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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.

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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. :-)

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Actually, back in the days when I was considering boosting my Integra, 1/2 bar (7 psi) was considered to be quite high boost considering the 10:1 compression ratio. –  Bob Cross Mar 13 '12 at 23:06
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For addingg a turbo to a factory NA engine, yes. 5-6psi is typical for those applications. However, factory turbo engines back in the early Integra days normally came with ~8:1 compression (7.8 was a particularly popular value) and hence supported the 10-12psi just fine. Lately that's been jumping up to 8.5:1-9:1 in factory turbo setups along with higher boost as engine design and controls have gotten better. –  Brian Knoblauch Mar 14 '12 at 13:09
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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:

  1. Base compression rate of the engine (see this question for related discussion).
  2. Octane of the fuel to reduce the risk of detonation (see point 1).
  3. The temperature and humidity of the ambient air.
  4. The temperature of the air after it passes through the intake path.
  5. 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.

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I've added some detail to my original post to address some of your questions –  wesanyer Mar 14 '12 at 1:07
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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.

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I've looked through the mentioned section in Heywood, and even read a good portion of the section on knock. I'm not looking for any more insight than that, i'd just like to know broadly what range or pressures engines are boosted to. Is 15 psi an upper limit, and is that usually reserved for performance vehicles? Or are greater boost values common? –  wesanyer Mar 14 '12 at 3:04
    
Fair enough. I think your best option is to compile a list of vehicles that fit your undersized engine category and see how much boost they run. Or even better find someone with experience equivalent to that survey. Unfortunately, that person isn't me. –  kahbou Mar 14 '12 at 3:23
    
@wesanyer Actually, can you give a couple of examples in that category of cars? I'm at a loss to think of any. Probably because I find ~1.3litre N/A engines with variable valve timing to be very drivable, although I'll admit that sporty driving often requires double-clutching/heel-toe into first. I blame the engineers for trying to stop people from accidentally selecting first. –  kahbou Mar 14 '12 at 3:29
    
That's the thing, I don't know any particular car models to reference. I do know from speaking to some Ford and GM engineers that both these OEMs are moving towards undersized vehicles with boosting, in order to meet tougher CAFE standards. I don't know of any that are currently on the market though. Are most stock boosted motors strictly boosted for performance? –  wesanyer Mar 14 '12 at 12:03
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