All About 2 stroke Oil Ratios. Interesting find

[Yamaha Tri-Moto]

I was Browsing the net and came across this, figured ive seen so much talk about oil ratios this may help some people out. I sure found it interesting.

*I did not write this, Spanky over at mx.com did*

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.

*less oil is better* People think that if they have a plug fouling problem or a lot of spooge, they need to run less oil. Wrong! Both problems are caused by rich jetting, and have nothing to do with the mix ratio.

*the oil in the fuel is what lubricates the engine* 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 as a mist by the spinning crankshaft, and the droplets are distributed by the air currents moving through the engine. Ever wonder why there are two small holes in the transfer port area of the crankcase, right over the main bearings? These are to allow some of the oil droplets being flung around inside the engine to drip down into the main bearing area.

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, unless you're just putt-putting around on trails without putting the engine under much load.

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.


__________________________________________________ _______________________________________________

Pre-mix ratios and power 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 (a modifed '86 YZ 250, mine) 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, and the burn-characteristics of the fuel with that much oil tended to be poor. 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.


As a side note, I no longer run 26:1, I now run 32:1. I'm not a young man any more, and I just can't push as hard as I used to, so I don't need as much oil now. 32:1 is enough oil to do what is needed for me now, since I'm getting slow...

[Troll 2]

Thats a good read http://www.3wheelerworldforums.com/s...ht=Pre-mix+101
http://www.motocross.com/forum/topic.asp?TOPIC_ID=21626









Then theres another way to look at it;



Another way of thinking found here; http://www.macdizzy.com/oil2gas.htm



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.


Anyway it's interesting.

[69HemiGTX]

I've used this jetting guide for many years now, and it always works. (PDF file)

[Yamaha Tri-Moto]

Thats a good read also.

[OldSchoolin86]

I've read that oil thing many of times and it just doesn't jive with my real world experiences. More oil has never equaled more power ever for me.

[The Goat]

This was the oil article... I don't remember reading it here but yeah

[Autophysn]

I would think things like altitude, temperature, and relative humidity have something to do with how the bike is set up. Point being is that information given by someone from a mountainous area may work for them, but not so well for a person who lives at sea level. Thicker air, usually means more power on a four stroke, however how does it affect a 2 stroke?

[mustangmachanic]

Yes it does. Air density maters wether its 2-4 stroke or propane, natural gas,
methanol, alcohol,or even diesel

[tw05tr0k35]

There is an article on oil ratios by Gordon Jennings here; http://edj.net/2stroke/jennings/ called oilpremix.pdf along with some of the other articles he wrote for cycle world and his book. Its old (1978 i believe) and the oil ball game has changed considerably since then. There are also technical papers about certain aspects of tuning 2 strokes, but i figured the oil article was interesting and relavent.

[TeamGeek6]

OMG thats absolutely right. Shocking to see advice on the Net that doesnt contradict the laws of physics.

"More oil has never equaled more power ever for me."
It cant make a great deal of power by combustion, but lack of it causes ring leakage which costs power.

WHy not just do like the ENGINEERS that designed it say- use THEIR ratio. They designed it in the first place.

With a nice hot ignition system, ive gone way WAY heavy on oil and it ran great (10-20:1 or so.)

[xomatosis]

This is a very good read, Honda does recomment 20:1 ration, says it on my tank. But that was in 1984 so maybe running 32:1 would better suit me cause I have lots of sploge coming out where my exhaust pipe meets the engine. It looks horrible...

[RideRed250R]

forgive me if I skipped someones reply but this is how it works, I have played with many engines and have had alittle ISH go down and this is how oil works.

the dude from mx.com is smokin a heavy stash, he is right in someways and absolutely wrong in others, if the oil in gas did not have lube characteristics till gas was evaporated then top ends would lock up all the time. He also doesnt discuss how you should be jetted to your pre-mix, if your going to run 50:1 jet to 50:1 go a main size up from what your running.

He is also a dumb if he thinks that less oil in fuel doesnt equal more horsepower, the reality is whenever fuel is mixed with oil, oil drops the octane rating so do the easy math and see that a 20:1 mixture will not have the same combustion efficency as 50:1. I personally believe 50:1 should only be used in alky configurations your dumping so much fuel into the cylinder that 50:1 is perfect and wont load up.

he touched on this somewhat but not entirely, you should mix your gas for your environment, during winter rides (70-60 degrees) I mix to 32:1, from 80-100 degrees I mix 20:1, you must keep a eye on what load is going to be placed on the engine and go from there. Such as, if I know iam going on a guys fast past ride I sometimes will mix to 20:1 just because I know I need the most lube I can get.

almost ever modern 2 stroke ATV was recommended at 20:1, its particularly because its a very safe mix, you can do pretty much anything and not burn up the engine, i believe 32:1 is just as safe, but you gotta realize that 2 strokes will never EVER have enough oil in them, its just nature of the beast. a crank by design wants to be bathed in oil and the same for a topend

[NINJA]

20:1 does make more power than 50:1 if jetted accordingly, I've seen it many times on the dyno. The oil also aids in cooling.

[David komatina]

So when u jet a machine for instance my 84 250r should you only jet the main? I have had problems with it for a long time i run a 32:1 mixture and at low rpms it dosnt run smooth it sputers and i hate it half too full throtle its a little better but seams to loose power i think it might be over heating a bit but i still dont think its jetted right i have had like four diferent shops jet it and it still not right can some one help me out i know the machine like the back of my hand but tuning it kicks my arse and i have to mutch money in it to blow it up. When i keep a even throtle it sputers except when im flyin and keepin the rs up but sometimes when im flyin down a strait strech it like burps then i halfto pump the throttle then it will go again its pissin me off someone help im runin out of paciance

[Pintowagon77]

I ride at sea level at the Oregon dunes, high rpm, and run Blendzall green label at 32:1. Thats one whole 16 oz to 5 gal. I have boyeseen reeds and an k=n filter, no air box. I went from a 130 main to a 128. I ride once a week: drag race, big! hill climbs, and long haul races. I have never fouled a plug. I bought the bike in 94 with original bore, 140psi with .0032 clearance. And only had to rebore it last season, the mechanic said the "sleave looked great, just to big... Stock piston could be reused with new rings in resleaved motor", i wont.
Prior to me owning it the original owned used "Ace's Saw Oil", not from aces hardware. Ace's oil was the best oild to run in saws, 70 yr old loggers swear by it, litterally swear! The oil was said to be to toxic and the company went under in 98 or so, based in Coquille Oregon. The oil came in little brown bottles with white writting. My friends grandmother was their accountant and had 6 cases of 30 bottles. This stuff is so good and trusted i was selling 12oz bottles for $25. It was a castor/ lead base, the lead was deamed to much of a hazzard in the woods. That is why, after running out of my 6 cases, I stayed with castor based mixes and Blendzall is that best.