Why do octane numbers jump from 87, 89, to 93? What happened to 91 in this progression? - Jinx
I put 91 in my car all the time.
I barely ever see 93. But, as you suggest, when I do see it, the pump usually doesn’t sell 91.
It varies from region to region. In the Rocky mountain states, 91 is commonly the octane rating associated with high performance. n most other areas, that would be 93. I believe the thin air is the reason for the difference.
It differs by brand too.
In our area, BP & Shell sell 87, 89, and 93; every station, company owned or independent seems to offer the same choices.
Phillips / Conoco sells 87,89, and 91 at every station.
Most of the cheapo convenience stores sell 87, 89, and 91 as well.
Why is there this variety in the US? In the UK we get unleaded or premium unleaded, the difference between which is apparently slim unless you drive a high performance car.
Sunoco offers 87, 89, 93, and 94 at most stations. They used to offer 86 octane around here, and I’ve seen 85 on occasion at other stations. There are a few indie stations that offer 100+ octane levels-- usually around race tracks/drag strips, but every once in a while, out in the middle of nowhere.
I think it’s just the wider variety of cars that we have in the US. For instance, I drive a VW Passat that takes a 91 octane (using the American method of R+M/2…I think the traight Ron value is like 95.) My roommate has a Honda Prelude that is a little bit more high performance, and takes 93. Most cars jsut take 87, though. I don’t know who uses 89.
Oh, and in my experience, around here 91 is the hardest to find. Most stations go 87, 89, 93. Some have 91 AND 93, and I haven’t seen any that have 91 but no 93.
Exactly. Higher octane numbers mean more resistance to predetonation. Less air means predetonation is less likely. Therefore, at higher altitudes, using a lower octane will produce the same* results as a higher octane at sea level.
*Discounting, of course, that there will still be less oxygen available to the engine.
ETA: In this context, “high performance” means “high compression”. I have a 5.7L Hemi, which I would consider high performance, yet the manual recommends 89. This is because, with the larger displacement, a very high compression becomes less efficient.
So that is why they sell 85 up here? I live in a town at 5800 feet, and have never seen that before moving here. It’s not going to do any harm to my car at this altitude, which the manual for says takes 87?
Exactly. At that altitude * you only get about 80% or less of sea level power. If you can’t hear it knocking (a rattling sound at full throttle) then you are not hurting the engine. With modern fuel injection you will also get better fuel economy due to decreased pumping losses and lowered aerodynamic drag.
-Kevbo, who has never lived below 5000’MSL.
- Assuming annual average ground level air temperatures. The performance loss for an aircraft operating at that altitude over the ocean would be less because the adiabatic lapse rate would provide much cooler (denser) air than typical ground level temperatures. Rule of thumb: 3.5% performance loss for a 1000’ altitude increase at constant temperature, or for a 10’C temperature increase. Normal lapse rate is ~2 deg/1000’ so an aircraft loses only about 2.8% performance/1000’ when climbing, but it is very important to mind runway density altitude with regard to takeoff performance.
Is there a wider variety of cars in the US? Do you have a cite for this?
Thanks! I’ve been meaning to ask; I did some Googling but didn’t find anything that explained it well. My car seems to be doing fine on 85, which I didn’t realize I was putting into it for a while (I was going for “regular” and not paying attention to the numbers, since I didn’t realize it went lower than 87!)
It’s my understanding that UK “regular” is 95 RON, and “premium” is 97-98 RON, depending on brand, with BP offering 102 RON at select locations, so it’s pretty much the same as North America. The lower octane fuels available in North America probably aren’t in the UK because there isn’t a great disparity of altitude, and the kind of large displacement, low compression engines common in North America are rare.
Just from looking at the badges of the cars in front on the run into work, most wouldn’t have been over 2l in size, petrol or diesel (and we’re talking about petrol here in this thread).
The funny part is that lower octane fuel actually has more stored energy per unit volume than higher octane. However, since the higher octane can be compressed more, we can utilize a greater percentage its stored energy because we can force the oxygen balance to positive.