First some real simple facts then for those of you who care to read on a
little deeper look and for those who want more click on the link at the end
of the article.
Fuels of today do not
last very long. According to the fuel manufactures the fuel is good for
about 3 months. I believe in our older cars 2 is closer to the mark. One
noticeable exception is Sunoco's 100 octane unleaded race fuel that they
sell at the pumps at select gas stations. The company claims a year and we
have to agree with them. So if you park your car either fill it up with 100
octane fuel or go to the local marine store and put some stable in
the fuel. Follow the directions on the bottle. Or you can have your fuel
system overhauled every year.
What most of us with
older cars worry about is Octane and Lead. Let's start with octane first.
Since as early as 1912 engineers have known that the limiting factor in
compression was engine knock. They just did not know what knock was. Most
people still do not. It is not pre-ignition, though that can cause knock and
knock can cause pre-ignition. Knock is actually post-ignition! Without
getting to technical it has to do with flame fronts in unburnt fuels and
gasses and how rapid the fuel is burnt in your combustion cycle.
Pre-ignition is usually caused by deposits & overheating.
Somewhere in the mid 1920's
they figured out that knock was not caused by those new electric starters or
by battery powered ignition systems. And from then to now they have been
refining the way they measure a fuels properties and it almost all comes
down to controlling knock. If your engine knocks your are going to self
detonate it. So as we clean up the fuels and cut down the emissions keeping
knock under control is still the biggest factor.
In very simple terms the
higher the octane the slower the fuel burns and the more complete the burn.
If I am burning slower I am also burning cooler which can allow me to do
things like increase timing and run higher coolant temperatures, all of
which give me more power. So, why don't we just have nothing but higher
octane fuels? Cost and waste are two huge reasons. To produce higher octane
fuels with out a lot of additives takes more refining, higher costs, and you
get less fuel per each barrel of oil, costs and waste. Lead was a great
additive for the fuel companies. it was cheap and did the job. It cooled the
combustion chamber and did the same as higher octane fuels for a whole lot
less money. Unfortunately it has a couple of major drawbacks. First it
builds up in the engines and on valves. Read your old repair manuals and
removing heads for cleaning was normal routine service. Cutting ridges out
of the top of cylinders to get the piston out was an everyday process.
Second it is very bad for every one and every thing. Air borne lead was a
major cause of poor test scores and learning disabilities in children.
Contact with it was a quick way to a slow death. So getting rid of it is a
good thing. But wait, what about lubrication properties? Being clean is much
better than the little bit of lube you got from your lead. Oh by the way
Jaguar starting putting hardened seats in their heads in 1948, Almost every
car has hardened seats after 1974 when they first started mandating lower
lead fuels. When catalytic converters showed up in 1978 they all have them
as lead disappeared from the fuels at the same time! For those of you with
older and especially pre-war cars the use of no-lead fuels will slightly
speed up your head's demise but most of the failure is poor metals and age!
When you have it re-done you will have a lead free engine unless your
machine shop is incompetent or dishonest.
Now here is something people
with older cars may find hard to believe but it is true. Because the fuels
are so much cleaner and the additives so much better today that it is not
unusual for an older car to run on lower octane today than it did new! If
you find you are fouling you plugs on high octane back up one grade and see
what happens. To get an idea of what octane to use for what engine here is a
little chart, now remember engine design, driving habits, engine state all
effect the octane requirements. And yes you may find you have no ping on one
company's fuel and do on another of the same rating. how and what the
additive package is makes a huge difference.
This chart is for normally
aspirated pre-fuel management cars. Interesting to see how much energy your
A brand new engine can run
on octane about 6-9 points lower for the first 12,000 miles or so an oil
burner may need as much as 12 points higher!
The cooler the temps the
lower the octane requirements. So you should be able to reduce the octane
requirements for your car in the winter. Very high humidity reduces octane
requirements too. The higher the altitude and the numbers go down too. Makes
sense as there is less oxygen up there. Av gas has a much lower boiling
point than car fuels, it is much colder up there, so using av gas to boost
octane can cause vapor locks in the right conditions.
The higher the rpms of your
engine the less knock is a factor as there is less time for pre-flame
reactions in the end gasses. The cause of knock.
timing and knock
numbers are based on a normally aspirated engine built in the late
seventies. An earlier engine design would may have slightly different
have a play in deciding octane requirements, cooling system, head and intake
design, piston design, engine materials..... Just upping or lowering your
octane is not the answer. To low causes damage and to high is wasteful and
cost more money. Shorten up tune ups also.
Start with two
things, your driver's manual recommendations and a good tune up. Then move
up and down one grade at a time until you fine the best combination of
power-fuel economy-and no knock.
For more info on fuels go here http://www.faqs.org/faqs/autos/gasoline-faq/part1/