Author Topic: Premix 101: Oils, Ratios, and Fuel  (Read 93 times)

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Offline blsnelling

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Premix 101: Oils, Ratios, and Fuel
« on: November 25, 2015, 10:03:47 pm »
I posted this over on AS and Kevin thought it would be a good resource to have here.  There are so many myths that float around about oil, fuel, and premix. I thought it important to share this here for all of our collective benefit. This is by far the best article I have ever read on the topic.

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Offline blsnelling

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Re: Premix 101: Oils, Ratios, and Fuel
« Reply #1 on: November 25, 2015, 10:03:56 pm »
Pre-mix 101

 OK, looks like it's time for a little pre-mix 101. I don't usually get into ratio discussions, because mix ratios are like religions to most people, and they tend to be closed-minded on the subject, but I'll put in my $.02 here anyway.

 There is a prevailing myth that less oil is better, and that the oil in the fuel is what lubricates the engine. Both are wrong. The engine is lubricated by the residual oil that builds up in the crankcase. All the oil in the fuel does is replenish this oil.

 The best way to determine if you are running enough oil is to check the level of the residual oil in the crankcase. If the ratio you run leaves enough residual oil in the crankcase to cover about 1/8" of the bottom of the crank wheels, then you are fine. If you don't have that much residual oil in your crankcase when you pull the top-end off, you aren't running enough oil for your riding style and conditions. With that said, to have that amount of residual oil in the crankcase at 50:1 (a ratio made popular by magazines and oil bottles), you can't be riding very hard, or your bike is jetted richer than necessary simply to deliver enough oil. I arrived at 26:1 for my bike with my riding style because that is the amount that gives me the proper amount of residual build-up. Small-bore engines require greater oil concentrations than larger engines to achieve the proper amount of residual build-up, because they rev higher and have higher intake velocities. Along the same lines, someone that pushes the engine harder, and keeps the revs higher, also needs to use higher oil concentrations to achieve the proper residual build-up.

 To understand why the residual oil is so important, you have to understand what happens to the oil in your fuel when it goes into the engine. While the oil is still suspended in the liquid gasoline, it can not lubricate anything. It has about as much lubricity at that point as straight gasoline. When the gasoline enters the engine, it evaporates, dropping the oil out of suspension. Now that the oil is free, it can lubricate the engine, but it must get to the parts to lubricate them. The way it gets to the bearings and onto the cylinder is by being thrown around by the spinning crankshaft. Some of the oil eventually makes it into the combustion chamber, where it is either burned, or passes out the exhaust. If the combustion chamber temps are too low, such as in an engine that is jetted too rich, the oil doesn't burn completely. Instead, some of it hardens into deposits in the combustion chamber, on the piston, and on the power valve assembly. The rest becomes the dreaded "spooge". The key to all of this working in harmony is to jet the bike lean enough to achieve a high enough combustion chamber temperature to burn the oil, but also still be able to supply enough oil to protect the engine. If you use enough oil, you can jet the bike at it's optimum without starving the engine of oil, and have excellent power, with minimal deposits and spooge. At 50:1, you simply can't jet very lean without risking a seized engine due to oil starvation.

 With the high oil concentrations that I use, I tend to get far more life from my cranks and rings than most of my friends that run leaner oil ratios. The high oil content also produces better ring sealing, so more of the combustion pressure is retained.

 One small point. No one ever broke an engine by using too much oil.

Offline blsnelling

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Re: Premix 101: Oils, Ratios, and Fuel
« Reply #2 on: November 25, 2015, 10:04:10 pm »
Pre-mix Ratios and Horsepower Production

 I have run Dyno tests on this subject. We used a Dynojet dynamometer, and used a fresh, broken in top-end for each test. We used specially calibrated jets to ensure the fuel flow was identical with each different ratio, and warmed the engine at 3000 rpm for 3 minutes before each run. Our tests were performed in the rpm range of 2500 to 9000 rpm, with the power peak of our test bike (an '86 YZ 250) occuring at 8750 rpm. We tested at 76 degrees F, at 65% relative humidity. We started at 10:1, and went to 100:1. Our results showed that a two-stroke engine makes its best power at 18:1. Any more oil than that, and the engine ran poorly, because we didn't have any jets rich enough to compensate for that much oil in the fuel. The power loss from 18:1 to 32:1 was approximately 2 percent. The loss from 18:1 to 50:1 was nearly 9 percent. On a modern 250, that can be as much as 4 horsepower. The loss from 18:1 to 100:1 was nearly 18 percent. The reason for the difference in output is simple. More oil provides a better seal between the ring and the cylinder wall.
 Now, I realize that 18:1 is impractical unless you ride your engine all-out, keeping it pinned at all times. But running reasonable ratios no less than 32:1 will produce more power, and give your engine better protection, thus making it perform better for longer.

 or this?

 Another way of thinking found here; Oil to Gas - First, Make It Last

 If you get the opportunity to take apart an engine very often you may notice that the bottom end of a two stroke almost always has a substantial amount of oil laying in the cases. Even in engines that have been run at leaner oil to gas ratios (50:1 to 100:1) there is plenty of oil. The other internal parts of the engine also are well coated wet with oil. It doesn't matter if the motor is air cooled or liquid cooled - the internals of the engine are still coated. Much of this oil may accumulate at idle and during periods of low rpm running. Once you get the motor spinning faster and it is under load, that extra oil in the case may finally have a chance to become suspended again in the fuel and air mixture and perhaps be burned.

 Many people will rev their engine to clear that oil (that plume of smoke they get at startup or after idle) and refer to it as "cleaning out" the engine. And that is exactly what is happening. That oil - which has become separated from the fuel has gathered in the cases and is standing by waiting for some serious turbulence to get it up the transfer ports and into the cylinder where it can finally be burned. Too much oil in the gas can lead to additional problems like carbon deposits on the piston crown and cylinder head, sticky rings, fouled plugs and wet drippy black gunk (unburned oil) coming out of the joints of the exhaust system.

 It is probably best to avoid sustained periods of idle, or very low rpm running under no load. It is also probably better to choose a lower gear (for instance 3rd at 5000 rpm instead of 4th at 3800 rpm) and let the motor spin faster when driving at slower speeds since there in less likely a chance of the oil not finding its way to the combustion chamber. It is my opinion that if you use top quality oils (synthetics are the best) you are only throwing away money and making more smoke by running more oil through the engine than it needs. I have had good luck by always using the same oil to gas ratio and the same oil brand.

Offline blsnelling

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Re: Premix 101: Oils, Ratios, and Fuel
« Reply #3 on: November 25, 2015, 10:04:21 pm »

 Let’s see … your bike is running on the rich side, so you put less oil in the gas to lean it out. Wrong.

 Or maybe your bike is running a bit too lean, so you figure that if you put more oil in the gas, that should take care of the problem. Wrong again.

 You would be surprised at the large number of riders who don’t have a clue what to run in their two stroke. I know; dozens of people write my DON’T ASK column asking that question.

 Many dirt bikers are mixing their gas at ratios as high as 75 to l, or even 100 to 1 with the new generation oils, in the belief that their bike will put out the most horsepower at a higher ratio. Riders who foul plugs all the time, are putting less oil in their gas/oil mix, in the belief that the oil is fouling the plugs, and many racers are trying to solve “too rich, too lean” problems by changing the gas/oil mix in`stead of the jetting.

 There are a few good reasons to run a fuel/oil mix at ultra thin ratios in a two stroke. High ratios such as 100 to 1 are usually environmental reasons, such as for outboard boat motors. The exhaust of an outboard motor goes directly into the water, and environmentalists are worried about the oil in the mix polluting the lakes and rivers.

 There’s a myth that the less oil you use in your gas, the more horsepower you get. Conversely, many dirt riders actually forget to put any oil whatsoever. We know of one guy who forgot to mix oil into his gas and actually rode it for two hours without seizing it. All the bearings were ruined and the piston was worn out, but it didn’t seize!

 Actually, you can get more horsepower out of a two-stroke engine with enough extra oil in the gas, because the oil provides a better ring seal and, therefore, more compression. People think that gas burns more efficiently with less oil, and therefore you get more performance. It almost makes sense if you look at that one statement alone.

 The seal of the piston is critical. If you remove the lubricants from the gas, the viscosity of the mixture becomes lighter and more prone to vaporization. With a lean mixture, there is less oil to seal the rings. The sealing of the rings has more to do with the performance of the engine than the possibility of having better-burning gas with an ultra-lean gas/oil ratio.

 The old fashioned two-stroke oil that was on the market years ago, was designed to be run at 20:1 and was basically petroleum with a few (very few) ad`ditives. Then, when high-per`formance oils came along, they cost more to make and sold for a higher price. They got into these high mixing ratios in order to jus`tify the higher prices.

 If you do foul plugs, it is more than likely caused by poor jetting, not a bit too much oil. If you get your bike jetted correctly, have a fresh plug and a strong ignition system, you won’t foul plugs.

 When the motor is idling, or at lower rpms, that’s when the machine has a greater chance of fouling a plug. Minibikes and 125s have even less chance of fouling plugs, because they are ridden at such high rpm. Because of the ultra high rpm, the load on a given part is much higher on a 125, than on an Open bike.

 Plugs should not foul at richer ratios if you are using high-quality oil in the mix. High-quality oils will have a good detergent/dispersing package that holds down the contaminants which produce plug fouling.

 A typical example: you go from a 50:1 ratio to a 20:1 ratio. Your engine will now run leaner, and you’ll have to make jetting changes. You’ll need bigger (in number) jets because the oil molecules are thicker and the flow rate (the amount coming through the jet) is less.

 Aha! The volume of fuel has changed. The oil takes up some volume that the gas used to occupy, so your engine is getting less gas and needs to be richened up.

 So which ratios should two-stroke gas/oil should be mixed? A properly jetted engine will run better, last longer and develop more power at a lower oil ratio than at a higher one. But what is the proper amount, and how do you know a quality oil from a bad one?

 The ratio a rider should use in his two-stroke will depend on the size of the machine and the type of riding being done. An 80cc racer will require much more oil in the mix than a 500cc play bike. The best bet is to consult the owner’s manual and follow the advice of the engineers who designed the motorcycle.

 As for which oil to buy, that depends on the type of riding being done. Someone who races will require a higher-quality oil for its superior ingredients and properties, than someone who only play rides and doesn’t put a lot of strain on his engine. A good, high-quality oil will cost more money than a poor-quality oil, because of the higher cost of ingredients, such as synthetic diesters and ash less detergent dispersing packages. Quality ingredients cost more money, and that makes the quality oils more expensive.

 Our advice then, is to buy a quality oil and run it at a moderate ratio. We’ve used 32:1 for many years. In race bikes that are ridden hard, we might go a trifle richer at say … 28:1. For a trail bike, 40:1 would be the way to go, assuming that you used a quality oil. If you own a mild-mannered bike, consider a 50:1 ratio.

 One of the things you should do, is run high octane gas with any two-stroke mix. When all of the two strokes (the old days) were developed, they all used Castrol petroleum oil at a 20:1 ratio and found that 92 octane gas had the octane reduced to 72 with presence of that much oil. Modern oils won’t affect the fuel quite as much, but if you started with 86 or 87 octane regular fuel, you can see where you’ll end with a very low octane mix. You could end up with a “pinging” bike.

Race gas? You don’t need it in your two stroke unless you’re a pro or expert, and most expert level riders are on the new generation four strokes.

Offline blsnelling

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Re: Premix 101: Oils, Ratios, and Fuel
« Reply #4 on: November 25, 2015, 10:04:31 pm »

 * Use only two stroke engine oil in two stroke engines. Do not use car engine oil like SAE 10W-30W, or the like. Two stroke engines burn oil and are designed to do this, and require the proper oil in the gasoline.

 * Mix the gasoline and oil thoroughly. One method is to take your gas and oil can to the gas station and mix right there at the pump. Fill the gas can about 1/3 full and then add the proper amount of oil, then fill the container. The gasoline pumping quite rapidly out of the nozzle mixes the oil and gas together quite well.

 * Shake the gas can vigorously before filling your gas tank. The oil must be suspended evenly in the mix, so the engine gets lubricated evenly. If the oil is not mixed thoroughly, the engine starves for lubrication, and the spark plug gets oil stuck on it.

 * Gasoline is also important. Head for your manual for types of gasoline and octane rating your engine requires. Some older engines require leaded gasoline. Most of the newer engines run on leaded or unleaded.

 * Once gasoline is mixed, use it. Don’t buy 10 gallons of gasoline and use five gallons. Gasoline allowed to sit gets stale and gummy. This gummy stuff sticks to carburetor parts and air passages, which eventually will restrict air flow, thus changing the air-gasoline mixture.

 * All the major manufacturers produce two stroke racing engines in their off-road motorcycles. Virtually all of them recommended 20:1 or 24:1 mix ratios. What the actual factory mechanics did at racing events was very telling. Their teams (admittedly not running "stock" engines) but were running engines putting out even more power for the displacement class, followed the same rules.

 * (1) The higher the RPM's the engine turned, the more oil they ran in the fuel. (e.g. a 125cc machine that routinely lived in the 10,000 - 13,500 rpm range ran 20:1 or 24:1 -- The 250cc engines that ran between 6,500 and 9,000 rpm ran 32:1 or 40:1, and the Open Class machines (251cc and up by AMA, but they were all 400+cc engines, usually 465's, 490's, or 500cc) ran 50:1.

 * (2) Additionally. Husqvarna did some testing in the mid 70's that was very interesting. They put 3 identical stock engines on a dyno and ran them for several days at varying RPM and load conditions. Then both motors were torn down and inspected. The engine running CASTOR based oil had the least wear, followed by the synthetic oil, and finally the engine running standard 2-cycle oil.

 * (3) A second test they performed was to run synthetic in 2 identical engines and one was run at 24:1, the other was run at 50:1 The engine that ran 24:1 had less piston skirt wear, and less rod bearing wear, but had the same main roller bearing wear as the engine run at 50:1.

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Re: Premix 101: Oils, Ratios, and Fuel
« Reply #5 on: November 25, 2015, 11:58:32 pm »
Thanks Brad that is a nice write up.    I might have even learned something.
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Re: Premix 101: Oils, Ratios, and Fuel
« Reply #6 on: November 26, 2015, 10:19:43 am »
Thanks Brad that was a good read. Thanks for sharing it with us too.
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