiency.

Regards,
-jjf

[SentraSE-R]

The ScanGauge IGN is not ignition timing BTDC, AFAIK. What tasdrouille said is correct. If I can keep IGN up in the high 30s and low 40s as much as possible, I can beat cruise control on a test route.

[tasdrouille]

It depends on the car. On VW TDIs the reported value is the actual start of injection value in degrees BTDC, negative values are ATDC. But on all gassers I've tested it, the higher the value, the more timing advance there was. Mid 40s is about as high as it'll get in the Elantra.

[SentraSE-R]

Now this discussion has me curious. The timing specs for my 1NZ-FE engine in my xB are 8-12º BTDC at idle. I just started it up, and the Scangauge IGN value stabilizes right in that range. So I now agree IGN does reflect timing BTDC.

So, I recall from the old days that under load, you had to retard your timing, or go to high octane fuel, to avoid detonation and engine damage. It stands to reason that under light load, coasting on the level or slightly downhill, timing could advance significantly, and give us our best mileage.

The 1NZ-FE has variable valve timing allowing a range of 60 degrees relative to crankshaft angle. Is that what allows the ignition timing advance up to 40+º BTDC?

[jfitzpat]

Originally Posted by SentraSE-R

So, I recall from the old days that under load, you had to retard your timing, or go to high octane fuel, to avoid detonation and engine damage. It stands to reason that under light load, coasting on the level or slightly downhill, timing could advance significantly, and give us our best mileage.

Now you see my confusion above. But I've been thinking about it. If you are an aggressive driver, high numbers are probably bad, because they suggest that timing has been retarded because of detonation.

But, if we start with the premise that you are trying to drive to maximize fuel economy, there is a factor I missed. The comment about the test track above made it click. A lot of times, you aren't even in 'light load', but overrun, essentially coasting with the injectors closed and O2 in the exhaust.

In another thread I posted some logs to show the trouble with OBD-II MPG calculations. If you look at the very first log, before I turned a bunch of traces off, you can see that timing advanced significantly when all the other indicators are for overrun. So, in that sense, 'high' is good. But, at the two ends of the log, when I was going up hills, higher would not have been good, but bad, so I'm still not sure if 'higher is better' always works.

-jjf

[rmay635703]

On the buick My FE was stuck around 27mpg on the highway instantaneous (terrible) I noticed my timeing wouldn't move past 25, threw in neutral, shut off motor, turned back on FE jumped after a few to around 37mpg instantaneous and timing was around 37 to think I wasted 15 miles driving at low fe when all it took was a restart.

[jfitzpat]

Originally Posted by rmay635703

On the buick My FE was stuck around 27mpg on the highway instantaneous (terrible) I noticed my timeing wouldn't move past 25, threw in neutral, shut off motor, turned back on FE jumped after a few to around 37mpg instantaneous and timing was around 37 to think I wasted 15 miles driving at low fe when all it took was a restart.

Forgive the engineer in me, but it's not 100% clear what actually was going on under the hood. The first question would be 'where is the FE reading coming from'?

If it is from OBD-II, it could be a chicken egg problem. From MAP/RPM/IAT or MAF you can calculate fuel used, but only if you know actual AFR. Modern cars run at stoich a lot, but not always. It is quite possible that a particular MPG gauge uses other clues, like sparkadv, to try to guess when other cases occur.

This raises the question, was the timing really indicitive of poor fuel usage, or suitable for the conditions at hand?

If you are getting MPG from something like injector pulse widths, then we can assume that the injectors never were going into overrun or coast and fuel was being used 'less efficiently' than your normal driving.

But this raises an alternate question, was it really a matter of 'resetting' the ECU, or was the ECU responding correctly to conditions that were altered by shutdown? This is where I find logging a wideband combined with OBD-II data pretty useful. Instead of 'it went away', I can look at the log and get clues for exhaust pressure problems, a clogged injector, bad shift points, etc.

I actually think that even a crappy wideband is pretty useful, because, like EGT, you are looking at the combustion results, what the ECU accomplished. But, unlike EGT, you aren't as easily fooled by abnormal combustion (though a slow wideband can still be fooled, because you are looking at a running average of measurements fed into a PID loop). A fast wideband is nice because you can see, say, individual misfires as spikes in the log. This stuff is impossible to see on a live gauge but when you look at a log you can spot things like a lean cyl or occassional spikes.

None of this says you are wrong, I'm just saying that without knowing how the FE reading was originally derived, and without any engine data to look back at, it is hard to know, for certain, what was really occurring.

-jjf