Biggest factor of fuel/oil that affects jetting

jaguar · Mar 28, 2019

At a ratio of 25/1 there is 96% gasoline and at 37/1 there is 97.3% gasoline which is only a 1.3% difference in how much fuel the engine is getting which is a very tiny difference.
What does make a big change is the different oil viscosities so that if you change your engine oil to one with a higher or lower viscosity then that will change the overall viscosity of the fuel/oil mixture. Let's compare Maxima Scooter Expert with a 40C viscosity of 56 to Super M with a 40C viscosity of 96. Gasoline has a 38C viscosity of .55 so with a 37/1 fuel/oil ratio the mix with Scooter Expert has a final viscosity of 2.05 and the mix with Super M has 3.13 which is a 34% difference which is huge. So even if there is a factor I'm failing to consider here then you still have to admit that it is the elephant in the room that everyone has missed all these years.
I don't know if you've noticed but on many different sites the engine tuners have recommended selecting a fuel/oil mix and to stay with it, implying that if you change any of the three elements then you'll change the jetting: fuel, oil, fuel/oil ratio.
I picked a moderate change in oil viscosities for this example but oils range from 36 to 120 or more. So my advice to you is don't worry about the fuel/oil ratio as much as the viscosity of the oil you are using and that when you change to another oil spend the time to check the jetting before riding it hard for a prolonged period of time.
Many engine oils with all their data, including 40C viscosity, is on my oil page at www.dragonfly75.com/moto/oil.html

jaguar · Mar 29, 2019

No matter how you slice it the final viscosity is much more which would require larger jets. That's easy to do with the main jet but not easy with the needle jet. If you know the viscosity of the recommended oil then try to stay close to that with any other oil you want to try. If getting the needle jetting right is near impossible then see what your oils 40C viscosity is and if its low then find an oil with higher viscosity, and if its high then find an oil with lower viscosity. That may solve your jetting problem.

Consider this: Amsoil Dominator has a 40C viscosity of 36, and Motul 800 Off Road has a 40C viscosity of 120. Huge difference.

Today I'm going to do a flow rate test using different fuel/oil mixes to see how close reality is to the calculations.

jaguar · Mar 29, 2019

I don't know if I did something wrong or what but my real life test showed the same flow rate between an oil with 52 viscosity at 40C mixed at 28:1 and an oil with 120 viscosity at 40C mixed at 40:1. (final mixed viscosities were 2.45 and 3.58)
The calculator at Poiseuille's Law Calculator shows that the flow rate should be proportional to the inverse of the two mixed viscosities. So I'm stumped on this. Theoretically the flow rate should change if the viscosity changes.

jaguar · Mar 29, 2019

It just occurred to me that maybe the high viscosity oil in this test has a "fudged" stat on what its viscosity actually is. So I need to make the exit hole bigger and do a comparison between the two oils. Its true viscosity needs to be 75 (and not the claimed 120) for the mixed viscosity of the two mixes to be equal.

jaguar · Mar 30, 2019

Aha, now the plot thickens. Months ago when I spent dozens of hours studying 2 stroke engine oils I had the feeling that it is a very deceptive industry in general although some of the manufacturers are honest. You can see on my engine oils page about different oils I won’t include in my fuel/oil ratio estimator spreadsheet due to some obvious deceptions of theirs.
I just performed a comparison test by timing how long it takes for 220ml of oil to travel thru a 25” long tube of 7/32” inner diameter. Maxima Premium 2 took 68 seconds and Motul 800 Off Road took 88 seconds. The temperature was 90 degrees F (32C) and so using an online calculator the oils viscosity at this temp should be 73.7 for Premium 2 and 179 for 800 Off Road. That is a ratio of 2.43 of Motul to Maxima. The ratio of oil travel times is 1.29 so if I multiply Maximas 73.7 by 1.29 to get Motuls true viscosity I get 95, not 179. In my studies I concluded that Maxima was the most trustworthy of the companies so I think Motul has fudged their numbers. They are claiming a higher viscosity than what they truly have. This explains why my previous test of flow rates for the same two oils was off.
So now I know that different oils viscosity will affect jetting needs but other than trusting Maxima we have no means of using published data to use in formulas to indicate needed jetting changes due to a change in engine oils (which I need for two of my spreadsheet jetting calculators). So I need to purchase each of the most popular oils and do comparison flow tests to see which ones are fudging their listed viscosities and which aren’t.

Rich Rohrich · Mar 30, 2019

Are you doing your testing with oils alone or are you testing them mixed with gasoline?

jaguar · Mar 30, 2019

I tested the oils alone.

Rich Rohrich · Mar 30, 2019

I think what you're doing is interesting. That said, I also think you'll have an incredibly tough time proving any sort of cause and effect correlation between a standalone viscosity test of oil and a repeatable impact on the fuel curve. I know you understand all of the stuff below but for the folks who may not have dug as deep into the subject of jetting it might be useful to remember a few things about jets, jetting and the ultimate goal air/fuel ratio in general.

Mikuni hex head jets are numbered by bulk flow, while Mikuni round head jets and Keihn jets are numbered by inner diameter in mm.

The flow difference between a Mikuni 175 hex jet and a 180 is about 6cc ~3%) . Keihn jets are numbered by jet id, and the difference between a 178 and a 180 is about .0008" (I'll let those so inclined do the math to determine the flow difference) .

Now to further confuse the issue and to get more to the point of how oil might impact "jet flow" keep in mind that air/fuel ratio is based on weight of fuel and weight of air, but jets essentially meter by volume. So we really need to know the weight of the fuel flowing through a jet to understand all this. In simplest terms fuel weight is a function of the area of the jet multiplied by the value of the square root of the fuel head pressure multiplied by the density of the fuel.

It looks like this:
weight of fuel = jet area * ( SQR Root (head pressure * fuel density)

None of the above takes into consideration the changes in fuel vaporization characteristics as you change altitude, which can have a profound impact on the final air/fuel ratio available in the combustion chamber ( the only place a/f ratio is really significant anyway) when the sparkplug fires.

As fun as it might be to look at jetting this way, the sad fact is the cross sectional area of two jets marked with the same number can vary FAR MORE than the difference in flow due to small changes in density . Good jets can vary as much as 5% cheap jets can be closer to 10% variance.

So I have no doubt you can find some specific differences in "bulk flow" through a jet based on the true viscosity of an oil. How that will ultimately relate to a change in the "usable" (and the word USABLE is critical in this case) fuel mass that the engine sees in the combustion chamber is likely to be open to debate.

Please don't read this as criticism, I'm interested in hearing more about what you ultimately find.

jaguar · Mar 30, 2019

I am going to import Super M (since name brand oils aren't available here in Paraguay) and do another comparison test to see if the change in mixed viscosity causes a change in fuel flow. Since both oils are by Maxima then I trust their listed viscosities.

Rich Rohrich · Mar 30, 2019

Super M is an excellent oil so it seems like a good reference point to work from.

jaguar · Mar 31, 2019

LAST TEST
There was enough of Maxima's Castor 927 left in my last bottle to mix with gas to do a comparison flow test with Maxima's Premium 2. Castor 927 has a 80 degree F viscosity 3.7 times that of Premium 2 but still the flow tests through a 1.2mm orifice were equal. The 927's mixed viscosity was 9.8 and Premium 2's mixed viscosity was 3.9 so this pretty much kills the idea that oil viscosity affects jetting. To make double sure I elongated the length of the exit hole from 2mm to 3.5mm and still they were equal. So obviously there is some exception to the rule of viscosity that I'm not aware of. It probably has to do with the low viscosity values or the properties of gasoline.

jaguar · Mar 31, 2019

I think I understand my mistake now. Since basically viscosity is resistance to flow then with low viscosity there is very little resistance and so even a viscosity of 10 means there is too little resistance to really matter. The oils viscosity was in the hundreds- that matters. So comparing fuel viscosities of 4 and 10 is like comparing next-to-nothing to next-to-next-to-nothing.

jaguar · Apr 16, 2019

FINAL SUMMARY: oil viscosity does not affect the flow rate through the main jet. The formulas and online calculators say it should but it doesn't.

I retested the viscosity between Motul 800 Off Road, Maxima Super M, and Maxima Premium 2 with a much smaller exit hose which gave better results. By this new method the Motul 800 has a listed viscosity about 5% higher than it should be. But my previous test of premix with Motul 800 vs Premium 2 still stands true in that both had about the same flow rate.

Biggest factor of fuel/oil that affects jetting

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Biggest factor of fuel/oil that affects jetting

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