[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.