BMEP vs. I see... Clearly

[QSHIPQ <QSHIPQ@aol.com>]

Regarding Effective and Static Compression Ratios:

Cold air gives more POWER from a given CR, but doesn't change it in absolute
pressure terms.  The easiest way to look at this is from a pounds of air vs
pounds of fuel (what most manufacturers look at anyhow, Fuel Injectors's are
rated in pounds of fuel per hour).  So, with a 7.5:1 CR and 10psi of boost the
Effective CR is 12.6:1, an absolute number.  TEMPERATURE is irrelevent in
these terms.  For HP, Power or Output, Temperature is VERY relevent.  An
engine with a 7.5:1 CR and 10psi of COLD air will have more HP/power/Output
than an engine with 7.5:1 CR and 10psi of hot air. Cold air means density, or
more Pounds of Air per Cubic Foot (so more pounds of fuel can be combusted),
than a Cubic Foot of Hot air.  26psi of boost pressure at the manifold is
claimed here by one.  That doesn't change the Effective CR, even with a
'huyege' massive IC.  The "rule of thumb" (exceptions so noted folks), is that
for every 11F degree drop in inlet temp, a 1% gain in HP can be expected.  So
B4B, an IC vs no IC shows some serious HP gains, a 75% efficient IC vs a 85%
efficient IC doesn't math out quite as well.

IC Efficiencies:
I suppose one could argue that at 26psi boost pressure, the stock IC might not
be adequate to handle the cooling needs of the turbo at 2.79bar output.  Yet
most folks have up to 2 bar mods tops (MC Effective CR of 15.1:1 hot or cold).
Using the stock IC core single pass B4B is a good and sound modification.
Larger turbo/more boost?  The stock IC core will not be very efficient, even
in single pass trim.  More from the pressure drop (Flow) across the core than
anything else.

My .02

Scott Justusson
'87 5ktqwRS2
'86 5ktqw
'84 Urq