Engine Limits - Psi, CFM, or horsepower?


A lot of people like to define an engine's limitations in terms of the maximum boost pressure it can sustain, but that's really not the right way to do it.

What happens in most of the cases when these limits are exceeded, is you either blow up bottom end stuff (ringlangs, for example) or you lift the head and/or blow up the head gasket. All of these problems are directly caused by too much pressure during combustion. However, maximum cylinder pressure is NOT determined solely by boost pressure, so we shouldn't use boost pressure to define the limits of a motor.

Another popular way to define the limits of a motor, are with a horsepower limit. While this is closer to being related to the true cause of motor problem, cylinder pressure, it still is not perfect. Horsepower is the net result of cylinder pressure over time, meaning that 300 horsepower at 5000 rpm is NOT the same peak cylinder pressure as 300 horsepower at 9000 rpm. Because of this, you can increase power by increasing top-end breathing capacity, without getting any closer to the limitations of the bottom end.

Cylinder pressure is actually most directly related to torque, because torque comes from the push on the top of the piston. In terms of what we can measure, torque is the most representative of cylinder pressure. Also, rpm is not an influence in the relation between torque and cylinder pressure. In general, if torque goes up, cylinder pressure goes up. (Interesting tidbit: thus, cylinder pressure, motor stress and load, and knock propensity are highest at the torque peak, which is also the VE peak).

In the end, I think that defining a motor's limits in general terms of horsepower, or maximum MASS flow, is the way to go. Torque is a better measurement, but nobody ever has payed much attention to it so switching over would be difficult.

Also, everyone knows that tuning plays a big part in all of this, because the quality of the tune plays a VERY big role in determining not only the maximum cylinder pressure, and also how it happens. Very sharp spikes in pressure can be more harmful to the motor than a nice smooth curve up to the same pressure. Also, knock/detonation is one of the primary killers of motors, because it causes excessive cylinder pressure oscillations:

 

Cylinder Pressure

 

Kyle Tarry 2004