For an engine that is boosted out of the factory, what are typical maximum and average boost values? Are they conservative in the 5-6 PSI range or do they get more aggressive getting up to 10-15 PSI? I'm more interested in what OEMs are doing with undersized engines to make them driveable versus more sporty performance enhancements.
There's a wide variation, but back in the '90's it was clustered in the 10-12psi range for a typical 4-cyl turbocharged performance model. Nowadays 16-18psi is not abnormal for high performance factory cars. Of course, you'll find some economy cars in the 5-6psi range, where they're using boost to let a little tiny fuel-efficient engine not be completely miserable to drive. :-)
This question doesn't provide enough information for us to answer it well as originally written. Some of the important factors determining what boost level is used include:
- Base compression rate of the engine (see this question for related discussion).
- Octane of the fuel to reduce the risk of detonation (see point 1).
- The temperature and humidity of the ambient air.
- The temperature of the air after it passes through the intake path.
- The application and related requirements such as reliability and longevity (a race car does not live as long between rebuilds as my daily driver).
A more specific question would help us address your particular points of interest. Which vehicle are you interested in? Are you thinking of upping the boost? What application to you plan to put this vehicle to?
I find Corky Bell's book to be an excellent (and entertaining) reference for background information on all of the above.
Bob Cross's answer.
And those are just the factors that are easy to quantify! There's also the combustion chambers' individual propensity to knock determined by geometry with spooky attributes like squish, tumble and swirl to consider. Then there is cycle to cycle variability and the effect of carbon deposits over the life of the engine, which will be different for each individual engine, let alone different models. I think reading any of engineering literature on knock would dissuade anyone from trying to give numbers for such an open ended question, even if a displacement and compression ratio was specified. It's much more complicated than not exceeding a temperature threshold.
I think reading something like Heywood's chapter on "Combustion in Spark Ignition Engines" gives a lot of insight into these factors. I'm assuming, of course, that you want the deeper insight.