Naturally aspirated and 87 octane...

[Sithspawn]

Originally Posted by dwx

Those are very different engines. Mazda runs 13:1 in their Skyactiv 2L but it only makes 155hp. It runs on 87, they have a 14:1 version which runs on 91. DI is the main reason they can pull it off.

Skyactiv certainly has my attention not for the power it produces, but the fact that they can get 87 octane to work on a 13:1 compression engine. One thing to notice is that the Skyactiv engine makes 4ft-lbs of torque more than the FA20. Same displacement, both DI, and yet the Mazda makes a touch more torque with a lower octane fuel and an additional 0.5 compression ratio. Personally, I find that extremely impressive. After reading a few articles on some of the things that they did to accomplish what they did, it makes sense. The funny part is that some of their strategies (like long tube headers) are something the aftermarket has been doing for ages.

So, back to the OP's question. Apparently, if you can reduce the residual exhaust gas remaining in the cylinders, you will have a better chance of being able to run 87 octane and possibly not have to pull quite so much timing. Because, after all, fuel doesn't ignite due to pressure: it ignites due to temperature. Pressure is how often the molecules collide and temperature is how fast they are moving when they do.

[cactus]

Originally Posted by dwx

Those are very different engines. Mazda runs 13:1 in their Skyactiv 2L but it only makes 155hp. It runs on 87, they have a 14:1 version which runs on 91. DI is the main reason they can pull it off.

Skyactiv motors are Atkinson cycle IIRC which lets one run a higher compression.

[arghx7]

Originally Posted by cactus

Skyactiv motors are Atkinson cycle IIRC which lets one run a higher compression.

The cams are parked in an Atkinson cycle position but they can dial it out with phasing. Basically they turn Atkinson cycle on or off when they want to.

[Element Tuning]

Yes you can tune it to run on 87 octane and I can't imagine it would be hard at all. The DI suppresses knock immensely and when I was first developing the Hydra EMS the best I could do on port injection only was about 20 hp less than oem with DI and port. So on 87 octane tuning with DI of course I bet we could make stock Hp but you would give up safety margin since more aggressive tuning on 93 gets +15whp.

So while it might not make sense to pay for a 87 octane tune if you have a user tunable engine management system like the Hydra and you do your own tuning, why not

[stugray]

"Because, after all, fuel doesn't ignite due to pressure: it ignites due to temperature."

Ummm.. ever heard of a Diesel?
And pressure and temperature are directly coupled in the cylinder of a car. remember PV = nRT? (as V {volume} gets smaller, BOTH pressure and temperature go up)

And "detonation" does not only refer to the "explosion" of the fuel when the spark goes off (as opposed to "deflagration" - what we want), but also the possiblility of PRE-ignition-detonation: When the airfuel mixture ingnites due to Pressure/Temperature BEFORE the spark goes off.

If you run too low of octane in this engine you might get detonation BEFORE the spark goes off.
If that happens, nothing you do to timing is going to save you from detonation.