Any discussion about octane invariably leads to statements from some cars’ owners that their engine performs better when they use the 91 or 93 (or higher) fuel blends in their vehicles.
For most modern, computer-controlled cars on the road today, this perception is more mental than it is factual. For classic car owners, octane can make a difference from an engine-efficiency standpoint; however, the octane rating of your gasoline has very little to do with the horsepower or torque output of your classic engine as is often alluded to in these conversations.
Octane is simply a measure of the fuel makeup, and its tendency or resistance to cause engine knock or ping when used under duress (higher RPM). The octane index rating is not based on a quantity of a chemical in the fuel mixture, but is a measure of the efficiency of the fuel blend, expressed as a ratio, relative to the efficiency of a pure hydrocarbon, which would have an octane index rating of 100 (or 100 percent). Because gasoline is made up of many different hydrocarbons, the octane rating is a comparison of the anti-knock characteristics of the blend relative to the anti-knock characteristics of a pure hydrocarbon with a 100 percent rating. Aircraft or racing fuels have a rating above 100 because the additives in the fuel raise the efficiency beyond that of a pure hydrocarbon.
Engine knock is caused when the fuel mixture ignites too early, often before the spark plug has fired. Knock often presents itself when there is an increase in engine RPM and cylinder combustion chamber pressures are also increased. The higher the cylinder pressure, the more likely the engine will knock.
Octane is measured by operating an engine under two different conditions and averaged to result in the rating you see displayed on the pump. The first method (R) is to test the fuel mixture for its anti-knock characteristics (as a percentage of efficiency to pure hydrocarbon) when the test engine is under load, the second test (M) measures the anti-knock tendencies when the engine is free-wheeling. The average of the two results is the percentage that is shown on the pump (R+M/2).
Fuel is required to meet minimum octane efficiency standards of 87 percent to be sold at the pump, with more efficient blends having an efficiency rating of 88 percent to 90 percent considered mid-range gas. Efficiency ratings above 91 percent get the “Premium” designation. Premium gas must be, by law, at or above 91 percent, although you do also see 93 percent octane ultra-premium at many stations.
Although higher octane can cost substantially more per gallon, it does not necessarily mean it is better for your car.
Higher octane gas is processed through additional steps that further refine the blend and cause it to burn more slowly than lower octanes. These additional processes are what contribute to the higher pricing, but that does not mean the higher octane will offer any advantage over other blends in many engines. Octane does not offer any better fuel mileage, increase engine horsepower, or make the engine start quicker. Higher octane only reduces the likelihood of engine knock or ping.
On modern computer-controlled cars with fuel injection, the computer is constantly monitoring fuel trim and detonation and making appropriate adjustments in the timing and fuel air mixture to compensate for engine knock. Most of these late-model engines have a sonic knock sensor installed in the cylinder block for just this reason.
As you go back in time to earlier fuel and ignition systems, the octane content becomes more important because the old point distributors and early electronic ignition distributors had only a vacuum advance to correct for engine knock. Exhaust gas recirculation systems were also in their infancy and were not as efficient as modern systems, so they had less effect on reducing knock as well.
Because higher octane gas burns slower, it is more resistant to knock when subjected to higher RPM and cylinder pressures. Compression ratios also factor into cylinder pressures. Higher ratios cause higher cylinder pressures and therefore cause the engine to be more susceptible to pre-detonation or knock.
The introduction of ethanol in fuels further complicates the octane debate. Ethanol has a higher octane rating than hydrocarbons and also ignites at much higher temperatures. Blending ethanol into pump gas will slow the combustion process and reduce the likelihood of engine knock. The delay in the ignition of the mixture, caused by the addition of ethanol, allows the fuel burn to occur while the engine piston is in the down stroke, when there is less cylinder pressure, and this reduces the likelihood of engine knock.
Ethanol can also be used as a method of increasing the octane of a fuel blend by lacing lower octane hydrocarbon-based fuels with higher octane-rated ethanol to arrive at the required octane index rating.
In summary, most modern vehicles do not require higher octane fuels, unless specifically expressed in your owner’s manual (read carefully, because there is a difference between higher octane being “recommended” and “required” in the manual). There are a few high-performance engines that were built with higher compression ratings or use higher RPM camshafts where 91 octane may be needed, but your average Subaru or V-6 Explorer will see no noticeable benefit from using the more expensive blends.
In classic V-8 muscle cars and vintage engines, a higher octane fuel is probably a good idea, but do not buy more than you can use quickly. The disadvantages of ethanol-laced fuels are most prevalent when stored inside your gas tank over longer periods of time. The higher octane fuels are slightly less efficient than the lower grades because the retarded ignition will lead to a little less overall power and a scant fewer miles per gallon, but the reduction of wear and tear on your engine should outweigh the extra cost of the higher-rated blends.
After my confessed acquisition
of a new car, I have received a number of e-mails on the subject
of new car break in,
because, it seems, for the first time ever, a lot of people have gone out and bought a new car thanks to these competitive
incentives the car industry has been offering.
My Porsche has pop up headlights, long since replaced on newer models by $1500 wrap around plastic lens systems.
If a headlight fails to open, or close, there's a little round knob under the hood that allows me to activate the headlight by hand.
It's not so easy, but in an emergency at least, it works.
As has always been the case, popular items fitted as standard features to expensive cars eventually filter down to even the lowest
of the low on the food chain. Even $15000 new cars now come, in many cases, with air conditioning, ABS brakes, windshield washers,
rear wipers etc, etc as standard.
An announcement has been made that very soon manual window winders and door locks may not be fitted on cars any more.
I probably don't have to tell many of you that the failure of an electric window in the down position is at very best, awkward and in
the worst case, as in a forty below snow storm or a torrential summer storm, immobilising.
The car has to be parked somewhere warm and dry until the weather will allow it to be moved for repair.
At this point. I'd like to enter a plea to the car companies to include in their designs a small access port in at least the drivers door,
if not all the doors, so that the windows can be wound down, or up, manually when necessary with a crank handle included in the tool kit,
or preferably clipped into the back of the glove box.
Now we get to a more disturbing trend which is the substitution of mechanical manual door locks with all electric systems.
I will agree that more and more of my clients hand us the keys and say "the key doesn't work, use the remote".
What they mean is, that they have never ever opened the doors with the key and now the door locks have seized solid.
This is about intelligent as never using your handbrake on an automatic transmission - until the day you HAVE to use it,
such as at a roadside safety check and you are left with a completely seized brake system that won't unlock as the cops drive away.
These "driver identification" systems, whereby a driver steps up to his vehicle and it recognises him/her and unlocks the doors
are fraught with possible failures.
I really don't know what to suggest if manual door locks go the way of the Dodo bird, but if you have no manual locks and no
window winders, pray you don't end up under water, having slid off the road into a river, or, like me with my rally car, end up upside
down in a ditch. I don't know for certain but I'm reasonably sure that these latest ideas to hand over ordinary manual operations to fully
automated electric actuators is going to increase garage profitability in the long run by quite a nice margin, because the solenoids, electric motors, remote controllers and their attendant batteries all fail more frequently than you can imagine.
The tow truck drivers are also going to love this latest trend.
I'm not enough of a Luddite to do a King Canute and try to hold back this technology.
My only plea is for manual emergency back up systems wherever possible, just like the slot that releases the shifter if the interlock
between the shifter and the brake pedal fails. And they do, often.
For the first time ever in my life, I've had the opportunity to try a front wheel drive car against an all wheel drive car in one of the harshest, snowiest winter we've had since 1971.
Both cars, by law, are fitted with snow tires and the 2014 Mazda3 came with a set of Nokias, in addition to the summer tires which are currently stored (on their sides) in my garage.
The Subaru Forester has had to sit outside to make room for a sleeping Porsche. Consequently, on at least three occasions, it has had to pull itself out of a snow bank so that all sides could be reached with a broom to clear off as much snow as possible. Equally, the Mazda has had to negotiate a snow bank at the end of the driveway, courtesy of the municipal snow plow, several times.
Neither car has had any trouble at all with negotiating deep snow drifts. However, the FWD car has to be driven straight out. Any attempt to turn the wheels before the snow bank is far behind, provides too much snow resistance and the car will bog down. Not so with AWD.
The roads have been consistently snow covered and/or icy for several months. In theory, the Subaru should have a major driving advantage, but in fact, putting the power on going round a corner makes the car tail happy.
Once you know this, you can have a lot of low speed fun driving a la Jack Brabham with the tail hung out all the time and a big grin on your face.
By comparison the Mazda with its good quality snow tires, is a stubborn under steering car. The only way to have fun is careful and judicious use of the handbrake to make the tail break loose. For the conservative driver, the FWD car will feel a lot more secure.
With concern for my knees and hips, I don't ski anymore, but if I did I might favour the Subaru. The late night Friday runs to the ski hill through unpredictable road conditions and the parking on a sloping and icy ski resort parking lot would give the advantage to the AWD car. How many times have I seen happy but tired skiers trying desperately to get their car out of a parking lot that has thawed all day and has now frozen itself into black ice.
Additionally, if I owned an all season cottage I would favour the Subaru.
But for suburban work, the extra weight and lower fuel economy, due to the rolling resistance of all the extra transmission parts, would make the Subaru an unnecessarily complex vehicle.
The one I have at the moment cost very little to buy and does not do much mileage, so its choice had more to do with availability than practicality.
If my next winter beater is an FWD, it won't make much difference for me.
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