3.5 V6 Atkinson Cycle engine and the gas grade parade extravaganza...

Higher octane gas means it burns at a slower rate (withstand higher temperature before igniting on it's own), period. It is made for higher compression engines. Higher compression engines create more heat and need slower burning fuel so it does not pre-detonate causing damage. Pre-Detonation is when the mixture explodes before the spark is fired, bad thing.

using low octane fuel in high compression engines IS BAD.

using high octane fuel in lower compression engines is just wasting money.

You may get slightly lower fuel mileage using high octane fuel in lower compression engines due to the fact the lower compression may not burn the ALL the fuel mixture before the power stroke is complete.

 

This statement is very narrow sighted and wrong at face value for a couple reasons. Compression ratio alone hardly dictates what octane fuel may be ran. Valve Timing, Ignition Timing, Injection method, Block Deck Design, etc all play a part.

Forced Induction vehicles generally have a lower compression ratio and often demand high octane. (STI has 8.4:1)

Direct Injection vehicles (like ours) have high compression ratios (ours is 11.8:1) and can either be tuned for low octane or high octane, thanks to the direct injection's inherent cooler cylinder temperatures, variable valve timing and accurate control over ignition timing.

 

After reading here about the top tier I tried Valero 87 . Mileage went down , tried another tank same thing . I went back to Walmart , filled up and mpg came back up to 22 ish at highway speeds . Buy freshest gas you can , who goes threw the most and you will be fine . I do wish we had QT in S.E. Oklahoma but we don't so , Walmart who is at about 30 Hr. delivery is as good or better than most . No issues after many years using Murphy Walmart fuel .
Chainsaws , weedeaters , 2 ZTR's , J.D. 4200 diesel , Tiller , 4 wheelers everything , no issues . Try to by freshest fuel you can . Never fuel when transport is unloading .

2 centavos -MM

 

Low compression because its a turbo car and makes boost. If it had higher compression on top of the boost you would have major detonation problems (and other problems too).

 

Correct, increased cylinder pressures from compressed air often demand lower compression ratios but again, there are other variables at play.

 

All of this talk in this thread about the same octane gasoline from this place or that place is sort of weird - I think? Around here (central Mo.) all of the gas from many different stations comes from 2 piplines, one in Jefferson City and one in Columbia. I have never seen a fuel tanker truck driver dumping any additives into their transport trailer.
This is not to say it is not done --buuuut I doubt it!?. Does the pipeline filler place change something for each truck --I doubt that even more!
What I am saying is, that I really doubt there is any real difference in one brand's 87 or higher octane versus some other brand's same octane fuel.
I think the oil companies' ad agencies are putting one over on the US buyers!

 

So you are in the belief that higher octane fuel will ignite at the same temp as lower octane fuel? Then you need to explain to this forum why there are different octane levels of fuel. Try running octane 87 in a high compression race engine and I bet it will not last long.

You touched on cooler cylinder temperatures which is a huge factor of when the air/fuel mixture will detonate.

 

Go stop in at a random Cenex on highway 50 and you may find 91 ethanol free. There is variability in suppliers, their additives, ethanol content and of course variability in gas station storage tank conditions, water permeation and fuel age based on volume and consumption.

 

I honestly have no idea how you came to this interpretation of what I said, and frankly don't know how to respond.

 

Simply put, it has to do with the temperature of the compressed air/fuel mixture as to when detonation will occur. You mentioned cooler temps going in to the cylinder. That will allow for higher compression and lower octane.

 

I should add that I live at 7000ft. My RS6 runs better with lower grade fuel, higher grade kicks codes for too rich. My 93 4 runner has the timing bumped up all the way mechanically and doesn't ping or knock on low grade. We don't have the air up here to take advantage of the higher grade but I ran it anyway. There are 2 gas stations in my town, the fuel doesn't have time to go bad although I get what you're thinking.

 

Direct injection's lower cylinder temps, open deck blocks pulling heat from the cylinder walls, variable valve timing adjusting at given RPM/Load conditions to optimize volumetric efficiency, timing advance's controlled burn with MTB factored in all play into whether or not you will detonate.

Clearly I'm stating that higher octane's detonation resistance lies in its ability to withstand higher temperatures compared to lower octane and as a result of these above variables, compression ratios can and have been increased in recent years while still managing to run safely on 87. To suggest that octane and compression ratios go hand in hand is fundamentally wrong.

 

Our trucks will run on any of the 3 basic grades you can purchase at local gas stations. If 1 appears to give you better mileage and run better then use it. It's your vehicle and do what makes you comfortable.

 

A particular octane rating will detonate at a specific temperature.

There are a whole host of ways to lower the temperature, intercooler, VVT, ignition timing, Combustion chamber design, etc. I agree with that. These things also change the compression ratio in the engine.

The Tacoma engine is not a CONSTANT 11.8:1 ratio. That is max CR, depending on ambient air temp, barometric pressure and air moisture content. There are many variables in the CR/Octane war.

 

Shell Canada says V-Power NITRO+ is ethanol free.
Shell USA does not mention ethanol.

 

At high altitude, you can get away with lower octane because of the lack of oxygen. That's why they sell 85 octane in places like Albuquerque, NM. A mile up from sea level 85 octane burns like 87 octane at sea level.

At high altitudes, you will also get better MPG and you'll have less horse power.

 

My dad used to live in New Mexico, and he used 85 octane for a decade. Zero issues.

 

Exactly, higher octane is not higher quality. I have been running Murphy Mart gas for 10+ years with no problems

 

Dear God....Yes, it is. It's a STATIC compression ratio.

You're speaking to DYNAMIC compression ratio which is a calculated result and you're confusing it with cylinder pressures, which are variable from the points you discussed....please tell me you don't build engines.

http://www.enginebasics.com/Advanced Engine Tuning/Static vs Dynamic.html

 

Also otto vs atkinson cycle in this engine changes the effective compression ratio by keeping valves open during a small portion of the compression cycle. Best thing for direct injection engines is to run ethanol free fuel. Direct injection fuel injectors run at very high pressure compared to port fuel injectors because a direct injector needs to overcome ambient pressure inside the combustion chambers in order to even get gas in there, much less cause any mixing of air and fuel. Because of this, direct injectors run better without the corn bits left behind in ethanol fuel. Eventually, there are little organic particles from the corn that don't get filtered out or fermented into fuel that can gum things up over time. Use ethanol-free, and you avoid these effects.

One of my other vehicles is a 2 stroke, direct injected Aprilia scooter and nowhere in the world is the difference between premium and ethanol-free 87 more apparent. Id say she tops out between 3 and 5 mph faster simply running ethanol free fuel alone.