Adding oil to gas reduce octane?

[pstephens46]

Still a little slow around here. On another forum a story was told of an Z250 owner adding oil to fuel for extra protection. I think that was what was implied. Have yet to get an answer for clarification. Does anyone know how or if 2 stroke oil affects octane? Where would one research such a question for a reliable answer? THT?

[boscoe99]

It has been slow hasn't it?

THT is the source for whatever information anyone wants to find. Ask a question and get 10 different answers. Choose the answer that is wanted.

I read years ago about failing piston rings allowing crank case oil to get into the combustion chamber and contributing to detonation. Theory being that crank case oil reduces the octane rating of the air/fuel mix.

To what extent two stroke motor oil is similar to crank case oil I don't know.

I have often wondered why two strokes use such low compression ratios in the first place. Maybe it is because oil getting mixed with the gasoline makes them more susceptible to detonation. The lower the compression the less likely for detonation to occur. Is that is the reason?

[boscoe99]

Oh, Yamaha specifically cautions folks to NOT add oil to gasoline in the HPDI models. Why? I know it is not needed but can it be harmful?

By the way, no knock sensors in the HPDI models. Why not?

[robert graham]

Maybe 2 strokes use lower compression ratios because they fire on every stroke and therefore produce more heat/higher temperatures in cylinders???

[zenoahphobic]

Two strokes have lower cylinder displacement/head displacement (compression ratio) because they inherently produce a greater fuel mixture concentration in ignition compression than a 4 stroke can.

Two strokes strokes are interrupted by ports that would make their "sealed " compression stroke, or displacement far less, could be 50% less.

It' s the ability to ram extra fuel/air through these ports above and beyond the cylinder volume. This also includes air/fuel that has escaped through the exhaust port being rammed back in.

So compression ratio tends to be dictated by different criteria between 2 and 4 strokes.

Modern times now have good oils that importantly do not burn, and emission imperatives require this oil to remain in the engine, and minimise escape to the atmosphere.
So I suspect excess oil (non vapourized, not burning) will add to the compression affecting detonation, which is really nothing to do with octane rating.

The octane argument that oil added to fuel, like other additives, was always BS. Clearly 50:1 and only what 10% of that 2% that actually burns is an insignificant amount, and really what is the octane of that oil anyway.

[rodbolt17]

actually octane rating is the rate of the fuel burn. the higher the octane the slower the burn. the slower the burn the less likely it will detonate(spark knock,head rattle whatever). so adding a standard 30 wt to the gas on a four stroke will tend to "increase" the octane as it slows the burn time.
used to happen a lot back in the day. my wife back then had an olds dynamic 88 with an ultra high compression 425 10.5/1.
use anything other than ethyl,old folks know, and it would rattle like a can when you accelerated.
sometimes you would get a batch of poor quality fuel. the cure was to mix a qt of 30wt non detergent with a gallon of gas and pour it down the tank.
have I seen an HPDI run on 50-1?
several times. does not seem to kill anything but it will smoke like a smokestack.