Octane Overkill

Hello,

Not being an engine expert, I figured I'd throw this question out to
this NG. I have a 1990 Civic LX, 1.5L DPFI 5-speed, and usually I put
regular unleaded (87 octane) in it. Every once in awhile I'll put mid-grade
(89 octane) or premium (92 or 93 octane) in it because I heard that doing so
will keep your injectors clean. (Is that true?) Once I really splurged and
put 100 octane fuel in it (only 5 gallons) and it did run pretty good. One
thing I noticed, is that if I run premium (93) in it several tanks in a row,
it will actually not run quite as good. It's a very minuscule difference,
but I can tell there is a slight decrease in performance. If I drop it back
down to mid-grade or even back to regular on the next tank, it seems to go
back to running better. Is this all in my head, or is it possible to have
an "octane overkill" on such a small 4-cylinder engine?

Thanx in advance,

Jonathan

 

The vehicle was designed to run on regular fuel. Using any other grade is
just throwing your money away.

Brian

 

High octane fuel is for high compression engines, to prevent detonation
(ping). Your owner's manual will tell you what grade to use - probably 87
octane. Rather than going to a premium grade (which, it's true, often has
higher detergency), stick with the recommended octane. Some brands have more
detergents than others, and they advertise that. (I like Shell/Texaco and
Chevron.) I had a Nissan 300ZX that would get balky - the clutch would start
to feel grabby - if I ran 76 in it for a couple months but would straighten
right out about halfway through a tank of Texaco. My other cars never seem
to care.

We also have a Toyota Prius, and on the Prius newsgroup others have warned
me it won't run right and will even set the "check engine" light if premium
grades are used. (The Prius has what Toyota calls a "modified
Atkinson/Miller cycle" engine - it uses valve timing to tame the 13:1
expansion ratio to a variable compression ratio.)

Mike

 

Prior to 1995, it may have been truer than not.

The oil companies used to put detergents into only their higher octane
fuels, as an incentive to get you to pay the price.

Around 1995, though, the feds defined a minimum level of detergency to
be put into ALL fuels. This dried up the oil companies' ability to
claim that "buy our expensive stuff, and your car will be cleaner and
run better".

Shell is back into that game now, though, with their V-Power. They
simply add MORE detergents to their 92 octane gas than is specified by
law.

This all being said, you can add a can of Chevron Techron to your fuel
every now and then and get the same results or better. And it's MUCH
cheaper than using Shell V-Power on an ongoing basis.

Not as good as you think. You wasted your money, AND you wasted fuel on
top of that.

That's right. The higher octane fuel won't give your engine as much
power as the lower octane fuel, believe it or not.

http://www.faqs.org/faqs/autos/gasoline-faq/part1/

 

What makes you think we are experts? We just have computers and
internet connections.

It is my understanding that using a higher octane than is called for
in the owner's manual is a waste of money. Modern engines are designed
to adjust the spark advance so as to avoid the "octane ping" that was
common with older engines.

On the other hand, it is true that some manufacturers claim the higher
octane gasoline ("premium grades") contains additives that may help
clean your injectors. You have to treat these claims the way you treat
all marketing claims.



Elliot Richmond
Freelance Science Writer and Editor

 

Waste of money. All modern gasolines regardless of grade have additives
to prevent injector problems...

 

increasing octane effectively = slower flame front propagation speed.
that reduces detonation in combustion chambers with poor flow/mixing
configurations, sharp angles & extended reach. good mechanical design
addresses these problems pretty much completely, therefore low octane
can be used without detonation and with ignition timing matched
accordingly. using a high octane fuel in a low octane timed engine puts
too much of the combustion energy out of the exhaust because it's still
burning on the down stroke, not expanding and providing push for the
pistons.

that said, you may notice a slight increase in power for a mid grade
fuel in that motor without a detonation sensor. partly because, in the
absense of a sensor, the engine timing used has to be a "compromise"
range designed to be safe and partly because of fuel chemistry.

here's an interesting post that touches on the subject:
http://www.digest.net/bmw/archive/v9/msg04942.html

it seems to assume that ignition timing is optimal for each different
combustion characteristic of the different blends described. you can be
assured that gas companies use all the cheap options for their lowest
grade of gas.

 

ive been using amoco ultimate in my 04 accord should i switch to regular??

 

Use the fuel that your owner's manual recommends.

In your case, 86 octane fuel is fine. Anything else, you're wasting
your money.

 

Unless your engine is knocking, there is no need to spend the extra
money for a higher octane fuel. Use what the owner's manual recommends.

I suggest reading:
http://www.ftc.gov/bcp/conline/pubs/autos/octane.htm
"The Low-Down on High Octane Gasoline"

 

Hey Mike
I thought you might be interested in some test that were run with high
octane E85 in a prius. Check out page 10 at
http://www.creedproject.org/stream_v7n2.pdf and observe a 20% increase
in power when the "Forbidden" fuel was tested.
I have made a bunch of post at The Temple of VTEC about this subject.
I post as cleverlever. You also might be interested in my patent
4,961,406 which demonstates how to change the burn rate of a given
fuel at low rpm

 

Only if you want to save money.

I once did a study when I was commuting 100 miles each way to work in the
mid 90's (late 80's GM Bonneville 6 cyl). I clocked my mileage and input
gallons closely for a month using 87 octane gas and another month using 91
octane. Both months were during summer, and no AC was used.

My mileage per tank was about 8% higher on average with the 91 octane. Of
course, the 91 octane was 10% higher cost.

 

Yes, they also got a check engine light. Cold start emissions were
higher, and overall MPG was lower. They did not perform any long-term
tests (ethanol can break down parts of the car over time).

The Prius is only rated to use E10.

 

this is great! an expert named
"jim beam" bought a computer and figured out how to wire it up. once he
read something on the internet and provided the following nugget of BS for
us to enjoy.

wrote in message

no. close though. the rate or speed of flame is the same regardless of
octane. increasing the octane does decrease breakdown components that tend
to ignite behind the flame front.

*dont take my word for it, buy a real book wioth real facts from real
research, not something some dude with a BMW once posted on the net.*

you are a bit confused. the design of the combustion chamber is indeed
important, but ignition timing and compression are the primary components of
spark induced ping. combustion chamber design is a very narrow area of
concern and for all intents, it does not mean much of anything when speaking
of octane. not anymore

really? using an octane over your requirements does not waste fuel. it
does not decrease efficiancy or power or anything. it will just cost you
more.

once an engine has had its octane requirements met, nothing, absolutely
nothing, will change as a result of the octane you are using. ie; if your
well tuned engine requires 87 RON MON, then it is the same as using 92 or 94
or 100. by definition, octane only has an effect on knock.

it must be true!

what are you trying to say? the "lowest grade" as it applies to octane is
not a qwuality judgement. there si not anything cheap or inferior about 87
vs. 92 octane gas. just its anti knock capabilities. for the record, major
petroleum companies use the same additive package across the board.

i suggest that you do some resarch of your own. i am informed becuase i am
a consultant for petro comapnies and get paid to read about gasoline. i
dont know much about cnacer, but then im not posting crap at the oncology
NG!

 

being tired and cranky,
just do a deja news search with "Joe Lang" and octane.

 

um, yes, it's less efficient.

 

An interesting link. Thanks!

My understanding is that octane is not specifically about burn rate, but
about ignition point. If the mixture ignites because of the heat of
compression, or if the compression heating is high enough that the reaction
propagates by the pressure front instead of the flame front, the fuel
mixture changes from a low explosive to a high explosive.
See http://www.howstuffworks.com/question90.htm

Mike

 

tell me more about combustion chamber design. as i understand it,
morphology influces gas flow, density, turbulence and pressure wave
propagation. this in turn does affect the stability of the resulting
cylinder charge on compression, hence octane matters. i'm /not/ an
expert on this stuff joe, but i do have some books on the subject, so
please, enlighten me further.

and please, let's keep the flame limited to the spark-induced variety.

 

Thanx for all the advice, folks. I appreciate it. I'll vote for saving
money and stick with regular unleaded (87 octane) from now on. On this
note, I have one other question: Does gasoline lose its potency if it sits
for awhile? The reason I ask is because I notice that when my tank runs
almost on "E" and I refuel, my car seems to be more responsive and run a
little smoother. Does a fuller tank cause better fuel delivery somehow, or
what? (I'm thinking the level of fuel in the tank shouldn't matter...that's
what a fuel pressure regulator is for...to keep the fuel pressure constant)
Why does it seem to run a little better right after I refuel, regardless of
brand of gas?

Thanks again!

Jonathan

 

If you're running your tank out every week or so like normal drivers,
gasoline doesn't lose anything. It's in your mind.

If you're filling up every 6 months or so, there could be a problem.
Gas does get old.