So if increasing compression makes for more power why did Mazda reduce the compression ratio for the diesel?
Although they did increase the ratio for the petrol. Can someone explain.
I'll try.
Lowering the compression ratio on the diesel allowed them to make the engine lighter, produce less NOX emissions (no urea after-treatment) and it allowed for a higher readline.
It also allows for more fuel to burn closer to the top of the cylinder stroke, improving the amount of work done by the fuel.
http://www.mazda.com/en/innovation/technology/skyactiv/skyactiv-d/
The power improvement for compression increase also isn't nearly as much as CX-sum wrote.
Going from 17:1 to 14:1 you'll only see a ~4% drop in power. Not hard to make up for that with more boost.
You might see a 4% increase in power for 1 point compression increase when you step up from 7.0:1 to 8:1 in a turbo engine, but once you hit higher compression ratios the increase gets lower.