You shouldn't use 86 or 85 grade gas, right?

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While I live in GA, I've done multiple road trips to places high enough to sell 85 octane gas. I've used it at altitude many times when not towing and if there were any performance issues they weren't enough to notice.

But 3 years ago I pulled a 4500 lb travel trailer to Colorado behind a F150 with the 5.0 V8. Without thinking I filled up in New Mexico with 85 octane which is sold as regular there. The 1st pass in Colorado was a STRUGGLE, and the transmission got pretty close to being hotter than I wanted. Even on relatively level stretches of road at 7000' I had to run in 3rd gear in many places.

We camped a few days between South Fork and Creed and burned up the rest of that tank without the trailer. For the next tank I used 89 octane, which is considered premium there. 87 is the mid-grade and it might have worked, but I wasn't taking any chances. The rest of the trip was much better. Much less rpm's needed to get over mountain passes and everything ran cooer.

After that I did some research. Based on what I've read they shouldn't be selling 85 octane even at higher altitudes. As someone else stated above it's OK with older engines with carburetors. But in a modern engine use 87.

The owners manuals in most vehicles don't state "use 87 octane". They state use "AT LEAST 87 octane".

 

89's their premium? Wow. I've got a European car that requires at least 91. Most places around here don't even sell 91; just 87, 93, and sometimes 89. I guess I'm lucky that the Rockies are a long way from here.

 

I ran pump e85 (with all its variability) in my 15.5:1 big block Chevy for years without issue. Modern heads and ignition systems make high cylinder pressures on comparatively low octane pretty tolerable. I wouldn’t regularly run 85 in a 4g, but it’s not a time bomb if you do so in leadville

 

As I’m to understand it, most modern vehicles that “don’t take” E85 are because the fuel system isn’t designed to deal with it. Ethanol is nasty stuff that ruins seals and hoses. I kind of wonder if they don’t have enough overhead in the ECU to not get upset having to enrich the mix enough to make up for the lower specific energy E85 has. Not sure about that. Resistance to knock isn’t the problem; E85’s great at that.

 

You're not wrong about Ethanol being corrosive to a lot of common rubbers.

Outside of that issue, on a lot of platforms, E85 requires two major changes - the first being the engine map to actually take advantage of the timing changes E85 allows, the second is fueling (Higher flow fuel pumps and, potentially, larger injectors.)

As an example of the latter, N54 BMWs are limited to about an E40 mix on the factory pickup pump, no matter what tune you run. Replacing the pump in the tank with a Walboro 450 lets you run higher Ethanol content fuel because the pumps just physically flow a higher volume of fuel than the factory option.

 

Interesting. That says to me that the lift pump in those cars is already more or less right on the edge of its flow rate. Google says E85 has 27% less energy than straight gasoline, so if it's only able to run E40 before maxing the pump out, it's gotta be running at like 85% capacity to begin with. I don't know about pumps from that era — I'm an E46 guy — but I do know that fuel pumps from the earlier era they ran M54 engines are not particularly long lived. That might explain it. Which reminds me...my E46 needs a fuel pump...

 

Lived in Colorado 35 years, 91 is premium, not 89.

 

I’m not advocating anyone use e85, I just mean that the historical knowledge of the limits of octane and cylinder pressures have changed a LOT over the past couple decades with new computer aided designs and DI

also, the rubber issues with ethanol are mostly a thing of the past. Nearly any fuel system rubber is now ethanol resistant.

 

How are the 4th gen on 87 vs 91? I always ran 91 in my 2nd gen because the slight power band shift made it so the gears weren't finicky ~75mph. 87 seemed to make it constantly want to downshift. Wouldn't have bothered paying for 91 if not for crappy transmission gearing.

 

Don’t run outside the recommended octane, esp a modern turbo engine.

The factory tunes to whatever gas it targets. A lot of sport car tuners can create maps for every octane level.

Octane has to do with timing, and it takes longer, hotter, whatever for 91 to combust than 85. Spark plugs even have hotter or colder steps for custom tuned applications to deal with the timing of the combustion.

So, yes you can run 85 when calling for 91, but you risk knock etc. And/or Pre or post detonations. Most likely not a catastrophic issue, but risk at damage or reduced life, yes.