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better mileage with higher octane? - Page 23

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Posted by y_p_w on January 4, 2007, 7:08 pm

Eeyore wrote:

I've heard of a typical RON 4 higher than (R+M)/2.  However - different
fuels will have different RON/MON spreads, so it's not a given that
it's going to be close to 4.  Pure octane has an RON of 100 and a MON
of 100.  So pure octane is 100 (R+M)/2.

Posted by E Meyer on January 2, 2007, 3:47 pm

On 1/2/07 7:11 AM, in article 4vv3viF1d8ujsU1@mid.individual.net, "dido"

There is qualified truth in your statement.  It depends on a number of
factors whether better gas mileage will result from higher octane gas.  The
number one factor being that the engine is designed for higher octane gas in
the first place.

Most modern engines have a knock sensor that detunes the engine when it
detects pinging, which is usually due to gas with too low an octane rating.
When it does this the mileage and performance are compromised.  Using higher
octane gas in such an engine doesn't necessarily guarantee better gas
mileage but rather it corrects the worse gas mileage that resulted from
running it on the wrong gas.

So, if your owner's manual says you should be using higher octane gas, it
should give better results if you do.  If it says you should be using
regular, premium gas will most likely be a waste of money.

Posted by Paul Hovnanian P.E. on January 3, 2007, 7:07 am

E Meyer wrote:

Right. If the engine's antiknock system isn't kicking in and retarding
timing, higher octane gas would be wasted.

It would be really nice what with all the brains built into ECUs if
they'd add a dashboard light that would signal the opreation of the
antiknock system. If you see the 'Bad Gas' light come on frequently,
higher octance might be of some value.

Driving habits (and other conditions) can affect octane requirements.
Some people might get away with using lower octane gas if they are
easier on the gas pedal.

Paul Hovnanian     mailto:Paul@Hovnanian.com
Madness takes its toll.  Please have exact change.

Posted by C. E. White on January 2, 2007, 4:04 pm

Whether or not you get better fuel economy with higher octane gasoline is a
function of the vehicle's PCM (Powertrain Control Module - aka, the engine
computer). If your engine has a knock sensor (or sensors) the PCM may be
able to adjust the engine parameters (mostly spark advance) to increase
performance and fuel economy when you use premium fuel. I've had a couple of
Fords with 5.4L engines. It was claimed that the use of premium fuel would
allow for a 5 to 10 HP increase and a 1% to 3% increase in fuel economy. For
both of these vehicles I ran premium fuel for a significant period of time
(5K+ miles) and compared it to similar periods when I was using regular
fuel. I saw no significant difference in fuel economy. Neither could I tell
there was any difference in performance, although I don't think I could
detect a 10HP increase without a dynometer. The results may vary for other
vehicles. I have noticed that many of the Japanese manufacturers have
started specifying premium fuel for their higher cost engines. I assume that
the major reason for this is achieving slightly better CAFE numbers. I am
confident that most of the vehicles would run OK on regular, perhaps with
slightly worse fuel economy and performance (you should check the owners

If you are going for maximum MONEY economy (as opposed to maximum fuel
economy), then there is little chance that purchasing premium fuel is a
smart decision. I doubt you will ever increase your fuel economy by more
than 3% by using premium, and since premium costs at least 5% more than
regular, it is a clear loser if you are trying to save money.

A word of caution - you can't really make any determination on the relative
fuel economy difference between regular and premium by comparing single tank
fulls of gas. Besides the inaccuracy of single tank full fuel economy
calculations, you must also consider the actual octane of the fuel mix in
your tank and how quickly the PCM adjusts to different fuels. The PCM will
not instantaneously advance the timing if you start using premium fuel. It
will make the adjustment over a period of time. So if you want to do the
comparison, I'd suggest making the measurement over at least 5 tank fulls of
each type of fuel. I'd also not include two tank fulls when you are
transitioning from one fuel to the other.


Posted by Eeyore on January 2, 2007, 6:25 pm

"C. E. White" wrote:

Most ppl call it an ECU.


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