Gas Octane

But there is no reason anyone will see more knock at higher elevation. Higher elevation equals lower cylinder pressure and lower octane requirements.

And as you must know there are limits to timing advance. My 360 and most small block mopars can take about 36 degrees. I can't go from 91 to 98 and benefit since higher advance doesn't help.

 

This is an inaccurate statment. More timming doesn't automaticaly equal more power. The optimal amount of advance is enough to ensure that the air/fuel mixture is completly burned by about 17 degrees ATDC. More or less than that and you will lose power. How much timing an engine needs to achive this burn rate is affected by many things, combustion chamber design, spark plug location, quench area and rod length to name a few. In fact the less timing you run while still able to achive complete combustion by about 17 degrees ATDC the more power you will make since the engine spends less timing fighting the expanding gases as the piston travels to TDC. Octanes only function is to slow the burn rate down enough to help keep the engine from entering a pre-ignition state which can then lead to detonation.

 

People are thinking about this the wrong way. This engine is built to deliver a certain amount of power. The ECU de-tunes when it senses knock. It's not that octane adds power, but that the lack of it will subtract power. That's the best way for the shade tree mechanics in the group to grasp this. You're absolutely right that octane doesn't make any engine more powerful.

It just happens that Toyota's published output for this engine is for 87 octane, for industry standard reasons I've already posted about, and that confuses things. People treat this number as rated output, and they quite rightly doubt that rated output can be increased by fuel choice. In fact, it's not rated output. It's just the output you get on 87.

 

I understand that I was confusing 2 things here, in my half drunken rant..

Octane requirements are less as you go up elevation, lets get that out of the way, if I insinuated something else I was wrong. Others are right.

However modern vehicles are designed to advance or retard timing based off knock, this allows them to maintain power at higher and lower elevation. Pretty much the system was designed for altitude changes.

As cyl pressure is lowered, knock is reduced, allowing the motor to advance the timing to compensate to maintain power.

I think someone saying use 87, you will never see a power increase or better MPG is just wrong. But that is my opinion. Some people in this thread are complaining about knock, and my answer is use better fuel.

 

My mentality is this, 91 is usually 20 cents more than 87, so for every 10 gallons I spend 2 dollars extra..I personally feel I get better gas mileage. So to me makes sense to get it... either way from a bone dry tank to full the roughly 4 extra dollars seems worth it to me.

 

thats how i see it as well.. i just get 93.

 

+2

 

Are you actually get better mpg with 91?

 

I think that depends how you drive. On 93 octane up the hill truck seem to have enough torque to hold on 5th instead of downshifting. So you cruze up the hill at 1500 RPMs instead of 2100 RPMs.

 

If this is true then higher octane might produce better MPG. The price difference between E10 (89) and Premium (91) is about $.25-$.45 per gallon.

 

PT Barnum stated: "There's a sucker born every day"

I'd think it would be more of concern that the spark plug(s) gaps
be adjusted (or plugs replaced) during scheduled maintenance...
as the SP gaps will change over time, use/wear.
This is not an exclusion of plug wires, as they may degrade as well.
I'd think this consideration would be more detrimental then a coupla' octane

Many folk more then likely don't get a true/accurate fuel use measurement/reading...
because they have increased the tire size over oem...
and haven't altered the gear ratio to compensate for that.
So the odo is inaccurate and the actual fuel mileage per tankful may be incorrect.

I also know that switching from one fuel octane rating
to another, demands that the cpu recalibrate each time
a different rated fuel is added.

I recommend sticking with what you choose to use...
and not bouncing back an forth between octane levels
as the cpu will need nearly 50 miles on the odo to recalibrate.

If yer junk starts to run badly with a lesser octane rating...
It's more probable that other things are culprit, IME

I have used the cheapest 87 Arco brand fuel I can purchase
and have for over the last 13+ years...
My junk runs perfectly fine...

well... until the plugs become worn anyways.

I re-gap the plugs every 10,000 miles just to be sure.
My plugs are replaced every 30K

87 for me most always...
but even E85 on occasion when I roll outta state.
Never had an issue.

but screw 'dat california summer gas blend
really screws us on fuel mileage

But heck... someone has to buy into the higher octane fuel
or the industry loses money...
just won't be me

enjoy yer expenditures

 

I was using 91 octane during the summer because my engine was pinging at 87 octane. With 91 octane, I didn't notice improved mpg's, but the engine was definitely more responsive & no more pinging. A few days ago, I switched back to 87 octane and no more engine ping .

 

shell 87 (silver) lower ethanol, better fuel efficiency. use premium every now and then and burn it up, clean out the system.

 

87 with NO ETHANOL works quite nice. I cringe when I can't find a place without ethanol and I have to fill up....

 

Here in Ohio I have found Shell 89 gives the best mileage. I don't think it is due to the
octane as much as there is less ethanol mixed in the 89 versus the 87. The difference
is about 1 to 1.5 miles per gallon more which makes sense to use it for 10 cents a gallon
more

 

You ran 85% Ethanol (E85) in your truck? That's a nice way to abuse/damage your engine. Now a Flex-fuel engine can run on E85 with out any issues.

 

No No, he's no sucker.

It's that CA summer blend that will fuck you up.

 

Three parts 87, two parts 91, one part 93 and a pinch of sugar

 

89,91. I strive to find gas without ethanol.