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hummingbird 04-14-2009 10:13 PM

Effect of fuel octane number on FE
I was using 93 octane fuel for quite some time, and I was getting about 45 mpg from my 1.5L 5MT City. (This was before jackrabbit pulses adoption).

Then I moved to 87 octane and adopted jackrabbit pulses. FE improved. But now I am experiencing that after the first jackpot tank netting 56.66 mpg, my FE has come down from the high tank, and is ranging ~52 for the next two tanks (the second is only half way through, but I can sense the trend from my fuel consumption trend).

Question is, was the jump in FE because my ECM was used to burning fuel in a certain way, and I gave it a shock by moving over to lower octane fuel? The ECM then adopted to the new fuel, and therefore the transient effect of the first tank is slowly settling to the new steady state parameters for the new fuel?

Alternately, was the cocktail of 93 octane fuel (~2 gal remaining in the tank before the 87 fill-up) and the 87 octane fuel resulted in better FE than any one of them individually?

Are temperatures a factor? the best tank came in Feb-Mar 09, where temps were kind of moderate ~60 to 70 deg F?

I am following the same routine through all these tanks, and am pretty consistent in the way I use the car, so that should not be a factor.

Christ 04-15-2009 12:27 AM

Yes, temps are a factor, because engine intake temps will increase in warmer months. This means higher chance for detonation.

Assuming you have a 12 gallon tank, your fuel's octane rating on your change-over tank would have been approx:

2 x 93 = 186
10 x 87 = 870

870+186 = 1056
1056/12 = 88

So you'd have had 12 gallons of 88 octane fuel, approximately. Simple averaging is all you need to determine a mix of octane ratings.

Yes, it is possible that the ECU had to experience a re-learn with the new fuel. Being OBD-II compliant kind of makes it difficult to speculate without actually knowing how the ECU compensates for potential knock (detonation).

hummingbird 04-15-2009 12:57 AM

Thanks for the reply, Christ! I have two points here -

1. With increase in temp I saw a decrease in FE. Should it not be the other way round?

2. The ECU mistook my ~88 octane fuel to be 93, and gave me waay higher FE. What did it do differently with that assumption that it adjusted for the later tanks? (If I know, I can mimic it, so I get back the FE) :D

Christ 04-15-2009 02:20 PM


Originally Posted by hummingbird (Post 97976)
Thanks for the reply, Christ! I have two points here -

1. With increase in temp I saw a decrease in FE. Should it not be the other way round?

2. The ECU mistook my ~88 octane fuel to be 93, and gave me waay higher FE. What did it do differently with that assumption that it adjusted for the later tanks? (If I know, I can mimic it, so I get back the FE) :D

1. Again, this deals with more things than just intake air temperature. Specific density of the air, dynamic compression ratio in the engine, etc - all affect detonation characteristics. Without more information, I can't say for sure, or even in thought, that higher intake temp would actually increase your efficiency.

2. The ECU may have kept telling the engine to run at higher timing specs, in a more efficient range than the ~88 octane fuel could normally be run at according to the ECU's programming.

This is about as much as I can tell you specifically... hopefully, someone who knows more about your specific vehicle can chime in.

TestDrive 04-15-2009 05:24 PM


Originally Posted by hummingbird (Post 97976)
1. With increase in temp I saw a decrease in FE. Should it not be the other way round?

2. The ECU mistook my ~88 octane fuel to be 93, and gave me waay higher FE. What did it do differently with that assumption that it adjusted for the later tanks? (If I know, I can mimic it, so I get back the FE) :D

Everything else being equal, on normally expects better FE in warmer summer temperatures. Since you switched from 93 octane to 87 octane (the first tank being a mix of ~88 octane??? the second tank being almost exactly 87 octane), everything else isn't equal. Looking at you fuel logs, I also see that you had it in for an oil change on the last tank. Any chance the guys servicing the vehicle bumped the tire pressure down from 50 PSI??? (This the very first thing to look at.) I also see that you've used the AC more often.

It's a misunderstanding to think the ECU mistook ~88 octane for 93 octane. Octane is basically a measure of how subject the air/fuel mixture is to pre-ingnition. As the piston compresses the air/fuel mix, it's temperature rises. If the temperature and pressure are high enough, the mixture spontaneously ignites instead of being ignited by the spark plug. The lower the octane number the lower the temperature and pressure at which the mixture pre-ignites. When the combustion of the mixture is induced by a carefully time spark, a wave of flame travels though the mixture from the spark plug tip to the top of the piston and cylinder walls. When the mixture pre-ignites, it happens more or less everywhere in the mixture at once. The burn is a lot quicker and resembles an explosion. Pre-ignition can quickly damage an engine. If the ECU detects that pre-ignition is (occasionally) occurring, it adjusts the spark timing to prevent pre-ignition occurring.

Anyway, if the tire pressures are still up and you really didn't use AC enough to account for the FE hit, then octane may be the culprit. To test this theory, at your next fill start with ~3 gallons of 93 octane then top off the tank with 87 octane.

aerohead 04-15-2009 05:29 PM

I just read your post,and input from Christ and I believe the timing may be the issue.All grades of fuel have the same energy content,and since the higher octane allows more ignition advance without detonation,and mpg is best at the highest timing advance,then the lower octane fuel may have triggered the vibration-sensor ( knock-sensor ) to retard the timing a bit to thwart detonation,and in so doing,move the engine into a less efficient timing regime.

theunchosen 04-15-2009 09:36 PM

The warm air is causing your engine to be more knock prevalent.

The colder air was allowing your engine to run on lower octane at the same spark location(roughly) without incurring knock(detonation in the cylinder as the gases expand and heat after the spark begins traveling towards the opposite side of the cylinder). In that sense the 88 octane would have helped as well as its slightly more capable of being at the higher compression or timing than 87.

Trying to replicate those results on 87 fuel is inadviseable. What your engine is doing is relieving stress in the engine by doing any number of things to reduce pressure in the cylinder at the cost of FE.

If you trick your engine into not doing that it will knock/detonate/ping more often and then you are probably shortening your engine's lifetime while also putting it within range of death on a particularly bad day. For instance lets say you are in a hurry for once and are accelerating kind of hard and your engine is experience knock because you tricked the sensors into not opening valves slightly early or retarding ignition. In this situation its possible for a component in your cylinder to overheat and create a glow plug. Then you are just a bad combination from pre-ignition and likewise a new cylinder and piston.

hummingbird 04-15-2009 09:40 PM

So, should I go back to higher octane fuel now that the air will stay warm for ~6 months...?

I mean I can then generalize this to a recommendation that one should use higher octane fuel in summer and lower octane cheaper fuel can work equally well in winter, maybe sacrificing a tank's FE in transition each way.

Wot u say?

theunchosen 04-15-2009 10:01 PM

It depends. It might be worth a test run on both. I gather you started hypermiling when you switched? if thats true you ought to get a baseline on both and then tabulate the extra cost of higher octane against the lower octane FE and cost.

I'm going to gamble FE losses are as someone else pointed out that it could be the tires or the AC as well. I highly doubt the higher octane is going to yield a greater savings for you. then. . .if you really want to be ecologically friendly lately premium gas is not selling as high in demand as regular and therefore as long as there is excess premium being created and not consumed thats not ecologically friendly.

That said I'm not going to pay for premium just because exotics and high compressioners stopped buying gas. Stick with low octane get a diagnostic unit for your car, check tires and cut down on AC as much as possible(lowering windows slightly is better than AC if it still keeps you comfortable(all the way down is still more efficient(on most cars) than the AC on full blast)). If that diagnostic unit tells you its encountering knock all over the place I'd switch though.

Is the car supposed to run premium? I kinda of doubt it. I am pretty sure its a not too distant cousin of our FiT, and they don't require premmy fuel. Yeah stateside Fits require unleaded as the only requirement(lowest grade of fuel available here)

hummingbird 04-15-2009 10:30 PM

Answering a few open questions -
1. Yes Oil changed with the service. As synthetic was 2.5 times as costly as unbranded mineral, I chose mineral.
2. Yes they got the tyre pressure down to 32 (habit slaves!) I insisted and made them take it back to the compressor and filled it back up to 50. Lots of weird glances endured... I check and top up regularly at each weekend using a digital gauge and battery operated pump.
3. AC use was not significant. It was only two / three instances, each not lasting 5 minutes before I got nervous about the FE hit and switched to window cracks. :D
4. I P&G regularly since Dec 08, (when I got to know about it on joining ecomodder). The change I had with the octane change was switching from granny mode to jackrabbit mode for my acceleration, on reading about the ABA testing done by SVOboy.
5. My car is almost identical to FIT (same platform), has 1.5L 5MT IDSI engine (not the popular Vtec variety), and does not require premium fuel. the premium fuel was touted to make the vehicle run smoother, so was using it.

Unchosen: What is the diagnostic unit you are talking about?

aerohead: Is the knocking thing equivalent to lugging?

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