Gas Octane

Could the MPG difference be because you are running the highway going to miami, & mixed driving @ home?

 

A little background- I have an A.S. as an "Automotive Technician" and at one time was ASE Master Cert. Does this make me a car guru, no...but I think it will help in making light of what people have posted here.

So knock sensors have already been covered. They are piezo-electric 'microphones' that pick up knock which is a certain frequency. This signal is sent to the ECU and retards ignition so the spark fires later in the cycle. Earlier someone was corrected saying 'detonation' was the correct term vs. "pre-ignition'. The correct term IS 'pre-ignition'. Detonation is a generic term such as calling pepsi ....coke. Everyone knows what you mean though the term is not correct. Detonation happens every ignition cycle.

So we know the low octane gas burns faster. When you introduce high intake temperatures, this accelerates the process more. Then throw in the spark and the fuel burns too quickly and the explosion is too early. This 'pre ignition' provides a downward force too early in the ignition cycle causing bent rods or cracking pistons. This happens most often when the intake air temperature is high. Combine high intake (ambient) temp, engine temp, and low octane fuel..and you begin the recipe for pre ignition. Thus....knock sensors. These microphones sense the pre ignition which in turn send the spark a few degrees later in the ignition cycle. Higher octane fuel burns slower, so even in higher intake temps, it can resist pre ignition, thus not having to retard spark timing.

Fuel map and switching octanes- Factory ECM's have whats called long term and short term fuel trims. Long term is the overall fuel map, short term is the adjustments it makes depending on immediate use/fuel/etc. So, in order to accurately decide what fuel is best, you would need to use the same fuel, from the same station in similar conditions for several tanks to get a truly accurate reading. Obviously the guys running 91 all the time are going to have better mileage because the long term fuel map is already adjusted.

Altitude- We all know higher elevation has less oxygen. Your O2 sensors are going to sense less oxygen in the exhaust, so its going to lean out a little in order to get the right mixture. In turn, higher elevation usually means lower intake temps, so it might compensate since cooler air is more dense. But take for instance in AZ where it can be hot and higher elevation...and you're definitly going to notice less power (less fuel because of less oxygen). But you don't necessarily get better mileage because you need more engine RPM to maintain speed.

So to simplify ...I'll recap some thoughts:

Higher octane won't really benefit you unless you're in higher ambient temperature driving conditions (for our trucks) such as summertime. Yeah, it 'might' run a little smoother, but could have a little placebo effect going. IMO, financially, the costs' are negligable.

If you're really that concerned about it, you need to run a few tanks of the same grade and brand of gas to really get an idea what works best for your area. Note that a heat wave or cold snap can effect the outcome, along with changing driving conditions.

I recently did a valve job and head resurface on a Kia that was owned by someone whom I'm sure used the cheapest gas possible. The valves built up carbon on the back side of the valve, and as the valve opened and close, it created a rough valve seat surface. Over time the valve seat would not seal, and the cylinders lost compression. Personally, I think over time using higher quality brand gas would prevent this. Also, I can see the advantages of a catch can on the PCV system in reference to this, though I don't have any personal experience on using one.

I tend to use whatever cheapo gas is available. But my truck isn't a daily driver either. My daily driver is a 2011 Ford Fiesta with a 11:1 engine and dual VVT. During the summer my mpg drops off at least 8-10% because of the heat (and AC use) here in AZ. I did try one tank of 91, but didn't maintain it long enough to see if there was any real improvement.

I hope this helps.

 

Thanks for the info. Lots of information. Pretty well explained IMO. I feel like my truck is highly sensitive to fuel types. Probably due to forced induction but I don't do any experimenting anymore with gas. I had some crappy gas about 8 months ago and my truck would fall on its face at times, pinged and sounded like complete sh&t. I feel like I even notice a difference between brand names even though its all premium that I use.

 

In his case....the added pressure (forced induction or increased compression ratio) increases the fuels tendency to pre ignite.

Something I thought about later, is how people have mentioned the truck runs smoother with 91 vs. 87. With our engines at a static 10:1 compression, it's possible that with 87, the engine is running in a constant state of retarded ignition. But thats just speculation.

 

Originally Posted by NW4RUNNER
Thanks for the info. Lots of information. Pretty well explained IMO. I feel like my truck is highly sensitive to fuel types.

X2

I have noticed the octane difference is more "dramatic" in the summer also. But due to ethanol in the e85 prefer to continue running 91 during the winter also since it is ethanol free. More often than not I can hear the difference in the tranny shift pattern when e85 is used - ours will downshift sooner on highway hill climbs when running low octane. Generally MPG increases about 2 mpg with the higher octane - which makes up for about $.35-40 at the pump. My wife also runs 91 in her car since she uses it so rarely - a tank of gas may sit for 6 months.

 

Octane [technically Octane Rating] is a rating of a fuel's ability to withstand heat & pressure without detonating (pinging) As compression ratios rise, greater octane ratings are required to avoid pinging or knocking.


Unless the engine has a higher compression ratio to take advantage of the higher octane fuel, there's NO benefit to be derived from using the more expensive fuel.


If an engine will run on 87 octane without pinging there's NO advantage to be gained by using 91 octane fuel. Convincing yourself that the more expensive fuel is better / makes more power / delivers increased mileage or makes you look sexier - is like saying the emperor's clothes are beautiful when he's naked.

Think of octane ratings like a ladder; if the roof is 10 feet tall you need a ladder that's about 12 feet long, a 30 foot ladder will work but it's more expensive, heavier and doesn't work any better than the 12' ladder. You can't take advantage of the extra length extending beyond the roof.

So, if you just want to spend more money the gas companies are happy to sell you expensive gasoline.

 

I've brought this up before but never got an answer, Toyota themselves used to recommend high octane fuel for the 4.0 tacoma but the manual was changed in 07 or 08, I forget when exactly. I know the 05-06 tacoma manual said something like "for best performance use...".

So why would toyota include that if it didn't make a difference, and what changed in the early to late year 4.0's to make octane irrelevant?

 

My '09 manual says that the stated mileage figures were for 91 octane

 

Doesn't elevation and performance/RPM matter as well. For instance, in our wakeboard boats we typically use cheap gas as we use lower speeds of 23 MPH, but in the competitive ski boats going 34-36MPH the speed and torque is much more important as well as precision. Where does this come into play?

 

Forgive my lack of knowledge but I was wondering if you can just switch octanes back and forth and its not a big deal? Should I just wait until my tank empties and then put the other type of octane in there?

 

You can mix it mid tank if you'd like. That's what knock sensors are for. I run the best fuel I can find, sometimes that means it's hand pumped out of a rusty 55 gallon drum, sometimes it's 100 octane race fuel. Truck runs fine on all of it, but seems to run best on non ethanol premium or race gas.

 

I run 85 (regular at higher elevations) in my truck with a tank of 91 thrown in every 3-4 tanks. I notice no difference in mileage or performance with my driving style. No knocking with either. If going down in elevation (currently 5800') I switch to 87.

I would love to be near a source of non-ethanol fuel. No such luck though.

*edited for grammer

 

I use 87

 

I mostly run 87 but it's ethanol free so it runs like a top.

 

Again this is speculation but someone mentioned the octane change in....'09? The only thing I can think of off hand is the dual VVT-i.

An engine can have a calculated compression ratio, then there's a dynamic ratio. I may be using the wrong terminology, but I'll give my best explination. When you do the math of the complete cylinder volume vs. the space left at top dead center, you have a calculated ratio.

Dynamic ratio is when the valves are open at various times, primarily the intake. The intake valve is still open for a few degrees after bottom dead center (BDC)to maximize intake volume because the air can only move so fast. Also the exhaust valve starts to open before the end of the combustion stroke to maximize exhaust exit. This action has little to no effect on compression ratio as that cycle is in the past.

Back to the intake stroke. When the intake valve is open those few degrees after BDC it's possible to loose a pinch of air as it comes back up. While I'm not totally versed on the Dual VVT-i, I guess it's possible that.....if pre ignition is detected the intake valve could stay open longer (i.e. couple extra degrees) to lower the dynamic ratio?

 

I had to select 91 octane with an asterisk ... in the Omaha area where I am currently working I always try to find non-ethanol gas and it varies from station to station what octane grade they sell that is non-ethanol. In most of the stations that have non-ethanol it is 91 octane, but if they have a lower octane grade that is non-ethanol I will fill up with that just for the monetary savings.

 

Check www.pure-gas.org - shows 35 stations in Colorado.


Is Englewood or Lakewood a Denver suburb ?

 

The oil companies only have the lowest and highest octanes at the pump station. I.e. If the station offers 87, 89, 91 and 93 octane and you choose 91, the pump mixes the proper amounts of 87 and 93 to end up with 91.