mpg on 93 vs 89 octane gas
05-29-12, 06:43 AM
#31
Tech Info Resource
iTrader: (2)
There is a breakover point for any engine regardless of engine management. My Supra was fine in California when I bought it and could run 93 in it. When the powers that be decided 92 was just as good as 93, I changed my coolant from 50/50 to 80/20 distilled water to Toyota red stuff and my fuel mileage (which had always been 18.x mpg around town) went from 14 mpg back to 18 mpg. When the powers that be decided 91 was as good as 92, there was nothing I could do to convince the engine to get more than 12 mpg around town. At that point, I put 4 gallons of 100 octane race gas to 14 gallons of 91 and got back to 18.x mpg. I also built a spreadsheet, and much to my satisfaction, the increase in fuel mileage offset the dramatically more expensive 4 gallons of race gas and my actual per mile fuel cost dropped over running straight 91.
The other key characteristic (and incorrect assumption above) - load is the only factor that matters. Low rpm or high rpm makes no difference. The wider the throttleplate opens, the more likely detonation will occur and it's simply related to the universal gas law - more air molecules, more temperature increase as the piston rises, more likelihood of autoignition.
So, if your normal driving pattern involves very little acceleration events and lots of freeway cruise, you may not see any difference at all between 87 and 93 as long as the ECM has enough correction for the few times you do accelerate and load the engine. In an ideal world, we'd run the lowest octane the engine needs all the time (or we'd eliminate all these stupid inefficient SI engines and run nothing but CI engines) but that's impossible, because we'd need 50 octane at freeway cruise and 110 octane at full load. There's no simple way of doing this without multiple fuel sources in the vehicle and all the necessary control apparatus and safety gear to prevent bad things from happening (like what happens if your 110 octane tank is empty when you really need it).
Interesting experiment, but ultimately you've only proved one thing - for your driving pattern there is no discernible difference in fuel economy between the regular gas you buy and the premium you have available. FWIW, I know engine builders in California who say the Cali 91 runs like the 87 we used to buy 15 years ago, and I trust their judgment.
The other key characteristic (and incorrect assumption above) - load is the only factor that matters. Low rpm or high rpm makes no difference. The wider the throttleplate opens, the more likely detonation will occur and it's simply related to the universal gas law - more air molecules, more temperature increase as the piston rises, more likelihood of autoignition.
So, if your normal driving pattern involves very little acceleration events and lots of freeway cruise, you may not see any difference at all between 87 and 93 as long as the ECM has enough correction for the few times you do accelerate and load the engine. In an ideal world, we'd run the lowest octane the engine needs all the time (or we'd eliminate all these stupid inefficient SI engines and run nothing but CI engines) but that's impossible, because we'd need 50 octane at freeway cruise and 110 octane at full load. There's no simple way of doing this without multiple fuel sources in the vehicle and all the necessary control apparatus and safety gear to prevent bad things from happening (like what happens if your 110 octane tank is empty when you really need it).
Interesting experiment, but ultimately you've only proved one thing - for your driving pattern there is no discernible difference in fuel economy between the regular gas you buy and the premium you have available. FWIW, I know engine builders in California who say the Cali 91 runs like the 87 we used to buy 15 years ago, and I trust their judgment.
05-29-12, 07:02 AM
#32
I didn't read the whole thread, so it might have been mentioned, but the OP is pretty risky running 89 with that crazy high compression ratio. There might not be any immediate, noticeable affects, but I'd be curious to see how the engine holds up in the long run.
05-30-12, 07:18 AM
#34
Pole Position
Thread Starter
Doh, I forgot to come back here and post that I decided to finish the experiment on 3/29/2012 and I went back to 93 octane. It was getting too hot with temps over 90F here now, so I didn't want to risk it. I'm also not driving much now--a paltry 650 miles/month.
Thanks for posting your experiences lobuxracer. I agree that the results from my IS 250 should not be used if you drive a tuned Supra or even an IS and drive it at maximum performance. My conclusion was if you drive sedately such as on freeway cruises, etc, and do not ever floor your car for max power, you can get by on 89 octane just fine. The engine does not knock and you don't lose any mpg.
What I meant when I said high load, low rpm, was that if you require a given amount of power from the engine at low rpm, you will need more throttle and place a higher load on the engine than if it were at a higher rpm, where it could produce the same power at less throttle and load. This is more relevant for manual transmissions. My IS 250 is an auto so if I press the gas pedal to ask for more power, it will just downshift and increase the rpm.
Thanks for posting your experiences lobuxracer. I agree that the results from my IS 250 should not be used if you drive a tuned Supra or even an IS and drive it at maximum performance. My conclusion was if you drive sedately such as on freeway cruises, etc, and do not ever floor your car for max power, you can get by on 89 octane just fine. The engine does not knock and you don't lose any mpg.
The other key characteristic (and incorrect assumption above) - load is the only factor that matters. Low rpm or high rpm makes no difference. The wider the throttleplate opens, the more likely detonation will occur and it's simply related to the universal gas law - more air molecules, more temperature increase as the piston rises, more likelihood of autoignition.
05-30-12, 07:34 AM
#35
Pole Position
Thread Starter
btw, I see you are in CO. What octane can you can buy there?
05-30-12, 10:26 AM
#36
could understand if you were leasing. (majority speaking, since there are more lexus leases than sales)
if youre in a 250 lease for $350-400/mo, a change from premium to regular actually makes a pretty nominal difference in the cost of ownership for those three years.
$0.40 delta in gas @ 1,000 miles/month = $20/month saved, or $720 over a 3 year lease. that equates to 2 months of lease payments saved at ZERO risk !! it really is money down the drain. if you were cross shopping lexus dealerships, would you go with the place that charged 38 months worth of payments or 36?
if youre in a 250 lease for $350-400/mo, a change from premium to regular actually makes a pretty nominal difference in the cost of ownership for those three years.
$0.40 delta in gas @ 1,000 miles/month = $20/month saved, or $720 over a 3 year lease. that equates to 2 months of lease payments saved at ZERO risk !! it really is money down the drain. if you were cross shopping lexus dealerships, would you go with the place that charged 38 months worth of payments or 36?
Last edited by ThermonMer; 05-30-12 at 10:43 AM.
05-30-12, 11:41 AM
#37
Well, it's only 20 cents regular to premium at most stations around here.... plus, Toymotas testing was mid-grade vs. premium, not regular... so now it's only 10 cents a gallon saved.
Your math also seems to be assuming 20 mpg...which is horrible for a 250... (it's not even good for a 350... I get 21-22 city, 28-30 highway).
So if we assume 25 mpg average, 1000 miles a month, you burn 40 gallons in a month.
10 cents a gallon saved using 89 instead of premium is a whopping $4 per month.... or a grand total of $144 saved over the entire 3 year lease....
Book the cruise!
Your math also seems to be assuming 20 mpg...which is horrible for a 250... (it's not even good for a 350... I get 21-22 city, 28-30 highway).
So if we assume 25 mpg average, 1000 miles a month, you burn 40 gallons in a month.
10 cents a gallon saved using 89 instead of premium is a whopping $4 per month.... or a grand total of $144 saved over the entire 3 year lease....
Book the cruise!
05-30-12, 11:47 AM
#38
i was actually being generous.
miami / miami beach - at least 40 cents difference for regular vs premium (3.70 vs 4.10). same mpgs for both. and i get 18 mpg at best, regardless of grade, so 20 mpg was generous. this is mixed heavy city / light highway / daily use of high story parking garages (14 mile per hour average). depends where you live, what type of traffic you have. my lease is $364/mo. so i would be at 2 month lease pmt difference for regular vs premium.
miami / miami beach - at least 40 cents difference for regular vs premium (3.70 vs 4.10). same mpgs for both. and i get 18 mpg at best, regardless of grade, so 20 mpg was generous. this is mixed heavy city / light highway / daily use of high story parking garages (14 mile per hour average). depends where you live, what type of traffic you have. my lease is $364/mo. so i would be at 2 month lease pmt difference for regular vs premium.
Last edited by ThermonMer; 05-30-12 at 11:52 AM.
05-30-12, 12:39 PM
#39
Generally speaking, 91 octane is the best you'll find. There used to be some places that carried 93 octane, but not sure if they still carry it.
05-30-12, 12:47 PM
#40
Lexus Test Driver
you poor saps that cant get 93 need to move down to the states that do all the refining. We get the good stuff down here in Louisiana,
87 reg, 89 mid, 93 premium.....I love it
87 reg, 89 mid, 93 premium.....I love it
05-30-12, 12:52 PM
#41
Well, at 5000+ feet you really don't need 93.
Hell, at sea level you don't need 93 for a stock 2IS...anything over 91 is pointless.
That said, premium around here is all 92 or 93, and I pay no attention whatsoever to which I get, since both are higher than needed.... My previous, far more modded cars, were another story.
Hell, at sea level you don't need 93 for a stock 2IS...anything over 91 is pointless.
That said, premium around here is all 92 or 93, and I pay no attention whatsoever to which I get, since both are higher than needed.... My previous, far more modded cars, were another story.
05-30-12, 12:55 PM
#42
Lexus Test Driver
tis true...but I dont have the 91 option so it is what it is. The extra 2 points of octane I get for no additional cost over the 91 sold elsewhere is a nice plus too, I guess. Plus I'm stretching my cars legs a little further than most with my MAPECU so I prefer to keep the knock in check
although, .I've never seen a 92 rating. Thats interesting.
although, .I've never seen a 92 rating. Thats interesting.
05-30-12, 01:10 PM
#43
Yup, we get 92 and 93 around here (though not at the same place), not sure why....
and for you, sure, you're one of the few who can actually get some use out of those extra 2 points 91->93 by bumping the engine timing a bit... it's not a typical use case though
and for you, sure, you're one of the few who can actually get some use out of those extra 2 points 91->93 by bumping the engine timing a bit... it's not a typical use case though
05-30-12, 01:33 PM
#45
Pole Position
Thread Starter
Well, at 5000+ feet you really don't need 93.
Hell, at sea level you don't need 93 for a stock 2IS...anything over 91 is pointless.
That said, premium around here is all 92 or 93, and I pay no attention whatsoever to which I get, since both are higher than needed.... My previous, far more modded cars, were another story.
Hell, at sea level you don't need 93 for a stock 2IS...anything over 91 is pointless.
That said, premium around here is all 92 or 93, and I pay no attention whatsoever to which I get, since both are higher than needed.... My previous, far more modded cars, were another story.
This point is to those people who just blindly stick to the manufacturer's recommendations. You can go below the recommendations depending on the operating conditions.