Percent of Ram Effect

Posted: June 18, 2012 in Induction Systems

An article from Mr. Steven P. Duray:

Percent of Ram Effect- The true useable amount of kinetic energy transfer to a pressure rise (boost). As a constant, 1 cubic foot of air at sea level is equal to 0.076 lbs.

PVEL = Velocity pressure in psi
p = air density in lb/cu ft
v = the car’s velocity in mph
PVEL = (p x v2) / 4311

This will help to calculate the effect of static head pressure on an intake tract.

EXAMPLE 1: Vehicle speed of 100mph, at sea level. What would the intake pressure increase be?

PVEL = (p x v2) / 4311
PVEL = (0.076 x 1002) / 4311
PVEL = 0.18 psi

Atmosphere = 14.7 psi, so at 100mph, using a system with a 100% perfect recovery rate, the intake tract would have a pressure of 14.88 psi.

NOTE: Static head pressure recovery rates can be affected by the design of the end of the inlet pipe. A straight cut pipe has a recovery rate of approximately 13%. A slightly flared pipe will have approximately an 85% recovery rate as long as the radius of the flare is 10% of the total diameter. A bell mouth end cap on a pipe will have approximately a 97% recovery rate as long as the radius of the bell mouth is 50% of the total diameter.

EXAMPLE 2: Let’s calculate the intake tract pressure rise using a straight cut pipe on example 1.

PVEL = (p x v2 x RREC) / 4311
PVEL = (0.076 x 1002 x .13) / 4311
PVEL = 0.023 psi

Atmosphere = 14.7 psi, so at 100mph, using a straight cut pipe to gather air, the intake tract would have a pressure of 14.723 psi. It is very clear that Recovery Rate (RREC) has a large effect on intake pressures at speed.

S.P. Duray 20120416

1. Richard Barber says:

I won’t lie, my head is spinning a little, but it does make sense. One thing I need to know is when I am ready to super charge my truck are you going to be willing to make the necessary calculations to allow my truck to give me an instant face lift?

Seriously, thanks for sharing bro.
Rick

• performancespecialties says:

Absolutely. Excellent excuse for me to take a road trip to the Carolinas.

• Steven P Duray says:

Thank you, sir. It may take a few times of reading and a bit of reflection, but its not as bad as it seems. Thank you for your post, it is much appreciated.