The Hendrick engine shop had four failures at Michigan. The 24 and the 14 reportedly both had valve spring failures. The worst was the 48, whose engine went south while leading with only six laps remaining. Jimmie Johnson drove the car up to the hauler and walked back to his motorcoach with his helmet on, not talking to reporters.
I don’t blame him, especially when you realize how close he got before the motor let go.
High, Sustained RPM
Michigan is one of the tracks where the speed at which the motor rotates stays constant throughout an entire lap. Watching the numbers from the television, most motors changed from only 7800 to 8500 rpm (revolutions per minute) throughout a lap.
Number of laps, or even miles are not the best way to gauge engine use because there is a huge difference between running at 8000 rpm and running at 3000 rpm. What’s important is how many times a part is called upon to do it’s job.
The valves (one intake and one exhaust) are raised and lowered by the rotations of the camshaft (as shown above). The camshaft is driven by the crankshaft. When we say an engine is running at 9000 rpm, we mean that the crankshaft makes nine thousand rotations every minute – or 150 rotations every second.
Here’s the critical part: The camshaft makes one rotation for every two rotations of the crankshaft in a four-stroke engine. At 9000 rpm, the camshaft is running at 4500 rpm, which translates to 75 openings and closing of the intake (or exhaust) valve every second. This means that the valve spring compresses and expands 75 times each second.
This is a linear phenomenon. If the engine runs half as fast, each of these things happens half as many (37.5) times each second. The faster the motor runs, the more movement, the more rubbing of parts and the more opportunity for pieces to break.
Watch the numbers this week at Bristol – you’ll see a much larger difference in speeds as the drivers slow down through the corners and accelerate through the straightaways. Even more importantly, watch the changes in engine speed coming up next week at Atlanta, where you’re going to see similar high, sustained speeds. The same issues will be in play for Charlotte and Texas. This may just have been a case of a box of sub-optimal valve springs, or the engine shop may have been trying a more aggressive setup in preparation for similar track in the Chase. I’m not worried – they’ll get it figured out (if they haven’t already).
By the Numbers
Let’s do a quick calculation. The race time was 2 hours, 46 minutes and 44 seconds to run 201 laps. There were 35 laps of caution, so (35/201=)17.4% of the race was run under caution and 82.6% of the race was run under green.
2 hours, 46 minutes and 44 seconds is 10,004 seconds. 82.6% of that is 8,263 seconds that were run under green. If we take an average of 8000 rpm, which is 66.6 revolutions of the camshaft every second, the average valve and valve spring went through half a million up-and-down cycles.
Jimmie Johnson ran a top happy hour lap of 36.323 seconds. Assuming an average of 8000 rpm, each lap at that speed adds another 2,421 cycles of the valve spring. Six laps means he was short 14,526 out of over a half-million cycles. Think about sixteen valves and valve springs that make well over a million (including practices) successful executions and come up short by a few tens of thousands.
No wonder Johnson didn’t want to talk to the press.