Re: Subaru fuel economy - Saab General Bulletin Board - Saabnet.com


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Re: Subaru fuel economy
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Posted by Dean (more from Dean) on Thu, 5 May 2016 08:02:05 Share Post by Email
In Reply to: Subaru fuel economy, Erik919kt, Tue, 3 May 2016 19:06:46
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When driving a turbocharged engine, it is rarely on boost. But to accommodate boost the compression needs to be lower. That decreases efficiency. Saab engineers were doing research on variable compression engines to try to get the best of both worlds, but the methods tested seemed to be impossible to realize. So you are mostly driving around in a lower compression engine that reduces MPGs and have to spend more money on premium fuel to support the brief moments that you are on boost. Premium fuel has less combustion energy than lower octane fuels. Economy is more than MPGs. The Saab turbo engines did have great weight and MPG advantages over larger displacement naturally aspirated engines of equivalent performance.

Many advances have occurred in engine technology in the last few years with modified Atkins cycles, variable separate intake and exhaust camshaft timing and direct injection [DI]. Unfortunately, DI engines get some soot backflow in the intake ports and they get carbon fouled and air flow can be restricted. Fort injection keeps that area clean, something that was happening that was not recognized at the time. These technologies were once restricted to expensive vehicles, but are now very mainstream.

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