Re: Back pressure (long)

[Matt Zajac <MTZajac@beloit.com>]

HI Marc
Thank you very much for the explanation. To see if I understand it correctly...
The pressure ratio (inlet/exhoust) has to remain the same throughout the rpm band. At low rpm naturally aspirated engine would have low inlet pressure thus has to have greater backpressure to retain proper pressure ratio. As rpm rises inlet pressure rises thus exhoust backpressure should drop...



MATT Z. (not Lori :) )

>>> <VW155mph@aol.com> 01/04 7:42 AM >>>
Hi Lori, if you're still out there...

To try and keep it simple, the amount of torque an engine produces is related 
to the pressure ratio at that rpm.  The ratio equals the pressure of the 
exhaust gases (p1) over the pressure of the inlet air (p2).  An efficient 
internal combustion engine would have a ratio of 1 (p1/p2 = 1) throughout the 
rpm band.  Unfortunately, engines do not have a static pressure ratio.

In order to produce an engine that will build torque down low, an exhaust 
with a good amount of back pressure will be needed to keep the ratio at one.  
This will in turn cause the car to suffer on the high end because as the 
inlet pressure builds, so will the exhaust to a point where there is to much 
back pressure to run efficiently.  

In order to maximize power on the high end, a larger exhaust with a lower 
back pressure will be needed.  In this instance, initial inlet pressure will 
be higher than the exhaust pressure, thus causing the car to run below the 
efficient ratio.  As the motor accelerates, it will approach 1, i.e. build 
more power as the rpms increase.

Vehicles with forced induction need to have an exhaust with lower back 
pressure so they can build power efficiently since inlet air pressure is 
increased by the pump.  Naturally aspirated vehicles would run inefficiently 
with such an exhaust because it would take a while before it would reach 1.  
On the other hand, if the engine is able to push the rpms, power would be 
available on the high end as the ratio approaches 1.  And since HP is 
dependent on torque and rpms, if you are able to push a similar torque to a 
higher rpm, the higher your horsepower.

Marc A. Brown
Speed Etc. Motorsports
Club GTI International (Northeast Region)
"Cogito Ergo Sum"
1995 Jetta GLX (well modded)
1998 New Beetle 2.0 (still stock)