[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
Re: Regular or Ethel ???
Virtual Bob wrote:
> On Sat, 28 Mar 1998, James Marriott wrote:
> >Not _always_ the case. Keep in mind: the higher the octane, the lower
> >the (volumetric) energy content. For example, my non-knock '87 4kq will
> >get worse mileage with 92 octane (R+M/2 method) than with 87.
> I thought they're suppose to be the same.
Well they're not. Virtually all hydrocarbons (propane, methane, butane,
diesel, kerosine, gasoline) have 19500 (+/- 250) Btu/lb. Refining to a
higher octane removes the more-dense items, leaving a fuel with less
energy content _per unit volume_. The "by-weight" is the same, but since
a gallon now weighs less--because we've refined out the dense stuff--it
has to have less energy.
> The only difference is their cumbustion rate -- the higher octane ones
> burns slightly slower than the one with lower octane rating.
Higher octane burns slower and has less tendency to pre-ignite.
> So, assuming engine (etc.) is in tip-top shape, an engine requiring 87 is
> fed a higher octane (and assuming it cannot change spark timing) will
> extract less power from the high-octane gas -- that is, the engine is not
> able to get the right amount (not more, nor less) energy from the
> high-octane gasoline.
Correct. Again, this applies to a non-sensing engine. My '87 4kq gets
WORSE mileage using 92 than 87. _Perhaps_ if I advanced the timing when
using the 92 the mileage may get better (BTDT, it doesn't). If a car is
rated to run on 89 octane, it WILL NOT make more power, mileage, or run
better on 92 octane. That is, using higher octane than is recommended by
the OEM won't help. Three caveats: higher-grade fuels may have better
additives (techron, etc), which helps keep things (injectors, manifold,
etc) happy; you may have to buy, say, 91 to actually get 89; if your
manual says "rated for 91, but runs on 87 (or similar)" you will see
more power on the 91. As was said previously, this engine will pull
timing/fuel/whatever to allow itself to run nicely on the 87.
> Conversely, if an engine requiring high-octane gas is fed a low-octane gas
> (still assuming it cannot change the timing), it will ping.
> Theoritically, if both engines are able to adjust their timing, both will
> get the exact same amount of energy out of same amount of (both types)
> gasoline... Assuming the engine is an ideal engine (which no human can
Nope. An "87" engine will _not_ make more power when fed 91. A "91"
engine _will_ make less on 87, either by knocking (non-sensor) or by
pulling timing/fuel/etc (sensor).
> In real-world, both (adaptable) engines will have slightly higher output
> with the higher-octane gasoline because in case of high-octane gasoline,
> the air-gas mixture is usually ignited slighty ahead of lo-oct gas in the
> engine rev (where piston is beginning to come down toward cranck shaft)
> cycle. The result is the high-octane gas will have slighty longer time to
> transfer the cumbustion energy into mechanical energy than the low-octane
> gas. Both produces the same energy, but hi-oct has better way of transfer
> it to engine power.
Nope. The "bummer" here is that the 92 _contains less energy_ than the
87. End of story.
'87 4kq, no sensor
'86 4ks, sensor