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Long term octane test (>100k miles using the wrong octane rating)
James Robinson wrote:
> Thomas & Sons Cycle Shop wrote: snip > A second consideration is fuel efficiency, since the anti-knock sensors > will retard the timing on an engine, when lower octane fuel is used in > an engine designed for higher octane fuel. The side effect is that fuel > efficiency will drop. That means that people might be thinking they are > saving money by using lower cost fuel, when they really aren't. > Further, the engine will lose horsepower, which is part of the reason it > was designed for high compression in the first place. > Not necessarily. In fact usually not the case. The octane requirement for spark ignition engines is a funcion of operating load. That is, as load increases, octane requirement also increase. That's the load effect. Then there is the speed effect. Generally, octane requirement DECREASES as engine speed increases (for engines with standard valve timing). What this all means is that at highway or around town cruising speeds engines are operating at fairly light loads. Here premium fuel octane is not needed. Even the "premium recommended" engines will be able to run optimal ignition timing. Only only during heavy accels or WOT operation will the fuel octane affect performance. But, since most people spend a low fraction of their driving time at high loads the net effect on fuel economy will be very small. |
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#2
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dyno wrote:
> There's the load effect (load up, octane requirement up)... > Then there's the speed effect (speed up, octane requirement down) ... There's also the altitude effect. The higher you go, the lower the octane requirement. O |
#3
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#4
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On Sat, 14 May 2005, dyno wrote:
> True. There's also the inlet temperature effect; temp up octane > requirment up. Oops, no. Ambient temperature goes up, octane requirement goes DOWN, and for the same reason that octane requirement decreases as altitude increases: With both increasing altitude and increasing ambient temperature, the intake air charge is less dense, therefore cylinder pressure is lower, therefore tendency to ping is less. |
#5
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On Sat, 14 May 2005 17:59:01 -0400, "Daniel J. Stern"
> wrote: >On Sat, 14 May 2005, dyno wrote: > >> True. There's also the inlet temperature effect; temp up octane >> requirment up. > >Oops, no. Ambient temperature goes up, octane requirement goes DOWN, and >for the same reason that octane requirement decreases as altitude >increases: With both increasing altitude and increasing ambient >temperature, the intake air charge is less dense, therefore cylinder >pressure is lower, therefore tendency to ping is less. You could be right, that it's not specifically intake air temp as the cause, but in many years of driving in the hot desert, I've had numerous cars and trucks that would run just fine on 87 octane in the cool months but would ping like crazy at well below WOT unless I ran 91 octane in the hot summer months. All had proper thermostats so it wasn't just a hotter engine. -- New service to compete with paypal Get $25 pre-registration bonus by following this link www.greenzap.com/25smackers4u |
#6
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Daniel J. Stern wrote:
> On Sat, 14 May 2005, dyno wrote: > > >>True. There's also the inlet temperature effect; temp up octane >>requirment up. > > > Oops, no. Ambient temperature goes up, octane requirement goes DOWN, and > for the same reason that octane requirement decreases as altitude > increases: With both increasing altitude and increasing ambient > temperature, the intake air charge is less dense, therefore cylinder > pressure is lower, therefore tendency to ping is less. Wrong. The change in density goes with the change in absolute temperature. But, the effect of inlet temperature is magnified during compression. The compression effect dominates over the small change in density. |
#7
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Thomas & Sons Cycle Shop wrote:
> In article >, dyno > > wrote: > >>>>As the load increases, the octane requirement increases . >>>>As the speed increases, the octane requirement decreases >>> >>>As the altitude increases, the octane requirement decreases >>>As ambient termperature increases, octane requirement decreases > > > There's also the humidity effect. > As the humidity goes down, the octane requirement decreases. > snip No, I think you have it backwards. High humidity is good for knock suppression, reducing octane requirement. |
#8
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"dyno" > wrote in message ... > Daniel J. Stern wrote: > > On Sat, 14 May 2005, dyno wrote: > > > > > >>True. There's also the inlet temperature effect; temp up octane > >>requirment up. > > > > > > Oops, no. Ambient temperature goes up, octane requirement goes DOWN, and > > for the same reason that octane requirement decreases as altitude > > increases: With both increasing altitude and increasing ambient > > temperature, the intake air charge is less dense, therefore cylinder > > pressure is lower, therefore tendency to ping is less. > > Wrong. The change in density goes with the change in absolute > temperature. But, the effect of inlet temperature is magnified during > compression. The compression effect dominates over the small change in > density. I had a Tempo that would run at the high end of the temp gauge when using 87 in the summer. If I switched to 92 (or better) the temp came back down into the normal range. In the winter I used 87 for easier ignition. Ken (MI) |
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