14 Degree Octane?

No, if it's not pinging, you're fine. But pay extra attention in very hot weather, which increases the engine's propensity to ping.

This is what I was told:

I have an '03 and that it has an "Anti-knock system." As explained to me the system will automatically retard the timing to prevent pre-ignition pinging. So if you use a lower octane the system will retard the timing and prevent the pinging, but you will lose the power you gained by advancing the timing.

That is what I was told, so that is what I'm passing along.

right on... the first hot weather in June usually reminds me to top up the tank with a higher octane. but, other than in June, July & August, 87octane is just fine... no pinging.

Pete Breemhaar Georgetown, Ontario

94BRG

No, if it's not pinging, you're fine. But pay extra attention in very hot weather, which increases the engine's propensity to ping.

Correct, but if Diane has set her timing to 14 BTDC, she presumably has a 1997 or earlier, with fixed base timing, hardwired ignition maps, and no knock sensor.

Those with 1999 or newer models should use whatever octane their owner's manuals recommend.

You mean, if you do not hear knock, I assume.

I think the common wisdom is that if you do not have audible knock accelerating at high load, (hill, high gear), you should be OK on the highway, where engine noise might mask the knock.

Leon

Perhaps slightly off topic, but my '03 and I live at 6200 feet in elevation. The manual recommends premium fuel, which, around here, is 91 octane. Our regular grade fuel is 85 and mid-grade is 87 or 88. I note that at lower elevations, 87 octane is the lowest you can buy ("regular - unleaded").

I am assuming that at higher elevations, the engine tends to run richer and therefore, needs less octane. My question is, can I "get away" with running my '03 on 87-or-88-octane fuel, locally a.k.a "Mid-grade?"

Alan MiniMi the '03 SE

Think for a minute or two-- The Miata has an altitude sensor. The computer controls the amount of fuel. The O2 sensor tells the computer how rich or lean things are. The computer "normally" sets a an A/F ratio of about 14.7:1 unless a loading condition calls for a rich or lean mixture. Less air in at a throttle (plate) setting means less fuel in when all is said and done. Less fuel and air in mean less power out. The throttle plate would need to be open more at altitude than at sea level to allow a given amount of air(and fuel) to produce the same amount of power out. Would not the engine need the same fuel octane? If not why not? (The effective compression is a bit lower?) (Remember we had to open the throttle plate farther to let in more air.)

(I'll admit I have an additional set of variables to deal with on my 99, due to a slight modification that produces up to 9-10 PSI of boost above outside air pressure.) (Hill? What Hill?)

OOOOhhhhh ! Chuck ! I loved that reply and you didn't even twist the knife when you were done! :-) I live at about sea-level and mild temps so I don't get into that too much, but that was a very nice reply.

Bruce RED '91 (with no "huffer")

Bingo. Now, explain why we get better gas milage at higher elevations, despite the engine working harder with higher throttle openings. Disregard turbocharged cars (nuthin' personal).

Lanny, Better miles at altitude, ??? less drag ???? :-)

Bruce RED '91 Hee, hee, hee.

For the same reason that smoking is safer outside an oxygen tent than within. There is less maximum oxygen concentration at the top of the stroke.

However, I found this summer that I had increased knock with the highest available octane even at altitudes of 6,000 ft.

Probably the drop in sold octane is more than that warranted by the pressure difference. Some regions (Missouri comes to mind) sell a maximum of 91[2]+ethanol even along the interstates at altitudes of about 1000 ft.

My problems might also arise from the fact that the temperature effect of a supercharger is not taken into account. Oxygen pressure is just one variable in knock. The temperature effect would also apply to normally aspirated cars.

You are not even going to see a proportional drop in pressure unless you drop the wastegate pressure when you go up.

The engine does *not* work harder. It has *less* power.

And higher throttle openings are favorable. It takes energy to create a vacuum. Which is why best fuel consumption at a given speed is typically in the highest gear.

Leon

It certainly works harder to climb mountain passes than it does in, say, Florida. Coasting on the downhills won't make up for it, either, when braking is required to stay on the road.

That makes sense. What about open-loop mode, though, which makes Miatas run very rich at full throttle?

To be honest, I don't remember if the % of oxygen changes with altitude or not. There is less oxygen and the other gasses per cubic foot of air. I would think that the lower pressure allows the fuel to vaporize more easily. Perhaps this may in part account for the better mileage reported by some.

As to torque and other mysterious things. It seems that an engine developing energy to move the car at a lower RPM has a bit longer to extract the energy (and heat) from the firing stroke than the same engine running at a higher rpm.

If you take a turbocharged aircraft engine (Piper Arrow for instance) and try to run the engine at maximum power out, (The prop setting controls the engine RPM) you will overheat the engine after a few minutes of operation. The additional heat will cause the engine to detonate. (Don't ask, 'cause I ain't gonna tell) The manifold pressure is quite high, about two x sea level or slightly more as I remember. And this is on high octane fuel!

Cruise at altitude sets the eng>

True. Do you have solid data that fuel consumption is less under such conditions than driving on a normal road at sea level, under, say, the same average speed? (Or whatever else is held constant.)

I am somewhat at a loss why this is relevant to the issue at hand?

In any case, in neither situation I would think full throttle is used most of the time.

Leon

It does. When you go up 90 kilometers. ;) I meant amount of oxygen per unit volume, not amount per unit mass of air.

Don't know. Sounds possible.

I would think the longer time scale would help most in achieving better mixing. The heat per unit time should be pretty much the same if the produced power and efficiency are the same. I would assume viscous effects in energy extraction would be negligible.

Only my own calculations under said conditions. Flogging mountain passes, I've gotten over 30 mpg. My normal average is around 28.

Hah, that's easy for you to say, Eaton-breath (old Johnny Carson allusion). My aspiration is of the normal persuasion. One climb out of a canyon required 100% throttle at 6500 rpm in 3rd gear for about 3 miles, without lifting once. We don't go to the mountains to sightsee, you know.

I alluded to an experience on my recent road trip that I couldn't figure out but was grateful for.

2 Miatas, a 97 and a 92, filled up with gas in Vernal Utah. Drove over two 9,000 ft steep, twisty mountain passes (Trucks, Motor Homes). Went through Grand Junction Colorado and ended in Moab Utah. Were over 4,000 feet elevation the whole time and got 35 mpg with both cars. Mine (97) was 341 miles on 8.8 gallons. The last 30 miles or so were nerve wracking, I must say. Both cars normally live near sea level and average 28-30 mpg. I like to fill up at 250 on the trip odo.

Certainly not scientific data, but interesting in that both cars got the same bump in mpg.

Steve Whisman

OK, since so far no one seems to offer a logical explanation of this obviously unexpected and maybe true phenomenon, let me be the first one to hazard a guess. :))

My guess is fuel cut-off. On my car, the engine computer cuts off the fuel completely as soon as you come back even a bit on the throttle (according to my dash-mounted WBO display). Coasting downhill certainly qualifies for that. You cannot get better than 0 mpg.

Of course, the car must work harder when going uphill, but the additional fuel required for the additional power output is not as much as you would expect. The reason is that with the throttle partially closed, the pistons must fight the throttle for air on the intake stroke. Opening the throttle more reduces this effort, making the engine more efficient. The thinner air also increases throttle opening.

The two together then offer a net gain.

The higher rpms in well driven twisty parts use more fuel, (either you need to fill the cylinder fully with air and fuel on each combustion stroke or fight the throttle,) but even there the thinner air helps increase engine efficiency. (While reducing its peak power, of course.)

Besides, I believe that while the truly twisty parts stick in one's memory best, they tend to be a fairly minor part of any trip long enough to get a reasonably accurate fuel estimate.

Of course, my car has a link ECU, but I am conjecturing that it is the same on the stock 92 and 97. I seem to have a memory that it had something to do with avoiding a particular pollutant.

Leon