Everything You Need To Know About Motorcycle Fuel

For many riders, putting gas in their bike is as simple as rolling up to your nearest gas station, putting a credit card in the slot, and filling up the tank. For the majority of riders, that’s all they need to know, other than what to do when it comes time to put their bike in storage.

Racers, and high-performance enthusiasts like myself, however, nerd out over the different ways we can make our machines better. From different exhausts to ECU flashes, suspension, and tires, looking for that performance edge keeps us up at night. This is what led me down the rabbit hole of motorcycle fuels. There are so many options out there, from several different companies. How do you go about choosing? My first thought was simply reading up about fuels the same way anyone else would – I’d Google it.

What I found was primarily targeted towards the car crowd or motocross racers and basically boiled down to which gas a certain team uses and why. While useful information (to some degree), I wasn’t so much curious about anecdotes but more about what goes into fuels (both pump and race) and why they are the way they are. There seems to be a separate language chemists speak when talking about fuels, and all of it is Greek to me.

To help make some sense of what both average street riders and racer folks like myself should concern ourselves about when it comes to fuel, I sought out experts to help talk me through the ins and outs of what all these terms mean and why they matter. Specifically, I talked to Bruce Hendel, Vice President of North America sales for VP fuels; Zachary Santner, Technical Specialist at Sunoco Performance Products; and Ron Finney, Technical Director at Renegade Fuels. As luck would have it, Finney is also a part-time college professor – the perfect guide to breaking down some of these complex topics in language the layperson (read me) can understand.

With over three hours of recorded audio from the interviews, clearly, we can’t republish all of it here, so what follows is an abbreviated version edited for clarity. Wait, did we say clarity? If there’s one thing to take away from these conversations, it’s that fuels are complex and confusing. What this offers is a better understanding.

Finney: Obviously, you want a quality product. With pump gas, they’re competing on price. It is a very competitive market, so there’s a lot of product variation, if you will, depending on how the markets are going chemical-wise. Pump gas is inconsistent and this is why. Still, there is a lot of misinformation regarding octane.

People say a higher octane slows down the burn rate of fuel. Well, there’s nothing further from the truth. As we add octane, we’re doing that on engines that turn higher RPM and have higher compression [ratio]. We run them harder. So, if the piston speed is increasing and we’re slowing down the burn rate, that’s going the wrong direction. So, you can figure out real quickly that it makes no sense. Having the right octane means that fuel will combust properly at those given cylinder pressures and temperatures, preventing detonation. So, that’s where it’s really key for riders to get the right octane value.

Or, put another way…

Hendel: The average street bike was made to run on pump gas. More octane doesn’t make more power. Something with more octane allows you to do things to your vehicle to make more power. So you can run a higher-compression piston. If you’ve got a turbo, you can run more turbo boost. You can advance your timing. So, you can do all those things to take advantage of higher octane.

From a pure octane standpoint, one fuel isn’t necessarily cleaner than the other. The only reason why you’d need more octane is because the engine requires it. So, to buy something with higher octane when you don’t need it isn’t necessarily a wise spend.

All pump gas is mandated to have a certain amount of detergent in it because it’s not very clean to begin with. Then a lot of the name brands that are out there, whether it be a Shell or a Mobil or whatever, even our branded gas stations have additional detergents that are added to it to basically give you a cleaner fuel.

Finney: If you look at your local gas pump you may have noticed the octane sticker says R + N/2. That’s research and motor octanes divided by two. In the US this gets us the octane value we see at the pump (Europe uses only the research octane number, which is higher). Neither one of them [research or motor octane] is indicative of a modern engine. The octane test was developed in the 1920s, using a single-cylinder engine, running at 600 rpm with 13 degrees of timing. That’s how we get research octane. Then we’ll speed it up to 900 rpm and we’ll advance the timing even more for the motor octane number. Neither of those is indicative for a modern engine that idles at 1200 rpm.

We tell folks, look at the motor octane number. You want to look at both but look at the motor octane number because the motor octane is more indicative or the more loaded of the two to make sure that you’ve got adequate octane to support the power levels that you make. But it is important to still look at the two because the difference, the wider that difference between motor and research is, the more inconsistent the fuel is going to be.

Let me give you an example. If we take two fuels, Fuel A is 120 research octane and 100 motor octane. It gives you an average of 110. Fuel B is 112 research, 108 motor octane. Also 110. Are they the same fuel? Not even close. They’re eight points difference on motor octane. So one will support a 14:1 compression ratio, while the other one will barely support 10:1. That’s the challenge with the fuels for our racers is things like that because they just see the pump number and think they’re there.

Santner: The ethanol in pump gas allows the refiners to have a lower octane gasoline portion. It’s a beneficial way to get the octane up. Ethanol isn’t hurting the octane at all. In fact, that’s a little backward – 91 octane gasoline mixed with 10% ethanol generates 93 octane. So, pure gasoline, 90, 91 octane, just pure gasoline mixed with 10% ethanol bumps it a couple numbers. The things that it does affect are its air/fuel ratio is different, so you need to burn more of it for complete combustion. So, you end up getting lower miles per gallon.

Hendel: If you use your motorcycle regularly the ethanol content in pump gas isn’t a big deal because it’s constantly being flushed through with new gas. Pump gas has a lot of gums and varnishes in general, and that’s why they have to add detergents to it. But if it sits long enough and evaporates, it leaves those gums and varnishes behind. That’s when you end up with problems in your fuel system.

The bad thing about ethanol is that it absorbs moisture. This causes all kinds of problems downstream in your fuel system if the bike is sitting for long periods of time. If you have a bike that doesn’t get used that often, it’s best to either number one, if you’re running pump gas, put a stabilizer in there. If you can drain it out, that’s the best thing you can do, especially if it’s not going to be ridden in a while. But because ethanol does absorb moisture, it causes a lot of rust and corrosion in the fuel system. That’s what happens. So, if you can use something without ethanol, especially if it’s something that doesn’t get used that often, it’s always best.

Hendel: We build fuels based on the fuel rules in any given racing organization. The AMA has different rules than FIM, and even within the same sanctioning body there can be different rules between events or disciplines. Then you have the club racers and hobbyists that aren’t tied to any fuel rules at all. We can – and do – make very different fuels for each application. Many times the same fuel will work just fine for a given machine, but it may not be optimal based on the use case, or it may even be illegal per the class rules. That’s why we have so many different fuels.

Generally speaking, you’ll find some common terminology as it relates to fuels. Especially performance-oriented ones. Here’s a breakdown of the words you’re most likely to see.

Santner: Simply put, it’s the density of the liquid, in grams per milliliter, where water is 1.0. The takeaway here is that lighter fuels will tend to burn a little faster.

Finney: It’s hard to paint in broad strokes because I can make a light fuel that burns slowly and vice versa. It may not perform at all, but I can make it. Gravity is important because it affects the burn rate.

Finney: Maximum cylinder pressure occurs at approx. 20 ATDC [After Top Dead Center], beyond this point, additional burning won’t result in increased cylinder pressure. If fuel burns slowly it will leave unburned hydrocarbons in the exhaust. This means that the engine will not generate maximum power. You’ve got about three to five, maybe twenty milliseconds max to burn the fuel. All the fuel has got to be combusted regardless of the engine by about 20 degrees after top dead center.

Hendel: A good example of a fuel with a lot of energy but is heavy is diesel fuel, for instance. Diesel fuel actually has a lot of BTU’s, has a lot of energy. But, it’s a very slow combustion process, which is why diesel engines are all very low RPM. So there’s a product that theoretically should be able to release a lot of energy, and it does, but you can’t run it in a higher RPM engine. There’s no way it would work. It burns too slowly.

Finney: The latent heat vaporization of a fuel is important because a proper burn will actually help cool the combustion chamber. I know that sounds crazy, but let me give you an analogy I use all the time. When you go to take a shower and you go in the bathroom and you turn the water on, everybody likes to crank it up. I like a nice, hot shower. You go in there, and you’re not cold. So, you turn the water on, let it warm up, take off all your clothes. You get in the shower, and you’re not cold. You take a shower. You’ve got the water hot. It’s steaming up the whole bathroom. That bathroom is probably 10 to 15 degrees hotter than it was when you got in. But what happens as soon as you turn the water off and open the shower door?

That flood of air cools you off and you’re shivering.

Yeah, you’re shivering cold because of latent heat vaporization. The moisture on your skin is evaporating, pulling the heat out of your body. When it changes from a liquid to a vapor, it absorbs huge amounts of heat in order to do that. So, the same thing happens in your engine. You burn that fuel, when that fuel is changing from a liquid to a vapor in that process of combustion, it’s absorbing huge amounts of heat, and that’s what is helping to cool that engine down. So, that’s where the combustion efficiency really helps because it can help keep the bike cooler. If it stays cooler, it’s going to be less detonation prone, and of course, the combustion process is going to stay more stable as well.

Finney: Reid vapor pressure is another test. The fuel is taken at 100 degrees, once it’s heated to 100 degrees it’s going to generate a certain amount of pressure in a closed cell. You need enough Reid vapor pressure so that the fuel ignites easily. The lower the Reid vapor pressure, the harder it would be to start in cold weather. But also, the better it would be in hot weather because it’s not going to vaporize and cause vapor lock issues. But that becomes a balancing act, too, because you’ve got to have enough Reid vapor pressure for the application, so the bike has throttle response, the fuel lights easily, and so forth. The RVP in racing fuel stays pretty consistent, and it matches up with cylinder pressure and temperatures to ensure proper combustion for a given application.

Santner: This number shifts a lot in pump gas because it’s directly related to the atmospheric temperatures. I live in the Pennsylvania area where we make Sunoco race fuels. In the summertime, it might be 7 PSI but in the wintertime it’s 12, because it’s so cold out they have to add butane and other things to the fuel to get it to evaporate, so you can start the engine easily when it’s 30 degrees outside. In California, you’re not going to see as much variation in that just because the temperatures are a little bit more stable. It is dictated by county and per state.

But as far as what this actually means, a high-revving motor is going to be consuming air quickly and you want quick throttle response. So, a fuel that has the appropriate Reid vapor pressure is important. It’s kind of hard to make any general statements on it, but one thing to look out for would be a fuel that is lower than 5 PSI is going to totally be like a dog for throttle response. It’s not going to want to be evaporating quick enough to satisfy the engine’s demand for fuel.

Finney: This is the process of heating the fuel and measuring how much fuel evaporates at specific points. These points are typically 10%, 50%, 90%, and the endpoint (where all the fuel has evaporated). The distillation curve of the fuel gives us specific information about how the fuel will perform in the cylinder when combusted. The general public doesn’t really care about these numbers much, but tuners will if they know what to tune for. A good race fuel will have its distillation curve tailored to provide the burn characteristics needed for a specific type of racing.

Santner: A characteristic of race fuels is that we take a more narrow cut of gasoline than what you get at the gas station. Something like a pump gas might have its final boiling or 90% evaporation closer to 350 degrees. Those higher boiling points correlate to heavier, slower-burning hydrocarbons. Not necessarily desirable because anything that’s slower burning has a higher chance of not completely burning and leaving behind carbon deposits in your engine or in an exhaust or on valves or anything like that.

In race fuels, we use more highly refined components that have these lower boiling points than pump gas. So, you get something that burns completely and is very clean and doesn’t leave behind a lot of carbon deposits. When you get that complete combustion, you’re getting all the energy out of it versus any little bit of carbon leftover, that was potential to have been horsepower.

Santner: Oxygenated fuels provide people a way to cheat, essentially, at the track. They could pour something in their tank and get a little bump in power if they knew and could adjust their engine to take advantage of that.

Finney: The fuels that have the real high content [roughly above 4% -TS], those are the ones that aren’t AMA or FIA legal, so we don’t care. These are for the guys that really want to run a really high-oxygenated fuel [for maximum power]. Then again, when you start seeing 8%, 10% [oxygen], depending on what they use, then you’ve got to be concerned with making sure your injectors are compatible, making sure everything in the fuel system is compatible. This is fuel that you don’t want to leave in your tank for a long period of time. You want to drain the thing back out. It could even create rust and stuff in the tanks because of the high oxygenation.

Hendel: Oxygen content helps with power. If I could put more oxygen in a fuel, it can make more power. Think of it as turbocharging your fuel. By putting oxygen in the fuel, I can now flow more fuel, so I’m going to make more power. Not only that, but I’ll have more torque and better throttle response too. So, when we’re making a more budget-conscious fuel and aren’t bound by oxygen restrictions, we’ll put more oxygen in it because that’s the cheap way to make power. But oxygen is only part of the equation since many sanctioning bodies limit the amount of oxygen content your fuel is allowed.

What we do is when you’re limited on oxygen, you try to use some other components that vaporize well, make power, but are not necessarily oxygen components.

MO: If oxygen is the cheap way to make power. Can you get more specific as to the expensive way to make power?

Hendel: I really can’t because I’d be giving away probably some trade secrets. But some of the things that we use in the level of the [VP] MR fuels and things like that, they don’t even remotely resemble pump gas. They’re almost all chemicals. Another thing is a lot of people think you’re just starting out with pump gas and you’re doctoring pump gas to make race gas. That’s not even close to being true. In the MR fuels, I can tell you it’s a blend of things that aren’t even remotely associated with anything that you would make for pump gas.

Lead was banned from pump fuels in the US starting in 1975 (and is banned entirely in Australia, regardless of use case), but the existence of lead in race gas is still prevalent today, though you absolutely should not use it in your stock street bike as it will ruin the catalyzers in your exhaust. The reason for lead’s staying power is simple.

Finney: It is probably the most effective way to get octane gain. That’s it. It has been thought for years that it’s there to lubricate valves. You’ve heard all of that. Valve seat recession and whatever. The fact is when you burn a fuel that has lead, it’s going to leave a little residue behind. Is that going to cushion the exhaust seat? Well, yeah. Any residue is going to cushion the seat. So, that was kind of a side benefit of it, but that was never why it was in fuel. So, the lead is in there to make octane. That’s why you don’t see unleaded fuels above about 104 or 105.

Hendel: Going back to our discussion about oxygen content, when you look at our high-energy fuels that are low octane and low oxygen, if we didn’t have lead, if you took the lead out of it, the octane would be so low you couldn’t use it. Some of the components that we use that are really good for energy don’t have very good octane. So, we need something in there to raise the octane up to a usable point. The scale doesn’t go up that high, but we make a leaded fuel that’s about 120 octane. It’s used for drag racing.

There are lead substitutes that can be used, but they have a lot of issues. They leave a lot of deposits, especially on spark plugs and cylinder heads and things like that. It’s just hard to manage. You try to manage some of that with detergents, but then you still have issues downstream with oxygen sensors and catalytic converters even if you try to use lead substitutes.

Santner: For the street gas thing first, I would say no matter what your octane rating is on your bike or what they call for, try to use the highest octane that’s available at the pump. My recommendation on that would be it’s not a spec that they have to meet, but what ends up happening on the pump side – Sunoco is an extremely large supplier of street fuels as well, and we’ve done quality control testing. What we’ve seen is the higher-octane fuels also tend to have less dissolved varnishes. This is a spec based on refining and stuff. So, for what you want out of it, the higher octane ends up being a little bit more refined, and that helps clean them up a little bit more. On the race side, I would say it’s important to use the same fuel if you’re using race fuel, and that will help eliminate a lot of variables.

Finney: I would say for the street guys running a good quality, ethanol-free product if you can. Otherwise, I tell folks all the time to try running 93 octane if it’s the highest grade you can get, and see if you feel the improvement of the bike. If you see no difference, then back it down because you’re not on the edge. A lot of the bikes, particularly California machines and stuff, are tuned pretty lean because they still have to meet emissions and stuff. So, they can definitely benefit from a couple extra points because they do tend to run leaner and hotter. So, run a good grade of fuel or try running some of the aftermarket fuel boost products. Everybody has got an octane booster in a can. Put a little bit of that in there for the street bike. It’s just kind of good insurance. It gets the octane value up a little bit. Gives it a little bit more protection.

Also, the one thing with detonation that I always share with people is people think, if I don’t hear it knocking, it’s not knocking. Well, if you hear it knocking, you’re way late to that party.

From the performance side, In that aspect, I would look for one of the oxygenated power sports fuels by any one of the major manufacturers. They’re going to improve throttle response. They’re going to improve performance. Depending on how far they jump, they need to look at what their maintenance tolerance is. Are they the type of rider that is a hobby rider and not really a hard-charging competitor? This will decide if you want to go through the added steps of draining your fuel tank after using high-performance fuel. For some people, that’s a deal killer.

Hendel: In general, the best thing, I would just recommend for street guys to just buy good quality fuel from a location that you know turns their inventory. If you’re going to buy it from a gas station, buy it from a busy station that turns inventory.

If you know you’re not going to run your motorcycle for months, there are a couple of things you can do. Number one, drain it all together. Start the bike. Make sure you run it dry. That’s the best thing. The second-best thing is to put fuel in there that you know will last, which is a performance fuel with non-ethanol. Then the last thing I would do is use some sort of stabilizer. Then when you put the stabilizer in, run it a little bit to make sure you run the stabilizer through the whole system. Don’t just put it in there and not start it up and run it through.

For the performance people, typically you don’t have some of the storage problems as you do with the guys that are running pump gas since you burn through it quickly. But if you do have some of the more exotic stuff leftover, you want to make sure to put it in a sealed container – not the plastic five-gallon jugs everybody uses because fuel permeates through plastic – away from UV light and in a temperature-controlled area. UV light breaks down the chemistry within the fuel and degrades the fuel extremely quickly – five to ten minutes is all it takes!

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