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Old 06-17-2008, 03:27 PM   #1 (permalink)
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Regular vs. Premium

Does running premium gas result in better FE than regular? If so, how much? With some quick, from-the-hip calculations, a gain of over 1.5mpg would be needed to offset the 20 cent difference between regular and premium.

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Old 06-17-2008, 03:30 PM   #2 (permalink)
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I doubt you'll see any difference. If your car wasn't made for it, then it prolly won't like it.
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Old 06-17-2008, 03:38 PM   #3 (permalink)
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If your car is fitter with knock sensors and can self learn ignition timing up to the knock limit, then it'll be a small improvement as ignition will be advanced to make use of the higher octane.

If there are no knock sensors then it'll make no difference..... unless you can manually advance the ignition?
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Old 06-17-2008, 03:41 PM   #4 (permalink)
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It won't matter in your car. Money out the tailpipe.

A car needs to have a knock sensor (or a driver manually adjusting the timing) to gain a theoretical benefit from higher octane (the computer will advance the timing to take advantage of the fuel, so torque will be increased).

But the higher octane only increases peak torque ability, so it probably won't make any difference to the average driver tooling around at part load.

Then again, if you exclusively Pulse and Glide (moderate to high load accel) an engine with a knock sensor & premium fuel, it may help.

Here's some good info about octane & fuel economy:

http://fuelsaving.info/fuels.htm
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Old 06-17-2008, 11:06 PM   #5 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
It won't matter in your car. Money out the tailpipe.
Yeah, shoulda specified I wasn't inquiring in regards to my '88 Accord. That'd be the proverbial golden ring in the swine snout. Thanks for the link, MetroMPG. Good info.
My OP was triggered by a discussion I had with a coworker regarding FE/octane/knock sensors. Theoretically premium should improve FE as the ECM advances the timing. I just wondered what the ecomodding community experiences in the real world...
Learned something new today; Some (most?many?) ECM's will advance the timing several times until knock is experienced, thereby determining what octane is in the tank, and adjust its spark table accordingly.
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Old 06-17-2008, 11:16 PM   #6 (permalink)
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well my experience has recently been running the middle grade, 89, instead of the usual 87. I do this as it doesn't... pop.. as much at idle, and idles a bit smoother. I know i'm long over due for all 6 plugs to be cleaned, but my mileage looks like it has steadily risen in the past few weeks since the switch to 89
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Old 06-18-2008, 02:35 AM   #7 (permalink)
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I haven't performed any scientific tests, but I usually see about 2-3 mpg better economy in my 07 Civic EX. As far as I know it has a knock sensor, and subjectively, the car feels spunkier off the line and doesn't run out of breath as soon when downshifting to pass someone on a 2-lane highway.

Run the numbers and see if it's worth it. I calculated the following figures that justifiy the extra cash up front:

Regular gas (here in Olympia ~4.25) @ 35 mpg highway = (4.25/35) $0.121
Premium gas (~4.45) @ 38 mpg highway = (4.45/38) $0.117

Therefore this equals less dollars per mpg. Even at only 2 better mpg the extra cost would be worth the mileage. One may spend more at the pump at one time, but one also visits the gas station less often. The effect is also amplified the worse the gas mileage gets. If with regular you get 15 mpg, then assuming premium is 20 cents more expensive, you only need an improvement of 1.26 mpg to equal the additional cost of premium.

Don't you love maths?
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Old 06-18-2008, 08:38 AM   #8 (permalink)
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Thanks, Superchow!
I'm pleasantly surprised to hear that premium gives you that much of an improvement. So, premium may be worth it IF your car-
adjusts timing in response to knock sensors
is in good running order
and
is subjected to P&G

I have an aversion to math, but it does come in handy...
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Old 06-18-2008, 09:56 AM   #9 (permalink)
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A friend* with a Yaris, who commutes up and over a 5,000 foot mountain pass, tested this. He got the best results with regular. Mid was a little worse, and premium was almost 5% lower mileage.

*bailout on cleanmpg
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Old 06-18-2008, 10:25 AM   #10 (permalink)
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I have a similar inquiry regarding premium...

Since premium comes at a, well, premium these days, we're wondering if we should try mid-grade and/or regular in the '06 TSX due to the inflated costs of this fuel (manual says "Premium Recommended"):

Engine Code: K24A
Compression Ratio: 10.5:1
Power: 205 hp (150 kW) @ 6800 rpm
Torque: 164 ftlbf (222 Nm) @ 4500 rpm
VTEC Engagement: 6000 RPM
Redline: 7100 rpm
Current # miles: ~36,000

FE is ~30 mpg for a calendar year, running premium, minimal technique. We tried a controlled experiment one Winter, but the temperature fluctuations were too erratic for good results.

My concern is that the timing will be retarded to the point of drop in FE. More pollutants is not the aim. We honestly don't need the engine's full potential. The reason we bought it was for the 4-cylinder in this vehicle class.

I have an OBD-II datalogger that can be hooked-up to store the following parameters with vehicle speed as the mandatory 5th paramater for a total of 4 optional at one time:

Engine Speed
Throttle Position
Coolant Temperature
Engine Load
Air Flow Rate (not supported)
Intake Air Temperature
Intake Manifold Pressure
Fuel Pressure (not supported)
O2 Sensor Voltage (up to 8 monitored)
Ignition Timing Advance
Short Term Fuel Trim (up to 2 monitored)
Long Term Fuel Trim (up to 2 monitored)
Battery Voltage

It's equipped with a knock sensor, so it would likely retard timing.

Since the last test, I found it was possible to calculate FE based on OBD parameters.

Finally, The Question:
Which parameters to choose?

RH77

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Last edited by RH77; 06-18-2008 at 10:33 AM.. Reason: Addl. Info
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