octane rating and engine performance

I'm not at all worried about following the manufacturers recommendation. One problem of autodidacts (most of us in most subjects) is we don't know what we don't know.

shelbyfv wrote:

Lynnmor wrote:
Some fuel stations, particularly those in
high altitude areas, offer fuels posted as
regular unleaded gasoline with an octane
rating below 87. We do not recommend
these fuels.

Thanks for posting this. I have a trip to the Rockies coming up and I have to admit I've never read the manual. I'll avoid the 85 octane. I have no credentials that would justify me second guessing Ford.

If you knew how ICEs worked you wouldn’t worry about second guessing anyone. If naturally aspirated, an engine, in general needs less octane with lower air pressure (altitude) for a given set of parameters. 85 is a fine sub for 87 at altitude.
Do you think they magically sell “bad” gas in the mountains?

For clarification's sake, I did buy the 85 octane. I could tell a difference but it's hard to differentiate exactly how much of a power loss because of the altitude vs fuel. I did do a considerable amount of driving without the trailer and I really couldn't tell much of a difference in either fuel economy or performance that couldn't be attributed to the elevation.

However, once I got back to Texas and out of the mountains and 87 fuel in Amarillo, I could tell a significant improvement in performance. I've made multiple trips to the mountains of CO and SD in the past and always bought the cheapest gas available without paying much attention. I honestly don't know why I noticed it this time, but I've got over 100K miles on my truck and have had no issues what's so ever based on my fuel.

I guess for my own peace of mind, next year, I'll try the 87 in the mountains to see if I can tell a difference.

Lynnmor wrote:
Some fuel stations, particularly those in
high altitude areas, offer fuels posted as
regular unleaded gasoline with an octane
rating below 87. We do not recommend
these fuels.

Thanks for posting this. I have a trip to the Rockies coming up and I have to admit I've never read the manual. I'll avoid the 85 octane. I have no credentials that would justify me second guessing Ford.

In my current truck and the truck on order, the manual specifically states to use 87 octane fuel. It calls out that in states where a lower octane rating is offered to NOT use them. In addition with the Eco Boost and the 6.2 I have on order, Ford recommends the use of premium fuel for max performance when towing or when it hot conditions. I only put about 6000 miles a year on my truck and it's used mainly for towing so, I use Premium. I would not use the 85 Octane fuel in my vehicles. Check your manual for the final answer on what is recommended by your manufacturer.

Lynnmor wrote:
I understand all that, I also know that knock sensors and related precautions are still utilized for protection from low octane fuel. If injection and turbo systems were capable of completely eliminating knock, we wouldn't even be having this discussion and Ford would say run whatever you want for fuel.

Ford does say to run whatever pump fuel you want that is 87 or higher, but they also say that 91 will add more performance to the engine.

I am not sure you understand how it works. The engine will run at the optimum timing until it detects knock. Then it will pull timing back to a point where it is not knocking thus determining power output. This is happening all the time while your engine is running and it is a constant back and forth depending on load, heat, and other variables.

So for example. Say you have 87 octane in your Ecoboost truck and you are towing a 10k trailer. The engine will be constantly adjusting timing to give you the most power it can on 87 octane without knocking. Lets say it can only give you 350 hp and no more without the engine having multiple knocking events per cycle.

Now lets say that all of the sudden 91 is in the tank and the knock sensors are not detecting any knock at the current timing of 350 hp so it advances it a little more increasing power output to say 360 hp. Remember, it is trying to run at optimal power levels so it advances timing a little more to 370 hp output if it does not detect knock at 360 hp. If it senses no knocking there then it will continue to advance timing as far as it can until knock is detected say 375 hp and then pull it back to a degree where there was not knock at 370 hp.

If you ever data logged the timing on the Ecoboost or any modern engine then you will notice that it is constantly adjusting timing multiple times a second. This is why engines like the 1st gen 3.5L Ecoboost had an advertised power rating of 365 hp on 87 octane and a non advertised power rating of 385 hp on 91 octane. It is not that it needs 91 to operate even when towing, but it will perform better if it does since 385 hp will pull a trailer better than 365 hp. Hence the reason why Ford recommends 91 IF you want better performance from the engine.

My 2011 Ecoboost manual says that for best performance when towing to use 91 octane. I have not been above 7000 ft yet pulling my TT but at 6500 ft I did not notice any problems with burning 87 octane. My TT is only 3200 pounds loaded so maybe the light weight helps. I think that when I get into the areas were 85 is the "normal" regular gas I will pay for the 87. The turbo doesn't care wht the elevation is.

I understand all that, I also know that knock sensors and related precautions are still utilized for protection from low octane fuel. If injection and turbo systems were capable of completely eliminating knock, we wouldn't even be having this discussion and Ford would say run whatever you want for fuel.

Lynnmor wrote:
We like to think that the engine can compensate for poor choices in fuel, but forget that the compression ratio cannot be adjusted. The Ecoboost engine is just one example of turbocharged engines that must lower boost, again to compensate for low octane fuels.

I don't know how Ford should write a manual, because even limiting words to non-technical confuses the masses.

I think you are applying the limitations of port injection to direct injected engines. Direct injected engines can run at higher compression ratios without the need of higher octane fuel because it can better control injection timing thus deterring knock or pre-ignition.

I would do some more reading up on how direct injection differs from port injected limitations. Here are some great articles to start with.

DI Engine Guide – Direct Injection

Explained: Direct Injection Piston Design and Tuning Theories

Notice in the first article where it states that direct injection engines can run a higher compression ratio than a port injected engine by ten percent. This is why other direct injected N/A engines like GMs 4.3L and 5.3L can operate on 11.0:1 compression ratios with 87 octane. The GM 6.2L has an even higher compression ratio of 11.5:1, but GM requires 91 octane fuel in that truck.

Boosted engines create their own atmosphere and turbocharged engines can compensate for altitude by just changing waste gate settings. They are really only limited by supercharger or turbo charger capacity. That's how some piston engine aircraft were able to operate at altitudes of 40,000 ft.