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I gotta dis-agree with you, Feets. You will never see bsfc numbers in the .4s with any turbo engine(with the exception of direct injected engines). It's not all that uncommon to see .45 in a well thought out n/a engine.
The myth of economy with a turbo engine comes from, as previously stated, a smaller engine that is capable of making the hp of a larger engine. If you were to use that smaller engine in boost all the time though, you would be no further ahead in economy. It's the time out of boost with a smaller engine that you see your gains in economy.



Listen to this man people, he knows what he's talking about.

There's no such thing as turbocharged economy for spark ignition engines. The only way it pans out is as Trendz mentioned, you match its output to a larger N/A engine and compare BSFC under light loads (4 cylinder turbo to 8 cylinder N/A for example). It only pans out because the larger engines have more friction and pumping losses.

There are a multi-tude of things you get dinged for in efficiency on a turbo engine.

1. You run less compression: we all know compression = power, but it also SIGNIFICANTLY impacts efficiency, in fact it's one of the largest and most well documented players in efficiency. More compression = more efficiency

2. You run less timing: while under light load (vacuum or low boost) the timing is the same-ish as an N/A engine, when under heavy load (high boost) timing is retarded 10-30 degrees easily, obviously having large impact on efficiency.

3. You run way rich: again while not under boost or under low boost maybe at stoich (14.7:1), but under heavy boost EASILY 10 to 11.5:1. You can't burn more fuel than air so that extra fuel is just going out the exhaust.

Plus all the other things Trendz covered such as exhaust restrictions etc.

So in short, if you want to build an economical v8, build a high compression N/A motor with low friction (think smallblock).