Octane vs milage
- Thread starter BLKNED
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The car should get the best mileage with the octane fuel it's tuned for. Higher octane fuel burns more slowly, meaning that you can add more timing and not get preignition. If the car is tuned for the faster burning lower octane fuel than the ignition will occur later than optimal and will not burn as completely and should then yield worse mileage. The fact is that the car has a pretty decent brain and will, while not in WOT, adjust for the fuel you run. So it shouldn't matter too much.
Dan
Dan
UrbanRedneck
New Member
- Mar 27, 2008
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^^^like he said.
Higher octane fuel actually has less heat energy, and is hardrer to ignite. If your engine needs it, you need it to avoid preignition. But if the engine does not need it, there is no performance benifit, and actually it will, in theory, reduce your power and milage because of it's lower heat energy and the fact that the ignition system will have a harder time igniting it.
The car is rated to run on 87, so it SHOULD run best on 87. Now I personally haven't confirmed this, but I have heard that the PCM can be seen retarding the timing (knock retard, observed on a scantool) on 87, which would indicate that the car is better suited for 89 (they can tell you to run whatever fuel you want, the computer will adjust timing to avoid preigniton, you will just loose power). If this knock retard is true, than the car should, in theory, run better and get better milage on 89. But judging by the engine's construction and compression, and the fact that the cited knock retard is only in the neighborhood of 3 degrees, I'd wager that the car has no use for 91 or 93 octane fuel, therefore no benefit from it. That is, of course, unless the engine is tuned for 93, which would involve more agressive timing strategy to take atvantage of the extra kock reistance of the higher octane fuel.
Higher octane fuel actually has less heat energy, and is hardrer to ignite. If your engine needs it, you need it to avoid preignition. But if the engine does not need it, there is no performance benifit, and actually it will, in theory, reduce your power and milage because of it's lower heat energy and the fact that the ignition system will have a harder time igniting it.
The car is rated to run on 87, so it SHOULD run best on 87. Now I personally haven't confirmed this, but I have heard that the PCM can be seen retarding the timing (knock retard, observed on a scantool) on 87, which would indicate that the car is better suited for 89 (they can tell you to run whatever fuel you want, the computer will adjust timing to avoid preigniton, you will just loose power). If this knock retard is true, than the car should, in theory, run better and get better milage on 89. But judging by the engine's construction and compression, and the fact that the cited knock retard is only in the neighborhood of 3 degrees, I'd wager that the car has no use for 91 or 93 octane fuel, therefore no benefit from it. That is, of course, unless the engine is tuned for 93, which would involve more agressive timing strategy to take atvantage of the extra kock reistance of the higher octane fuel.
I know about octane. I guess I should have been more specific. I was wondering more about custom tunes. I plan on ordering a CAI and tuner from Brenspeed after I pickup my car and was wondering if I will see any benfit to running the 93 tune vs the 87 tune, obviously with corresponding tunes. I'll be taking a 2000 mile round trip a couple days after I pick up my car, this will be in July when gas will easily be $4 a gallon. Sorry for the confusion with the question. TIA
Ahhh, yeah, no real idea there. Technically lower octane fuels do have more energy per unit volume, so you should have slightly better mileage. At that point though, the financial difference of increased mileage is probably about the same as the 10-20 cents a gallon you save by buying 87, so a good 87 tune would probably be the way to go. Also, the tune will probably increase you mileage over stock.
Dan
Dan
LV51FER
New Member
Is that right? Over here in the UK, TOTAL sell 97 octane fuel (your 91-93) and GUARANTEE better mpg. I always understood that it had a higher energy value.
CBus 06 GT
Active Member
Octane rating is indirectly related to energy per volume. You can have a high octane fuel with high energy per volume and a low octane fuel with low energy per volume. The octane rating simply tells you the fuel's resistance to knock/pre-ignition/detonation. Whether the fuel has more or less energy per volume depends on what the refiner uses to achieve the desired octane rating.
As to our S197 GTs, I have seen from multiple sources that our cars do like 89 octane over the 87 the car is rated for (dyno comparisons and the above reference to knock showing up on the scan tool). Is it enough to raise the MPG to offset the extra $.10 per gallon for 89? I don't have a clue. IMHO, I am quite happy with the mileage I get (running 89) compared to the truck I used to drive (25 vs. 18 highway, 20 vs 16 mixed) If you are coming from the world of compact 4 cylinder cars, you obviously won't feel the same way.
As to our S197 GTs, I have seen from multiple sources that our cars do like 89 octane over the 87 the car is rated for (dyno comparisons and the above reference to knock showing up on the scan tool). Is it enough to raise the MPG to offset the extra $.10 per gallon for 89? I don't have a clue. IMHO, I am quite happy with the mileage I get (running 89) compared to the truck I used to drive (25 vs. 18 highway, 20 vs 16 mixed) If you are coming from the world of compact 4 cylinder cars, you obviously won't feel the same way.
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