Why do I
need premium gas? Can't I use regular?
Will premium gas make my car go faster?
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When the spark plug in the cylinder of a gasoline engine fires, a tiny ball of flame begins to expand in the combustion chamber. If all goes well this expanding flame front will continue to burn evenly until all the gasoline vapor in the chamber has been burned. If the gasoline is of the wrong kind however, or if the spark plug has fired too soon, or the compression ratio is too high, instead of burning evenly the gasoline explodes, combusting all at once with damaging force. Sometimes this undesirable detonation can be heard as a "ping" or "knock" in the engine, but it can occur without being heard. The octane rating of gasoline is an indication of its ability to resist detonation. The higher the octane number, the less tendency to knock, and of course the higher the price.
Engine knock is bad news. If it happens often it can literally put holes in your pistons. I bought a Jaguar XK-E in Denver once, for half what it should have sold for because some fool had advanced the ignition in a deluded attempt to recover the power lost because of the Mile High City's thinner air. Three pistons had holes in them the size of 50 cent pieces!
In many newer cars, the effects of running on regular gas when you should be using premium are less serious because modern ECUs have knock sensors that can detect detonation and which can retard the engine timing until the knock goes away. The price you pay for this is considerably less power from the engine.
Until recently this
article in this FAQ
read as follows--
"So the bottom line is, use the grade
of
gas your owner's manual recommends.If your car only needs regular, it
won't produce any more power with premium gas. If it needs premium, use
premium, because regular will cost you power and maybe an engine
overhaul. "
But lately Honda seems to be engaging in
a marketing ploy. Premium fuel is expensive, and people aren't inclined
to buy cars that need it, so trusting to the efficiency of its engine
knock sensors Honda has recently been recommending regular gas for some
of its products that could really use premium! In those cars, the
owner's manual says use regular, but premium really will get you
another ten HP or so! How do you tell? Well short of a dyno test, check
your compression ratio. As a rough rule of thumb (there are a lot of
variables) a 9.2 to 1 compression ratio Integra 1.8 litre engine only
needs 86 octane, while a 10 to 1 VTEC engine needs 91 octane to produce
full power.
Hold
on now, just what is "octane"
anyway? I read my Honda's Owner's Manual and it says to use 91
"RON".
What's
that? Isn't that the same as what the pump sticker says? Not
necessarily. That 91 RON is the same as 87 pump octane. The numbers
quotes in the paragraph above are actually pump octanes.
There are two methods used
to determine
the octane rating of a gasoline: Motor Octane Number (MON) and Research
Octane Number (RON). RON is the higher of the two. Almost everywhere in
the world, a gasoline is referred to by its RON rating. Here in North
America however, the MON and RON are added together then divided by two
to get what we call the Anti-Knock Index, or AKI, or "pump octane". So
the formula is:
(RON + MON) / 2 = AKI
That's why 91 RON is the same as 87 AKI (pump octane).
91 AKI would be more like 96 or 97 RON. Britain has recently
re-legalized leaded gas, and "4-star" 97 RON is available once again.
If you're interested in finding out more about RON, MON, AKI, and a LOT more than you ever intended to about gasoline in general, click here for the excellent and engrossing Gasoline FAQ