I'm considering getting a turbocharger for a my car, 95 civic sedan d15b7 engine. and then I had a thought.... ....How will my mileage be? worse, better, same? And whether or not it will pass smog. Will it not, or will I have to adjust the psi before I smog it to get it to. Not sure which turbo, still looking, but holding off depending on the answers to my question.
-Thanks!

 

if you have to smog it, go with greddy. it is smog legal. you should be able to pass without changing anything aroud. as for mileage. how hard are you going to drive it? it really depends. a turbo has the potential to dramatically help your mileage. it raises the volumetric efficiency of the engine, letting it make more power with less displacement. a turbo is basically free power. when an engine is running, it uses about 30% of its power that its making to propel the car, the rest is being wasted as heat. i think its about 40% is lost through the cooling system, and the other 30% is lost as heated exhaust gases. so if you can use some of that wasted energy to drive a pump to pump more air into the engine, then its pretty much free power. now, if you are constantly mashing the accelerator, then the engine starts injecting WAY more fuel in to compensate for the extra air you have coming in. so obviously, it can hurt mileage. ive known people who hate the mileage on their turbo cars, and some who say they fill up half as much as before, so it can go either way. i'd say, as a general rule, it will hurt your aggressive street mileage, and help your cruising freeway mileage.

 

I'm not sure if you are interchanging engine efficiency with volumetric efficiency. They are not the same. In most engines nowadays, VE% is around 80% give or take. This is a measure of how much of the engine is filled with combustibles at every cycle. For example, we have a 4 cylinder 1.6L engine.. that means we have 0.4L per cylinder and at say 80% VE that means we only have 0.32L of fuel and air in the cylinder at every cycle.

A turbo increases VE% by forcing more combustibles into the same space. It is typical for turbos to get effective VE% above 100%. Air takes up a lot of the space in the combustion chamber. A turbo compresses the air making it take up less space allowing more combustibles to fit in the same space.

A turbo will not increase gas mileage because it hasn't really done anything substantial to increase the ENGINE efficiency.