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The turbo engine, and turbo engines in general have lower compression ratios to cope with the compression air under boost. The boost pressure also raises the temperature which makes things more knock prone as well.
The octane requirements for the engine are driven by the basic design of the engine and its operating parameters such as boost and temperature etc. But there are also other factors. Deposits in the combustion chamber can create hot spots which act as glow plugs which can lead to preignition and knocking. Deposits in the injectors can create poor spray patterns and lean mixtures in some cylinders which also promote knocking... and hard starting.
High octane fuels are formulated with more additives to fight such deposits. Lower octane fuels will create more deposits and increase the engines acquired octane demand. But with the N900 turbo engines you will also need to use fuel system cleaners, even with the higest octane pump gas availble in you market. Techron Conentrate is widely available in the US and is effective. I suggest using it every 8000 miles or 3 times per year. Often the lower volitility summer fuels will create injector deposits which make for hard starting with the first cool weather in the fall where the summer fuel is still being provided. So this is a good time to use the Techron.
You also need the correct spark plugs. Use NGK BCPR6ES11'2 gapped to .039. The DI system will nor work right with many other plugs or 'trick' plugs. So don't experiment. Non resistor plugs are thought to risk damage to the DI system.
So it is all about knocking conditions. The issue is more than fuel as you can see. The DI detects conditions that will promote knock and will avoid knocking by fuel enrichment, less agressive timing and failing that will limit the boost pressure. So lower octane fuel will reduce power and increase engine deposits. The octane does not have to be super high. Some folks report that they seem to get good power and boost on mig grade fuel. But with modifications that increase boost should be used with the higher octane fuels. This is especially true when a manual boost controller (MBC) has been installed which removed the ECU's failsafe mechanism of reducing boost pressure.
But in general, if one did not care about engine power, the system will run fine on low octane regular fuel. The ECU and DI on the Saabs is probably the most adaptable and refined system available.
Consider using a full syntheic oil, Mobil One 10W30 suggested, and better filters. Castrol Syn-fake(tm) is not considered an option.
How many miles?
Trany?
Any mods as purchased?
95SET, 140000 miles
posted by 207.43.19...
, Wed, 20 Mar 2002 11:13:38
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