Generally the maximum boost, absolute pressure, is fixed, i.e., a spring on a wastegate holds the wastegate closed until enough molecules push it open. Therefore, a wastegate set to 15psi at sea level would open at 29.7 psi absolute at sea level. Therefore it would open at 29.7 psi absolute at an altitude where, say, atmospheric pressure was, say 8.7psi. At that point you could argue you were getting 21psi boost and you would be right, but the engine is still only seeing 29.7 absolute, same as at sea level. The compressor, however, would be spinning much faster to achieve the higher pressure ratio (29.7/8.7 vs 29.7/14.7) and would therefore be heating up the air much more...either your intercooler could handle the extra heat or the engine would ping and the electronics would dump the extra boost, retard timing, or both. Most likely, the turbo would be incapable of reaching the same boost levels at 10-12K feet that it could at sea level...most turbo systems are not designed to achieve maximum boost at 10-12K.

In olden days the turbos on piston fighter aircraft were designed to achieve rated boost at 30-40K feet...they were WAY oversized at sea level.

There are some control systems that work on differential pressure, i.e., they give you for example 10psi boost, period. Such a system would give lower absolute pressure at altitude compared to sea level.

If the Saab is edging out a Subie, the Subie has a smaller turbo or is closer to its altitude limit.




posted by 68.55.18...

• my buddy claimes one thing , but I differ, Help solve., das, Sun, 29 Jun 2008 12:14:35
  • Re: my buddy claimes one thing , but I differ, Help solve., lms, Mon, 30 Jun 2008 05:59:45
  • Your friend and you are both correct, just wrong words, Nahum Cook, Sun, 29 Jun 2008 14:24:34
  • Issue is gauge pressure vs absolute., SJohnB, Sun, 29 Jun 2008 12:35:09 <-- Viewing This Message

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