2 stroke oil lean or rich?

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ccdhowell

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There is a thread going on another board I frequent dealing with 2 stroke oils. There is a differing of opinion so I thought I'd pose the question here.

Say I run a 2 stroke atv at 50:1 premix and jet the carb acordingly, if I switch to a 32:1 premix with another type of oil, will I be running leaner in the engine since I've reduced the actual amount of fuel going in in relation to the oil? In the gas can, more oil = richer, but the question is, when it goes in the engine does this richer oil ratio actually lean out the engine? And, does modern synthetic 2 stroke oil contribute, or diminish from the btu of the exploding fuel/air mixture inside the cylinder?

Here's a link to the thread: thread in question

I figured the guys on this board would know. If we get some good, factual info, I'm link the other thread into this one so my pals on the other board can take a look. Chris
 
Yes, in theory a richer "oil/fuel" mix will result in a leaner "fuel/air" mixture. BUT, it's not as big a deal as some people make it out to be. For example your 50:1 premix is 2% oil, 98% fuel. Switching to a 32:1 premix your 3.125% oil and 96.875% fuel. You have reduced the fueling by 1.25% with the new mix. Lets say you carb uses a 250 mikuni hex main jet, in theory you would want to increase its' size 1.125% to "compensate", a 253.125 size jet would be needed. This isn't even a 1/2 jet size bigger and would be a waste of time. Even on engines that are jetted right on the edge of lean it takes at least a 3% change in fueling to really see any difference in performance or egt temps from my experience. Also this whole exercise in math makes a assumption that the oil is not mixed with the fuel and does not burn, which goes back to your btu comment. So in short 50 to 32 no biggie, switch from 100:1 to 20:1 you might want to watch the mixture. Oil in the fuel has more effect on the octane and weight of the fuel (ability for vacume at the nozzle to pull it), then any reduction in btu value. At least that the way I understand it.
 
Thing is...I suggest you guys who do your jetting using a math exercize actually try it out. You may find that there are other factors that weigh in on the equasion that makes your math seem lacking. Or at least you may see that the need may not be shown on paper, but the real world shows the change to be a very real thing.

The operative phrase here was "and jet the carb accordingly".
If you are running a 50:1 ratio, and have the carb metered to an optimum burn of fuel and air, the change to a mixture of more oil requires a change of jetting...or else the optimum charge at combustion is no longer. More air is added to the properly jetted mixture and it is now leaned past optimum. Pencil lead or no pencil lead.

Granted those who are not jetted to optimum will see very little difference, but for those who are... get out your brass jets, and put away your lead pencils.

If you do insist that your math is sufficient, then be sure to factor in the evaporative properties of the particular fuel. Or does pencil jetting work for all fuels?

I'm sorry...the simple percentage reduction or addition of oil is only one factor to consider, and is not enough to make the pencil a viable jetting tool.
 
changing jetting for the oil ratio is almost not practical unless you were in some world championship racing or something.
just run the thing. thats what its made for.
 
If you think it's not important,try it ina 2stroke running at elevations of 8-10K feet. BIG DIFFERENCE.
 
Quote:

changing jetting for the oil ratio is almost not practical unless you were in some world championship racing or something.
just run the thing. thats what its made for.

Click to expand...

When you hear some sort of bonehead remark like:
"Just put ony oil you want into your engine, and run it...that is what it's for" would you not immediately have a good feeling for the person who made such a statment?
Would you not feel right away that the person was either being sarcastic, or had very few clues as to what he was talking about?

When I see such a statement, it tells me alot. Such as perhaps the person should qualify his statement as nothing but an opinion, with few facts that he could possibly back up his assertions with.
dunno.gif
 
"Thing is...I suggest you guys who do your jetting using a math exercize actually try it out"

The math example was not meant to be a formula for tuning or substitute for actual verification of heat in the parts (plugs, piston crowns, etc). It was just a way of showing the fueling is not changed as much as hyped on relatively small changes in oil ratio. There are many more factors involved then posted when actual calculations of fueling are made, and jet numbering is not always in proportion to the result. I mention mikuni hex jet because their numbering works the best for simple calulations like this. Round jets and keihen require additional consideration.
 
I always enjoy playing with the jet settings on a 2-stroke engine to ensure proper operation. I really can’t tell you about your 1:32, most modern 2-strokes are designed to run a 1:50 or 1:40 mixture and most 2-stroke oils compliment this in their literature. Unless you are running in the races where you will be revving an engine past the 14000rpm mark my experience tells me that a 1:32 oil ratio will gunk up the engine in the long run causing sluggish performance. 1:32 in most of my applications has resulted in oil being spewed the other end, so I usually cut down to a 1:40 for initial break and 1:50 for the rest of its life. More important is to make sure that you check carb, too lean will cause an overheat and seizure, too rich will cause sluggish performance. I do this by reading the spark plug and making adjustments accordingly, however it is not a skill that I have learned over night.
 
It is not the desing of the modern 2T engine that changed, but rather the recommendation changed in accordance with the available oils out there. Now and then, the ratio recommended is more of a CYA exercize than anything.
Unless the OEM recommends a specific brand and type of oil to use, then suggesting a ratio is just that, a suggestion.

There is alot of difference in a castor bean based 2T oil and a synthetic ester based oil. Both can be run at the same ratio with drastically different results, in both performance and cleanliness.

Making a blanket statement that a 32:1 ratio will eventually cause sluggish run is simply not based in fact.

I have a small bore 2T that the OEM recommeded a 40:1 ratio.
OK fine, the very same mfg used to make 20:1 recs. as well as 60:1 recs. It is nothing more than a guideline. and nothing to do with intense testing at certain ratios. There is simply no way they can do that with the vast array of oils available.
BTW...
That small bore 2T that I have is run with a ~20:1 ratio all the time and the power valves are clean at rebuild, as well as very little carbon deposit on the piston, and the engine is far from sluggish with virtually no spooge or smoking. That is all a function of the way the bike is JETTED at all circuits, and how the bike is ridden.

I do agree that from the evidence I've seen that the larger bore engines can utilize less oil with similar results, but no matter what bike or what oil, the jetting needs to be sorted out or you are just blubbering around in 2T la la land.

Problem is, many run around in 2T la la land all the time, and when it comes time for suggestions, they refer back to their engine that isn't even close to metering the fuel properly. And they make their recomendations from what they are experiencing, rather than from what science has to say about the issue.

Motor oil is also disucssed by the multitudes who have few clues. It is a very similar exercize to this one, and many times it ends up being an endless circle of folks passing along bad misinformation and home-brewed theories that are far from fact.
 
I agree wit the last two posts and came looking for correct information. I, personally, currently run at 50:1 with Mobil 1 MX2T. When my little stash of that runs out, I'd like to try some of Amsoils Sabre at 100:1 to see how that would work. Some guys on the board I referenced at the start of this thread are running the Sabre at 100:1 in their atvs with positive results.

I agree that a change from 50:1 to 32:1 is a very small actual change in the fuel going into the engine. Now let's talk oil. Guys run all kinds of stuff, synthetic, castor oil and various blends. What are the different burn characteristics inside the combustion chamber for the various oils? I would think that the castor oil would more readily ignite, don't really know I've never run it, but mention it for the sake of discussion. I would also guess that the higher flash point of the synthetic oils would mean they contribute little or not at all to combustion energy, with the blends somewhere in between.

Being an oil guy myself, I've never really stuck with one brand of 2T premix oil. I've wondered all over the synthetic and blend brand spectrum. Haven't had a problem to date, but I don't push mine too hard too often either. But I am curious about the fundamental approach that oil ratio and oil type have on jetting and engine power. Does one type of oil(castor, synthetic, or blend) have a power advantage over any other type of premix oil?
 
Migration time through the engine is one factor that comes into play when considering ratio. The amount of time the premix and the amount of oil that gets "flung" out of it as it travels through the crankacase is directly related to the rpm of the motor. Generally speaking high rpm engines require more oil to achieve the SAME amount of lubrication a slower rpm engine could have with less oil in the mix. Engine design also comes into play. Example, some engines may have a bearing on the far end of the crank that may be drip fed or durability testing have show will get starved with lower oil ratios. The "fix" even though the top end cylinder and ring are getting plenty of lube, is to increase the oil you put through the motor. Another example the other way is small engine manufactures moving to less oil for enviromental reasons. They may get away with reducing the oil ratio because they are using coated pistons and nicasil bores and loosen their clearances up a little to prevent seizure. Power advantage? most tests show more oil in the mix = more power, to a point, but depends on engine. Example. Some has to do with ring design, number and tension. Tests that clearly demostrate one ratio making more power may not be repeatable on a different design engine. case in point, dykes style rings vs semitrap, nicasil vs iron bore, etc.
 
Thanks wileyE I found that quite informative. There is a much quoted test of 2 stroke atv engines that concludes the oil mix ratio for highest horsepower is 16:1 because of decreased friction with the higher oil ratio. I didn't really believe that myself. I have mixed at 20:1 and found my engine to be sluggish and lacking top-end power.
 
Castor bean premix should be used only by those who keep the throttle pinned and the engine in the upper ranges of RPM. More tentitive riders will find that castor oil products will create a lot of carbon and power valve buildup.
That said...a castor oil does a great job of lubrication, but as mentioned, the engine needs to be run hard and fast the majority of the time.

Experimenting with ratio is a #@$%! shoot. You will never get the engine running optimum unless you chose an oil, and a ratio, and stick with them. Once you have chosen, then study up on how to jet the carb properly.

Those who actually do study up on jetting, and have a full understanding of how it works, and how to accomplish optimum combustion, will find that they have a much better running engine than their pals.
BUT...realize also that jetting correctly means jetting to the style of riding you do. From what I am seeing, you should probably stay away from castor bean products, unless you only ride for the smell. (don't laugh, many do)

Gimmick 100:1 oils are a mistake, but that is for people to either find out for themsleves or not. Fried crank bearings is an expensive lesson though.

wileyE's last post couldn't be more spot-on..and should be read again...and understood.
But, those who have studied how to jet properly (and WHY it works) will know this.

The 16:1 thing you heard, is spot-on as well. Shifter cart guys run 16/18:1 on a normal basis just for that reason.
More protection, and more power. The secret is to get the right amount of air/fuel into the combustion chamber when it fires.
A two-cycle rider can benefit from the proper jetting of the bike engine no matter what...and those who are into "world championship racing"(?) may employ even futher methods of finding the optimum air/fuel mixtures at all circuits by mounting an oxygen sensor on the exhuast that gives them a more precise readout/picture of the combustion process.
 
I'm one of the guys from 3WW that ccdhowell linked to in the first post. I'm not a mechanical engineer, or a chemical scientist. But I am a rider, a racer, and a self tuner. I've studied dozens of SAE tech papers and books on 2 stroke engines. I will probably never call myself an expert on the subject but would like to think I'm a little bit better educated on the subject then the average joe riding back and fourth to the mailbox.

With that being said, here are my thoughts:

On a high performance engine (Pretty much anything in the motorcycle/jetski/snowmobile/outboard/kart/ field built in the last 30 years) with a properly dialed in carb more oil will make more hp, and provide better lubrication. The first part is the increase in power. This is in my opinion caused by the increase in BTU from the additional oil, the increase in better ring seal, more complete reed valve or rotary valve seal, and reduced friction in the form of piston side load and more importantly main/wristpin/rod bearings.

The second part is the lubrication. There is a lot of debate on this area in particular, mainly synthetics vs dino oils. In conventional oil tests, where the application would be for a four cycle engine, the synthetic most definitely lubricates better. The 2 stroke engine differs a little bit though. Most synthetics are ashless. This is mainly for environmental reasons IMO. So when a synthetic oil burns in the 2 stroke engine, its pretty much just gone. The castors on the other hand, after they are burnt leave a light very fine ash that acts as another stage of lubricant. Think of something similar to graphite. I cannot recall the exact study where I read it, but castors for the most part burn in stages. When a synthetic burns, its pretty much gone. So think of it like this. The synthetic has a superior lubrication property until its burnt, and then the castor takes over. Some guys on another forum I visit, PlanetSand, have done dyno runs with Synthetics vs Castors and have found that the synthetics make 1-2hp more on the lowend rpm range, and then tapers off, where the castor is the opposite. This is all at the same mixture ratio/gas and proper jetting as indicated by an EGT.

I believe that you can adjust and jet for nearly any amount of oil in the gasoline to prevent carbon build up and "goop". Some oils, yes, will be easier to jet for, or less inclined to do this then others. And contrary to everything I've read, and everything I've ever been told the heat range of the spark plug I'm running has made a big difference in how well the engine burns whatever is in it.

Now the other part is the myth that more oil in the gas makes a leaner fuel/air mixture in the carb. Believe this only if you also believe that the oil is not burnt, or that the oil cannot mix with the air.
 
Quote:

Now the other part is the myth that more oil in the gas makes a leaner fuel/air mixture in the carb. Believe this only if you also believe that the oil is not burnt, or that the oil cannot mix with the air.

Click to expand...

Everything you have stated makes good sense, up until this.

OK, when the 2T pulls at the fuel and the air, it travels through orifaces of fixed sizes. The amount or "charge" of fuel and air that comes through the orifaces is decided by the physical size of those orifaces and other metering devices at the carb, as well as somewhat by the viscosity of the fluid. So, with X size of orifaces, there is X amount of fuel and air in the charge. (Some would argue that a 12.5:1 a/f ratio would be sought after.)

When we change how much space in the charge is taken up by fuel, say like adding 2T oil, there is less fuel going into the charge than there was with no oil. So, without changing the size of the metereing devices, we have essentially leaned the combustion charge simply because there are less fuel molecules entering the charge, and have been replaced by oil molecules.

Now, an engine that has the proper orifaces and devices, to meter the fuel charge so that there is an optimum fuel/air ratio at the time of combustion, will also see very little of the oil residing in that final combustion.
In the optimum running engine, the oil will have fallen out of suspension from the fuel once it enters the engine case and becomes part of the internal charge. The fuel atomizes into a vapor and enters the combustion chamber with it's companion air...while leaving the lubricating oil fallen out of suspension and recondensed on the metal parts to migrate and protect area's outside of the combustion chamber.
 
Quote:


In the optimum running engine, the oil will have fallen out of suspension from the fuel once it enters the engine case and becomes part of the internal charge. The fuel atomizes into a vapor and enters the combustion chamber with it's companion air...while leaving the lubricating oil fallen out of suspension and recondensed on the metal parts to migrate and protect area's outside of the combustion chamber.

Click to expand...


Thanks for the welcome
patriot.gif


I have not heard of this before...Are you saying ALL the oil separates from the charge, or only a small portion? I'm not sure if I agree with the idea, although most every bottom end I've ever split had some gas/oil residue in the crankcase. I believe that the complete mixture passes throughout the engine and is subsequently burnt, then released, then partially forced back in, reburnt, repeat, etc.

I had said on 3WW I thought, depending on the oil, that you might have to richen up the jetting when going from a very lean oil/fuel mix to a very rich one (like 8:1) but that was in regards to the extra heat in the cyl from the increased BTU of the added oil.
 
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