Octane rating does not relate to the energy content of the fuel (see heating value). It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where the octane number is raised by blending in ethanol, energy content per volume is reduced.
I copied this from Wikipedia octane rating. Don't run e85 use the gasoline that your vehicle specifies, that goes for any vehicle, the manufacturer makes an engine to run on a specific fuel, contrary to popular belief putting premium gas in a car made for reg doesn't make it perform better.

 

No, it will ruin your fuel pump and fuel injectors as well as lower your MPG by a good 15% or more.

 

When I use 87 octane my MPG drops.
I only use it when travelling and the only gas station is in a small town that does not sell enough premium and it is stale in the ground.
I had that happen once on a off roading trip, pulled up to the pump, brushed the cob webs off of the pump and filled the tank, truck ran like crap, it ran, but ran like crap.

 

Correct, it dries the rubber out thats why those E85 comp vehicles have stainless steel lines and viton, silicone hoses and seals.

PLUS in order to take advantage of E85 or ETOH fuels you have to increase the flow. On alcohol dragsters etc. the fuel lines can be 100% larger. So extrapolate you need from there.

I used to own a racing fuel company here in Dallas many moons ago and we sold VP fuels. my best additive was Propylene Oxide.

I know that you use naptha and other things to increase octane numbers.
What you have to understand is that high octane fuels burn slower, lower octane fuels burn fast.

 

And that is why when you use regular gas your MPG decrease and you loose power, because the ECU retards the engine timing to compensate for the fuel burning to fast and not providing enough power.
Instead of the spark plug firing milliseconds after top dead center it fires significantly later to try and take advantage of as much of the power stroke as it can.

I know you already know that DLife, but the original poster may not.

 

YUp you bring up a very good point the OP needs. I have used lesser grade fuels in winter.

 

Whoa! Hold the horses. Since when did spark plugs fire AFTER top dead center? Bad info here.

Think of your base timing. Is it set to before TDC (BTDC) or after TDC (ATDC)? That's right, it's BTDC. Then think of timing advance - the timing is advanced (BTDC) versus retarded (ATDC). When you're cruising under light load, that's when you see the most advance (40+ degrees BTDC) to help burn as much of the fuel as possible. When under heavy load, the timing is backed down - to avoid detonation/pinging - but the timing is still BTDC.

The gasoline needs time to burn and create the energy needed to force the piston back down at the right time - thus why it's called "timing". If you have the plug firing after the piston is trying to go back down, that engine will have very little power. That's why engines have their timing set to BTDC in order to "time" the burn and create the most power.

There are so many factors linked to the "right" timing - compression ratio, head chamber flow characteristics, piston style (pop-up/domed, flat top, dished), camshaft profile, etc. that there's not a one-size-fits-all timing number for all engines.

I can't speak to modern day electronics and how the timing is changed by the ECU. I suspect that the ECU, via the O2 sensors, will increase timing for higher octane fuels so that the engine has longer to get the fuel burned enough to keep the exhaust readings in check. Knock sensors will tell a computer that the timing is too advanced, and the ECU will back the timing down till the pinging stops.

So many people here are convinced that 92/93 octane is required for our engines for various reasons (manual says so, internal engine issues, etc.). My distributor'ed 3.9 with the base timing bumped from the standard 6 degrees to 10 degrees does just fine on 85 octane (the lowest octane in these parts, versus 87 octane at lower elevations). I don't get pinging/detonation. This tells me that the engine doesn't realistically need the higher octane. My ECU doesn't control the timing (14CUX). If low octane were bad, it'd be evident.

Now, OBDII systems have control of the timing and there is likely a lot going on - it'd be neat for someone with a recorder to run a tank of low octane and a tank of high octane and compare readings. It'd be interesting to see proof of the ECU dialing back the timing when the engine is burning low octane, as is being claimed.

 

Yes and no.
Spark ignites, begins process ignition, fuel burns. Fuel burns a variable rates and btu's. The longer a fuel burns (typically) the more energy it gives off. Then throw in the things we use to cOntrol IGNITION, when the fuel ignites.
That's all we control. Hotter spark makes for a better ignition, that's it. It does not cOntrol the rate at which fuel burns. The fuel we use dictates the amount of time it burns compounded by compression and air flow or swirl which can also dictate the flame travel after ignition.

The OP is discussing the types of fuel to use.
Our computers get more energy out of higher octane fuel.
Lower octane fuel won't let our motors produce as much well rounded power response and emissions.

The manufaturers want the mOtor to produce 100% of its capability.

 

At higher elevations lower octane can be used with no ill effects.
Now take your tank of 85 octane and your set timing and come down to sea level and lets see what happens.
Now add to that that some 3.9's are actually a lower compression ratio and do not require premium fuel, they are 8.35:1 instead of the higher 9.35:1 that all other Land Rovers are with the 3.9-4.0 engines.

Using my Ultra-Gauge I ran a 2 tanks of regular in my truck keeping track of my MPG, I lost 1-1.5 MPG by the end of the 2nd tank of fuel.
Keeping a eye on timing is a PIA because it changes with every hill that I go up or down, as well as other factors, like the wind blowing, if I'm drafting a semi or that same GD Prius is drafting me, intake air temp, etc.

If you want to go cheap on fuel thats your buisness, personally I'll go cheap on wiper blades and spend the extra $3 per week to fill my truck with the REQUIRED octane rating fuel.

And thank you for the correction on the TDC and BTDC.

 

That is a good point on the distributered 3.9s, you won't lose the power due to ecu changing timing, and the dfference in "power" from lower to high octane is probably negligible. As long as no pre ignition is happening.