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Re: question about engine longevity
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Posted by Barry Harvey [Email] (more from Barry Harvey) on Wed, 5 Aug 2009 08:50:36 Share Post by Email
In Reply to: Re: question about engine longevity (long), xhawksaab, Tue, 4 Aug 2009 21:41:37
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Let's think about what wears in an engine: the rings, crankshaft journals and bearings, connecting rod pins, and pistons.

The rings mostly wear proportionately to the total number of strokes, i.e. total miles. If the piston gets very hot as in sustained high-power events, they can weaken and lose springiness and compression. But a high-power engine typically has oil spraying from below that keeps the piston edges cool enough. Anyway, it's pretty hard to maintain full power for any amount of time, so thermal capacity of the metals keep things relatively cool during bursts of power.

Crankshaft journals only wear appreciably when the oil is contaminated with particles or lacking altogether. Proper maintenance keeps oil clean. Lacking oil happens mostly when hard cornering clears a low oil level away from the sump pick-up tube. It could also happen when lugging the engine at too low an rpm, which allows low oil pressure with heavy torque. Our turbos don't provide much boost at such low rpm's. For the most part it's the crankshaft's bearing that take damage and wear, being faced with sacrificial soft metals.

Connecting rod pins normally don't wear much, but they can be broken by overly high rpm's or detonation of the mixture. Our engines are rev-limited to fairly low rpms. The engine control electronics effectively prevent almost all detonation. Perhaps lugging the engine might damage the pins.

Finally, the piston crowns can be broken by sustained high power combustion or detonation. At 250HP this doesn't happen frequently, but can above this power.

So, all in all, the Saab factory-tuned engines should last the same for any tuning. Lifespan is about oil and miles.

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