Octane rating advice

Iso octane is added to gasoline for the sole purpose of inhibiting detonation, aka ping or knock. High compression or forced induction engines can produce more power. But they have an increased tendency to detonate as the cylinder pressure is higher. So a higher octane fuel works for them. The goal is for the flame front from the spark to advance uniformly through the cylinder creating maximum combustion. Bottom line is the lowest octane that doesn’t cause spark knock is the right one. Or the one on the owner's manual.

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That’s why there is little to no power available via tuning for 87 octane. Timing is already very aggressive, almost like Toyota optimized it for the required 87 octane… We really mucked up a 3rd gen thread. Lol.

 

Why can't it be both?

 

Camshafts play a big role in all this stuff too.

 

Did I not say basically that in the second paragraph that you quoted. I know how turbochargers work. I've utilized them in everything from cars, to Kenworths, to Freightliners to twin engine aircraft.

If you're cruising at 50 mph with no headwind, in whatever gear the '22 10 speed would be in, I can almost gaurantee that the boost gauge will be showing no boost. At that point you're running at atmospheric pressure, regardless of elevation.

 

How high up are you? I’m at 6000 and I run 85 without issue.

 

I ran 91 in Colorado for a solid year and saw zero difference in MPG and no difference in performance. I did this after reading another thread on here where there was discussion about how higher octanes provide these benefits.
If people are seeing these benefits by running premium then good for them but I didn’t see it.

 

I’ve never noticed any benefit in my 2007 5.7L in running 91 Octane at average elevations of 3500-6000ft. I have noticed the truck runs stronger at lower elevations when I travel to the east and get out of the high plains and Black Hills. That 3% per 1000ft starts to make a difference when you’re gaining 10% power or more.

 

Yeah but you’re saying to sacrifice engine performance. I can guarantee that in 3rd gen boost is needed in many situations, but especially at highway speeds. So it doesn’t matter the elevation, the minimum recommended octane should be used because it’s more efficient and how the engine was designed. So whatever the absolute manifold pressure is at 50 mph and top gear, 87 octane will give the best performance over a lower octane fuel.

 

Yep.

Positive manifold pressure as you point out will be present in a turbocharged engine, at least at any altitude any road in the world offers. That is after all that be of the things turbos do is compensate, very effectively, for loss of air density at altitude. If the engine and turbo combination can make even 0 PSI of manifold pressure vs. vacuum then whatever the elevation and ambient air density will be compensated for; and the Octane rating of 87 will be needed.

The guys chiming in thinking they’re going to not be in boost and therefore can run lower octane fuel are not being realistic.

For starters the turbine side on the turbos here are small, made to spool up and make boost at modest engine speeds, exhaust gas flow, and volume. This maximizes available torque at useful truck engine speeds. Plus this is a computer controlled wastegate turbocharger, when the ECU decides it needs “x” amount of torque in a given gear at a given speed it’s going to close that wastegate and make boost. The driver has little control over that if they’re just cruising around at highway speeds with cruise control set. If the programming is to hold a higher gear and let that low end torque work when possible the engine will constantly be in and out of boost as needed.

I guess if a guy wants to never use cruise control, or accelerate hard enough to cause the ECU to close the wastegates he can try it out just to save a bit of money on fuel. Problem is they’re probably still not going to succeed, and frankly if money is that tight they probably should reconsider buying a new truck to begin with.

 

Boostane is like 15 cents a gallon I think for a 3 point bump

 

6500’

 

I’ve run plenty of 85 Octane 100% gasoline in the 5.7L at 3500’. Switched over to 87 Octane ethanol blend years ago after determining there wasn’t enough advantage in fuel savings to justify the higher cost. The truck ran fine on it though, no issues at all.

 

This I agree with. Anytime I drop in elevation I feel like I’m driving a different vehicle.

 

That’s the primary reason I’m interested in the Gen 3, forced induction gives me all the rated power (or so close it’s an insignificant difference).

 

The reason you have 86 available and not 87 is based on your altitude also. Run the 86. It will not lessen your performance. Your altitude, however, always will play a factor on performance. The numbers Toyota specs are for 87 octane at sea level.

Where I live, at sea level and sometimes literally below sea level, pumps have 87, 89, and 93.
I’m actually in NM at the moment (grew up here) and of course you guys have 86, 88, 91.
You can probably even still find 85 octane in Albuquerque (used to be that way way back), but they’re pushing mile-high.
Anyway, Ill be topping off with 86 while here.

And yes, as alluded to already, most people out there in the world think they are doing better by paying for premium but these are the same people who think that the higher number will also make their car faster. They have no clue how octane ratings work or what they mean.

It is somewhat odd that a forced induction engine is tuned for for lower octane, but thats just how they set everything up and modern tuning is done on automatic parameters. 20 years ago, they’d have said 91/93

 

Just read through more of the comments. The air/fuel ratio controls on these things is run so tight and constant (probably milliseconds) that small differences in octane at different altitudes is not going to be a factor, even under boost. The ECU is constantly adjusting but is “default” to an 87 “tune” if you will. What Toyota should have said is to run “Regular” unleaded, instead of saying “87”

 

I never claimed that the engines were sparking at the optimal time. I said that retarding the timing means the engine was not sparking at the optimal time.
Also, according to the testing DAP has done, the timing is at 0-2 degrees on 93 octane, meaning the "optimal" octane is probably around 94-96.

The ecoboost is a very different engine. And the introduction of forced induction changes things considerably. I also wonder how you think you know your engine is adjusting so quickly? Do you have a scan tool hooked up and read the data in real time? Because otherwise, you would never actually know what the ECU is doing.

The problems with ethanol are more about the government subsidies and increased corrosion issues. The fact is that it is also less energy dense than gasoline. 30% less to be somewhat specific.

You are mostly right. The engine will still hunt for peak timing. But it won't have to swing so far to do so. Just because you may not reach MBT with 91 doesn't mean you should just run 87 or whatever else. That's like saying "well I can't eat an optimal diet so I might as well just eat snickers bars and donuts".

Maximum brake torque isn't necessarily the goal here. MBT is generally only achieved at wide open throttle. For a daily driven vehicle, you would never tune for MBT anyway. You tune to run efficiently at normal loads for normal driving, and let the ECU make adjustments as the loads and RPM increase.