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Old 10-12-2008, 05:53 PM   #31
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I keep a log of my mpg for all of my vehicles in an excel spread sheet. I log octane for every fill and created formulas to average by each octane. My truck gets about 1 mpg less on 93 octane fuel than 87. On the other hand my 79 yamaha motorcycle was averaging 25.3 on 87 and 42.3 on 93. The sticker on my bike says it requires a minimum of 91 octane Leaded gas.
The way I understand octane is, the higher the octane, the less volatile it is for ignition. That way you will not get pre-ignition in high compression engines.
I would say that the 93 octane in my truck ignites too slow. Which would cause most of the fuel to be burned as the piston is already traveling down, wasting a lot of the power to compress the air/fuel mixture.
I could be wrong, but that is my best guess..
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Old 10-12-2008, 06:51 PM   #32
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Higher octane fuel ignites and burns slower than lower octane. The peak pressure of combustion is lower but over a longer period of time. In engines designed to work with the different pressure curve you can get more power with timing advanced to expolit the longer sustained higher pressure of combustion.

Generally speaking this required higher compression ratios, which in the sixties with leaded fuel approached 12.5 to 1. These same engines would quickly knock themselves into shrapnel with regular fuel.

In a car designed to run on regular fuel, premium is a waste. Many newer engines with knock sensors will run on regular but the timing will be more retarded that the same engine with premium.

My SLK owners manual gives specific instructions to avoid the use of regular except in emergencies. 8.8 to 1 compression with 7 pounds of boost.

My wife uses only regular in her Murano, which also recommends premium. Maybe one of these days I will see if the mileage improves with premium.

This is all my opinion but I think of the combustion properties of premium and regular in the same way as the burning rates of different gunpowders.

Pistol powders are very fast burning, they generally are used in straight walled cases and short barrels. Rifle powders are much slower burning and are mostly used in bottlenecked cases and much longer barrels. If you used the same weight of pistol powder in a rifle, you could easily blow up the rifle.

regards
gary
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Old 10-12-2008, 07:13 PM   #33
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Now there's some good explanations. Unfortunately, there's nothing new there. The only new (to me) information is that it actually results in reduced MPG.
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Old 10-12-2008, 07:59 PM   #34
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Now that it's cooling off here in upstate NY I am noticing quite a significant drop in my mpgs.
Others have already made a number of good suggestions. However, one that hasn't been mentioned yet is tire pressure.

When the air gets colder, you really have to watch your tire pressure and add air if/when needed. The reason for this is that air has higher pressure when its hot, and less pressure as it cools. So even if/when you aren't actually losing any air (and most tires/wheels leak at least some), you will be losing "air pressure" as the outside weather cools. And as has been pointed out many times in this forum, as your tire pressure drops, often so does your FE.

So when the weather starts getting cold, it is important to routinely check your tire pressure and add air (to bring the pressure back up) as needed. Otherwise, you FE will suffer "in the winter" due to the lower tire pressure you are running with.
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Old 10-13-2008, 07:20 AM   #35
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Others have already made a number of good suggestions. However, one that hasn't been mentioned yet is tire pressure.

When the air gets colder, you really have to watch your tire pressure and add air if/when needed. The reason for this is that air has higher pressure when its hot, and less pressure as it cools. So even if/when you aren't actually losing any air (and most tires/wheels leak at least some), you will be losing "air pressure" as the outside weather cools. And as has been pointed out many times in this forum, as your tire pressure drops, often so does your FE.

So when the weather starts getting cold, it is important to routinely check your tire pressure and add air (to bring the pressure back up) as needed. Otherwise, you FE will suffer "in the winter" due to the lower tire pressure you are running with.
Yes, tire pressure makes a significant difference and you will loose more than a pound or two when it gets real cold out.

I just picked up a set of plugs and will be changing them today. (30,000 miles on them)

Also I am running full syn fluids in the motor, a/t, both diff's and t-case. I suspect this does only make a very small difference, but at least it has better lube qualities than dino. I read somewhere that it takes 10 miles to completely warm up the entire drivetrain in cold weather. I am not sure if this is true, but I tend to believe it, as it seems logical. Your best FE in the winter will be after your drivetrain is warmed up to normal running temps. I also think the syn fluids help with this.

On cold start up in the winter, I do not let the motor run for 20 minutes. Rather 30 seconds or so at the absolute most. After that I start moving to warm the entire drivetrain at the same time and I turn on the heated seat to the low position and only run it until my butt is warm and shut it off. That is not long and by that time I am starting to get good heat.

I do not run the defrost unless I need it. When you turn on the defroster, your a/c runs to dehumidify the air. All vehicles do this unless it is a really old one. While it works great and keeps your a/c system properly lubed in the off season, I only use the defrost when I really need it. I installed the in-channel type vent visors so I can run with the windows cracked open and not hamper aerodynamics and help with defrost.

I am sure you guys all know this stuff so I apologize for my long post.
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Old 10-13-2008, 08:36 AM   #36
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I read somewhere that it takes 10 miles to completely warm up the entire drivetrain in cold weather. I am not sure if this is true, but I tend to believe it, as it seems logical. Your best FE in the winter will be after your drivetrain is warmed up to normal running temps. I also think the syn fluids help with this.
Synthetic fluids are known for reducing friction and better cooling (through imrpoved heat transfer). Those qualities, if true, would make me think that the drivetrain would warm up slower with synthetics (though they would also mean less frictional loss to begin with, of course).
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Old 10-13-2008, 07:01 PM   #37
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Your Amsoil dealer might have an old brochure with the story of an airport service vehicle, perhaps a fuel truck, from the part of the country where zero degrees in winter is considered warm, that not only had a rear end failure but the oil had frozen to where it actually broke the housing.
Amsoil cured the problem.
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Old 10-13-2008, 08:43 PM   #38
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Synthetic fluids are known for reducing friction and better cooling (through imrpoved heat transfer). Those qualities, if true, would make me think that the drivetrain would warm up slower with synthetics (though they would also mean less frictional loss to begin with, of course).
I think that is an interesting thought, but I am not going to go back to dino. In fact, the a/t fluid and t-case fluid from the factory is synthetic per factory spec. All the other fluids I switched myself to synthetic.
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Old 10-13-2008, 10:23 PM   #39
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I think that is an interesting thought, but I am not going to go back to dino. In fact, the a/t fluid and t-case fluid from the factory is synthetic per factory spec. All the other fluids I switched myself to synthetic.
I think your theory is backward. Synthetics transfer heat better so the gearboxes would heat up to running temperature faster.
On the other hand........so what?
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Old 10-14-2008, 07:22 AM   #40
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I think your theory is backward. Synthetics transfer heat better so the gearboxes would heat up to running temperature faster.
On the other hand........so what?
If is transfers heat better, then the items generating the heat would have it transferred away more quickly, and therefore would take longer to warm up (though, importantly, the oil itself would warm up more quickly). If it transfers heat worse (in other words, if it's a better insulator), then the gears would warm up faster.

As for the "so what", I was pointing out that the logic used for "Your best FE in the winter will be after your drivetrain is warmed up to normal running temps. I also think the syn fluids help with this." was broken.

The practical upshot, I'd guess, is that the it takes significantly longer to warm up, with lower friction generating less heat all that better heat transfer draining the heat away from the gears and oil to the housing...but of course, that whole lower friction thing is really what you're looking for anyway. All this talk of heat transfer and time to warm up is in pursuit of lower friction, is it not?
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