Subaru Turbo Engines

Interesting! Do you have a turbo boost guage? (If so, was it factory-installed, or after market.) If not, how did you get the measurements. Being a numbers freak, I'm starting to regret not getting the turbo gauge.

Niels

Here in the Northeast US it's 87, 89, 93 (or occasionally 94 -- mostly Sunoco) R+M/2 (average of "Research" and "Motor" methods) IIRC R+M/2 is generally equiv to 91 RON (Research Octane Number) I believe "100 octane racing gas" is 100 RON, not R+M/2 so about 97 or 98 R+M/2 perhaps?

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The point of higher octane isn't to improve mileage so much as to improve power output (though a slight mileage increase is theoretically possible). Higher octane means increased resistance to detonation -- which is the gas / air mixture exploding before the spark plug tells it to. Such explosions, which are also called "knock" because of the sound made when they occur, are

*very* hard on the engine because they occur while the piston is still making its upstroke. The force of this early explosion tries very forcefully to push the piston back down from whence it came.

Of course, the entire purpose of putting an explosive mixture into the combustion chamber is to push the piston down, but the timing and valvetrain are designed so that this happens later in the piston's stroke. All of the stress-bearing components inside the engine -- the connecting rods and pins, the crank, and the crank bearings -- are designed to accommodate this explosion at just the right point in the piston's stroke. When the explosion occurs early, these components all have to continue pushing the piston up through its stroke, but against a tremendous resistance. An occasional detonation won't destroy an engine. But over time, it will definitely take a toll. (In addition to the use of higher octane, cooling the intake charge also helps to reduce detonation and therefore increase power output. This is the reason for the water spray on the STi's intercooler.)

While any car with a knock sensor should theoretically see a benefit from higher octane, it generally grants the most benefit to those engines with very high internal cylinder pressures. Those would be cars with high compression ratios or some sort of compressor on the intake (be it a turbo or a directly-driven supercharger).

Those with turbocharged engines don't need to panic if they've been running low octane gas. All modern engines employ knock sensors that can do any manner of things to stop knock once its detected. The only affect of running low octane is that your engine's computer might not be allowing maximum power output.

I run 87 octane unleaded in all of my cars except the Audi, which gets the highest octane available at whatever pump I happen to be using to fill up. My new turbo-charged Subaru will also get the highest available octane.

- Greg Reed

Just adding a few things.

If I recall, it's a combination of increased pressure in the fuel/air mixture combined with residual heat that can cause the early explosion. Forced induction or higher compression ratios will cause an increase in pressure. I think carbon deposits cause a slight increase in compression ratio, and a larger increase in residual heat - i.e. they get hot (and may prevent heat transfer?).

The other thing is that some people think that higher octane rating means the fuel burns slower. I've heard that high octane racing gasoline actually burns faster, but resists spontaneous explosion better than ordinary gasoline.

And it's been a while, but there was something about a "second flame front". The explosion from the spark causes the an increase in pressure before the flame spreads, and the fuel/air mixture may explode in a separate part of the combustion chamber. Thus two explosions crash into each other - making the knock that we hear. The ideal is a single explosion gradually and evenly spreading from the spark.

I found some good articles, but this one from Car & Driver is easy enough to read (+ another decent article):

The knock sensor(s) doesn't per se do anything except alert the ECU that detonation (or something that sounded like detonation to the sensor) has occurred. The ECU will respond to its programming by retarding the engine timing to prevent recurrence of detonation.

This will normally be all fine and dandy, but if the engine is designed to run on 91 or 93, and it's got a tankful of 87 (which is more prone to preignition than 91 or 93, given the same conditions), the ECU *may* run out of adjustment range on the timing to prevent recurrence of detonation, depending on the conditions.

At that point, the ECU programming says, "we gotta stop this preignition!! cool the intake charge, with more gas!", and the injectors spray more and more fuel into the cylinders. See where this is going?

Excess fuel begins to wash the cylinder bores clean of oil...a really bad thing. Google (detonation timing bore wash) and you'll see pages of stuff.

The concept of "It says 93, but it'll run fine on

87" is risky with today's engines.

If you have to fill up somewhere on 87 in your WRX or STi, do what you have to do but don't push the engine...stay off the boost as much as possible and get good gas in there, ASAP.

Steve

Steve,

Thanks for the explanation. My dealer had mentioned that in a pinch, use the cheap stuff if you ABSOLUTELY have to. He never told me 'why'.

Has anyone ever been to a gas station where premium was unavailable? NEVER have I seen such thing.

Niels

I've seen it occasionally...usually a confluence of events, like finding an open station in a rural area off the interstate late at nite, on a big holiday weekend.

Steve

Wow!! I didn't realize just how much of this stuff I'd forgotten. Thanks for the additional details.

- Greg Reed

to add to steve's comments,

the subie rex ecu has a "high det mode" (aka mode B, mode 2) in which if ignition advance cannot be retarded enough to prevent knock, more serious measures are taken.. dumping a LOT of fuel, and restricting boost to wastegate pressure. hopefully your ecu will never have to do this.

ken

You mean they're sold out or never had it in the first place?

Good thread guys, excellent information - Thanks. I think this also applies to those with EJ25 DOHC engines that may be wondering what to do to keep from blowing their head gaskets. The lesson being that better gas would keep engines with weaker gaskets from breaking sooner.

Sold out.

Australia evidently as some of the worst fuel in the world - baring third world countries of course. We have 98 RON in the big cities, but there is none between Adelaide (where I live) and Melbourne - a trip of over 1000 Kilometres. There is also none between Adelaide and Darwin - 3000 Klm's.

Rod

Eek.

According to the following web page (which doesn't claim to be exhaustive), there is 98 available between Wellington and Auckland in at least Levin, Palmerston North, Taupo, and Hamilton.

As long as you can manage at least 200 km (250?) on a tankful there should be no problem with polluting your engine with that inferior 96 octane stuff.

-- Bruce

Rod, you should really tell the oil companies that. They really do give a shit - honest!