Where I now live, unleaded premium is 91 octane. We live at an elevation of around 7000 feet. The other day, I noticed that I was pretty much running on empty. The remote gas station I stopped at was out of premium, so I put in a couple of gallons of mid-grade (88 octane) to get me home. Strangely, I noticed the car was more responsive, with no knocks or pings -- even on hard acceleration. So my question is, for what reason do I need the 91 octane premium gas?

BTW, the engine is stock with about 35K miles on it.

 

It's responsiveness was probably a placebo effect more than anything.

When you're at 9k rpms, you need all the predetonation resistance you can get.

On top of that, you still had some 91 in the tank so putting a few gallons of 88 would probably net you an 89/90 octane anyway.

 

If your mpg doesn't suffer then go for it.
Levi

 

Most likely the difference is the oxygenation level of the fuel (I.e. Percent of Ethanol or MTBE). The difference has changed your Realized Air/Fuel ratio which you are feeling on your butt meter. Try www.racegas.com/article/19 . This outlines Sunocos recommendation for tuning jetted carbs with 10% oxy fuel now very common.

Compare Ethanol/MTBE levels of the two fuels......

Utah

 

um...because Honda said so.

higher octane fuel is more resistant to detonation. the ECU retards the timing to prevent detonation. this means less power. You can feel this on a really hot hot day with the A/C running on full, sweating your balls off. After stopping at that red light for 60 seconds and heat soaking the intake and engine bay, you're cooler, but as you set off the car will feel sluggish. Higher octane fuel resists this better than lower octane fuel.

At least this is how I see it. And also, because Honda said so.

darcy

 

I got stuck in the middle of nowhere with low fuel and the only station I could find just sold 87 otane. I put in 2 gallons , enough to get me home. It drove normal, not any better but not any worse. I assume my timing wasn't being affected much in the driving conditions I was under.

87 octane has more BTU's and burns hotter compared to 91, as long as timing isn't being retarded due to knock it can work, I don't advocate it but it can be done, as soon as timing gets retarded then power drops.

 

It never gets really hot outside where we live now. And my AP1 does have what has been called the "dolebludger mod" on its air intake which I designed when we lived in hot Okla. where I had trouble with heat soak in the summer. I don't know whether that mod is still available to see with the "search function" this board, but it really worked for me. It was a DYI job that actually incorporates some ram air function. No, the 88 octane did not reduce fuel efficiency, And yes, when the engine was properly warmed, I tried flooring it in 6th gear at 45 mph up a pretty steep grade here in Colorado, and no ping or knock. Then, on the straights, I ran it up to 9K rpm. Still with no ping or knock. When I got home and turned it off, there was no hint of "dieseling".

As far as increased acceleration and performance is concerned, I confess that I have not put a stopwatch on it. But after driving performance cars for 56 years, I think I can actually feel it. And My AP1 DOES perform better on the lower octane gas.

Now I have talked with oil company experts and execs (who were a dime a dozen in Okla. where I used to live) and was told that high octane gas is meant only to prevent ping, knock and "dieseling" on high compression engines, and that "octane" really is an ignition retardant -- for use when it is needed. When it is not needed, it actually reduces performance. I don't know what is true about the octane issue, and that is why I posted here. I do know that we also own a couple of Mercedes for which the owners' manuals say to use premium, but dealers in Okla. and here in Colorado have told me to just use regular. I've done so, and they run great.

As my S2K is over 14 year old, I have no fear about voiding any warranty, because there isn't one any more. But, of course, I don't wan't to damage a 35K mile engine. So comments on my post are certainly appreciated. Just remember that my driving of it happens between 6500 and 11000 ft. in altitude, where the air gets rather thin, so compression ratio is decreased.

 

Lower octane fuel is more efficient/ignites easier and generates more explosive combustion, so in theory as long as the car isn't knocking and the timing is the same, the car will make more power on lower octane fuel. Its entirely possible you experienced this, especially at a high elevation where there is less air per fuel being burnt. It should be noted that high elevation demographics do sell gas at a lower oct then a typical sea level. My mom lives at 7000' elevation in Taos NM and 86 OCT is their standard fuel. Less air = less combustion energy. It should be noted that 91 is still the min recommended from Honda, so use lower OCT at your own risk. I have played with a few tanks of mid grade in years past as a experiment, and I felt the motor was more responsive also. But I think in more cases when driven hard at sea level, timing will eventually be retarded, and any slight benefits will then be negated. With that I run 92OCT since thats the best we have and saving 1$ a fill up isn't worth the risk vs possible benefit to me. I want a safe fuel for a broad spectrum of conditions, when the car is heat soaked, to driven hard.

 

Oh, I agree that if I were at sea level (or anywhere near it) I would need 93 octane or above. And indeed, I don't mind paying a few cents for 91 octane here where I live (the house is at 7400 ft.). Here, my S2k driving is from 6500 to 11000 ft elevation, and, by comparison, the car feels sluggish on the 91 octane compared with the 88 octane. And it gets better fuel economy too. Not trying to save any money. Just noticed that the car runs better up here on lower octane. And as I mentioned, there is no problem with heat soak because it does not get hot here, and my car has a very effective CAI. And thanks for yours and all of the replies.

It has been a while since I posted on this forum. Used to post frequently. Guess I learned so much here that I didn't need to learn anymore. But I will be back!

 

At your altitude you're fine.

As an FYI I have put nearly 200k on two different vehicles from Honda rated at 91oc. Both high compression motors. These were both at sea level. mpg did not suffer and you might only have known if I told you. Actually in one case I got 32 mpg freeway on a car rated for 28 and regularly 25.5mpg on a car most only saw 23mpg.

Modern cars from the 90s on can take different octanes without issue. They pull timing, adjust a/f and if boosted, psi. It does take a sensor input after burn i.e. Knock. If you stay lower oc all the time even less issue.

If you think your Honda is special, you can dig up articles from Porsche stating their 13:1 could run on regular without issue. Car companies are worried a fuel mix up will cost them a lawsuit. Their tunes are never so aggressive as not to be able to take poorer grades. This makes for happy owners!

We run our 3rd Honda, 3.7v6 on 89 or 91. We are at 5k feet. Cannot tell the difference.

FYI fuel grade does not void warranty. You could pump 91 and the station actually serve up 87 oc. Outside your control.