Re: Cheap Horsepower-Ram air (fwd)

[Dave Weiss <dzweiss@worldnet.att.net>]

Orin Eman <orin@wolfenet.com> wrote:

> From the diy_efi list...  there is a thread going on about Ram air
> there too.  Note the last paragraph!

> Orin.

>> "Ram air" just uses the Dynamic Pressure at the front of the vehicle 
>> to aid engine breathing.  The formula is simple, 1/2 the fluid density
>> times the velocity squared (in suitable units).   This works out to
>> about .04 psi boost at 50 mph, .17 psi at 100 mph, and .7 psi at 200
>> mph; pretty feeble boost (although not as feeble as the
>> previously-mentioned cooling fan), but what the hey it's nearly free. 

Geez, a sanity check-- I hate it when that happens! :)

Similar formula as for a pitot tube-- based on kinetic energy change of
air pushed by the car.  Density, of course, isn't constant here, but
integrating dP/Rho doesn't change the results much under these
conditions.

In theory, the pressure increase of a ram should be _less_ than this,
because the air going into the scoop doesn't achieve the velocity of the
car at that point-- it's flowing through the aperture.  There is flow
from the adjacent higher-pressure regions (where the nose is solid) to
the scoop, though.

A sophomore- or junior-level engineering student should know this stuff
cold.  Apparently I've been doing control systems and software too long!

Dave Weiss
'91 V8 5-spd
'93 90 CS UnQ