Switch from 50:1 to 60:1?

[liv2day]

So, your gut feeling of a change of 50:1 to 60:1 making sfa difference is correct.

Great job on the math; I started to get a slight ice cream headache (lol), but I like the flow of the logic.

I'm going to get into the carb this weekend and see what's what - been putting it off and need to do it anyway. I think an interesting test would be to try 60:1 and see if there's any noticeable difference - might do that too on the next ride.

Thanks for all the responses gents

 

[noobi]

Very Well Done! What are your thoughts on the difference in viscosity?

In respect to what specifically?

Material properties of miscible fluids are typically additive.
In the same way that density was additive for the previous example.

Gasoline isn't particularly viscous around 0.4-0.7 cSt at 40C, infact its generally less viscous than water.
Where as 2 stroke oils are around 45 cSt at 40C.

So the difference in viscosity is about 90 times in favour of the oil.

Lets consider a mix of 50:1, or 2%
0.98x0.4 + 0.02x45 = 1.29 cSt, so you've tripled your viscosity by adding 2% oil.

As for your previous comment on the reduction in octane by adding oil, that is again due to the additive nature of miscible fluids, the gasoline has a octane number of say 90. The oil has a much much lower one, probably zero, but lets say 4 for the sake of the example.

So when you mix at 2%, 0.98x90 + 0.02x4 = 88.3.
So by mixing at 2% you have reduced the octane rating by ~2 points.

Thats why you will have read about reduced octane ratings in premix.

 

[Zman]

I was thinking that a more viscous fuel would require a higher amount of airflow to be drawn through the carb, resulting in a leaner condition.

 

[Fred1956]

I run Amsoil Dominator (not Interceptor) at 50:1 with no spooge but I'm also using a Leo Vince with spark arrestor. I believe Interceptor has a higher flash point than the Dominator, that coupled with the Q could be your issue.
BUT I agree if throttle response is good then carry a paper towel and wipe the sponge on occasion.

 

[mjtursky]

Correct Oil ratio

This is getting pretty deep into the science. My opinion on this is not to run higher than 50:1 oil ratios. You will give up engine life with mixtures higher than this. When testing outboards with variable injection, the engine can tolerate leaner oil mixtures (up to 100:1) but it will require richer mixtures (40~50:1) to maintain proper lubrication to critical parts of the engine. Since we are running premix on our GasGas I would recommend staying with 50:1. Once this is defined you can focus on the correct carburetor setting to optimize performance.

 

[liv2day]

I believe Interceptor has a higher flash point than the Dominator, that coupled with the Q could be your issue.

I ran Dominator initially, but switched to Interceptor after reading what I think to be the exact opposite...lol. I haven't double checked yet, but I think the Dominator has a higher flash point as it's more meant for racing. The Interceptor is targeted to the trail rider who isn't reving the engine as much (again - if memory serves me...which is always a big if now...lol).

When I have time this weekend, I'm going to pull the carb and check the jetting....need a base line

 

[andoman]

Wait: I'm confused...

As for your previous comment on the reduction in octane by adding oil, that is again due to the additive nature of miscible fluids, the gasoline has a octane number of say 90. The oil has a much much lower one, probably zero, but lets say 4 for the sake of the example.

Isn't that backwards? Since octane rating is a measurement of resistance to combustion (specifically used to analyze flame front propagation) wouldn't the oil bring a HIGHER overall octane rating to the mix? Especially one like Motul 800 with a flash point 3 times higher than the petrol it is mixed with?
You assumed an octane rating of 4 (perhaps as low as zero) for the oil: wouldn't that low of a rating indicate a dangerous propensity to spontaneously combust?

I apologize to all for hijacking the thread earlier. Cudos to fred, mjtursky, and livtoday for helping to get us back on topic.
Having said that, I still appreciate noobi's willingness to explain things. Though I have a hard time following the science, I find it absolutely fascinating. Thanks.

 

[wence]

My 2012 has no spooge problems and I am using GRO at 50:1.
I have the bike jetted slightly rich (safety), and have great throttle response everywhere.
Cheers Mark

 

[gasser]

I run Amsoil Dominator (not Interceptor) . I believe Interceptor has a higher flash point than the Dominator,

Actually Domiminator has the higher flashpoint but the difference is very small and both are on the lower end of the flashpoint spectrum of the top synthetic oils. I have never ran better oils, yes there are some that are just as good but none (here in the U.S.) can I get as cheap as Amsoil.

 

[noobi]

Isn't that backwards? Since octane rating is a measurement of resistance to combustion (specifically used to analyze flame front propagation) wouldn't the oil bring a HIGHER overall octane rating to the mix? Especially one like Motul 800 with a flash point 3 times higher than the petrol it is mixed with?
You assumed an octane rating of 4 (perhaps as low as zero) for the oil: wouldn't that low of a rating indicate a dangerous propensity to spontaneously combust?

I apologize to all for hijacking the thread earlier. Cudos to fred, mjtursky, and livtoday for helping to get us back on topic.
Having said that, I still appreciate noobi's willingness to explain things. Though I have a hard time following the science, I find it absolutely fascinating. Thanks.

No problem, I haven't needed to explore this stuff mathematically before, so its interesting for me too.

You've kind of jumped the gun a bit by saying octane is a resistance to combustion, although that's an effect of higher octane numbers. Its a reference to the activation energy of the combustion reaction that must be overcome by work done on the mixture. Work is done on the mixture by compression, it loosely obeys PV=nRT, and as volume decreases pressure increases, but temperature also increases. A lower octane number mixture requires less work to be done, so the activation energy is overcome at a lower pressure than a higher octane mixture. Hence the resistance to combustion.
Now when the spark goes off, there is a localised hot spot where the spark has arced, and that is what overcomes the activation energy and causes combustion.

Now oil doesn't combust under compression alone, so an octane number doesn't apply. So when you dilute the gasoline with oil, the overall mixture volume has a lower octane number because the oil doesn't contribute to it.

Also, flash point does not mean ignition, specifically it means the point at which it becomes a combustible vapour. So an oil still requires an ignition source once in a vapour state. Gasoline doesn't necessarily.


As for viscosity increases, I would say it is possible that you could have a reduced flow through the jets if the viscosity was increased by a large enough amount. I feel that that would be incredibly difficult to model.

Hope that helped a bit.

 

[andoman]

Thanks! That was very well put and helps my understanding greatly.

 

[mjtursky]

A little more

In my prior post i forgot to detail that you can run 100:1 at idle or under light loads on a 2 stroke but heavy loading requires 40~50:1. I also have the luxury to buy gas at the airport which has high octane so I don't need to worry about oil influncing the octane. But I release now a lot of members maybe limited on what kind of fuel is available.