I just filled up with 92 after finally depleting the 87 I added many moons ago. I can say with absolute certainty that the engine is now much quieter at lower RPMs such as when cruising (9 to 11 o'clock on the power meter), a little quieter at noon, but not much difference when at high RPMs up a hill. I don't have either a decibel meter or OBDII device to measure the RPMs, but there is definitely a noticeable difference.
The general rule among aviation folks is that at 8,000 feet MSL (above mean sea level) any normally aspirated (non-turbocharged) internal combustion engine will only be able to make 75% of it’s rated output (which is rated at sea level). As Landshark mentioned, higher altitude=less oxygen=less fuel=less power.
I respectfully disagree. Manufacturers recommend 87 octane because the engine was specifically designed for that fuel. Unfortunately there is a lot of misunderstanding about the octane rating of fuels and unfortunately some people think that putting more expensive higher octane fuel into a vehicle will somehow magically make it perform better. The truth is that higher octane fuel usually has slightly lower energy content than standard 87 octane gasoline.
Sounds like you don't have much experience burning gas in your car. Do you think your Clarity is noisy at high RPM?
The Clarity recommendation specifies the minimum octane acceptable to be used: There may be better, or worse operation within the range. The interactions of the knock sensors, O2 sensors, spark timing, and injector timing in modern engines can dramatically affect the operation / efficiency of the operation. The old-school wisdom that higher octane only matters in high-performance engines is no longer true. The old-school guidance would require high-octane for the high compression ratio of the Clarity. Octane operation is also dependent on altitude: Low octane will operate better at higher elevations. You used the word 'usually', which makes this truth very difficult to evaluate. Non-ethanol fuel is sold with a higher octane, and this would 'usually' have a higher energy content. In the modern engine, energy content of the fuel is only one factor in mpg.
What makes noise coming coming from an engine? If the octane is too low then you can hear knocking. Otherwise the valves make the same sound and the accessories make the same noise (eg. none for the Clarity PHEV). Does higher octane gas generate less noise because its lower-energy explosions are quieter than the big-bang explosions 87-octane gas generates? If high-octane gasoline produces lower-energy explosions, it would seem the car would have to increase the revs to produce the power the ECU was demanding. What is the noise-maker that gets louder on 87-octane gas?
This is about first gen Chevy Volt that recommended premium gas: "Premium gas will maximize the fuel economy when the engine is used, GM powertrain spokesman Tom Read said. In an emergency, regular gas can be used on the Volt, but fuel economy will be compromised, and the engine may become noisier. " Source: https://www.cars.com/articles/does-the-chevy-volt-require-premium-fuel-yes-and-no-1420663197814/ Clarity recommends regular as a minimum, but enough people have tested premium and found that the engine works smoother. In US market there is strong aversion to premium gas for non luxury cars. It is basically a sales killer, so many manufacturers recommend regular but in reality their cars work much better with premium, for example my 2016 VW 1.8T (regular was recommended) ran like a tractor with 87, but much better throttle response with premium. I never put another drop of regular in that car after trying premium.
I think people are generally challenging two things 1) is the engine knocking or otherwise retarding timing, otherwise it is unlikely higher octane would result in any benefit 2) how would premium make the engine quieter? I'd imagine any incremental additional power would be nominal and wouldn't drastically decrease rpms, which is really only way sound would decrease as otherwise the ICE is still doing all the same things on any fuel level. I also think people have their own thoughts/ feelings and are unlikely to be swayed away from whichever side they are standing.
Another way of interpreting the recommendation of “87 Octane or higher” that is provided in the manual, is that it may be there to inform the owner that it will not harm the engine if 89, 91 or 93 Octane is used. If the guidance was simply “87 Octane” an owner might have reservations about using fuel with any other Octane rating. It certainly isn’t necessary to use anything other than 87. There is no information in the manual that says a higher octane fuel will improve fuel economy, provide improved engine performance in hot or cold temperatures or high elevations, or provide more power. There may be measurable or perceived benefits from higher octane fuel. It won’t harm the car to run 87.
I appreciate your concern but I have done many road trips in my Clarity and have been using the gasoline almost exclusively for the past month. I would say that qualifies me enough to make a subjective observation. But thanks. No more noisy than my 2013 Camry Hybrid.
I will concede that a modern engine is capable of making certain adjustments that will take advantage of higher octane fuel. However, if you do get any improved efficiency/better mpg from that it will be pretty minimal. In my part of the country you would pay about 25% more for premium gas than regular and in my opinion most modern engines that are specified for 87 octane or greater will not pay you back with a 25% increase in efficiency by simply switching to premium fuel. And altitude is simply lower absolute atmospheric pressure which means that even a higher compression ratio engine at altitude cannot squeeze as much air/fuel into the combustion chamber as it can at sea level. 100 octane aviation fuel (commonly referred to as ‘100LL’) has absolutely no ethanol in it whatsoever. However, if you do a little research you’ll find that, nominally, it has a lower energy content than 87 octane gasoline. And most of my research has shown that 87 octane auto fuel and 92-93 octane auto fuel have virtually identical energy content.
Is anyone claiming there are one or more engine components that make more noise at a given engine speed on 87-octane gasoline? If so, what are those components? Alternatively, is the contention that the Clarity's engine is quieter on premium gas simply because it revs slower? If so, nebulous audible comparisons are unnecessary because an OBDII scanner can reveal the engine speed when going the same road speed on the same stretch of road with two different octane gasolines.
I was wondering myself if this is the case. But again, I do not have an OBDII meter to test with. After reading the new replies I somewhat regret bumping this thread.
The merits of 'premium' gas has been controversial for decades. When this topic first came up here, I spent a fair amount of time googling whether higher octane fuel can reduce engine noise. Certainly if this were true, there would be research studies that address it. I found one or two oblique references, but remained unpersuaded because there was not a preponderance of evidence either way. I know a couple of people attempted to 'test' this by using an iPhone-based sound meter (hardly a scientific instrument, and hardly a controlled test). I may have missed something, but I would encourage you to research this topic. If it is 'real', somebody has certainly done some legitimate research. Would the Clarity engine somehow be unique in this regard? I doubt it.
