Turbo question

[HarryD]

I have always wondered why the base Compression Ratio for a Turbo is less than a Normally Aspirated type engine. Anyone know?

[Tim Walsh]

this is off the top of my head but I think I remember it being something along the lines of b/c the charge is greater in a turbo engine there's a higher chance of pinging. Therefore they bump down the compression to combat the pinging (predetonation)

[thabaer]

Yup, pressure+heat=boom (w/o needing a spark).

Turbodiesels heat the air charge above ignition temps without any fuel in the chamber, then use high-pressure fuel injection to squirt in the fuel at the appropriate time.

Detonation from heat and pressure created by compression in the cylinder make it difficult to supercharge a high CR gas engine. A lower ratio produces less heating due to compressing the air-fuel charge and lets the supercharged intake system stuff in the bigger AF without having it ignite before its time. More advanced control systems are allowing higher base ratios but you still have to deal with the additional heat and mass of a pre-compressed charge getting even hotter when squeezed in the cylinder.

The factory CR in my '91 200q20v turbo is 9.3:1; but when people put bigger turbos/injectors on them, they will often drop the cylinder ratio when going over 500hp. Some aftermarket kits also increase CR but you need to pay special attention to charge temps and mixture when doing that to avoid having the heat from the extra crunch burn holes in pistons.

Here's a recent PP thread about CRs and efficiency:
The effects of higher CR