The 24 and 14 reportedly succumbed to valve spring failures. The 48’s engine went south while leading — with only six laps remaining. Jimmie Johnson drove the car up to the hauler and then 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
The engine rpm remains pretty much constant throughout a lap at Michigan. The television numbers show you that most engines go from about 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 a hundred miles at 8000 rpm and running those same hundred miles at 3000 rpm. The important question is: How many times a part is called upon to do it’s job.
The rotating camshaft raises and lowers the engine valves (one intake and one exhaust). The crankshaft drives the camshaft. When an engine runs at 9000 rpm, the crankshaft makes nine thousand rotations every minute — that’s 150 rotations every second.
Why RPMS Matter
The camshaft makes one rotation for every two rotations of the crankshaft in a four-stroke engine. At 9000 rpm, the camshaft runs at 4500 rpm. That translates to 75 openings and closings of the intake (or exhaust) valve every second. In turn, the valve spring compresses and expands 75 times each second.
If the engine runs half as fast, we’re looking at 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. Next week at Atlanta, you’ll see more high, sustained speeds, as you will in Charlotte and Texas.
The Hendrick failures may just have been 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. They’ll figure it out — (if they haven’t already).
By the Numbers
Let’s do a quick calculation. The drivers ran 2 hours, 46 minutes and 44 seconds to complete 201 laps. With 35 laps of caution, (35/201=)17.4% of the race was run under yellow, which means means 82.6% of the race was run under green.
2 hours, 46 minutes and 44 seconds is 10,004 seconds.
From this calculation drivers ran 8,263 seconds under green. I assume an average of 8000 rpm, which is 66.6 revolutions of the camshaft every second, which means that the average valve and valve spring complete half a million up-and-down cycles.
Jimmie Johnson ran a top happy hour lap time 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.
NOTE: I refreshed this blog post on 8/17/2021 to clean up the writing.