This is on the mind of almost every Porsche enthusiast, whether you’re looking to purchase a car or are a weekend track warrior guilty of shifting your car from 4th to 3rd instead of 4th to 5th. We’d like to dig into one of the reasons why avoiding high rpms is so important and the reasons to avoid it.
Piston weight.
Understanding the concept of compression and exhaust stroke is important to understanding why high rpm happens. When a piston travels on a compression stroke, both the intake and exhaust valves are closed, which creates compression. Think of this as a cushion for the piston. When a piston travels on an exhaust stroke, the exhaust valve is open to evacuate the exhaust from the controlled burn and the intake valve is slightly open (valve overlap). When the exhaust valve is open and the intake valve is on overlap, the compression cushion goes away. Without that compression cushion, the piston wants to leave the engine. This increases the weight of the piston based on inertial force. This force exponentially increases with rpm as the crankshaft and connecting rod work to pull the piston back after expelling the exhaust from the cylinder.
For example, in an M96.03 engine with the factory set rev limiter of 7300rpm:
· With a rough piston weight of 1.9lbs and a stroke of 3.07 inches, at 7300rpm the piston weight reaches 4,413lbs. At 7300rpm, this happens 121 times a second.
· The range for over-rev weights can be much higher. For example, at 9000rpm, the piston weight reaches a whopping 6,709lbs. At 9000rpm, this happens 150 times every second. Let that sink in.
· Disclaimer. There are a few ways to calculate over-rev weights. However, regardless of how precise the numbers are, there are still thousands of pounds of inertial force on internal components.
Why should this concern you?
Such force leads to internal components being put under stresses that they were not designed for which could lead to stretching the connecting rod bolts, breaking a connecting rod or even cracking or breaking the piston. This is why it is so important to pay attention to engine rpms. You should have your car scanned for over revs pre-purchase, think twice about flashy tunes that increase the factory rev limiter, and last but not least, avoid the 4th to 3rd 'money shift'.
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