Ignition Timing & Octane with ScanGuage

[OriginHacker21]

I finally got a ScanGuage for my car and would like to do some testing on if more octane increases my ignition timing to take advantage of 91 octane over 87 octane.

Only problem: it looks like the ignition timing changes all the time with the gas pedal (mountains all over here) so how would I accurately compare? I'm going to run my tank of 91 as low as possible and fill with 87.


What to do & how to accurately compare ignition timings with different octane with scanguage? Do I need to run more than one tank to know?

Thanks!

 

[OriginHacker21]

My Vehicle - Pontiac Sunfire 2004 2.2L Ecotec 140 hp 5 spd manual. Compression ratio: 10:1.

Drive to work is 110 miles one way with plenty of steep mountains. Several 10 minute uphill stretches are at 90 engine load according to ScanGuage. From MPG observations at fuel pump I see ~2MPG better with 91 octane than with 87.

 

[1 FMF]

how does the scangauge work exactly, what does it report?

ignition timing changes with rpm, what you would need to do for a relatively accurate compare is find long uphill straights and run the car up it at a steady speed. Doesn't have to be steep. Run it up in 5th or 4th or in whatever gear depending on what speed is practical. Keep it at a lower rpm, without varying gas pedal, where the engine starts to lug but keeping it from downshifting. You should see the difference in ignition timing advance under this condition between 87 and 91, which also accounts for mpg increase. The higher octane would allow the engine to have more power at that rpm and wouldn't need to downshift as early. You will probably also notice with 91 octane the car will run uphill at lower rpms easier than with 87. Have a camaro 6speed manual, I notice the same thing going up hills in 6th at 1200 rpms, 50mph. For a given hill the car climbs it in 6th with little additional gas pedal with 93 octane, when I run 87 the engine looses rpm unless I give it considerable pedal but it is obvious I need to downshift.

Another ways to compare would be to accelerate up hill (the same hill preferably) in a high gear under same conditions for both octane grades. Try to be scientific about it, don't do the same hill with 87 but with another person or groceries in the car, or a 1/4 tank of gas vs a full tank, etc.
Also pick a gear where the car will accelerate up a given hill for both octane grades, and put the gas pedal to the floor for as long as possible and observe timing advance or any knock retard. Also take into consideration outside weather temps, humidity, and engine temps.
It would be cool if you are able to get good observations with just 87 vs 91, then use an octane enhancing additive with 87 or 91 and see if there's a relative difference.

 

[Rickey]

I agree with what 1 FMF said. And I wonder if the ScanGuage can monitor the knock sensor in any way? good luck Rickey.

 

[Master ACiD]

in order for the timing to advance or retard you need to have a knock sensor or some sort of fuel monitoring device which sences what type of fuel is in the tank. most cars dont have these devices.

 

[Rickey]

quote:

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Originally posted by Master ACiD:
in order for the timing to advance or retard you need to have a knock sensor or some sort of fuel monitoring device which sences what type of fuel is in the tank. most cars dont have these devices.

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Most if not all OBDII vehicles (all post '96 are OBDII) are equipped with a knock sensor. Rickey

 

[Matt_S]

The parameter you want is knock retard. This is a measure, in degrees, of how much the timing is being retarded due to knock. Zero is ideal. Anything else means you have some knock. If you can achieve 0 KR on 87 octane, there's no reason to go higher. On the Monte Carlo forum I'm on, a lot of guys will monitor KR as they mod the engine to make sure everything's working properly.

 

[Matt_S]

From what I see on the scangauge.com site, no, it doesn't appear to support KR. If someone you know has a scanner, such as a GM tech II tool, or something else capable of monitoring OBD readouts, you should be able to check that out.

 

[SHOZ]

What I do is see what the bottom end for timing is at WOT from standing still. If the timing bottoms out to the low end of the tables then more fuel, higher octane or less timing is needed. More fuel will almost always fix the problem verses the higher octane. Especially with an ethanol blend gas.

 

[OriginHacker21]

I cannot see much of a difference in IGN (Ignition timing) between 87 octane and 91 octane in the city. Mainly because the gas pedal never stays the same so IGN is always changing. *I have yet to do 1 FMF's testing suggestion (going up hill, etc...). Will do that next tankfull of 87.


My biggest thing is from what I've read, 91 octane has more cleaning additives (Chevron w/ Techron), and my knock sensor will increase timing to allow a greater MPG in the long run. Will I save money? Maybe not, but I am giving my car the best.

I've recorded better MPG when going through the mountains with 91 octane. I think it matters more then (more load on engine). But on highway trips across the USA on flat highway - 87 octane should be good.

2.2L Ecotec has compression of 10:1 - seems like 87 would be a low octane for it as other cars with same compression require 91 octane.

Comments welcome!!

 

[Patman]

Like others have mentioned, it's knock retard that you're looking for.

Also, you want to look for it at full throttle. You will also notice that knock retard shows up more in the higher gears, when the engine is under load. So you could end up with no knock retard at all at full throttle in first gear, but then get a couple of degrees in second and 5 or 6 in third. You obviously want to eliminate it entirely, but if you can at least get rid of knock retard in first and second gear, that's the most important thing.

If you're getting no knock retard at all in first and second gear with 87 octane, I wouldn't even bother using premium gas, as you really won't get much better performance out of it.

 

[OriginHacker21]

Is it possible to detect Knock Retard with that ScanGuage I have?


Using 87 octane would save me twenty cents a gallon here in California...

Just don't want to harm my engine via knock retard, etc...

 

[brianl703]

Set up the Scangauge to display engine RPM, thottle position sensor %, engine load and of course engine timing.


When you are on a lightly-travelled road or some other place where it's safe, note the throttle position sensor (TPS) %, engine load/speed and timing.

You should then be able to repeat these conditions with another tank of gas by getting the TPS%, engine speed, and engine load readings the same. Then notice how much the timing has changed.

Note: You may want to repeat the test several times with each tank of gas just to get an idea of what the normal variation in timing is. Then you'll be better able to tell if the gas is making a difference.

There are two other factors which may affect engine timing, they are coolant temperature (do this with a fully warmed engine) and intake air temperature (do this when outside temperatures are consistant from day to day).

 

[OriginHacker21]

So, if I drive very conservatively (not Grandpa'ish but not fast)... then using 91 octane isn't necessary and won't harm my engine. Only if I was WOT all the time would 91 octane help?

I seriously think the increase in MPG with 91 octane comes from me driving more conservatively because I paid more for the gas. The "Thought mentality" thing.

Probably just safest to go with 89 for peace of mind and for no spark retard in car?

 

[ALS]

I have mine set up with gallons per hour, ignition timing, engine load, and throttle position. Since I'm looking for better gas mileage these read outs are the most important for me to be looking at. The compression in my I6 24 Valve 2.9 liter is 10.7 to 1.

 

[OriginHacker21]

Have you done testing between 87 octane and 91 octane?

 

[The_Jeff]

Total spark advance is a "generic" OBD-II spec and consumer scantools can read it.

Knock retard (from optimum) is proprietary, and harder to come by with reasonably priced computer interfaces.

I like the idea of going up a hill at mid-throttle in a higher gear for comparison. At WOT you get tons of fuel dumping (which can cool combustion and suppress knock), no EGR, and the revs climb quickly. IDK about your ecotec but saturn 1.9s start ignoring the knock sensor at higher (3500?) RPMs.

If you want to see something cool, have a friend rap on an accessory bracket with a small hammer while you watch the scanguage with the engine idling. The knock sensor will see the rapping and cut your timing.