gas octane question for PB F150

[EricR]

Every vehicle produced in the past 30 years has an auto retard capability to prevent pre-ignition and knock that can damage the engine. Higher than the specified octane rating fuel provides zero gain but does help the profits of struggling companies like Exxon and Shell and Chevron.

For some reason all these developments over the past 50 years in engine technology have escaped most people. Follow the recommendations on the stickers on the vehicle and in the owner manual and then you will have no worries and not waste your money.

An excerpt from the 2021 user manual:

Some fuel stations, particularly those in high altitude areas, offer fuels posted as regular unleaded gasoline with an octane rating below 87. The use of these fuels could result in engine damage that will not be covered by the vehicle Warranty.​

For best overall vehicle and engine performance, premium fuel with an octane rating of 91 or higher is recommended. ​

​

The performance gained by using premium fuel is most noticeable in hot weather as well as other conditions, for example when towing a trailer.​

Sponsored

 

[imnuts]

It's not at all clear that advancing the spark enables more power. The purpose of spark advance is to ensure complete combustion and max cylinder pressure at TDC from the burning of fuel. In other words there is no more energy extracted by advancing the spark because of higher octane. The basic Otto Cycle only extracts more power if there is a higher compression ratio and higher octane fuel enables use of that by compressing the mixture more without detonation. If you advance the spark beyond the complete combustion point, you get pinging & knocking since you are now compressing a fully burned (& far higher pressure) mixture.

How exactly is the engine compressing the air/fuel more when the piston travels the same distance in the cylinder every stroke? If by higher compression, you mean changing the ignition point, that isn't changing compression, but may change cylinder pressure. Either way, if the engine is getting more complete combustion, your extracting more energy from the fuel, thereby making more power. It likely won't be enough for anyone to feel, but a dyno would confirm.

 

[Gros Ventre]

How exactly is the engine compressing the air/fuel more when the piston travels the same distance in the cylinder every stroke? If by higher compression, you mean changing the ignition point, that isn't changing compression, but may change cylinder pressure. Either way, if the engine is getting more complete combustion, your extracting more energy from the fuel, thereby making more power. It likely won't be enough for anyone to feel, but a dyno would confirm.

No, my point is that the engine needs to be a higher compression engine to use the benefit of higher octane. In other words, just running high octane fuel does not improve power output. The engine compression ratio calls for a specific octane rating. Using higher octane than that is just pouring money out the exhaust pipe. Octane measures resistance to detonation (self-ignition), that's all. So why can gas companies get away with selling 85 octane at high altitude? They're banking on you remaining at or near altitude. Since the inlet atmospheric pressure goes down with altitutde, effectively the engine cannot attain cylinder pressures that would cause detonation. But, suppose you load up with 85 octane and immediately drive down to sea level on the same tank? You risk detonation at high powers. On the other hand today's engines have detonation detection and will start to retard the spark minimizing detonation.

 

[JExpedition07]

It wasn't always.
In 2018, the small print on Ford's 3.5 Ecoboost ratings for the Raptor stated 93 octane. For the non Raptor it stated 87.

I don't know what it is currently, but as you stated, maybe it's all 93 now.

You know I noticed on Fords website there is a note under the 3.3, 2.7, and 5.0 that say power ratings are based on premium fuel but that note isn’t there on the F-150 page for the 3.5 Eco BUT… the Expedition 3.5 DOES say premium fuel required for the 3.5 and it has even less power than the F-150. Fords site is missing so much info….and I just found yet another blank spot lol

 

[Samson16]

It's not at all clear that advancing the spark enables more power. The purpose of spark advance is to ensure complete combustion and max cylinder pressure at TDC from the burning of fuel. In other words there is no more energy extracted by advancing the spark because of higher octane. The basic Otto Cycle only extracts more power if there is a higher compression ratio and higher octane fuel enables use of that by compressing the mixture more without detonation. If you advance the spark beyond the complete combustion point, you get pinging & knocking since you are now compressing a fully burned (& far higher pressure) mixture.

How exactly is the engine compressing the air/fuel more when the piston travels the same distance in the cylinder every stroke? If by higher compression, you mean changing the ignition point, that isn't changing compression, but may change cylinder pressure. Either way, if the engine is getting more complete combustion, your extracting more energy from the fuel, thereby making more power. It likely won't be enough for anyone to feel, but a dyno would confirm.

I'll take a stab at that. Higher octane fuel actually takes more heat to combust than low octane fuel, so higher octane fuel will withstand greater heat caused by compression and reduces the chance of fuel pre-ignition. When the fuel/air mixture explodes early while the piston is still moving up it has to fight through the expanding gas pressure until it reaches the top and can then go down. This robs power and in severe cases can damage the engine. Why spend a fortune on a truck and put cheap gas in it. If it's recommended for towing what does that tell you? It's better all the time. If hard times come and we need to eat(who am I kidding I'd have sold the truck by then...I think), 87 it is, but until then 93 if they have it please.

 

[Gros Ventre]

You know I noticed on Fords website there is a note under the 3.3, 2.7, and 5.0 that say power ratings are based on premium fuel but that note isn’t there on the F-150 page for the 3.5 Eco BUT… the Expedition 3.5 DOES say premium fuel required for the 3.5 and it has even less power than the F-150. Fords site is missing so much info….and I just found yet another blank spot lol

Yeah... Ford could be a tad more specific... (wink, wink, nudge, nudge...) The engine is designed for a specific minimum octane rating, that's 87. I don't have a problem with them saying when towing a heavy load, summertime, steep road, lower RPM, use a higher octane. That allows for wear and off-design things they can't predict... But at the end of the day, my read of the manual says the design point of the Powerboost engine is 87 octane.

 

[Samson16]

No, my point is that the engine needs to be a higher compression engine to use the benefit of higher octane. In other words, just running high octane fuel does not improve power output. The engine compression ratio calls for a specific octane rating. Using higher octane than that is just pouring money out the exhaust pipe. Octane measures resistance to detonation (self-ignition), that's all. So why can gas companies get away with selling 85 octane at high altitude? They're banking on you remaining at or near altitude. Since the inlet atmospheric pressure goes down with altitutde, effectively the engine cannot attain cylinder pressures that would cause detonation. But, suppose you load up with 85 octane and immediately drive down to sea level on the same tank? You risk detonation at high powers. On the other hand today's engines have detonation detection and will start to retard the spark minimizing detonation.

And since PBs are most certainly turbocharged, how does pressurizing the incoming air/fuel mix effect the overall compression force if any?

 

[Davexxxx]

From the owners manual.

"Your vehicle operates on regular unleaded gasoline with a minimum pump (R+M)/2 octane rating of 87.
Some fuel stations, particularly those in high altitude areas, offer fuels posted as regular unleaded gasoline with an octane rating below 87. The use of these fuels could result in engine damage that will not be covered by the vehicle Warranty.
For best overall vehicle and engine performance, premium fuel with an octane rating of 91 or higher is recommended. The performance gained by using premium fuel is most noticeable in hot weather as well as other conditions, for example when towing a trailer. See Towing a Trailer. "

https://www.fordservicecontent.com/...ingEnabled=False&userMarket=USA&buildtype=web

 

[Gros Ventre]

I'll take a stab at that. Higher octane fuel actually takes more heat to combust than low octane fuel, so higher octane fuel will withstand greater heat caused by compression and reduces the chance of fuel pre-ignition. When the fuel/air mixture explodes early while the piston is still moving up it has to fight through the expanding gas pressure until it reaches the top and can then go down. This robs power and in severe cases can damage the engine. Why spend a fortune on a truck and put cheap gas in it. If it's recommended for towing what does that tell you? It's better all the time. If hard times come and we need to eat(who am I kidding I'd have sold the truck by then...I think), 87 it is, but until then 93 if they have it please.

The issue isn't cheap gas. The issue is the only thing octane rating does for you is tell you how well the engine will resist detonation or self ignition. It's a rating. Your engine is designed (eg compression ratio) for a certain minimum octane rating. So long as the engine sees that rating, it'll operate fine. You can use "cheap gas" or spend more on "Top Tier" gas as your budget allows. Top Tier fuel does bring some benefits via additives etc. But when you select the octane button... buying higher than needed is spewing money out the exhaust pipe.

 

[Gros Ventre]

And since PBs are most certainly turbocharged, how does pressurizing the incoming air/fuel mix effect the overall compression force if any?

The engine is designed for a certain compression ratio. The design operating full power point assumes a certain maximum atmospheric pressure (sea level or maybe lower if you're in Death Valley), a specific inlet air temperature, with the turbos full on providing a certain bottom dead center cylinder pressure and temperature. From there the compression ratio yields a peak (before spark) cylinder pressure and temperature. The required minimum octane rating will prevent detonation (self ignition) given those design parameters.

 

[Snakebitten]

You know I noticed on Fords website there is a note under the 3.3, 2.7, and 5.0 that say power ratings are based on premium fuel but that note isn’t there on the F-150 page for the 3.5 Eco BUT… the Expedition 3.5 DOES say premium fuel required for the 3.5 and it has even less power than the F-150. Fords site is missing so much info….and I just found yet another blank spot lol

I'll see if I can find the same sheets for the 2018 Ecoboost. It stated 87 and the Raptor stated 93.

But Ford does, and marketing wise should, keep some separation between the Halo truck and it's sibling.
Although they couldn't do anything about that 570ft lbs of torque that the mighty Powerboost has.

The aftermarket tuners have pretty much proven that Ford could release the 3.5 Ecoboost at any power output they desire to.

 

[Snakebitten]

Anyone monitoring OAR and KR and Timing advance can clearly see how the 3.5 Ecoboost responds to 93 VS 87.

It's not smoke and mirrors

 

[Gros Ventre]

Anyone monitoring OAR and KR and Timing advance can clearly see how the 3.5 Ecoboost responds to 93 VS 87.

It's not smoke and mirrors

I have no doubt the engine responds differently with different octane levels. The smoke & mirrors comes in when one thinks they get more power out of the engine with higher octane fuel.

 

[JExpedition07]

I don’t own an EcoBoost, but I can tell you I have seen dyno results and the 3rd and 4th gen DI versions of the 5.0L V8 pick up significant power running E-85 on the stock tune file just filling up with corn juice. The engine makes more than its rated power figures and most definitely advances ignition timing and actually allows full cylinder load. Many throw the crank torque rating to the wheel on E-85 aftermarket files which means you are picking up 40-50 lb ft. On 87 the 5.0L won’t even let the cylinders load up to their potential because pre-ignition occurrs at its high static 12:1 compression. The valve timing both bleeds compression and the ignition timing backs off. If you fill it with 93 or E-85 (105 octane), that goes out the window.

The 3.5 EcoBoost is twin turbocharged, I can tell you with 100% certainly it is not allowing maximum cylinder load with 87 octane. It is certainly making more power on 93 octane than 87. Is it worth the money? That’s up to you. Certainly not required.

 

[dolsen]

No, my point is that the engine needs to be a higher compression engine to use the benefit of higher octane. In other words, just running high octane fuel does not improve power output. The engine compression ratio calls for a specific octane rating. Using higher octane than that is just pouring money out the exhaust pipe. Octane measures resistance to detonation (self-ignition), that's all. So why can gas companies get away with selling 85 octane at high altitude? They're banking on you remaining at or near altitude. Since the inlet atmospheric pressure goes down with altitutde, effectively the engine cannot attain cylinder pressures that would cause detonation. But, suppose you load up with 85 octane and immediately drive down to sea level on the same tank? You risk detonation at high powers. On the other hand today's engines have detonation detection and will start to retard the spark minimizing detonation.

You’re considering only static compression ratios and not dynamic compression ratios. The only way you would not change dynamic compression ratio is if the valves closed at BDC (assuming the cylinder reaches ambient air pressure at BDC) for both 87 and 93 octane. I don’t know the cam profiles of these trucks so I don’t know exactly where the valves close in relation to the piston position, but I can pretty much guarantee it isn’t at BDC. Using higher octane allows “advanced timing”. This means that the valves will close closer to (but still after) BDC when using higher octane fuels than when using lower octane fuels (aka retarded timing). Advancing the timing and closing the intake valves sooner means less air is pushed out of the cylinder giving a higher *effective* AKA dynamic compression ratio. It sounds complex, but really the theory is pretty simple

Variable timing is kind of a “pseudo” variable compression ratio

Sponsored