Jeep Compass octane

[Tom Del Rosso]

The Compass manual recommends 87 octane. Is that just for power or would
it be damaged by regular?

[Tom Del Rosso]

Post by Tom Del Rosso
The Compass manual recommends 87 octane. Is that just for power or
would it be damaged by regular?

That should read 89.

[Indira]

Post by Tom Del Rosso

Post by Tom Del Rosso
The Compass manual recommends 87 octane. Is that just for power or
would it be damaged by regular?

That should read 89.

Assuming USA Octane ratings (which are the average of research octane and
motor octane values), it won't make a difference in almost all situations.

The octane rating is merely the tendency to ping (or not ping) which
happens when the compression ratio or heat or load is such that the flame
front proceeds too quickly.

The spark timing can be retarded in almost all (if not all) modern engines
to prevent any damage from occurring (due to knock sensor input &
electronic ignition system output).

The worst that will happen in most modern engines is slightly retarded
timing under maximum load (e.g., accelerating up a hill or towing a load).

What will really happen in almost all circumstances is... nothing.

[Snag]

Post by Tom Del Rosso

Post by Tom Del Rosso
The Compass manual recommends 87 octane. Is that just for power or
would it be damaged by regular?

That should read 89.

You might see a slight decrease in gas mileage and power due to the
slightly retarded spark timing . Modern electronic controls will
optimize it all .

[Indira]

Post by Snag
You might see a slight decrease in gas mileage and power due to the
slightly retarded spark timing . Modern electronic controls will
optimize it all .

Years ago I ran a test on a vehicle which stated it needed premium and I
couldn't "measure" any difference whatsoever in gas mileage, even as I
agree fully with your premise that there "should" be a "slight decrease" in
gas mileage.

I think the reason I couldn't measure it was that the measurement system we
typically use isn't precise enough because the decrease is too slight. We
measure by the typical method of what I refer to as "gallons used" tests.
a. Fill the tank
b. Drive normally until you need to fill the tank again (note the miles)
c. Fill the tank again (note the number of gallons used & divide miles/gal)

In my experience, this "gallons used" method isn't precise enough to
measure the "slight decrease" you'd expect from lower octane rated fuel.

When they really test gas mileage, they put a known WEIGHT of fuel in a
plastic bag attached to the fuel lines, and run that and then weigh the
fuel after a set number of miles. Our gallons-used test is less precise.

Also, we don't normalize the type of driving, where in general, it's my
understanding, that the only time the "wrong" gas should knock is in when
the spark is advanced which doesn't happen all that often in normal
driving.

Sure, if you race everyone out of a stoplight, you'll be advanced, but most
normal driving doesn't use spark advance all that much (although all are
advanced - but I'm referring to extra degrees of spark advance situations).

[Snag]

Post by Indira

   You might see a slight decrease in gas mileage and power due to the
slightly retarded spark timing . Modern electronic controls will
optimize it all .

Years ago I ran a test on a vehicle which stated it needed premium and I
couldn't "measure" any difference whatsoever in gas mileage, even as I
agree fully with your premise that there "should" be a "slight decrease" in
gas mileage.
I think the reason I couldn't measure it was that the measurement system we
typically use isn't precise enough because the decrease is too slight. We
measure by the typical method of what I refer to as "gallons used" tests.
a. Fill the tank
b. Drive normally until you need to fill the tank again (note the miles)
c. Fill the tank again (note the number of gallons used & divide miles/gal)
In my experience, this "gallons used" method isn't precise enough to
measure the "slight decrease" you'd expect from lower octane rated fuel.

This method is accurate - if you go like 10 tanks the differences
average out . One tank at a time will give you a "close enough" mileage
number .

Post by Indira
When they really test gas mileage, they put a known WEIGHT of fuel in a
plastic bag attached to the fuel lines, and run that and then weigh the
fuel after a set number of miles. Our gallons-used test is less precise.
Also, we don't normalize the type of driving, where in general, it's my
understanding, that the only time the "wrong" gas should knock is in when
the spark is advanced which doesn't happen all that often in normal
driving.
Sure, if you race everyone out of a stoplight, you'll be advanced, but most
normal driving doesn't use spark advance all that much (although all are
advanced - but I'm referring to extra degrees of spark advance situations).

Your understanding is outdated . Modern controls will optimize spark
advance and fuel mixture for the best possible combustion . Now my '86
GMC pickup with old fashioned distributor CAN be over-advanced and made
to ping . A note about that - the original distributor did have a
mechanism to retard spark if a knock was detected . But the block ,
heads , and connecting rods are about the only stock parts left .