I’ve been a big fan of Cecil’s for more than 25 years. I’ve even had one of my questions answered. But last Friday’s column on octane was the first time in all those years that I’ve been disappointed by an SD column.
Not because it was bad, wrong, or poorly written. It just didn’t rise to the level of a Straight Dope topic. This isn’t one of those deep, dark mysteries that require the prodigious talents and brainpower of the Master. It’s been covered to death all over the place. In fact, Click and Clack answered the question on this week’s Car Talk on NPR! And I’m sure it’s on their Web site, too.
I suppose that in 31 years Cecil’s entitled to one or two less-than-stellar columns. But I hope he’ll do better next time.
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It’s also rather reminiscent of Una’s recent report on diesel and gas. I understand when the classic columns run in themes (they’re arranged thematically in the books), but this seems a bit odd.
Cecil doesn’t just deal with deep dark mysteries, but also with common misperceptions. I’ve often seen him write a column on less than cosmic topic with the explanation that the same question had been coming in the mail regularly lately. This may be one of those things. Yeah, it’s a slam dunk, but many folks are still confused by it.
But in any case, before reading the column, I had the vague idea that premium gas had more power in it than mid-grade or regular … but now I know better. With today’s gas prices, I’ll now save about three bucks at the gas station every time I fill up. Not bad. Esoteric mysteries are fun, but this column was really useful. A thousand blessings on Cecil’s … um … on Cecil’s manhood!
What wasn’t mentioned is that ball lightning is what happens when you use higher octane fuel than in called for, and the Tunguska event was the result of an unleaded shortange which forced everyone to put diesel fuel in their cars.
The timing of the column on premium vs regular and Una’s staff report on diesel is purely coincidental. The Staff Reports are prepared many weeks in advance, and that was the next one in line.
There have been occaisions when Ed Zotti (who selects the Staff Reports) has jumped the queue slightly to get some co-ordination with Cecil’s column – the most recent was that Mathochist’s guest Staff Report on Fibonacci numbers jumped the queue slightly to coincide wiht Cecil’s Report on the golden ratio phi
That didn’t happen in this case, it was sheer happenstance.
Cecil sometimes tackles the really big issues, and he sometimes wants to have an easy time of it, and so takes on a topic that doesn’t require much strain or effort.
I enjoyed the treatment of octane ratings but would like to add some detail that might be useful. The distinction between detonation and pre-ignition was pointed out in 1906 by Bertram Hopkinson and more fully investigated by Sir Harry Ricardo in the 1930’s and '40’s while pushing the performance limits of British aero engines for fighter aircraft.
As you point out, detonation occurs when all or part of the unburned mixture ignites due to heat and compression, leading to the well publicised “knock” or “ping”. However, the flame front may be initiated by either the normal spark OR pre-ignition from a hot spot in the cylinder. Pre-ignition (before spark ignition) may occur without detonation and hence go undetected. The result is increased cylinder wall and piston heat which can destroy the engine as the pre-ignition comes increasingly early in the cycle. The only immediate symptom is gradual loss of output power.
Detonation and the silent killer pre-ignition are separate processes which can occur in any combination.
Ref: The High Speed Internal-Combustion Engine, 4th ed, Sir Harry Ricardo, Blackie & Son Ltd.
Looking in my owner’s manual (well, okay it’s really my brother’s)
I find the following: “Your Ford is designed to operate on Regular
gasoline of not less than 98 octane.” (From the late sixties.)
I seem to remember a similar manual for a 1966 or so Mustang
that expected 102 octane gasoline. What the deal here? Has
there been some kind of deflation where the octane ratings of
decades past have slipped down? Is the 100 octane “Regular”
of my childhood equivalent of to nowaday’s 87 octane? What
should I put in my old cars? I’d have to buy gas at the airport to
get octane ratings in the triple digits.
Many months to a year ago I watched a history program about the Battle of Britain which claimed that English pilots had an advantage over the Germans because of the higher octane gasoline supplied by the United States. Since increased octane does not deliver more power, was the program correct that it was an advantage or was it flat out wrong?
I think, but am away from my library, that the issue was that the prop-engined planes could suffer from knock (especially if they used superchargers to boost their power and operate better at altitude) which was so bad as to either limit the altitude they flew, limit their boost - or even knock badly enough such that engine operation was impacted. A higher octane fuel would have allowed them to operate at higher levels of boost, and thus have more power and more altitude capability with fewer problems. But I won’t swear to that 100%.
With higher octane fuel you can design/optimize engines to run at temperatures/compression ratio’s that are more extreme than otherwise might be the case. A more powerful or higher performance engine may require higher octane fuel (or indeed suffer or trash itself on lower octane stuff) but the fuel is not the reason for the higher performance (higher octane enables rather than delivers higher performance). The Germans actually built a few super-performance engines designed to run on high octane but these weren’t pragmatic for the Luftwaffe as a whole given their fuel supply situation.
The US, in one of its earlier steps in embargoing Japan (pre WWII) cut off deliveries of aviation fuel, defined as octane numbers of 87 or more. As it turns out all of Japan’s engines were designed for (and perfectly content at) 86 octane. So that portion of the embargo had 0 military utility while still pissing off the Japanese.
Not flat out wrong. Higher octane allows you to design an engine
with higher compression ratios. The higher compression gets
more power out of the gasoline. That power is also in the lower
octane stuff (as The Perfect Master tells us) but vintage engines
couldn’t extract it without knocking themselves to bits.
And consider the increases in efficiency over the years. Cars
these days get substantially better mileage than previously. That
extra energy was wasted before. So you might say that those
German airplanes were burning more gas for the same amount of
go in exchange for octane tolerance. After all, they could burn the
high-test stuff with impunity while their more picky adversaries
wouldn’t tolerate the more common grades.
Even then, things are more complicated for airplanes than for
cars. Weight is more critical for flying machines so increasing
compression isn’t obviously more efficient. Higher pressures
need thicker metal. And reliability becomes an issue. Aircraft
engines spend a lot more of their lives at full throttle. And a
breakdown is a little more difficult to deal with when one can’t
just pull over. Where would you rather break a piston, over
water (like the Channel) or enemy territory? Neither sounds fun.
And then there is aerodynamics. A higher compression engine
might weigh more for the same horsepower but have a slimmer
outline. Sleeker cowlings mean less drag and more speed.
Then there is water vesus air cooling, blah blah blah…
Anyway, like I said, it gets complicated. Higher octane is rightly
associated with higher performance, even if the joules per liter
or gram isn’t any better.
“Octane” is not a precise scientific measurement. It essentially means, “This gasoline burns as though it were x% made up of the chemical octane, C[sub]8[/sub]H[sub]18[/sub], or, more specifically, isooctane, alias 2,2,4-trimethylpentane: (CH[sub]3[/sub])[sub]3[/sub]CCH[sub]2[/sub]CH(CH[sub]3[/sub])[sub]2[/sub].” An Octane Number of more than 100% means the “burns like” effect is even further along the scale than 100% isooctane would be. “Burns like”, of course, is not an entirely quantifiable attribute.
There were two different traditional ways of getting the Octane Number, called “Research Octane” and “Motor Octane”. Both involve running the fuel in a highly specified engine under highly specified conditions. With typical modern automotive fuel, the Motor Octane number will be about ten points lower than the Research Octane number. In much of the world, the Octane Number posted at the pump is the Research Octane Number, but, in the US, the “Anti-Knock Index”, which is the average of the two, has been displayed at the pump instead for the last two or three decades. So Octane Numbers from the 60s will be on a different scale.
More precisely, it means that it burns as though it were x% isooctane and (100-x)% n-heptane (the octane rating a mixture that is x% isooctane would vary all over the place depending on what the other (100-x)% is).
Note that at one point in time, there was a difference. Some 30 or so years ago, detergents were only added to Premium gas. My mechanic advised me to “once in a while run a tank of premium through the engine to help clean it out” and he swore by either Chevron or Mobil- and Mobil was slightly cheaper.
In the few months I’ve owned my new car, I’ve been putting in premium gas (93 (R+M/2)), because it has a turbo-charger. I already knew premium <> better gas in all cases, but premium should be used in high compression engines. If a turbo-charged engine isn’t high-compression, Idon’t know what is.
Yesterday, I decided to RTFM. According to VW, I should be using 91 (R+M/2). The only flavors availible to me are 93 or 89, so is it better to go up two points or down two points?