Common Tread

What gas should I put in my motorcycle? The skinny on octane rating

Mar 08, 2019

Religion, politics, and preferred engine oil brand are never to be discussed in polite company.

Add fuel octane to the list. Advice on what to run in what motorcycle is dispensed as freely in America as fuel itself, with varying levels of understanding as to when fuel of a given octane rating should be selected and used.

gas pump
All gasoline is not created equal. Photo by Kenneth Stone.

So there’s a short answer and a long answer to the question asked in the title of this article. The short answer is, “Use whatever the manufacturer of your engine recommends. Fuel of that rating will give the best performance and economy at the lowest price.”

The long answer, of course, is the same as my answer for most motorcycle questions: “It depends.” What kind of engine do you have? What level of computerized operation does it offer? What mechanical modifications, if any, have been made to it? Do you value power, or economy? How deep is your wallet? Let’s get into the thick of things!

What is octane?

Octane is actually short for “octane rating” or “octane number,” a figure calculated through a few different methods. The number indicates how resistant the fuel is to detonation, which is when the air and fuel mixture begin burning prematurely due to heat and pressure, rather than beginning with a spark introduced at the optimum time by the spark plug.

In the United States, we usually calculate our number using the (R+M)/2 method, which we then call AKI, or Anti-Knock Index. Fuel at American pumps usually ranges from 87 to 94, though you may find higher or lower numbers from time to time. Basically, higher octane means the gasoline will only ignite under higher temperatures and pressures.

What's the problem with detonation?

Well, if the burning of air and fuel happens too early, it will occur while the piston is still moving upwards in the cylinder. That’s sometimes too early to make power (and often actually robs it) and in severe cases, can damage the engine. The piston, rings, and ring lands usually take heavy abuse from detonation, and damage can even extend down into the bottom end of the con rod, wrecking big end bearings. “Detonation causes increased pressure, and thus resistance, and that resistance also causes heat,” says good friend and chemical engineer Andy Toback.

It manifests itself as a pinging or knocking sound. If you’ve ever heard a diesel knocking away, it’s the same sound. (And basically the same cause, too. It’s just a characteristic of the way a diesel operates.)

Why are different octane ratings necessary?

High octane helps combat detonation that naturally occurs with high compression ratios, which produce higher cylinder pressures and are more prone to detonation than the same engine with a lower compression ratio.

Gas pump face
High-test, baby. Photo by David Falconer.

Ah! So high octane is just for high performance, then?

Traditionally, high octane fuels were used in large engines with high compression ratios, because high CRs foster the conditions that support detonation: high pressure and high temperature. So generally, an engine with higher compression will run with lots of timing advance to give that detonation-resistant fuel time to burn at high revs. Since all that gave the high-octane gasoline time to burn, the mixture could be run “fatter” than necessary, which is perfect for acceleration demands.

“In tuning for best power, your goal is referred to as RBT and LBT, which stand for Rich Best Torque and Lean Best Torque. The mixture required to reach those targets in a normally aspirated motor is between 8 and 16 percent richer than the actual stoich value (14.7:1 air to fuel, the “perfect” burn in terms of efficiency) of the fuel being used,” says Michael Goni, owner of Standard Cycle, a dyno tuning outfit.

All the factors that came with high compression happened to demand modifications that were good for making lots of power. High octane, you see, is necessary to support an engine with a high compression ratio. The important takeaway here is that octane itself does not cause the power, it merely allows an engine to make power if it’s built to do so. Using high-octane fuel in a standard-compression engine won’t give you one more lick of power. I contacted a technical specialist for a manufacturer who’s a good friend, but needs to remain anonymous. His reply in regards to high-octane go-juice in a low-comp motor in search of power? “Often it has quite the opposite effect. It takes more energy to ignite the fuel, resulting in power loss.“

You should be aware that higher compression ratios cause more power to be extracted from a given portion of fuel. You should also be aware that for maximum power and efficiency, ignition timing should occur so that fuel finishes burning around 20 degrees ATDC — but finding when to start burning the fuel (the amount of advance) is a bit of a moving target, depending on heat, engine load, and engine speed.

The symptoms of either too much ignition advance (causing the incoming fuel charge to fight the piston while it’s still on its way up the bore) or too much compression relative to fuel of a given octane is the same: detonation. My OEM buddy summed that up nicely, too. “Ever stand on the rear brake pedal and load the engine (engage the clutch slowly while adding throttle) and hear that knock, knock, knock from deep within the engine? That’s detonation. ”

Tuck that thought into the back of your head, because we’re coming back to it shortly.

So how have octane choices affected engine design?

That’s a good question, and one that requires lots of information to answer even somewhat correctly. First, fuel availability affects what is built for a market. A built race motor may be designed to use 108 octane fuel, which is easy enough to procure before an event, but since that octane number is rarely seen for sale at dispenser pump in America, it would be difficult to use an engine built like that in a street machine. It would certainly be a sales flop.

Also, fuel is not the same in every country. If fuel of a greater octane rating is the only thing commonly available in a given locale, an engine not built to take advantage of it is effectively burning a more expensive fuel with no resultant benefit. (But with no harm done, except to the rider’s wallet.) Incidentally, if you are wondering about race gas (and all the different varieties of it that exist), the advantage is also related heavily to consistency in addition to quality. Goni summed this up nicely.

"The consistency of the overall pump gas formulation is not as strict as lab-mixed racing gases and the blend will change depending on the time of year and weather, as well. One of the reasons race gas is favorable, whether it is high or low octane, is its consistency and the data sheet provided will have information that will help a tuner calibrate things for best performance."

K1600B gas
Proper feeding of the wild motorcycle can often prove a daunting task. Photo by Lemmy.

Effectively, race gas offers more choices and more consistency, allowing enterprising custom engine builders and tuners to whittle down allowances they might have to leave in for lower quality fuel (or fuel of unknown quality).

On the factory bike front, high-octane fuel is used in similar ways for slightly different outcomes. Motorcycles differ from cars in terms of their packaging. In the context of our previous question, consider that a high-performance, hi-comp engine that’s very small has some desirable characteristics. It will offer better power than a comparable unit with lower compression, or alternately, harmful tailpipe emissions could be reduced. Another potential advantage is that a physically smaller engine may be used in a given application instead of a larger, heavier, less efficient engine making the same power. As a design consideration, packaging is more important in motorcycles than with larger vehicles. A smaller engine with higher output is desirable in many applications.

Remember when I mentioned higher compression ratios extracting maximum power, and I also mentioned the elusiveness of the beginning of an ignition event? Engine designers used to build engines with a healthy compression safety margin in them, in order to keep customers happy and prevent warranty claims from going through the roof.

Enter the knock sensor.

A knock sensor is basically a device that uses the engine as a vibrating diaphragm. It interprets acoustic events characteristic of knocking, and reports findings back to the engine control unit. When knock is detected, the computer can very rapidly retard the ignition timing some, keeping the pinging at bay.

“The knock or Ion sensor will pick up all noises and pressure waves and the calibration in the ECM will determine what frequency is the correct one to start pulling timing away from the base map and it will hold the adjusted crank angle for a preset time. It will then attempt to return back to base timing. If detonation is still occurring when it returns, it will repeat the cycle until it sees it's safe to stay at base timing,” Goni explains.

This technology allows engines with higher compression ratios to be built with much less concern that damaging detonation could wipe out an engine, because the engine control unit can delay ignition (retard it a bit) to occur closer to the piston arriving at TDC. That flexibility means that many engines can now take advantage of higher octane fuel to produce more power if it is used, or extract more power from lower octane fuel, or even better, allow the use of both. It also means that the healthy margin of error that used to be included could be made much, much smaller.

Effectively, the engine doesn’t “know” what octane you loaded your bike up with. Instead, it simply advances the ignition as much as it can, trying to make the engine run as cleanly and efficiently as possible. (It just so happens that an engine that is burning its fuel most efficiently is also often at its power peak, as well.) Higher octane fuel will simply allow more ignition advance to be added, and also allows the engineers to run higher compression ratios than they would without a closed-loop feedback device like the knock sensor.

“The stock engine management system on any vehicle will have a base timing map that will be calibrated at the factory to produce maximum brake torque as possible assuming fuel of the octane specified, with some safety margin built in," says Goni.

“There are compensation tables built into the calibration that take information about intake air temperature, coolant temperature, gear position, and manifold pressure. With that information, the EMS will remove or add timing from that base map situationally. Things are a bit different, say, from a cold start in the driveway and crossing the desert two-up with loaded saddlebags.

“The knock sensor is there just in case. Pump fuel can vary so much from pump to pump, and season to season. Sometimes the wrong gas gets poured in or maybe you are stranded in a one-horse town and they only have regular when your motor requires super,” says Goni.

This means some bikes are a bit flexible: You can run high-octane fuel for max power, if you want to pay for it, or if you want to save a few bucks, you can use a lesser quality fuel — it just depends on what the engineers designed it to do.

So there’s no use spending a few more bucks at the pump to buy “the good stuff,” huh?

Well, not really. At least not in terms of octane rating, anyway. One thing that’s important to know is that fuel differs not just in octane, but also in additives. You may have heard of Top Tier gasoline. This is a gasoline performance standard designed and supported by some automotive manufacturers. Effectively, these are voluntary standards that specify fuel with a certain amount of detergents to prevent fuel deposits from forming on valves and fuel injectors.

Some feel that those deposits coming from fuel without detergents can simply be removed through periodic use of items like fuel injection cleaner. Others feel that keeping the deposits at bay from the start using Top Tier fuel is a better method. It should be noted that the EPA in America requires a certain amount of detergent additives in gasoline. Top Tier advocates posit that the minimum is not enough. Many of them are automakers who had seen persistent fuel-related problems, indicated by repair records.

Gas pump
I seem to take this shot a lot. Photo by Lemmy.

I mention this because if you want to help yourself at the pump, this is a definite and proven help for your bike. Is it “worth it?” Depends, I suppose, on your outlook, use, budget, and probably some other factors, too, related to repair. (I asked Andy what his take was on the situation. I knew his answer before he got it out. “The more important thing is to keep ethanol out of there.” Typical Andy.)

And there are a few more monkey wrenches to toss in here as well. Some “premium” gas contains more additives than Top Tier gasoline, so you’re getting not just the higher octane, but also detergents and perhaps other additives, like stabilizers. On the flip side of that coin, there are brands of fuels that are not Top Tier, but are still of very high quality.

And of course, as discussed, you can absolutely buy race gas, even fairly low-octane stuff, just for the consistency, quality, shelf life, and superior handling processes. Contrary to popular opinion, there is plenty of street-legal, fairly low-octane race gas available for purchase.

That’s a bit of the rationale behind the suggestion to use what your manufacturer suggested, and nothing more. The folks who designed your bike’s engine are aware of what is available to you to buy at a pump and built your motorcycle with those options in mind. You’ll have a hard time finding a motorcyclist who ran what the factory recommended and had trouble... but if you can get your hands on better fuel reliably, the factory probably left a little room for you (and folks like Goni) to eke out a little more from their motorcycles.