temperature and octane

[mjoekingz28]

Just as an example, Stihl recommends high octane (89 or 91 I cant remember) for use in their 40:1 trimmers. When I was running them working for the city we always had 87 in them, but I wasnt there in the hottest of months (March-early June).

Lets say I have a sportbike that requires 93. Would I be safe to run 87 in all but the summer months. My question, I guess, is since pinging is the reason I believe some engines require high octane (high compression) and they only ping when they get hot so they're just playing it safe by recommending high test all the time?


Sorry I don't know how to word it correctly, but I think you get the point.

 

[Shannow]

90% of the time, you don't need squat in terms of Octane rating.

You are operating at part throttle, and peak compression temps/pressures are way low compared to a sea level full throttle run.

That's the sad thing about spark ignition vehicles, that the majority of the time, they could run on Kerosene, and you need to provide the octane rating for the couple of pedal to metal times per day.

The Oldsmobile water injection systems made sense, except that they didn't say fill tank with water/meth

 

[tomcat27]

I run 93 in all my 2 cycle engines every since my boat mechanic recommended it years ago.

 

[Kuato]

Octane is a measurement of how quickly or slowly the fuel burns. The higher the octane, the slower the burn.

So in a high compression engine, you need higher octane since it starts to burn (spark occurs) as the piston is still coming up to full compression; then it continues to burn as it forces the piston away in the power stroke.

In a lower compression motor, the timing is such that ignition usually occurs after the piston has started moving down, so the fuel can burn more quickly.

IME, using too high octane can cause carbon fouling, and as we all know too low can cause detonation. One of my brothers used 93 in a car he had which required 87....after a year or two his performance went down and he found that that plugs were all fouled up. I bought the car from him, and went back to 87 and after a few tankfuls the performance began to return.

 

[Corvette Owner]

Outside temperature has little to do with octane requirements. Ignition timing and compression have everything to do with it. Modern engines can often change timing enough to "get by" with lower octane. Basically it is the temperature INSIDE the cylinder that determines octane requirement (over simplified).

Hard to believe that running higher octane "carboned up" an engine, as usually higher octane gas has more cleaning agents. Technically, it is true that higher octane gas has less energy (due to octane supplements), but the Otto Cycle is too inefficient to use it all. Maybe it was that change in driving styles that caused the change?

 

[Rickey]

First carbon build up is usually not as damaging as detonation.
(I'll take excessive octane every time)

(for illustration)
With no knock sensor or pre-ignition both regular and premium are ignited when the spark plug fires.... ignition timing point is unaffected by octane.

Burn rate is UNAFFECTED by octane.
Octane rating is determined the fuel's auto-ignition point.

Lower octane fuels tend to behave more like diesel fuels regarding auto-ignition point.
Pinging, spark knock, or detonation occur when the pressure wave created by the primary ignition event sets off a secondary ignition event (one example of auto-ignition) in another part of the combustion chamber.
The resulting opposing flame fronts then collide creating a pressure and thermal shock wave in the combustion chamber(ping).
Of course the results do not improve the long term health of the engine.

It is generally agreed that a "small" occasional amount of spark knock will not do any harm. However heavy repeated detonation can disassemble an engine in short order.

I would run 93 year round in the sportbike. No exceptions, no arguments. It's cheaper than picking up the pieces.

With loud exhaust it is hard or impossible to hear detonation.
I have experience here.....I paid the price for this "education" twice.

Rickey.

 

[Rickey]

Originally Posted By: Corvette Owner
Outside temperature has little to do with octane requirements. Ignition timing and compression have everything to do with it. Modern engines can often change timing enough to "get by" with lower octane. Basically it is the temperature INSIDE the cylinder that determines octane requirement (over simplified).

Hard to believe that running higher octane "carboned up" an engine, as usually higher octane gas has more cleaning agents. Technically, it is true that higher octane gas has less energy (due to octane supplements), but the Otto Cycle is too inefficient to use it all. Maybe it was that change in driving styles that caused the change?


+1

 

[Kuato]

Originally Posted By: Corvette Owner
Hard to believe that running higher octane "carboned up" an engine, as usually higher octane gas has more cleaning agents. Technically, it is true that higher octane gas has less energy (due to octane supplements), but the Otto Cycle is too inefficient to use it all. Maybe it was that change in driving styles that caused the change?



Perhaps "carboned up" is the wrong term to use; I didn't want to put excessive detail in my first post, but now I see that it's needed.

My brother reported that with 93 octane it was initially smoother; then after about a year and a half he said the performance had fallen off. I got the car with 93 in it, and ran 93 for awhile then went back to 87, as designated in the owner's manual. It pinged and tried to detonate if you got on the gas, especially in higher gears. I used several cycles of blended 89/93 with gas treatment, then moved to 89, then blended 89/87 and finally back to 87, with gas treatment all the way through. Once it was ok with straight 87, I tried the 93 for a 1/4 tank and it did indeed make it smoother. Then I ran 87 in it for the remainder of the time I owned it - with no ill effects whatsoever.

Just use the fuel for which the engine is designed.