school me on 85 octane

[LandCruiser]

New resident of the great purple state of Colorado and have been finding 85 octane in all the gas stations as the "regular" gas blend. Manual says 87 is the minimum for the truck.

I've been told that 85 octane is for older vehicles because it doesn't contain ethanol, but at the same time it says "all gasoline products contain up to 10% ethanol" on the pumps...

Can someone tell me exactly what the reasoning behind 85 octane is up here? I haven't seen it in the southeast, on my cross country trip 85 and 86 turned up somewhere in northwest Tejas.

 

[KidVermicious]

85 is the low here in Utah too. Higher elevation = less air density, which leads to less compression in the combustion chamber. Less compression = lower octane needed to prevent detonation.

 

[1engineer]

85 is the low here in Utah too. Higher elevation = less air density, which leads to less compression in the combustion chamber. Less compression = lower octane needed to prevent detonation.

This is correct - to a point. If you are running an older vehicle with a carburetor then yes, lower octane gas will be beneficial at altitude. With today's vehicles having fuel injection and ECU systems the lower octane is not beneficial at all and could cause real problems. The reason Toyota and most other OEM vehicle manufacturers have a cutoff of 87 octane fuel has to do with the knock sensors and their range. Almost all modern cars have a knock sensor that measures the fuel detonation (keeping it simple here) and adjusts the mixture accordingly to maintain optimal mixture to prevent knocking. Well, these sensors operate within a range and the low end of that range is 87 fuel, hence the warning labels.

Point is, if you run 85 octane in a modern vehicle you will get, at best, worse fuel economy due to unburned fuel and an ECU that can't regulate the "knock." Run 87 and be done with it. Just because they sell 85 and it's cheaper does not mean you have to buy it! Oh, and there is Ethanol on all different octane levels too.

 

[JohnJerk]

85 is the low here in Utah too. Higher elevation = less air density, which leads to less compression in the combustion chamber. Less compression = lower octane needed to prevent detonation.

This is correct. 85 is 87 up here with regard to protecting against pre-detonation. No worse fuel economy.

 

[KidVermicious]

This is correct - to a point. If you are running an older vehicle with a carburetor then yes, lower octane gas will be beneficial at altitude. With today's vehicles having fuel injection and ECU systems the lower octane is not beneficial at all and could cause real problems. The reason Toyota and most other OEM vehicle manufacturers have a cutoff of 87 octane fuel has to do with the knock sensors and their range. Almost all modern cars have a knock sensor that measures the fuel detonation (keeping it simple here) and adjusts the mixture accordingly to maintain optimal mixture to prevent knocking. Well, these sensors operate within a range and the low end of that range is 87 fuel, hence the warning labels.

This confuses me. Are the modern sensors actually sensing the amount of octane present? Because if they're like the ones I know and just "feel" for pre-detonation, then it shouldn't matter which octane you're actually running as long as the motor isn't knocking. And it's less likely to knock the thinner the air gets, so there's less need for octane and he should be able to run 85 regular just fine.

If your fancy newfangled sensors are actually measuring the octane, then I'll take my ignorant butt back to the Grandpa Trucks forum and shut up. :cheers:

 

[1engineer]

This confuses me. Are the modern sensors actually sensing the amount of octane present? Because if they're like the ones I know and just "feel" for pre-detonation, then it shouldn't matter which octane you're actually running as long as the motor isn't knocking. And it's less likely to knock the thinner the air gets, so there's less need for octane and he should be able to run 85 regular just fine.

If your fancy newfangled sensors are actually measuring the octane, then I'll take my ignorant butt back to the Grandpa Trucks forum and shut up. :cheers:

Haha no, the sensors are not measuring the actual octane levels but they do have limits and those limits can be exceeded with poor fuel. Here is a decent explanation:

Higher octane ratings correlate to higher activation energies: This being the amount of applied energy required to initiate combustion. Since higher octane fuels have higher activation energy requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise known as autoignition or detonation.

The compression ratio is directly related to power and to thermodynamic efficiency of an internal combustion engine (see Otto-cycle). Engines with higher compression ratios develop more area under the Otto-Cycle curve, thus they extract more energy from a given quantity of fuel.

During the compression stroke of an internal combustion engine, as the air / fuels mix is compressed its temperature rises (PV=nRT).

A fuel with a higher octane rating is less prone to auto-ignition and can withstand a greater rise in temperature during the compression stroke of an internal combustion engine without auto-igniting, thus allowing more power to be extracted from the Otto-Cycle.

If during the compression stroke the air / fuel mix reaches a temperature greater than the auto-ignition temperature of the fuel, the fuel self or auto-ignites. When auto-ignition occurs (before the piston reaches the top of its travel) the up-rising piston is then attempting to squeeze the rapidly heating fuel charge. This will usually destroy an engine quickly if allowed to continue.

There are two types of induction systems on internal combustion engines: Normally aspirated engine (air is sucked in using the engine's pistons), or forced induction engines (see supercharged or turbocharged engines).

In the case of the normally aspirated engine, at the start of the compression stroke the cylinder air / fuel volume is very low, this translates into a low starting pressure. As the piston travels upward, abnormally high cylinder pressures may result in the mixture auto-igniting or detonating, which is why conservative compression ratios are used in consumer vehicles. In a forced induction engine where at the start of the compression stroke the cylinder pressure is already raised (having a greater volume of air / fuel) Exp. 2 Bar (14.7Psi), the starting pressure or air / fuel volume would be 2 times that of the normally aspirated engine. This would translate into an effective compression ratio of 20:1 vs. 10:1 for the normally aspirated. This is why many forced induction engines have compression ratios in the 8:1 range.

Many high-performance engines are designed to operate with a high maximum compression, and thus demand fuels of higher octane. A common misconception is that power output or fuel efficiency can be improved by burning fuel of higher octane than that specified by the engine manufacturer. The power output of an engine depends in part on the energy density of the fuel being burnt. Fuels of different octane ratings may have similar densities, but because switching to a higher octane fuel does not add more hydrocarbon content or oxygen, the engine cannot develop more power.

However, burning fuel with a lower octane rating than that for which the engine is designed often results in a reduction of power output and efficiency. Many modern engines are equipped with a knock sensor (a small piezoelectric microphone), which sends a signal to the engine control unit, which in turn retards the ignition timing when detonation is detected. Retarding the ignition timing reduces the tendency of the fuel-air mixture to detonate, but also reduces power output and fuel efficiency. Because of this, under conditions of high load and high temperature, a given engine may have a more consistent power output with a higher octane fuel, as such fuels are less prone to detonation. Some modern high performance engines are actually optimized for higher than pump premium (93 AKI in the US). The 2001 - 2007 BMW M3 with the S54 engine is one such car. Car and Driver magazine tested a car using a dynamometer, and found that the power output increased as the AKI was increased up to approximately 96 AKI.

Most fuel filling stations have two storage tanks (even those offering 3 or 4 octane levels): those motorists who purchase intermediate grade fuels are given a mixture of higher and lower octane fuels. "Premium" grade is fuel of higher octane, and the minimum grade sold is fuel of lower octane. Purchasing 91 octane fuel (where offered) simply means that more fuel of higher octane is blended with commensurately less fuel of lower octane, than when purchasing a lower grade. The detergents and other additives in the fuel are often, but not always, identical.

The octane rating was developed by chemist Russell Marker at the Ethyl Corporation in 1926. The selection of n-heptane as the zero point of the scale was due to its availability in high purity. Other isomers of heptane produced from crude oil have greatly different ratings.

I "bolded" a few areas of interest. Bottom line is the "knock sensor" has an operating range and the ECU can only r e t a r d the ignition so far to prevent auto detonation. Toyota has set a minimum octane level of 87 (as have most other OEM's) so operating limits will be within their range.

Edit: Here http://www.aaa.com/aaa/006/EnCompass/2007/mar/mar_AutoTalk.htm is a link explaining the Colorado situation and why it has 85 Octane.

 

[nevada]

85 is the low here in Utah too. Higher elevation = less air density, which leads to less compression in the combustion chamber. Less compression = lower octane needed to prevent detonation.

this is correct.

This is correct - to a point. If you are running an older vehicle with a carburetor then yes, lower octane gas will be beneficial at altitude. With today's vehicles having fuel injection and ECU systems the lower octane is not beneficial at all and could cause real problems. The reason Toyota and most other OEM vehicle manufacturers have a cutoff of 87 octane fuel has to do with the knock sensors and their range. Almost all modern cars have a knock sensor that measures the fuel detonation (keeping it simple here) and adjusts the mixture accordingly to maintain optimal mixture to prevent knocking. Well, these sensors operate within a range and the low end of that range is 87 fuel, hence the warning labels.

Point is, if you run 85 octane in a modern vehicle you will get, at best, worse fuel economy due to unburned fuel and an ECU that can't regulate the "knock." Run 87 and be done with it. Just because they sell 85 and it's cheaper does not mean you have to buy it! Oh, and there is Ethanol on all different octane levels too.

whahh? that's news to me.
im pretty sure it adjust timing, not fuelling.

and what was said above, and below is correct.

This is correct. 85 is 87 up here with regard to protecting against pre-detonation. No worse fuel economy.

yes, also this.

 

[1engineer]

whahh? that's news to me.
im pretty sure it adjust timing, not fuelling.

I did not say anything about "fuelling."
Let me put it simpler:

If your engine is knocking (uncontrolled detonation) the modern knock sensor detects this, sends a signal back to the ECU and the ECU then r e t a r d s the timing to get rid of the condition. At high altitudes, running 85 is probably (maybe) OK (even though Toyota says no) but at lower altitudes running 85 could cause pre detonation that cannot be regulated by adjusting the timing. I said "could" not will.

Again, bottom line, run what the OEM says (min of 87) and you will be fine. Just because "your buddy" or "my dad" or even "I heard" does it does not mean it's right. I posted hard data for my side of the debate. Please post your data to the contrary.

 

[Scott@Hightech]

This is correct - to a point. If you are running an older vehicle with a carburetor then yes, lower octane gas will be beneficial at altitude. With today's vehicles having fuel injection and ECU systems the lower octane is not beneficial at all and could cause real problems. The reason Toyota and most other OEM vehicle manufacturers have a cutoff of 87 octane fuel has to do with the knock sensors and their range. Almost all modern cars have a knock sensor that measures the fuel detonation (keeping it simple here) and adjusts the mixture accordingly to maintain optimal mixture to prevent knocking. Well, these sensors operate within a range and the low end of that range is 87 fuel, hence the warning labels.

Point is, if you run 85 octane in a modern vehicle you will get, at best, worse fuel economy due to unburned fuel and an ECU that can't regulate the "knock." Run 87 and be done with it. Just because they sell 85 and it's cheaper does not mean you have to buy it! Oh, and there is Ethanol on all different octane levels too.

As everyone else said, this is mostly incorrect. The knock sensor retards timing, and the range is very large you will not exceed it before the truck ceases to run.

The lower octane is due to the lower atmospheric pressure(higher elevation). This causes compression to be much lower on engines. The lower compression requires a quicker burn. This is achieved with the lower octane fuels.( this is a vague explanation, and not %100 "textbook" wording).

The reason manufactures require a certain grade of fuel is almost strickly due to emmissions standards. Their goal is to acheieve the cleanest burn at the lowest cost, while considering performance as well.

 

[phillyd2]

Don't know about all that fancy schooling stuff but recently driving 1200+ miles around Colorado I used 85 without noticeable issues. Since the engine was not loving the hill climbs I tried going back to 87 and even 92 for a few tank fills to see if I got any better results. Sadly no. Long term? Listen to the egg heads :grouphug:

 

[JohnJerk]

I did not say anything about "fuelling."
Let me put it simpler:

If your engine is knocking (uncontrolled detonation) the modern knock sensor detects this, sends a signal back to the ECU and the ECU then r e t a r d s the timing to get rid of the condition. At high altitudes, running 85 is probably (maybe) OK (even though Toyota says no) but at lower altitudes running 85 could cause pre detonation that cannot be regulated by adjusting the timing. I said "could" not will.

Again, bottom line, run what the OEM says (min of 87) and you will be fine. Just because "your buddy" or "my dad" or even "I heard" does it does not mean it's right. I posted hard data for my side of the debate. Please post your data to the contrary.

Where is he hard data talking about elevation effects?

 

[1engineer]

As everyone else said, this is mostly incorrect. The knock sensor retards timing, and the range is very large you will not exceed it before the truck ceases to run.

The lower octane is due to the lower atmospheric pressure(higher elevation). This causes compression to be much lower on engines. The lower compression requires a quicker burn. This is achieved with the lower octane fuels.( this is a vague explanation, and not %100 "textbook" wording).

The reason manufactures require a certain grade of fuel is almost strickly due to emmissions standards. Their goal is to acheieve the cleanest burn at the lowest cost, while considering performance as well.

So I am scratching my head here, wondering how my post is incorrect and wondering how I said the knock sensor DID NOT cause the ECU to change timing.

Scott, I like you and all that but please give me some hard data on your "emissions standards" quote from above and also the range of efficient adjustment of timing. R e t a r d i n g timing simply causes your vehicle to be less efficient, which will happen if you run 85 at lower altitude. The engine is having to make an adjustment that it would not have to make if you are running 87.

EDIT: Ahhh, I now see where my wording "Low end of the range is 87 Octane" can be misconstrued. Should not have worded it like that. How about "Low end of the range could be exceeded and degradation of performance is possible if the fuel is outside of manufacturer specifications" or some shit like that.

ANOTHER EDIT: Yep, "changes mixture" should not have been there either. I meant "changes timing." I need to get out of this conversation lol.

 

[nevada]

adjusts the mixture accordingly to maintain optimal mixture to prevent knocking.

I did not say anything about "fuelling."
Let me put it simpler:

If your engine is knocking (uncontrolled detonation) the modern knock sensor detects this, sends a signal back to the ECU and the ECU then r e t a r d s the timing to get rid of the condition. At high altitudes, running 85 is probably (maybe) OK (even though Toyota says no) but at lower altitudes running 85 could cause pre detonation that cannot be regulated by adjusting the timing. I said "could" not will.

Again, bottom line, run what the OEM says (min of 87) and you will be fine. Just because "your buddy" or "my dad" or even "I heard" does it does not mean it's right. I posted hard data for my side of the debate. Please post your data to the contrary.

above, what else beside fuel could you be talking about with, "adjusting the mixture"

face it, you were wrong. such is life. move on.


at higher elevations, lower octane fuel is sufficient to avoid engine knock, and thus isn't even offered in places like Denver.

 

[nevada]

So I am scratching my head here, wondering how my post is incorrect and wondering how I said the knock sensor DID NOT cause the ECU to change timing.

Scott, I like you and all that but please give me some hard data on your "emissions standards" quote from above and also the range of efficient adjustment of timing. R e t a r d i n g timing simply causes your vehicle to be less efficient, which will happen if you run 85 at lower altitude. The engine is having to make an adjustment that it would not have to make if you are running 87.

EDIT: Ahhh, I now see where my wording "Low end of the range is 87 Octane" can be misconstrued. Should not have worded it like that. How about "Low end of the range could be exceeded and degradation of performance is possible if the fuel is outside of manufacturer specifications" or some shit like that.

what are you talking about? 85 isn't even available at lower elevations.

 

[1engineer]

above, what else beside fuel could you be talking about with, "adjusting the mixture"

face it, you were wrong. such is life. move on.


at higher elevations, lower octane fuel is sufficient to avoid engine knock, and thus isn't even offered in places like Denver.

Yep. Brain signals are not transmitting to my fingers today and apparently not my eyes either as I had to go back and re read that post 15 times before I saw my mistakes. Geez what a day. I really do understand how this works. Honest.

 

[thebwlife]

I've run 85 (which is normally "regular") everytime I'm in the mountains - I've done this on all my vehicles not requiring premium and never had a problem.

Normally coming out of the mountains on highway 550 from Durango CO to Albuquerque NM probably averages 5-6000' and on 85 oct I see my highest mpg averages of 22-23 mpg at 65-75mph (normally 5 over the speed limit).

 

[KidVermicious]

If your engine is knocking (uncontrolled detonation) the modern knock sensor detects this, sends a signal back to the ECU and the ECU then r e t a r d s the timing to get rid of the condition. At high altitudes, running 85 is probably (maybe) OK (even though Toyota says no) but at lower altitudes running 85 could cause pre detonation that cannot be regulated by adjusting the timing.

This meshes with my understanding, thanks for clarifying. I suspect that Toyota says no because there's so many variables regarding what altitude has low enough air pressure to allow lower octane without causing knock, and they don't want to be responsible for Sally Soccer Mom melting her pistons from running really low octane at 1000 feet because it's "high altitude".

I bet LandCruiser would be fine running 85, and if he notices lower MPG or pinging on acceleration he can just drop a bottle of octane booster in there and switch to 87/88 on his next fuel up. Will the 5th gen computer throw a code when it detects knock? My '85 doesn't, it just retards the timing way back and assumes you like it that way.

 

[Greg_Canada]

You will be fine running 85 as long as your are at elevation. they write 87 because that is what 95% of north america has available. The only issues you will have with it, is at lower elevations you have less power. Modern electronics are very good and can compensate for whatever gas you are running.

 

[eddiebx]

Yep. Brain signals are not transmitting to my fingers today and apparently not my eyes either as I had to go back and re read that post 15 times before I saw my mistakes. Geez what a day. I really do understand how this works. Honest.

I think what my neurologically challenged friend is trying to say is...just because 85 octane can work in higher altitudes just fine, does not mean it is "better"?

The way I see it is, while yes 85 octane is fine and dandy on top of the mountains, what about once you get back down and still have half a tank of 85 to burn at sea level? (super theoretical, I know CO is generally at a high elevation and probably can't get back to sea level in one tank, lol)

Whereas if you threw in 87 octane it would be fine down at sea level and at the peak of the mountain?

Now a question for the smarter people...does running 85 octane in high altitude make it run better than 87? Or, is it one of those things where because it's safe enough to do, might as well safe a few pennies and do it? Just curious

The lay person like me would've just figured it really makes absolutely no difference...kind of like running 87 and 89 at sea level.

 

[1000MPH]

We're at 4500 ft and don't get the 85 octane here...or 93 super.
I'm crazy, I actually thought it was the same gas but rated lower at elevation.