octane rating and engine performance

4x4ord wrote:

noteven wrote:

4x4ord wrote:

noteven wrote:
I drove a company F150 3.5 ecoboost that cost less money per mile for fuel running 91 vs 87.

This is from the owners maual for the ecoboost engine:

We recommend regular unleaded gasoline with a pump (R+M)/2 octane rating of 87. Some stations offer fuels posted as regular with an octane rating below 87, particularly in high altitude areas. We do not recommend fuels with an octane rating below 87.
To provide improved performance, we recommend premium fuel for severe duty usage, such as trailer tow.


Maybe the engine in your company F150 is all carboned up from using the wrong fuel. If the engine is healthy there is no way its going to be saving you money using premium.

So Shell V-power 91 carboned up a ecoboost during our economy test? Those dirty rats!

We interpreted the manual wording above to mean 87 is design minimum, don’t use 85, and engine performance can be optimized using 91. Seemed to work.

In today’s age of consumer protection a company would be quickly harassed for “requiring” a more expensive propellant when they can program an engine to run on “regular” so the consumer can choose.

It makes sense that to get peak performance (as in power output) out of a boosted engine higher octane fuel would be required... so long as the fuel and ignition timing maps are tuned to aim for the higher cylinder pressure that can be optained with higher octane fuel.

Detonation occurs after the spark has ignited the fuel mixture and the cylinder pressure rises to a point where fuel can ignite spontaniously ahead of the flame front. Higher octane fuel is able to resist detonation.

Doing a fuel economy test you are running down the road at 60 mph running part throttle conditions ... the spark advance and cylinder pressure are reletively low and high octane fuel will not be not be required to run the optimum timing advance. On an engine desugned to accept regular fuel there is no way I'm going to believe high octane fuel will return an increase in fuel economy that will offset the additional cost of the fuel. High octane fuel is more likely to cause poorer fuel economy, if it is not needed, due to its slower burn rate.

High octane fuel has little to do with “burn speed”. It has all to do with the ability to resist detonation. And if you look at tuning maps you will notice that timing is highest or most advanced (for best torque) at cruise which gives you the most power at the least throttle opening which equals best mileage. For the best mileage you need the coolest intake air (timing subtracted for hotter air) and using fuel with a high enough octane rating to resist detonation for your engine type/load/altitude etc.