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According to Will higher octane fuel improve gas mileage? higher octane fuel does NOT notably improve fuel efficiency.

Will higher octane fuel, increase power while towing?

I looked online and found two different answers.

Will using a higher octane fuel than required improve fuel economy or performance?

It depends. For most vehicles, higher octane fuel may improve performance and gas mileage and reduce carbon dioxide (CO2) emissions by a few percent during severe duty operation, such as towing a trailer or carrying heavy loads, especially in hot weather. However, under normal driving conditions, you may get little to no benefit. source

And the opposite.

Avoid Premium Fuel

One secret to lowering your overall fuel cost is to just use regular gas, unless your owner’s manual specifies a higher octane gas. Most RV's don't benefit from burning high octane fuel, and therefore you will pay at least $20 more per tank for no additional performance. source

The top answer by Brian Knoblauch in the gas mileage question says, it might have less power.

higher octane fuels actually contain slightly less energy (they just offer a more controlled burn that higher compression engines can take advantage of).

I am planning to use a 2014 Honda Pilot to tow a 4,000 pound trailer over the Rocky Mountains, a couple of percentage points in power increase (or decrease) could make a huge difference.

Will higher octane fuel impact the available power in common v-6 regular gas burning engine?

  • 3
    This is a great question – Zaid Apr 10 '16 at 11:11
  • Indeed, great question – DucatiKiller Apr 10 '16 at 17:46
  • Update, I used regular 87 octane to cross the continental divide twice, both times at 70MPH no problem. In South Dakota I was driving into 30 - 40 mile head winds, I tried filling up with premium fuel, but there was no notable difference in performance. – James Jenkins Apr 29 '16 at 21:00
  • Higher octane fuel contains slightly more energy ; may be about 0,5 % because of slightly greater density. – blacksmith37 Sep 11 '17 at 20:25
18

That is a great question. I believe the true answer here is "it depends".

Like so many other things you can do with your vehicle, if your vehicle cannot use the higher octane fuel, it won't benefit from it. Something to remember about octane ratings is, the higher number does not mean it is more energy dense. The higher number indicates the fuel is actually harder to burn. This seems counter intuitive, but it makes sense if you think about it. Higher octane fuel is used to counteract pre-ignition or knocking. This condition occurs when the fuel spontaneously ignites due to any of several reasons before the spark occurs. This gives the noise which we commonly call "pinging" or "knocking" and can cause damage to an engine if it happens too often and too much.

Since the advent of electronic fuel injection, pre-ignition happens a lot less often. Knock sensors which detect the knock, will pull timing from the ignition. When this happens, the power output is brought down as well.

If your engine experiences pre-ignition while towing up over the Rockies, it will definitely have more power from higher octane fuel than if the ignition is pulling timing. While the higher octane fuel is not giving it more power directly. If the engine needs it, the higher octane fuel will provide the means for which the engine can maintain the power it should normally have. Pre-ignition can occur due to greater stress on the engine, towing is a prime example of greater stress, especially when doing so up an incline.

  • My Honda engine is probably a different beast (K20A) a high compression motor designed to run on a variety of Octane ratings. I understand that ECU uses knock sensor and tries to optimise power under full throttle. So running on a higher octane makes a difference for it. See youtu.be/gQghB4asSnI . For your towing needs note the torque commentary @4m36s – Jayfang Apr 11 '16 at 14:02
  • @Jayfang - Absolutely and is why I stated "it depends". Great point and appreciate the add. – Pᴀᴜʟsᴛᴇʀ2 Apr 11 '16 at 18:25
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Engine control systems in typical cars do not generally change the spark advance to add more than expected, the engine is designed to run at a particular efficiency for a given octane, so changing it from whats specified by the manufacturer isn't a good idea.

If there is sub optimal conditions and the engine management detects a problem, fuel mix or knock detection etc, it'll retard the timing and/or switch to a second fuel and timing map that is designed for safety while the problem exists.

So if there is a problem and its being caused by pre-ignition ( before the spark, after the down cycle) a higher octane which is slower to burn might help.

If its detonation, a few things can happen, the piston can pop out the side of the engine, break seals, etc, different engines handle detonation in different ways, and some detonation is more severe than others. There are engines that cannot take any detonation, and there are some that can for a while.

Detonation is bad, if its happening then something more severe is happening and a slightly higher octane probably isn't going to do much.

Some pre-ignition can be cured with a higher octane, and it'd be ok if it was only caused by severe load outside of the normally expected running of the engine.

Without reprogramming the ECU for a different air/ful mix and spark advance you won't get benefits from a higher octane than spec'd unless you're in a sub optimal burn cycle, which either means excessive load or a problem, and you're unlikely to see much there, slightly smoother running perhaps.

The injectors are programmed to spit out a fixed amount of fuel when you're in open loop which is where you'll likely spend most of the time running.

So if per cycle the same amount of fuel is being injected into the cylinders, the amount of fuel being used would remain the same, regardless of the type of fuel. This table is fixed in the ECU and usually the only feedback there is the knock detection, have to generalise here because there are some engines that don't follow that convention.

You're not putting more or less fuel into the cylinder because its a different octane, the fuel pressure is the same, the time the injectors are open, is the same. The ECU in open loop is using the same timing/mix.

Some cars do have wide band and can run closed loop upto WOT, but they're less common.

6

Two great comments already; couple of things to add.

The reason why we have Octane types is because the lower-octane fuels tend to ignite at lower temperatures (ie. hot metal parts inside the combustion chamber, rather than the spark plug firing) or lower compression pressures (ie. the compression fires the engine like a diesel engine works).

The octane itself historically was a measure of how much of the chemical "octane" was in the fuel; octane is a relatively long-chain hydrocarbon for gasoline. The longer the H-C chain, the harder it is to ignite. (Which lights easier? Propane or candle wax?)

Detonation is a huge power waster, besides being destructive. Think about it: the piston is trying to compress the air fuel mixture, and before the spark plug fires, the fuel/air mixture starts to burn, trying to pound the piston back down before it gets to Top Dead Center. Combustion pressures exceed the engine's design and blow holes in pistons, pop cylinder head gaskets, break connecting rods, etc.

6

... As the air gets thinner at higher altitudes, your air/fuel ratio is going try to stay the same (emissions, performance, fuel efficency) if you're under the control of the ECU in closed loop mode.

Consider the Mass Airflow Sensor, which detects the total mass (not volume!) of air entering the engine by a hot wire resistance sensor.

Note that I have no experience with Honda EFI systems, but I used to work on engine management systems on ships. (Of course, on ships, altitude was rarely an issue...) And I've got lots of experience towing cars and trucks.

So now the engine's under a huge load and it's less powerful (less air = less fuel = less power...). Supercharging was invented for piston-engine airplanes, just to keep the power up at altitude.

You're going to be pushing the engine very hard doing this:

Raising your engine temperature (hot weather, heavy loads: towing, aerodynamics, hills) exacerbates the pre-ignition of the fuel. If your temperature gauge is higher than normal non-towing driving, you're making pinging more likely.

If you're running thinner air, the A/F ratio should adjust... until you hit open-loop mode (basically some part of the map is outside of parameters). The "CHECK ENGINE" light will come on. And the ECU will probably dump fuel into the engine to avoid detonation... do that for too long and your exhaust system (catalytic converters especially) will be overheating. More subtle but possibly more damaging than a glowing red hot exhaust system would be pinging for a long time because the fuel delivery and timing can't be optimized to avoid it.

Most modern engines have a compression ratio over about 9.0:1, certainly at the point where Mid/High Octane can make a difference. But in normal operation (not your case here), the ECU can probably compensate enough and avoid detonation, allowing most cars to "get away with" running 87 Octane. Charliex noted the excessive load problem. View the altitude power reduction as an excessive load, view hill-climbing as an excessive load, view overheating as an excessive load, view the trailer as an excessive load.

Conversely, if the engine's compression ratio is low enough, and it's not at risk of overheating, pouring Mid/Premium gasoline into it a waste of money. My favorite engine is the Chrysler Slant-6; there's an example of a low-tech stout-as-nails old engine that only pings on 87 Octane regular unleaded if there's a timing or overheating problem.

You're likely to heat up the engine a lot. Harder-to-IGNITE high octane fuel is your asset. Remember, as I noted, detonation wastes a huge amount of power, so even if there were a minor tradeoff in the number of Joules/L of fuel, you're still going to benefit - power is nothing without control. (Think hydrogen bomb vs. Mr. Fusion on Doc Brown's Delorean.)

If the owner's manual recommends higher octane fuel (especially in this circumstance) use it. If the temperature gauge moves at all above what's normal to your vehicle, use it.

Get a full tuneup; I'd recommend your brand's dealership. Tell them what you're planning to do and that you want a checkup. Go over the cooling system and the fuel system with a fine-toothed comb.

And while you're at it, make sure the transmission, steering, brakes, and tires are in impeccable shape. And the brakes and tires on your trailer.

Look at a turbodiesel pickup truck with a 5th wheel trailer for this task. If that's not practical, take your time, leave the air conditioning off, and have a contingency plan. The premium fuel feels like cheap insurance.

Lawrence

5

There has been a lot of good information shared here, especially by Lawrence Wade (from my perspective). But we're slightly left hanging about the two closely-related fundamental questions from the OP:

Will higher octane fuel, increase power while towing?

and

Will higher octane fuel impact the available power in common v-6 regular gas burning engine?

Strictly speaking, no, higher octane fuel will not increase power while towing, compared to "regular" octane levels.

Secondly, if we assume the engine is operating as intended, in good tune, and not overheating, no, higher octane fuel will not impact available power in a normally-aspirated engine such as your Honda V6.


However...
That's not to say that premium high-octane fuel isn't recommended. Since loss of power and possible mechanical damage may occur if the engine isn't running within spec, high octane fuel is cheap insurance to make sure that the adverse operating conditions - like a heavy towing load, and climbing mountains - don't lead to pre-ignition or detonation.

The answers here have already established that pre-ignition and detonation are bad for your engine, and why, and the aggravating factors that can lead to those states. (They're distinct conditions, but not mutually-exclusive ones.) But as long as those conditions are not happening, then premium fuel will do nothing for you.

This relates to how your other two example answers from the question about economy are not in fact opposite responses - they're both correct, with importantly nuanced differences.

For most vehicles, higher octane fuel may improve performance and gas mileage and reduce carbon dioxide (CO2) emissions by a few percent during severe duty operation, such as towing a trailer or carrying heavy loads, especially in hot weather. However, under normal driving conditions, you may get little to no benefit.

This is correct because the author is describing conditions that push the operating parameters of your engine toward the edges of its envelope, and a little pre-ignition may occur as you're hauling that trailer up Pike's Peak. Having high-octane fuel will preserve your power output (such as it is - high altitude will still have reduced it a lot) by preventing pre-ignition. Thus, it may "improve performance" compared to regular gas if you usually lose power and performance while using regular gas.

But it doesn't inherently add energy or power over regular fuel. And as they say, under normal driving conditions, there's pretty much no benefit of any kind. I take this to simultaneously imply that the engine fuel mapping and ignition (and general state of "tune") is all well within normal limits.

One secret to lowering your overall fuel cost is to just use regular gas, unless your owner’s manual specifies a higher octane gas. Most RV's don't benefit from burning high octane fuel, and therefore you will pay at least $20 more per tank for no additional performance.

This statement is making no reference to marginal driving conditions, and so is implicitly assuming "normal operating conditions" for the vehicle, and comes to the same conclusion as the other answer: no benefit.

And while we're at it, this:

higher octane fuels actually contain slightly less energy (they just offer a more controlled burn that higher compression engines can take advantage of).

...is also correct, although the energy margin is very small; less than 1%.

While the deep engine theory, and organic chemistry, and physical chemistry, and thermodynamics of it all is a fascinating topic, and I would gladly add to it if there seemed to be a need, in the end our result is the same:

Under the specified conditions, assuming a properly-running, naturally-aspirated engine system, higher-octane gasoline does not improve towing power.

As soon as you add turbocharging, overheating, dirty injectors, fouled plugs, or other conditions adverse to properly controlled ignition, then you've changed the equation, and you might need to step up the octane rating to prevent problems.

Practically speaking, there's a fair chance you'll encounter pinging/knocking conditions during your trip. I recommend using high-octane gas as insurance during the mountain legs. If you really prefer to stick to regular gas, take a bottle of octane booster as a backup option.


My background: FAA certified Airframe and Powerplant mechanic.
Also, I love my turbo-charged Subaru in the mountains.

Safe travels!!

  • Well, I had more to say, but it kept "looking like spam", so since I'm new on this branch of StackExchange, I had to axe some. Let me know if you want some organic chemistry details! – Alpinwolf Apr 24 '16 at 10:52
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A lot of people get confused with octane ratings. The octane rating is the fuels resistance to detonation, ie, the fuel / air mix igniting just due to pressure. When this happens the resulting burn is very violent and can easily cause major engine damage. When the fuel / air mix burns normally it pushes down progressively on the piston; when detonation occurs it is an explosion which is more akin to hitting the piston with a hammer.

On their own a higher octane fuel will give no more power in general. Higher octane fuels tend to have a very marginally lower energy content (after all, the additives used to increase the octane rating are themselves not good fuels). Further one way to increase the octane rating of a fuel is to just make it burn a bit more slowly (so cylinder pressures increase more slowly).

If you have a high enough octane fuel to avoid detonation then you get no benefit from a higher octane fuel on a basic engine.

However there are edge situations where the octane rating is borderline and high load situations can trigger detonation (eg, climbing very steep hills).

There are exceptions.

If your vehicle has a knock sensor and the ECU can adjust the ignition timing based on the feedback from this then it is possible that it can take advantage of higher octane fuel to an extent to get more power (and possibly better fuel consumption from using less fuel to get the same power). The difference is likely to be fairly minimal.

More common (but probably not on an RV) is a vehicle with a turbo and a knock sensor, allowing boost to be increased to take advantage of the higher octane fuel. The difference here can be significant (one vehicle we have you can readily feel the difference in power between European 95 RON and 98 RON fuels as boost is increased).

Of course, the other option is that you manually advance the ignition timing (either remapping, or on older vehicles just turning the distributor a touch) and / or increase the boost to take advantage of the higher octane fuel. But doing this and then being forced to refuel with lower octane fuel puts you at risk of serious engine damage unless you readjust it first.

So overall, it depends. On a basic engine in a low state of tune there will be zero benefit. On a modern normally aspirated engine there is the possibility of a tiny benefit (but I doubt it is enough to compensate for the higher cost of high octane fuels). On a turbocharged engine there is a good chance of improved power and probably higher fuel economy (if you can resist making use of the higher power available).

protected by Pᴀᴜʟsᴛᴇʀ2 Sep 11 '17 at 20:31

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