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Old 11-29-2020, 01:57 AM   #61
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Originally Posted by Velosprout View Post
Read your owners manual. As per the online 2018 E450 manual:

“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.”

Ford built the vehicle; I would follow the owners manual instead of the dealers advice.
Exactly. I do run the 87 at the higher elevations where it's the mid grade because the high octane isn't needed at altitude. They don't say that in order to keep it simple.

At sea level up to about 5000' I use the 91. I can tell a difference and definitely get at least .5 mpg more. I'm always towing when RVing.
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Old 11-29-2020, 11:18 AM   #62
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Originally Posted by grumpyoldman View Post
For those of you that state that ethanol increases the octane of the gasoline, that is true. Knowing that, the refineries produce a lower octane fuel, which is cheaper to do, so when the ethanol is added the octane comes up to the advertised number of 87,89,or 91. You are not getting a higher octane than what is advertised. Also high octane gasoline is not regular gasoline with more additives. It has to go through additional processing at the refinery to increase the octane. That is the reason that high octane gasoline costs more than low octane gasoline.


If you have 87 octane that is supposed to have 8 percent ethanol from the factory. And you take the same fuel and put in 4 percent unintended extra ethanol you will have higher than 87 octane.
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Old 11-29-2020, 12:06 PM   #63
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In the old days of carburetors and fixed ignition timing (you adjusted the timing yourself and it remained fixed until you adjusted it again), avoiding preignition was the limit on advancing the timing and leaning mixture.

Preignition happens when gas-air mixture ignites due to HEAT and COMPRESSION before it is supposed to. Obviously, this is usually when your engine is running under heavy load (like towing up a grade).

Higher octane gas suppresses preignition through needing slightly more heat and pressure to self-ignite AND slower spread of the flame front.

Modern engines have knock sensors and computer regulated timing to prevent preignition. So if you have knocking with a modern engine - your issues can be knock sensor(s), computer programming, and insufficient octane when your engine is running hot and at high power.

Easiest cure is to increase octane to see if that alleviates the problem. You only need the increased octane when under heavy loads.

Because I learned this flying super-charged, carbureted, air-cooled piston aircraft where the pilot monitors and controls engine temp and mixture, when I'm towing I increase the octane of the gas I buy as a preventative measure. Around town, not towing at 6K altitude 85 octane is just fine. Towing, I go to the manufacturer's recommended 87 octane minimum so I don't have to worry about the knock sensors and computer retarding timing. When towing below 2K altitude, I'll go up to 89 octane as a precaution because of the increased internal pressure and because the gas engine has 235K miles on it. Although the engine temp gauge stays in the same place, I can feel the heat pouring out of the engine compartment when I stop on a warm day after towing at 75mph across Western Texas. The higher octane gas may not be critical but it gives me peace of mind under the circumstances.

just my experiences
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Old 11-29-2020, 05:57 PM   #64
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Quote:
Originally Posted by Velosprout View Post
Read your owners manual. As per the online 2018 E450 manual:

“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.”

Ford built the vehicle; I would follow the owners manual instead of the dealers advice.

My 2016 E450 manual does not have this statement, just recommends 87 octane. I went to Ford online and found that this statement recommending 91 octane started in 2018 owners manuals. Anyone have any idea why this changed in 2018, can one assume that this applies to pre-2018 as well?.
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Old 11-30-2020, 11:07 AM   #65
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My guess is that is when Ford re-formatted the V10 from 363 HP and 420# of torque down to 320 HP and I think 457 ft. lbs of torque. However, those stats covered the V10 used in motor homes, one of four different V10's I believe used in various Ford platforms.
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