Regular Unleaded or Premium Unleaded?

[Pskhaat]

My recursive redirect response:

https://forum.ih8mud.com/showthread.php?postid=755620#poststop

Basically, yes theoretically it should be more efficient, but consensus is the contrary. As for your 2nd question if not answered in the above link, O2 mass per any volume of air, dealing with partial gas pressures. Man I hated chemistry.

 

[double0jimb0]

Strangely, I am studying for an Internal Engines Combustion exam at this very moment, so I thought I would throw in my 2 cents. This could all be b.s. for all I know....I have not passed the class yet.

As for emissions, in an gas burning engine, I dont believe unburnt hydrocarbons are the main issue. The main source of pollution is from NOx production which is the result of dissociation from very high peak temperatures. In general, the higher the engine performance, the hotter it runs, if all else is equal. That being said, if running premium gives better performance, it is running hotter.

The ECU will dial back lambda values when you use regular. The lower a lambda (ratio of air-to-fuel) value means less fuel is being injected which results in a slower and cooler burning, therefor lower probability of autoignition. This also corresponds to less NOx and CO formation due to less dissociation.

Well, that probably does not help to much, but I felt it was a good study break.

First post BTW!

 

[a990dna]

Well, that probably does not help to much, but I felt it was a good study break.

First post BTW!

Sounds good to me, you get an "A+".

 

[spressomon]

I, like others here, run 91/92 premium octane fuel. I have experimented with 87 octane only to discover a few less miles per tank and less power. So I run what I have learned best for the truck and also it happens to be what Toyota recommends: 91/92 octane. (coincidence ?)

 

[Landpimp]

FWIW, I switched back from 89 to 91, been running 91 for over a month now, I have noticed NO improvment in milage and no difference in power............so back to 89 I go.

 

[Pskhaat]

The lower a lambda (ratio of air-to-fuel) value means less fuel is being injected which results in a slower and cooler burning, therefor lower probability of autoignition.

That's contrary to what I would expect I guess. I would guess that a more lean condition in the combustion chamber may lead to higher temperatures?

 

[uzj100]

Guys,

I am running 91 again and it makes a difference from a power and gas mileage standpoint on my 99 LC.

Basic readers digest is that the truck does not seem to be trying so hard. With 87 it seemed liked the bad boy was always trying hard.

Just my opnion.

uzj100

 

[Greg B]

FWIW, I switched back from 89 to 91, been running 91 for over a month now, I have noticed NO improvment in milage and no difference in power............so back to 89 I go.

Me too; same scenario.

 

[stewbo]

I've been running 89 for a year or so.
I'll switch to 91 to see if I can feel a difference.
I drive in the city mostly.
I'm running up to Colorado the 1st week of the new year.
So that will give me 1600 miles , there and back, to test the 91 octane theory.

 

[tabraha]

Guys,

I am running 91 again and it makes a difference from a power and gas mileage standpoint on my 99 LC.

Basic readers digest is that the truck does not seem to be trying so hard. With 87 it seemed liked the bad boy was always trying hard.

Just my opnion.

uzj100

That's exactly what I experience too. To me there looks like there is no truly scientific answer. I think it is VERY vehicle and usage dependant and each person should get their own data to decide from. My data is cut and dry and collected for me by me. It simply is more cost effective for me to run on premium.

Another interesting difference is what "premium" and "regular" are. Pimp describes seeing no difference between 89 and 91. Neither of those are regular or premium at alot of stations in SC. Both of those are midgrade depending on the station. When I buy regular I get 87 octane. When I buy premium I get 93 octane. It is a far greater spread than 89 and 91. This is yet another reason why I think these results are turning out to be very situational and theories and physics in broad strokes aren't gonna give a answer that works for all of us.

 

[hoser]

I've switched back and forth between 87 and 91 octane and have concluded there is a difference in power (it is not just psycological). I have not experimented with 89 yet. I need all the power I can get when running 35's.

 

[spressomon]

Maybe the elevation I live and use my LC has something to do with the fact I can definitely feel the difference between the 87 and 91 octane.

 

[rzpapp]

The 2UZ-FE V8 engine has 9.5:1 compression ratio which is the main reason Toyota recomends 91 octance. You see if they did not they would have to state the HP under regular octane 87 and that would be lower. If you run 87 the knock sensor will detect premature combustion (if it occurs... lower altitudes and hot weather make it worse). It then will send a signal to the ignition computer to set the ignition (ie the spark) early in the compression cycle so that the the compression expolosion won't happen. instead a normal ignition (use the spark plug) happens.

Knocking or (compression ignition or the diesel effect) only happens when there is a large amount of fuel present... typically when your press down hard on the gas... So what will happen is that in acceleration modes the fuel will not be as efficently burned if the ECU detects knocking and retards the ingition as described. Then end result is lower power and lower gas mileage.

Now this ONLY occurs if you engine is experiencing knocking. And I am not sure how long the computer takes to reset. But I can imagine if one is taking a LONG trip in the desert when its +100F in a fully loaded truck up and down hills knocking would occur on 87 and the ECU will retard the timing, and you would experience lower gas mileage.

This is exactly what I have seen. I have driven between AZ and CA in the summer and noticed about a 1MPG difference on average using 87 vs 91 (actually 92 in CA).

Power wise I think it would be really hard to tell. Cause the loss of pwer would only be at teh very top end... so unless your flooring and the conditions are right (ie hot weather .... etc) would you notice the power difference.

IMHO it does not matter much if one uses 91 or 87 ... gas is GAS there is good gas and bad gas (ie dirty, its got water etc...) but the octane ratign does not deterime that... bad gas is usually the result of poor puming system at the gas station which would allow contaimiants or water to collect or justa bad batch from the refiner... its awalys best to buy gas from a HIGH volume place (that's why I like COSTCO gas, not to mention they usualy have the best price in town) so that you get lots of turn over in the tanks in the ground.

I typically get 91(92) in the summer and in the winter I will alternate 87-91(92) every other fill up or go with 87 for two in a row sometimes... depending on my mood.

 

[rzpapp]

From what I understood that was EXACTLY how they got mid grade GAS. The pumping systems mix regular and premuim. I don't think stations get delivered 3 kinds of gas... just 2.... anyone know for sure

 

[Pskhaat]

No, I don't but I can find out. I am pretty sure it is a mixture thing as well though.

 

[DMX84]

Rzpapp, Very well put. The ECU will remember your octane count (Knocking). It will always pull timing quickly (to avoid damage) and be slow to advance it back. If you disconnect the battery it will erase all its memory on this setting therefore setting it back to the factory fuel maps & timing.
On my car I had a custom chip made that will re-set the “octane” count every time the car is started. This helps eliminate problems with the bad gas scenario, plus on the car I run a large turbo and need all the advancement I can get.

The 2UZ-FE V8 engine has 9.5:1 compression ratio which is the main reason Toyota recomends 91 octance. You see if they did not they would have to state the HP under regular octane 87 and that would be lower. If you run 87 the knock sensor will detect premature combustion (if it occurs... lower altitudes and hot weather make it worse). It then will send a signal to the ignition computer to set the ignition (ie the spark) early in the compression cycle so that the the compression expolosion won't happen. instead a normal ignition (use the spark plug) happens.

Knocking or (compression ignition or the diesel effect) only happens when there is a large amount of fuel present... typically when your press down hard on the gas... So what will happen is that in acceleration modes the fuel will not be as efficently burned if the ECU detects knocking and retards the ingition as described. Then end result is lower power and lower gas mileage.

Now this ONLY occurs if you engine is experiencing knocking. And I am not sure how long the computer takes to reset. But I can imagine if one is taking a LONG trip in the desert when its +100F in a fully loaded truck up and down hills knocking would occur on 87 and the ECU will retard the timing, and you would experience lower gas mileage.

This is exactly what I have seen. I have driven between AZ and CA in the summer and noticed about a 1MPG difference on average using 87 vs 91 (actually 92 in CA).

Power wise I think it would be really hard to tell. Cause the loss of pwer would only be at teh very top end... so unless your flooring and the conditions are right (ie hot weather .... etc) would you notice the power difference.

IMHO it does not matter much if one uses 91 or 87 ... gas is GAS there is good gas and bad gas (ie dirty, its got water etc...) but the octane ratign does not deterime that... bad gas is usually the result of poor puming system at the gas station which would allow contaimiants or water to collect or justa bad batch from the refiner... its awalys best to buy gas from a HIGH volume place (that's why I like COSTCO gas, not to mention they usualy have the best price in town) so that you get lots of turn over in the tanks in the ground.

I typically get 91(92) in the summer and in the winter I will alternate 87-91(92) every other fill up or go with 87 for two in a row sometimes... depending on my mood.

 

[Coastal]

In college I worked at what is called a tank farm where gas is piped into a city for distribution then stored in tanks, then picked up by the trucks and delivered to stations. There are definitely 3 grades of gas, each stored in a different tank and each put into a different compartment on the tanker trucks - If I remember correctly a big tanker carries 11k gallons and has 3 compartments - one about 5.5k, one about 2.5k and a 3rd about 3k (I could be off here, but you get the idea). Each city/region is different, but at the time I was working, different companies used different refineries in differrent parts of the state (but several used the same and their pipelines). So in some cities / regions Chevron and Exxon gas is the exact same stuff made in the same refinery and shipped in the same pipeline until it gets into their trucks for distribution where it gets each companes differrent additives (including detergent & octane boost etc etc). The different grades of gas are all sent thru the same pipeline (including diesel if i remember correctly) but you are talking about such a large volume that grades and types of fuel mixing is not an issue at all.

I use 87 octane in my 100 and will continue to until i hear pings or knocks. From everything I have read, If you are not pinging or knocking under a load run the lowest octane you can.

 

[tabraha]

I think it varies as to whether it gets mixed or not to get to midgrade. I think some pump stations do mix but I know a couple that only offer regular and midgrade so that makes that one easy to figure out.

 

[SWUtah]

99- LC Toyota recommends 91 octane so I use 91 octane
2004 - 4runner V8 Toyota says 87 octane so I use 87 octane
2003 - 350Z Nissan recommends 91 octane so I use 91 octane
I also use Top Tier Gas and I also run Mobil 1 5w30 in all of them and I also run synthetics through out the driveline.

 

[turboale]

I was told that the pre 00s require premium and the 00s and on got a different ECU that pulls the timing. That could be why you guys are getting different results with the lower octanes. Thoughts...?