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Old 05-26-2022, 07:41 AM   #1
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85 vs 87 Octane for big gassers

After being on the road for 3 weeks heading east from So Cal, we're seeing ever decreasing prices. But we're also seeing lower octane than what we have in California. The low priced spread outside of California seems to be 85 octane. Stations always show the lower priced. What irritates me is the delta between 85 and 87 octane. Usually $.20 a gallon. But a few are as much as $.60 more for 87 octane. Now that makes me wonder. Why 85 octane and is it OK for the big gassers?

Used it once in the Jeep and it didn't make a difference. Talked to a BMW owner who told me that's all he uses.

Anyone able to say if/why 87 is necessary? That's what the Jeep owners manual recommends. What's the downside to 85 octane?
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Old 05-26-2022, 08:09 AM   #2
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Assuming a non turbo engine 85 is fine at altitudes above about 3500 feet. Your engine loses 3% of it's HP per 1000 feet. As you lose HP the octane you need goes down as well. Up on that 8000+ foot pass you could have 65 octane gas and not be able to tell the difference.
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Old 05-26-2022, 08:12 AM   #3
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85 vs 87 Octane for big gassers

I would continue to use 87 because of the heavy load most RV engines are under.

Higher octane is used to control engine pre-ignition (knock) which can be very damaging to the engine.
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Old 05-26-2022, 08:16 AM   #4
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I have used 85 octane in my 2004 8.1 Workhorse engine, when available, since purchasing the coach new in 2003. I have experienced no problems with the engine in doing so. The engine now has 77k miles on it and runs great. Most of the higher altitude mountain states are where you find 85 octane fuel. I ran across it in Utah and several other states on my first trip east from California in our then new motorhome. I do not hesitate to fuel up with it.
And yes, I know the owners manual calls for 87 octane for the coach.
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Old 05-26-2022, 08:22 AM   #5
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Low octane in southwest gas stations

Different engines require different octane. Check your chassis owners manual. My Nissan Pathfinder requires 87 octane.

Many newer engine ignition systems can compensate for lower octane than they are designed for. This results in less power available.

If the engine system is not able to compensate, "pinging" can occur. This may eventually blow a hole in the piston top.

I make it a point to patronize stations that provide at minimum 87 octane. The stations that offer lower octane are price baiting.
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Old 05-26-2022, 08:25 AM   #6
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Octane is a rating that defines a fuel's resistance to engine knock. Sometimes called pinging or pre ignition. Prolonged engine knock can cause engine damage. However most if not all modern fuel injected engines like in your motorhome have sensors that detect engine knock and adjust the engine to reduce the knock. These adjustment also reduce engine performance.

Note: OCTANE RATING is NOT an indication of fuel QUALITY!

A higher octane rating indicates more resistance to engine knock. There are several factors that influence an engine's need for higher octane fuel. Among them are valve and ignition timing, head design, the designed cylinder pressure etc. Atmospheric pressure is also part of the requirement for for octane rating. Internal cylinder pressure changes with at atmospheric pressure.

Atmospheric pressure is dependent on elevation relative to sea level. The higher you go the less atmospheric pressure. Researchers and the federal government have determined that at higher elevations 85 octane fuel performs the same as 87 octane fuel at sea level.

Therefore the western mountain areas sell 85 octane fuel. Assuming your engine is designed for 87 octane fuel at sea level, 85 octane fuel at higher elevations saves you money and performs the same at higher elevations as 87 octane fuel performs. Be aware that your MH engine loses about 3% of its power for every 1000 ft of elevation increase. It loses this power regardless of the octane fuel you put in the tank!

I live in Colorado. I use 85 octane fuel in my MH and Jeep. Both specify 87 Octane fuel. The Jeep has ~190K miles on it and still runs perfectly! I have a high performance Plymouth that has an engine designed for 91 octane at seal level. This engine has no computer to adjust it. If the fuel octane is too low you can HEAR the pinging. At 7000' where I live I can run 85 octane fuel and get no pinging. However if I go to Pueblo where the elevation is less that 5000' I can hear pinging unless I put 87 octane in. By the time I get to Lamar at ~3000' I have to put 91 octane in to stop the pinging!

I hope this relatively long explanation is helpful.
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Old 05-26-2022, 08:55 AM   #7
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The lower the octane number is, the more flammable the fuel is. (just the opposite of what most people think)
Lower octane is more prone to pinging/detonation/pre-ignition. This is not quite as big of a problem in modern engines as the computer will advance or retard the ignition timing as needed.
A lot of people have the idea that a higher octane number equals better gasoline which equals more horsepower and that just isn't so.
Higher horsepower engines usually have higher compression and bigger camshafts that require high octane to prevent pre-ignition.
My guess is that probably on the 2005 and certainly on the 2016 engines, you will see very little (if any) performance difference in the octane rating. you may see a small economy difference, but even that would be minimal.
I hope this helps.
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Old 05-26-2022, 11:33 AM   #8
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WARNING: E-15 gas waiver is a BAD IDEA for ALL RVerís

I wouldn't use 85 octane gas in my MH. Here's another type of gas I wouldn't use that the EPA is going to issue an emergency wavier to use this summer. E-15.
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Old 05-26-2022, 11:56 AM   #9
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Vacationers, beware: Bad gas can damage your car

https://www.usatoday.com/story/money/cars/2013/05/29/bad-gasoline-low-octane-too-much-ethanol/2369579/
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Old 05-26-2022, 12:06 PM   #10
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I have experimented with lower octane gas. It doesn't matter much at higher altitudes. With newer engines unless you have a some way of viewing live data from the engine the knock sensor (for one) might retard performance to deal with the lower octane in ways you couldn't really tell without test equipment. With such a computer system you could be giving up quite a bit of engine performance without ever realizing it with just anecdotal experience.

I suspect my rather low compression old 7.5 would run OK with 85. But since its power to weight ratio is already a bit edgy I don't think I'd mess with it. Plus it's just one year too old to give me engine datastream info. Now with a V10, its knock sensors, and OBD II data output I would sure check it out. With my scanner recording data while climbing hills and such. If I found the timing was being retarded much on hill climbs with the lower octane I wouldn't use it. Of course I'd have to do some baseline testing with 87 before I could make that call. "Seat of the pants" testing wouldn't quite do it for me.
I can't speak for the GM engines, I haven't messed with them.
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Old 05-26-2022, 12:06 PM   #11
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I have used 85 octane several times in my W22 8.1 engine while on trips to places that didn't have 87.

Worked fine for me.
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Old 05-26-2022, 12:08 PM   #12
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I would never fill up my RV with 85 octane fuel. I don't stay at altitude for a full tank of gas. So when I come down to a lower altitude, and have half a tank of that, I would expect problems. Also I have had problems with my Onan generator running on anything but 87 octane with no more than 10% ethanol. Just my .02
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Old 05-26-2022, 12:24 PM   #13
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Quote:
Originally Posted by Persistent View Post
The stations that offer lower octane are price baiting.
Just not true. As many responses here have said, gasoline grades sold at higher altitudes generally are a couple of points lower in octane, because the higher octane simply isn't needed at altitude.
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Old 05-26-2022, 04:34 PM   #14
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All I can say is that my 8.1L does not like 85 octane when I have tried it out west in the mountain states, I get knocking issues even at elevation, and end up stopping in the next town to top off with 89 or 91 octane to fix the problem.
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