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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




Actually, filling the cylinders with boost increases the effective compression ratio.

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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.




If you are actually in high boost your looking to make power and fuel economy is clearly a non-issue at this point. A large naturally aspirated engine at WOT making the same power under the same conditions will also have poor economy too.

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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.




The same applies to any naturally aspirated engine under WOT.

I have run small engines and big engines, I did what I'm told can't be done (towed with turbo 4 engine for 50,000 miles!) and if I had a choice I'd take a well built small displacement engine that was turbocharged.

Excellent mpg and more power is just the flick of a switch away.

My turbocharged 4 got better mpg while towing a car than my current truck gets empty.

That's real world.

The other thing you have to calculate is, is it worth it?

Slapping a turbo on an existing package isn't going to magically get you better mpg or car makers would be all over it. And how long do you have to drive the thing before you make your money back from the mpg savings to just payoff the turbo setup?