I have a 89 4x4 V1500 Burb that I have had since new with a 40 gallon
tank. You cannot burn it dry and typically I have 6 to 9 gallons left
when I run in low on trips between fills. The fuel pump is cooled and
lubeb by fuel in the tank so it is not wise to run it low all the time.
I have gottn a little over 600 miles for a fill a few times on long
trips if I run it low (averaging 17 to 18 mpg) and 500 mile with a god
reserve it no trouble at all.
On my '98, no way.
600 miles on a tank is about max, but I think about filling as it approaches
500. Typical MPG on a round-trip to the beach (about 190 miles) is
14.something. That's with a rooftop box and roof racks.
Per other threads, it doesn't do the fuel pump any good to run the tank almost
dry - supposedly because it relies on the fuel around it for cooling.
Try better octane fuel as in a 9 to 1 CR or higher engine, 87 can murder
the mileage as the ECM retards the spark to control kncok. I can
consistantly get 17 to 18 MPG on trips with my 89 4x4 burb with 175 K
miles on it now. I never use 87 and raely use 89, it is usually 91 or
better in it as performance and MPG suffers when I use lower octane fuel.
Ever crunched the numbers? i.e. is the additional cost per gallon cover
My '98 gets even worse mileage than most others report - but I lay some of that
to all the junk I have on the roof.
OTOH, somebody somewhere opined that the occasional tank of premium would have
some effect on the vehicle's onboard 'puter and improve mileage when running 87.
I am surprised as i always get 18+ mpg on steady trips and with 92000 miles,
i have never used anything but ARCO 87 octain gasoline. I have been through
all the western states and never a miss.(except a battery connection
1989 ford fs bronco
2000 xj cherokee
1988 xj cherokee
55cj5 with a chevrolet 350
an FC!50 jeep /302 ford engine
all on 87 octane
Yes I have, never got over 15 on 87 when it was new and performance
sucks with 87 too. Even if MPG was not better I would still use it
because it runs much stronger and crisper on better gas. It is not like
the old days when it was obvious when you needed better gas (engine
knock was apparent) Today, with knock sensor and ECM retarding spark it
is stealling your MPG and performance from you quietly because it only
knocks now when it exceeds the abilty of your ECM to control.
Also by what logic is a occaision tank of 87 going to improve 87
performance and MPG?? As soon as you fill it up with 87, the ECM here
the knock and goes back to retarding spark the instant it hears it. The
only reason that there is a knock sensor is so to limit consumer
complaints about motor knock on 87, no because of emissions or that 87
is the best fuel. If 87 and 89 was removed from market, (especailly 87
which was intended for engines with about 8 to 1 CR or less in mid 70's)
they could even remove knock sensors for a lot of engines
I meant to say a occasional tank of 91 octane improving 87 performance..
The ECM hears the knock as soon as low octane fuel hit engine and the
edge is lost. I recently picked up a real time graphing scanner to
monitor engine performance (airflow. manifold pressure, fuel flow, times
and so on) and I am going to test drive some new vehicle with it plugged
in and "listen" to the engine on low octane fuel. On my 2000 5.7 is show
no spark knock retard at all at any time (because I use good hi octane fuel)
Correct me where I am wrong, I know you will. To the knock sensor and
retarded timing. A knock sensor senses detonation before it reaches a level
that will cause damage. It retards ignition more than actually needed. If
optimal total timing is 31 degrees, when it senses knock it will cut back to
about 25 degrees. Instead of slowly rolling back it instantly jerks it back.
Could have went to maybe 30 degrees and controlled knock. That is where you
loose so much fuel mileage. The key to getting maximum effect is living on
the edge. Running a engine almost to detonation. I've spent countless hours
jetting a carb, reweighting/springing a distributor and adjusting timing
trying to live on the edge. Sucks when you fall off the edge. The lower the
octane rating the higer the BTU output, right or wrong. Higher octatne gas
is made to controll detonation in high compression engines. The higher
octane is less volatile. Why don't auto makers program the spark and fuel
curve for low octane fuel. Is it emissions? What am I missing. If it is
simply smog laws and you don't live where they test, I dont'. Spend a
couple bucks, polish up those combustion chambers and piston tops, install a
cooler thermostat, reburn that prom and live on the edge with that low
You are partially correct but maxium power and economy is achevied when
peak pressure is achive at a certian point in the power stroke of the
engine and lower octane fuel that has to have its spark retarded lower
the peak pressure and changes the point at which it occurs which lowers
efficency and peak power. When the ECM hear lnock, it will retard it a
lot (15 to 20 degress and then slowly start dailing some back in until
the point you mention is found but by its presence and need to retard
the spark, you are reducing engine output and MPG at any given speed. My
300 K3500 shows between 30 and 35 degress of advance in normal driving
and almost 20 at a idle and shows no knock activity at all and it runs
good but then like I said, it never sees 87 octane fuel either.
I could not agree with you more as power will increase because injecting
fuel at last second into chamber it cools mixture allowing higher CR
ratios without knocking and more power and better efficency.
Furthermore, less fuel is lost to savanging os gasses when valve overlap
so more MPG gains too. Overall when it hits main stream, the line
between diesel and gas economy if going to get a lot blurrier and diesel
will loose more of their appeal.
David Johnson wrote:
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