mixa86

I totally disagree with the 87 is a devil if dealer requires premium. Until this car i never filled up super. My e39 530i and my 05 Legacy GT turbo all required premium gas. None of those cars knocked ever, nor did i ever have issues because of the gas i was using. As far as gas mileage i never saw an increase or felt the power difference on WOT. Even on the legacy WOT seemed the same! I am not sure why i decided to use premium for this car only but this is what it is. Also when i took loaners from lexus, i only put 87 in those cars, when i had is350 loaner that car felt just like mine no difference on 87.

UniSlayEX

Exactly right. We don't know what the long term effects are yet. But I'd advise you start doing it right and since your girlfriend went cheap on the fuel, or better yet ignored the recommended fuel, I would advise you check other service preformed on the vehicle and make sure they were preformed properly. (I.E. oil changes, filters, etc.)

Kurtz

iTrader: (1)

And again- were you datalogging the computers?

Because if not then you have no idea if they ever knocked.

This isn't a 70s pinto where you can obviously hear and notice when the engine is knocking.


Knock sensors and ECUs are very good anymore. They'll hide it extremely well.

It doesn't mean it's never happening.

I actually have datalogged in cars, including ones tuned for higher octane fuel (including cars I've tuned myself too)... and the knock happens even if you don't notice it. (and you generally never do- as I say, modern cars are really good about catching it and pulling timing)


Now, if you don't plan to keep the car more than 50-100k miles, and aren't racing it, you probably don't give a crap... (other than you are likely to get provably more power and better mileage with the fuel the car requires, but it'd take more than "feeling" to prove it- you'd need to datalog under controlled conditions for more than 1 or 2 tanks, and if not racing you won't miss the extra ~10 hp or whatever)



I mean, there's literally no motivation at all for a car maker to "require" a higher octane fuel than the car actually needs... cheaper fuel would be a selling point... so it's not like they're saying you need premium for their benefit.

ibidu1

iTrader: (1)

Kurtz,

I have never data logged my 250. I do have access to one, I may do it one of these days.

My question is this, you are claiming 87 octane will knock. Do you have data logging to prove it? Because I am not seeing a loss of power.

From my understanding, the o2 sensor measures heat of the exhaust, and leans and richens the mixture as needed. If the vehicle is knocking wouldnt that change the exhaust heat?

06isDriver

"The sensor does not actually measure oxygen concentration, but rather the difference between the amount of oxygen in the exhaust gas and the amount of oxygen in air. Rich mixture causes an oxygen demand. This demand causes a voltage to build up, due to transportation of oxygen ions through the sensor layer. Lean mixture causes low voltage, since there is an oxygen excess."

http://en.wikipedia.org/wiki/Oxygen_sensor

Because we know the concentration of oxygen molecules in our atmosphere it makes it rather easy to determine.

It doesnt measure heat....otherwise they'd probably call it a heat sensor.

Toymota

Hey just for the record, I don't want to encourage this behavior of going WOT or racing on reduced octane in cars tuned for premium. Car and Driver already did a test back in 2001: http://www.caranddriver.com/features...results-page-2 . The E46 M3 was 6.6% slower on the track on 87 regular.

In my experiment with 89 mid-grade I specifically limited my driving to light throttle, mostly highway cruising and cooler temperatures in winter. These conditions made it less likely for the engine to knock, so I could get away with using 89 octane and my mileage was the same.

Toymota

Temp, mixture, etc may all be related but the primary method of dealing with detonation and knock is ignition timing. You should read the attached PDF. Then it would be great if you could use that data logger.

Kurtz

iTrader: (1)

seems a couple other folks have already addressed most of the questions for me (and quite well too) but I'll just mention no I have not datalogged an IS250 on 87 octane specifically... the reason I'm saying it will knock eventually (esp if driven hard at all) is because for one that's the only reason it would say "Requires 91 octane fuel" several places on the car and owners manual.

Otherwise the requirement would make no sense and actually hurt sales of the vehicle because it "needs" more expensive gas.

And as explained, it's mainly ignition timing (combined with engine compression) that's causing the requirement- the timing is programmed to work properly with 91, and if it doesn't get it it will eventually knock and retard the timing.

Or you can just read the PDF Toymota attached, which explicitly states the car will retard timing on lower octane fuel (and goes into some detail about it)

p0lo8

Some people here just think that they are smarter than Lexus engineers... Use premium, don't worry and be happy

jgbackes

iTrader: (2)

Truth vs. Fact. If you want truth, read some Nietzsche. Facts on the other hand are based on scientific principals.

http://www.differencebetween.net/mis...act-and-truth/

It is a fact that your engine will run better on 91 octane.

BTW, the O2 sensor and the knock sensor are just that, sensors. These sensors do not directly change mixture or engine timing, they are inputs to the ECM. The ECM controls mixture and engine timing.

JayRo354

91+ octane all the way. I like the silence in the cabin without having to listen to the knock from the engine. The car just runs smoother.

adrr

I run 91 in my car but here's some info i picked up from car talk not sure if its valid or not on our cars You need to run lower octane gas in higher altitudes if you have carburetor otherwise you risk damaging your engine for some reason. Mountain states carry 85 gas because of this.

When i had my G35, i noticed a big difference in gas mileage when filling up in Vegas with non ethanol gas. Used to be able to get MTBE gas in California buts that changed 10 years ago. I assume the reason for this is that ethanol has less energy for volume than gas but it has higher octane, its around 100 octane. Also my G35 owners manual stated notto use any gas with ethanol in it but that was kinda hard living in California which banned MTBE.

ibidu1

iTrader: (1)

Thanks for the link! This explains how the ECU learns a persons driving habits and stores it in memory, and adjusts engine timing as need be for optimal performance.

I am happy with the performance, mpg of my 250. Would it run better on higher octane? Of course it will! But, I have logged 110k miles with out any issues (knock on wood) so I will continue to use it.

Kurtz

iTrader: (1)

Mountain states have lower octane fuel because with thinner air your effective engine compression is lower, meaning you don't need as high an octane.

And I don't think any major car maker has even made carburated cars in the US in over 20+ years at this point

ibidu1

iTrader: (1)

Correct me if I am wrong, but I think you have it backwards. Winter fuel has more MTBE (i think its called), and Ethanol which increases octane, and I believe raises the oxygen in the mix.