Do not use premium fuel on prius!!!

winterized gas drops the mpg by 4-6. Premium recovers about 3

 

So, on a fixed valve timing naturally aspirated engine that's calibrated to run on 87 at sea level, I would have no qualms about running it on 85 at high altitude... as long as I knew I wasn't going to lower altitudes on that tank. The reduced intake air pressure will counteract the lower octane nicely.

On a variable valve timing engine (like the Prius), however, I'd be more reluctant to do that. It depends on the engine, but it's possible that they're advancing the intake cam to keep more of the intake charge in the cylinder, to compensate for the altitude. This means less power is lost at altitude, but it also means that you're closer to sea level cylinder pressures and heat. Therefore, lower octanes aren't safe if the engine's strategy is to do that. There are other strategies possibly being used, however, like matching cam timing to load (advancing it as load increases), and that would be safe to use lower octanes (but remember to get higher octane fuel into it before descending). I'm not sure what the Prius engines actually do, but it's best to play it safe and run 87, IMO.

Forced induction engines, for what it's worth, are where you should never go lower octane. Many of them will try to maintain a specified absolute manifold pressure, and therefore are insensitive (or at least are less sensitive) to altitude, increasing boost to compensate for altitude effects.

 

I want to close the loop on the oil viscosity question, because it's more important than the 87-93 octane battle, which won't affect anything but your wallet.

This is the G3 section, and the recommended viscosity for the ZW30 is 0w20 (synthetic) if memory serves.....but pretend that I'm wrong and look it up for yourself. It's in the manual, and it might be different for the G2.
Here's the thing.
All Prius engines are liquid cooled, and their temperature is regulated by a thermostat.
This is sooooooo critical for this motor that unlike almost every other car on the planet, there is no engine temperature gauge in a Prius.

OK....
So let's presume that the normal operating temperature inside a running G3's ICE is 180-185 degrees (f).
You KNOW what that means, right?
That means that in Texas, on I-10, running at 85-mph, during the summer, and with 4 Texas-sized adults as cargo your ICE will be operating at...................(wait for it!)......................180-185 degrees (f).
Just like it would be in Buffalo, NY in the winter, crunching along some back alley, in the dead of winter and with 1 95-pouund college student at the wheel.

Don't overthink this.
While I don't think that you would wreck your car by clubbing up to a 5w-30, I don't think that you would derive any benefit from doing so either.

 

And another consideration is that the 0W20 means that when cold, it flows like a 0 weight oil (which will get oil to the engine components faster - especially important in a hybrid that's cycling the engine), and when hot, it flows like a 20 weight oil.

Hot is going to be basically the same temperature anywhere, as @ETC(SS) pointed out. It's cold that's the difference - in Texas, a 5W20 may be fine (although you really want a synthetic, and at that point, why NOT run a 0W20?), whereas in Minnesota, a 5W20 is a really bad idea.

 

+1

 

In North America yes, around the world, Australia for example, a much fuller gamut of oil weights are indicated as acceptable. I can't recall from memory, if 0W20 is still preferred.

We're off topic tho, this thread is gas octane.

 

Yeah, but that's really a non-topic....or it SHOULD be.

Again...the manual is pretty clear, even if the way we measure octane isn't.
Most people don't even know what the octane rating for fuel measures exactly - and two things do not help this argument:
1. Branding by evil oil companies. They call it "Premium" ....or "Hi Test"....which leads the average consumer to think that it is "better" for your car than plain old regular gasoline....which it is but only if your engine is prone to pre-detonation.
2. Overly complicated octane rating systems by evil governments that try to keep evil oil companies from looting drivers.

OK....so a gasoline's octane rating is a measure of its resistance to knock as compared to a particular pair of hydrocarbons. Isooctane, an especially knock-resistant variety, is 100 on the octane scale; at the other extreme, n-Heptane is 0.
Two procedures with differing test conditions determine a gasoline's Motor Octane Number and its Research Octane Number. The average (MON + RON)/2 is the one we call "pump octane" displayed on the service station pump.
Remember.....this has nothing to do with energy content. In fact, depending on its blending, a premium fuel may actually contain less energy per unit volume than regular, although in the real world I think I read where 93 octane fuel is a little more energy dense....further exacerbating the arguments.
Prius engines run at relatively very low RPM as compared with normal engines.
That's why in a Prius the tachometer is located in the same place as the temperature gauge.

Rather than trying to calcuguess RON over MON divided by the number of cylinders times RPM on Tuesday mornings when the gas station tanks are the coolest it's a lot easier just to use Regular gas from a regular gas station.


The whole thing about Priuses is that you're supposed to drive them somewhat like an ordinary car and they will reward you with extraordinary efficiency and better than average reliability (especially if you check your oil regularly.)

Otherwise?
You're probably missing out on one of the better things about rolling a Prius...................

 

Totally on the same page. You know the minutia of why, I just stumble through with the "executive summary", lol.

There was a Canadian TV consumer watchdog show "Market Place" episode, on the myths and misnomers of higher octane gas. Bottom line was that 90% of the cars on the road don't need higher octane, and those for which it is recommended do not bad on regular, because the car's computer can retard timing to compensate.

There is a strange disconnect between this, and the equal billing the various octanes have at stations.

 

However, retarding timing can hurt efficiency, so that isn't necessarily what you want.

Really, use what the manufacturer recommends, unless you know enough about the car's tuning to know why the higher octane was recommended, and then you can make an informed decision from there. (I'm guessing that, for instance, the Gen 3 variant of the 2ZR-FXE in Australia's requirement of 95 RON is a side effect of the importer just copying European specifications without regard for what's actually needed, as in North America, that engine calls for 87 AKI (or 91 RON), and I have a suspicion that it probably calls for 90 RON in Japan.) Conversely, something that recommends 91 AKI in the US probably will actually run better on it.

And, regarding the original topic, higher octane than recommended shouldn't cause a problem unless your ignition system is marginal.

 

Because higher octane takes more oomph to ignite?

The one time I had trouble with gas was decades back before alchohol a common additive. Filled up a carbureted Accord at Mohawk (home of the green gas) that added alcohol. The car stumbled at idle; never went back there.

 

The entire point of higher octane is that it has higher resistance to ignition, so, yes.

 

The funny thing is, if you ask most anyone on the street, and (sadly) gas station attendants, they'll tell you everything but that.

 

We know the maximum intake advance for Gen2 and Gen3, and the numbers are not high enough to create sea-level-like cylinder pressures while at high altitude. So this isn't a problem for those eras. I haven't hesitated to use 85 octane in my Prii when traveling in the zones where it is available.

I haven't been following the Gen4 close enough to notice its intake valve adjustment range and maximum compression ratio details. Do others have those details?

 

It's really not that complicated. By either of those two test methods, the octane rating of a particular gasoline recipe is simply the percentage of isooctane in a mixture of n-heptane and isooctane that provides the same resistance to knock as that gasoline recipe. At least that was the classical definition, invented many decades ago. Extrapolation was needed to rate fuels with knock resistance better than that of 100% iso-octane (aka, 2-,2-,4-trimethylpentane).

 

I think this is why my mileage decreased when I tried high octane has in my Prius. Because it has higher resistance to ignition, not all of it combusts, and so mileage is reduced.

However, a friend of mine says that if you leave the fuel in the vehicle for a long time, such as some Chevy Volt owners do, you should use high octane gas because the ethanol in the fuel will eventually evaporate, which will reduce the fuel's octane. Has anyone else heard this?

 

The 2011 - 2015 Volt recommended 91 octane. 2016+ recommend 87 octane. GM's statement was that the use of Premium allowed them to tune the generator to be more efficient, something like 5% - 10%. By their figuring that meant saving more money than buying the cheaper stuff. From day 1 GM had been back peddling that decision saying things like "the world won't end if you use regular but it won't be as efficient so we recommend 91." I don't recall GM ever putting a statement out there that premium lasted longer than regular. Apparently they redid their math in 2016 and found 87 octane is more efficient, but my bet is the marketing guys explained to the engineers that recommending premium was a turn off for consumers.

As for ethanol evaporating, I don't think so. The Volt, like my 2016 Prius (not sure when Toyota introduced it, it might have been on the Gen 3), uses a sealed and pressurized fuel tank to limit evaporation. In the Volt (and I'm assuming the Prius) there is no evap canister. I trust GM and Toyota to not be knowingly allowing anything from the fuel to evaporate to atmosphere because that would piss off the EPA. Clearly not everyone plays by the EPA's rules (VW) but I have no reason to suspect that the system the Prius and the Volt use does this. It is a system that was engineered and agreed upon by multiple OEMs working together.

The Volt tries to go into "Maintenance mode" every 6 weeks (don't quote me on that, this off the top of my head) to burn some gas. If you keep telling it not to go into maintenance mode, eventually it won't ask and just does it. The system is designed to force you to replace all the fuel at least once a year. So if the fuel in your Prius is over a year old you may have cause for concern.

 

in the gen3 there IS an evaporation canister (that's where the leaks, responsible for the last recall, show up) /

afaik, ALL gas deteriorates (regarding the octane number)/
lately I'm fueling up maybe every 4 month, so to make sure I still have gas with the recommended octane number after a few month, I fill up with premium/
(& no, I didn't notice ne negative side effects yet (but to be honest, no positive ones neither)
$4.- more in 4 month per tank - I guess I can live with that//

 

What actually happened is that the Gen 2 Volt's ICE uses direct injection, which greatly reduces octane sensitivity of the engine, as it can inject fuel only when needed.

Based on the power outputs and RPMs, I suspect GM isn't running any Atkinson cycle at peak. So, the 10.5:1 compression of the old engine and the 12.5:1 compression of the new engine are most likely true compression, not just expansion.

 

No...Never heard ethanol evaporates from tank. Since the tank is enclosed it should not matter too much.

Also, big No on resistance to ignition causing less MPG. Higher octane has higher resistance to "auto-ignition" which means higher octane will wait for your spark plug to make a spark. Once your spark plug makes a spark in the vapor/air mixture inside the cylinder...KABOOM you get a mini-explosion. Failure to ignite would be missing a cylinder and extreme rough engine. Low octane fuel knocking is when the explosion happens a little before the spark, because lower octane tends to pre-ignite without the spark. In the extreme case, low octane is diesel fuel at ~20 Octane, diesel needs no spark.

Your Premium could cause lower MPG if it contained less energy (eg; 100% ethanol is high octane but low energy). But in California (RFG) you should expect similar MPG for Reg to Premium.

 

Well, you've also got ignition delay, not just resistance, coming into play. So, the fuel could light off late, which would result in work being wasted (the fuel still being burning when the piston hits BDC). I'd think it could adjust ignition timing based on that, though, because it'd know how much it needed to adjust from the crank or cam sensor.