turbo questions...

[SaintlyCharBoy]

well, i've been playing with ideas in my head (scary) and i was wondering why it seemed that noone plays with relatively high compression low boost applications

it would seem that the compression would help to buffer the lag to full boost, even if it were relatively small

i never plan on breaking the low 300 hp mark, as such i don't think i'll ever need to see the crazy numbers that PSI and Boosted are running

just curious what y'all think/ know about the idea

yes i have Maximum boost, and have read it
how are we ("we" being normal humans not living in the ultimate automotive shop) supposed to be able to find the volumetric efficiency and exhaust gas flow rate of our motors?

just curious
-Sean

[boosteds14]

well the reason is because even with a high compression NA motor, the combustion is so hot. hot spots will occur which could cause very bad things. yes u can do it and some ppl do it but there is a limit of boost u can run.

[DSC]

From what I've read, you will be able to make more power by increaseing boost than increasing compression.  Given 5psi on a low and high compression engine, sure the high one will make more power...but the low one will be able to take more boost and a 7psi low c/r motor will probably make more power (and with less stress ont he motor) than a 5psi high c/r motor.

Keep in mind i'm just making up numbers...

[luke91]

It is entirely possible to make high power with the combination of high compression AND high boost, provided every other factor (ignition, cam, and valve timing, fuel mixture etc.) is nearly perfect.  It is especially possible with Methanol, or Nitro as a fuel.  In a gasoline engine, volumetric efficiency has to be perfect, and there is little room for error.    
It's not practical, though.  DSC is right.  You'll gain more power by replacing compression with boost, essentially.  The combination of high compression and high boost is useful for a purpose-built racecar to get that "extra edge", but not for a street car.  
Especially in a street car, it's just better to keep compression low, and boost high.  
It can be done, though.  
--luke

[Jeff240sx]

Physics on this tells all.
When you increase pressure of the combustion chamber via piston height, you bring the piston closer to the spark, and cause hot spots like boosted brought up.  That's a bad thing.  And I don't know so much about compression, so I wont dwell on it for fear of giving misinformation...
But I do know boost!
If you get the series from SCC called "Suck, Squish, Bang, Boom"  you will be told how to find volumetric efficiency.  It was either Sept 2000 or 2001, but I'm thinking 2000.
Anyway... when you increase boost, you are adding virtual displacement to your engine, based on 1 atmosphere pressure making stock displacement.  That means that at 14.7psi, standard air pressure, your engine sucks in 2.4 liters of air.  When you boost, you add pressure, which can be converted to "bar," with 1 bar bieng 14.7psi.  I am running 4.35 psi, or .3 bar boost.  This means that I am running the standard 2.4 liters of air sucking plus another .3, or 30% of the standard displacement.  This happens because the engine is squeezing much more air than originally possible into the cylinders, and the overall ammount of air in there equates to my motor bieng a 3.2 liter engine.  And boosted, at his 24psi or whatever it was is the equivalent of a 6.34 liter engine.  This is why boost over compression is better.  Nowhere else can you make that kind of power.
-Jeff

[luke91]

Two more points:

The more air is compressed, the hotter it gets.  When you have forced induction, you are already heating the air inside the cylinders more than a NA engine is designed to handle.  Add compression to that, and one of trwo things is going to happen:
Either the increased heat in each cylinder will cause detonation, burn as piston, melt the headgasket, etc.
or
The sheer force will blow the head gasket, crack the cylinder wall, etc.  
More often than not, it's the heat that kills an engine.  

Also:
High comperssion pistons are more often "dome" shaped.  This forces more air around the piston, and stresses the piston rings.  
A low compression piston is flat on top, distributing cylinder pressure more evenly.  
This is why most high compression engines burn a little oil.  
Examples:
Honda Prelude, S2000, type-R.  

--luke