ENGINE RPM - Singing like a soprano in a four-part chorus

Engine RPM is a hot topic when the discussion turns to front engine top fuel dragsters. It is common to hear of numbers ranging from 9500 to over 10,000 RPM being tossed around. Those are startling high numbers for large cubic inch supercharged engines, and even more so when you compare them to the 2200-2400 RPM your daily driver registers on your way to work each day. Let’s take a closer look at RPM in general and try to put some perspective on the RPM being recorded by the AA/Fuel Dragsters.

RPM is an abbreviation for Revolutions per Minute. It is a standard reference point of the total number of revolutions the crankshaft of the engine would rotate, if held at the observed level, for a period of 60 seconds. However, just to make things a little confusing, the engines in drag race cars do not operate at a constant RPM, and certainly not for a full minute.

In the course of a run the engine in an AA/Fuel Dragster will see a wide range of RPM. When it is idling at the starting line the RPM will be in the 2400-2600 range. When the driver steps on the throttle the explosive sound you hear is actually the engine jumping up to near 8000 RPM. This change from idle to the new RPM occurs in less than 3/10 of a second. That is why it sounds more like an explosion than a simple rise in RPM.

Top Fuel Dragsters do not have a transmission. They utilize a ‘high gear-only’ direct drive. Because of this they must initially slip the clutch a small amount in order to help move the car from a standing start without taxing the engine too heavily. As the clutch begins to engage it will pull the engine RPM down approximately 400-600 RPM from its high point. All of this happens in the first second and a half of the run.

As the car gains speed, and the clutch slips decreases, the engine RPM begins to climb. On a typical 250 mile per hour run the engine speed will peak at around 9500 to 10,000 RPM. Why the wide variation? It is because of tire slippage. These cars are limited by rules to the size of tire they can use, and the size of wings that can be installed to generate downforce to help traction. The tires are actually spinning as the cars cross the finish line. The amount the tires are slipping dictate the variation in engine RPM at similar speeds.

Now that you understand the range of RPM in which the engine in an AA/Fuel Dragster operates during a run, let do a little math and look at RPM from a different point of view. We will use the data from an actual 5.85 second, 250 MPH run with a maximum RPM of 9800 to illustrate our point.

If you were to add up the RPM recorded at each sampled increment during the run, the average RPM between the time the driver stepped on the throttle, until he lifted, would be 8281 (2717 min / 9800 max). However, if we look at just the time from when the front wheels triggered the lights at the starting line to start the clocks, until they stopped the clocks at the finish line beams, the average RPM would be 8441 (7432 min / 9754 max). Why the higher average number? Because the engine had that explosive moment to jump up in RPM before the clutch engaged enough to move the front wheels. In essence, the engine got a bit of a head start.

Now here comes something you might not have expected. If we use that 8441 average RPM number we can calculate that the crankshaft in this engine only rotated 823 times in the 5.85 seconds between the starting line and the finish line. Now that number doesn’t sound quite as scary or radical as those larger RPM numbers, does it? But how about if we tell you that the crankshaft was rotating at an average rate of 141 times per second, or that when it was at maximum RPM it was rotating at a rate of 162 times per second? Those numbers start sounding a little spooky again, don’t they? An 80 pound crankshaft, with a full compliment of rods and pistons hung on it, spinning over at that speed does stretch the limit of our ability to comprehension.

Granted these numbers are somewhat extreme, but they do occur each time a front engine top fuel car makes a run. Hopefully this little example with give you a better appreciation of what the crew chiefs have accomplished, and will now make it a little easier for you to understand the next time you hear someone say “She was running over ten thousand in the lights”.

Perception is all in how you process the numbers.

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