Saab newbie

Yes. The numbers are calculated differently here, we average the number you use (the theoretical) with a measured number which is always much lower. So the octane listed here differs from yours, a lot, for exactly the same gasoline.

Dave Hinz

Yup, "low lead 100". Obviously not for cars with a cat.

Correction:

There hasn't been a 2-stroke SAAB *manufactured* since... well, a long freekin time.

;-)

-Fred W

That's right. The excuse is that octane requirement is lower, because of less dense air. And that is true, for normally aspirated engines. Unfortunately, it does not apply to turbos, as far as I have understood.

BR,Z

Why wouldn't it apply to turbos?

Turbos make their own air denser...

-- MH '72 97 '77 96 '78 95 '79 96 '87 900T8

Did one have to mix the fuel and oil for the 2 stroke Saabs or did they use a separate oil tank?

Yup - would depend on the turbo control as in whether it's relative to atmospheric presssure or an absolute figure in bars.

Graham

Yes, a turbo charger is indeed a compressor...

However, I still fail to see the logic in lower octane at altitude is okay for naturally aspirated engines but not for turbo charged ones (suggested by Zon and I guess now Graham).

Octane is a measurement of how high the ignition point of gasoline is... nothing more. Altitude is not an "excuse" to use lower octane. I'm not a refiner but I doubt octane is directly related to the cost of refining the oil. Because of the altitude, the air pressure is lower and therefore things heat up at a lower temp -- where I live at

6,200' water boils at 209 deg F and hence gasoline ignites at a lower temp too -- which leads to the 85 - 91 ratings.

Turbos are actually more an asset at high altitude than sea level... and this is mutually exclusive of octane level. For example, why do you think commuter aircraft are turbo charged? Because they do make already thin air denser for combustion... but again, it is a stretch of deductive reasoning to say because things get hotter at lower pressures that octane is therefore a derivative pressure.

There's still something you're missing here. Turbochargers effectively raise the compression ratio when under boost, higher compression engines require higher octane ratings to prevent detonation due to heated air from the compression, in a turbocharged engine the intake air temperature is higher by nature and it gets hotter still when compressed in the cylinder. This remains fairly constant regardless of atmospheric pressure, while in a N/A engine the compression is effectively lowered with the thinner air. Turbos are more susceptible to detonation under any circumstances and are not affected much by altitude, N/A engines are less prone to detonation at higher altitudes so lower octane is permissable. That said I have yet to come across a production N/A engine that wouldn't run just fine on plain old

87 octane, you can't advance the timing as far and won't get as much power but in engines with a knock sensor there's no fiddling, just less power produced on lower grade fuel.

Not missing anything and I think you failed elementary physics... I understand a turbo charger is driven by exhaust gasses and those exhaust gasses are indeed hot. But when a gas is compressed, any gas, it gets colder by nature... not hotter!

Um, no, when you compress a gas it gets hotter, when it expands it gets cooler. How do you suppose a refrigerator works? The compressor compresses the refrigerant until it liquifies and runs through the hot condenser which is cooled by ambient air, and then passes through an orifice tube where it it evaporates into a lower pressure gasseous state and absorbs heat out of the compartment. For that matter about a Diesel engine? Are you trying to say Diesel fuel spontaneously combusts when you freeze it?

And I did very well in physics all the way up through college thank you.

Because the turbo will boost until it gets the air density it wants, regardless of local atmospheric pressure. With naturally aspirated, you get what you get and that's all there is. So, a turbo with an air mass meter will be even more dramatically better than a naturally aspirated model, at altitude.

Dave Hinz

What an odd way to participate in a group discussion; by insulting someone who is trying to explain something to you.

It depends. The early ones (1949-1962) are all "mixers", one quart of oil to 8 gallons of gasoline. Starting in 1963, oil injection became an option (in the "GT-850" engine). Separate tank which holds about 5 quarts of oil (with a sight-glass on the side), driven by the crank, and injects the oil onto the main bearings which then splash-lubricates the rest and is burned as normal in a 2-stroke. Not sure if mixers continued to be sold to the end (was the Shrike engine a mixer, or injected? I don't have one).

While an injected engine is more convenient, it's definately fun to watch reaction from people at gas stations when you pour a quart of oil into the gastank before filling. My usual line is "Well, it's never had an oil change, so I just dump it in here and things take care of themselves."

Nothing better in the winter than a 2-stroke Saab 96. Not even my c900.

Dave Hinz

One hot, bright summer afternoon (British summer, maybe 30C with not very dry air) I saw a small (10 litre?) propane tank builders had just been using to heat tar. The lower half of the tank was frosty, neatly marking the part still containing liquid gas.

And anyone who has used a bicycle pump can attest to how warm it becomes, quite quickly.

The fuel was mixed at the filling station. The car had only one tank.

/Bengt

This sentence is ambiguous, but I read it as; lower octane requirement does not apply to turbos, and I consider that a correct statement; a turbo makes a high fuel/air mix pressure and consequently high RON to prevent engine knock.

Be careful now....

No it doesn't.... What do you think the intercooler is for? When a gas expands, it cools. That's causing the puff of mist you see when you open a bottle of a carbonated drink; the carbon dioxide in the bottle is under pressure and it when it expands to atmospheric pressure, it draws heat from the surrounding air, cooling it down locally, so moisture from the air condensates in a puff of mist. The same thing happens when you open the valve on a butane or LPG tank; the liquid gas expands, drawing so much heat from its surroundings that ice will form on the valve.

To prevent knock when the turbo comes on, the compression ratio in a turbocharged engine is only 1:8 or so (vs. 1:10+ in a NA engine) . The higher the MON/RON the higher the APC computer will allow the turbo pressure to be.

-- MH '72 97 '77 96 '78 95 '79 96 '87 900T8

Because the liquid gas (under high pressure) is expanding to near atmospheric pressure and drawing heat from its surroundings, cooling it down to -0C, freezing the moisture in the air. Same things happens in carburetors, and LPG evaporators, these sometimes freeze up. The gas boils, turns from liquid phase to gas phase, like boiling water. Boiling needs heat.

Yes, but that's also because of friction in the pump valves etc.

-- MH '72 97 '77 96 '78 95 '79 96 '87 900T8