I tuned my engine for lower octane fuel

[twj347]

The DIT engine in my Subaru is designed for 91 octane fuel. I figured I would try to save some money on fuel by making a tune for 87 octane. The price difference between 87 and 91 octane fuel is about 12-15% around here, so there are some decent savings to be had.

Lower octane fuels will cause engine knock at high loads. Modern engines will automatically adjust for any knock that is detected by retarding timing, but the engine will still experience more knock as the ECU constantly tries to advance timing back to normal levels. Retarded timing will result in less power and efficiency, but this will only be required at higher engine loads.

I used engine management software to reduce ignition timing. At lower loads, below 35-40% engine load, the engine is not knock limited, and therefore no changes to the tune had to be made. At higher loads, timing is retarded by 1.5 to 2.5 degrees at most load points. I'm not sure how much power is affected by this, but I've seen rules of thumb that suggest a 2-3% reduction in power per degree of timing. In most normal driving, loads are low enough that ignition timing, and hence fuel economy, is unchanged.

I've been running 87 for the last few tanks, and the engine is just as knock-free as my previous tune. Power loss is not very noticeable, and fuel economy hasn't suffered noticeably. Between this and my other engine-tuning tweaks (especially lean burn), I'm saving over 20% on fuel costs.

[teoman]

That is awesome. Congrats man.

If you look around, some have used water injection to overcome knock.


I do not own a subaru. But if I did, i am sure I would appreciate a mini tutorial on how to do what you did. Software, visualization, exactly what you changed.

Keep up the good work.

[twj347]

Here are a couple of pics showing what has changed if you want a visual. The first is the actual ignition timing. The second shows the difference between the base 91 octane base tune and the lean-burn 87 octane tune. The positive (red) values show where ignition timing was increased in the lean burn range. The negative (yellow) values show where ignition timing was decreased allow for 87 octane fuel.

Ignition Timing

Ignition Timing Delta

There are coarse knock learning tables that define how much ignition timing is retarded after knock events. I used these tables as a guide for determining how much ignition timing to pull, then fine tuned the values a bit based on knock readings during driving. The software used is Accesstuner Race.

[teoman]

The imgur is not working for me on an iphone.

[Ecky]

Generally you'll see either "Premium fuel recommended" or "Premium fuel required" under the gas cap door. I understand the difference is that "recommended" are vehicles whose knock sensors will pull timing and safely operate with lower octane gas, while "required" are those that will be damaged by running regular.

Out of curiosity, is there any advantage to re-turning a vehicle where premium fuel is only recommended?

[slowmover]

I tuned my last big V8 (7.0L +) for higher octane. Also increased thermostat temp from 185 to 195F. Had to rework mechanical & vacuum advance, plus change carb fairly much. But it was worth more than 10% around town. And more fun. Asctge quality of gasoline is nothing like it was in the 1960s (not octane), it was a practical necessity. Not just FE.

[19bonestock88]

Originally Posted by Ecky

Generally you'll see either "Premium fuel recommended" or "Premium fuel required" under the gas cap door. I understand the difference is that "recommended" are vehicles whose knock sensors will pull timing and safely operate with lower octane gas, while "required" are those that will be damaged by running regular.

Out of curiosity, is there any advantage to re-turning a vehicle where premium fuel is only recommended?

Oftentimes, the timing curve is pretty conservative and some cars can get away with a smaller constant retard on timing than the PCM retards timing upon knock detection...

Example, if my car could handle 24* of timing advance running wide open on 91octane but pulls 12* out when it detects knock, it might run better on regular if timing were backed off to 19* of advance to eliminate knock... would yield more advance on the cheaper fuel...

[cRiPpLe_rOoStEr]

Originally Posted by Ecky

Out of curiosity, is there any advantage to re-turning a vehicle where premium fuel is only recommended?

If you could live with a slightly decreased performance rating for the sake of using the cheapest fuel available, even if any MPG improvement becomes minimal...

[twj347]

Originally Posted by Ecky

Generally you'll see either "Premium fuel recommended" or "Premium fuel required" under the gas cap door. I understand the difference is that "recommended" are vehicles whose knock sensors will pull timing and safely operate with lower octane gas, while "required" are those that will be damaged by running regular.

Out of curiosity, is there any advantage to re-turning a vehicle where premium fuel is only recommended?

I suspect the only difference between "premium required" and "premium recommended", is that in the latter case, the manufacturer is confident enough that their knock sensors and knock control strategy are sensitive enough that no significant damage from knock will occur if higher octane is not used. Other manufacturers might want to have the option of voiding engine warranty if low octane is used.

My user manual says that premium is required, but that 87 octane can be used on occasion if necessary. The fact of the matter is that any engine will constantly try to increase timing up to a set maximum, to learn how advanced timing can be without knock. This results in occasional knock on low octane fuels. The stock tune on this engine is very conservative with timing at high loads when poor fuel quality is detected. This pretty much ensures that the knock learning is done at lower loads where knock won't do any real damage.

Knock at low loads is said to be pretty inconsequential, but it's probably best if it's tuned out. I'm also able to run more optimal ignition timing for better power and economy. The stock knock control is so conservative at higher loads that it would result in quite a bit less power if I just ran 87 octane on the stock tune.