When the Clarity is not in EV mode

Agree that the smooth and silent EV operation makes the engine noise all the more offensive, especially when the little engine really gets going. It's not too bad when HV stays in "gear mode", where the engine is mechanically coupled to the drive train. HV mode tries to maintain the battery about where it was when HV mode was selected, plus or minus a couple of miles. In high demand situations, such as continuous high speed highway driving or a long uphill grade, HV mode gets behind and tries to catch up by satisfying the current power demand plus make extra power to put back into the battery. A couple of techniques I use to stay out of or break this cycle:

* Plan initial switch to HV mode when power demand is low, so that the engine has time to warm up before higher power is needed.
* Do an "HV reset". That is, change to EV mode briefly and then back to HV to reset the target battery level that HV mode is trying to hold.

I find that I don't mind the engine running when I am on the freeway because I can barely hear it over the road and wind noise, and it's usually a pretty steady sound. When I most notice the gas engine is when accelerating from stop signs or stop lights in HV mode as that is when you can't miss it. Fortunately for me I almost always have enough EV range to cover all of the surface street driving that I do, which is where I really want to be in EV mode if possible. On days when I know that I will be driving more than my EV range will cover, I switch to HV when I get on the freeway, then switch back to EV when I am exiting. Sometimes I forget to switch back to EV when I get off the freeway, but the Clarity has a built in audio reminder system for that, of course the audio reminder being that the first time I accelerate from a signal light after exiting the freeway I get an earful of engine noise.

Actually the engine seems quieter now on surface streets since I switched to premium gas. I realize that makes no logical sense, but many people have reported that using premium in the Clarity reduces engine noise so I tried it and it does seem to be the case. There are those who think it's just a placebo effect and that people are just imagining that it is quieter. I can't prove them wrong but like others my experience is that it does seem quieter when I run 93 octane vs. 87. And I use so little gas that the cost difference is minimal. To make sure it's not a placebo I suggest while still using regular gas try and pay closer attention to the engine sounds and try and build into your memory how loud it can get. Maybe also use a sound level app on your phone, even though those will have limited accuracy at least it is less subjective. Then try higher octane and see if there is any change.

As for why this would be the case, all cars adjust their spark timing to compensate for different octane fuel. Normally when you put lower octane than recommended the car will adjust the spark timing to avoid knocking, but this affects performance and fuel economy. Using higher octane than recommend normally has no effect other than on your wallet. However in the case of the Clarity using higher octane seems to (allegedly) make for a quieter ride also with no other side affects on performance or mpg. The Clarity owners manual recommends 87 or higher, so it won't hurt anything to use higher octane. Except for hurting your wallet a little.