Diesel vs gas......................

Do you know how much of an effeciency increase a properly installed turbo will get for a gasser? I was reading a "wheels" article a while ago that claimed that almost any gasser can gain around a 20% jump in efficiency when a turbo is installed, but that sounds high to me.

Depends what they mean by efficiency. Volumetric efficiency of a normally aspirated SI engine will be about 80-90%. A turbo will bring that up to at least 100%. So that number would seem valid.

If they're talking about fuel consumption, you wouldn't get anything (assuming you're constrained to running at stoich with a modern, aftertreatment-equipped engine). The higher air flow would have to matched with the same relative amount of fuel.

However, if you were an engine designer and were using a turbo to get similar horsepower from smaller displacement, than you'd get the same fuel consumption at high load, high speed regimes of the turbo and probably a 10% reduction in fuel consumption elsewhere due to the smaller displacement.

Again, I only deal with production type engines that have to go into factory vehicles so I'm not as well versed on the ins and outs of racers and other vehicles without emissions and fuel economy contraints. So maybe someone else will prove me uninformed. That would be cool.