Just found this great video explaining why 2~3 gallons of E85 mixed with 12 gallons of 91 octane makes so much more power in the Skyactiv engine. And the power is right in the daily driving power band.

 

Odd, that’s not what the video says. The horsepower advantage in the tests is due to the higher compression ratio, yet the SkyActive engine has a fixed CR and a fixed boost value. Adding the ethanol will increase the fuel’s octane rating, which might result in a horsepower increase if your current fuel requires the ECU to pull timing to prevent knock, but if your current fuel and ambient temperature doesn’t require the ECU to pull timing, you won’t see a hp increase, just a lower mpg.

When I drag raced my blown Pontiac I added a few gallons of toluene to the tank to increase the fuel’s octane, which allowed me to run more boost and reduce my ET. I suppose I could have added ethanol, but the toluene still worked very well.

 

The 2.5 l naturally aspirated skyactiv engine is actually a variable compression engine, it uses cam intake timing to vary the compression ratio. That is how they are able to run 87 octane in a engine with a advertised compression ratio of 13 to 1.

One of the interesting takeaways from that video, is that the octane increase is not linear, you get the most octane boost with the first couple of gallons you add.

it's also very interesting that direct injection engines get the most cooling effect from running E85.

 

Found a link for a online calculator to show you what octane can be achieved with different mixtures of E85.

https://www.morepowertuning.com/emix

Mix 2 gallons of E85 with 13 gallons of 91 and achieve a octane rating of 92.9

 

If the 2.5NA is a variable compression engine, does that mean there is a power bump to be had using 91/93 vs 87 octane? Will the ECU adjust?

 

I'm curious myself about this question as my girlfriend has the 2.5NA. I know that the Turbo benefits over 4k RPMS with higher octane gas, but I thought that the 2.5NA didn't.

 

Usually the owners manual would state this, but they don't so I'm skeptical about the whole variable compression thing. It seems the only reason that would be a feature is if it has a benefit from a higher octane fuel.

 

This is a quote from the article in the link below about how the skyactiv engine has variable compression:

"They knew that a higher compression ratio would *increase fuel efficiency and economy, but that detonation would be a problem if the temperature and pressure in the combustion chamber was too high for regular gasoline or even *premium pump gas.

Their solution was to reduce the temperature and pressure inside the combustion chamber by carefully managing the running compression of the engine via VVT, by precisely timing and *aiming the direct fuel injection pulses into the combustion chamber, and by extracting heat as quickly as possible from the engine with a long tube 4-2-1 exhaust manifold."

link for the quote:

https://www.techshopmag.com/mazda-s-...iv-technology/

 

If the NA motor produced more power with alternate fuels, why wouldn't Mazda advertise it? They do so for the 2.5T so it makes no sense they wouldn't for the NA as well. I had an Audi 5000 way back when with a 5 cylinder that had higher output on premium gas.

 

Just curious as to why anyone would buy a daily driver vehicle and mix E85 with the fuel for it.
Instead of just buying a vehicle with the HP they want in the first place.