Every return to a restrictor plate track brings suggestions about how we might eliminate the restrictor plate. Restrictor plates serve the very necessary function of limiting car speeds at Daytona and Talladega so that the cars stay on the ground. The negative is that they remove throttle response. One suggestion from some readers that I hadn’t heard of before suggested that NASCAR could just change the rear-end gearing parameters to shift the power curve and reduce horsepower that way. Will that work?
The amount of horsepower an engine make depends on the rotation rate of the engine. The faster the engine runs, the more combustion events and the more power generated. This works up to a point, because if you rotate the engine really fast, you start having problems getting enough air into the engine and the power goes down. The graph below is for a typical unrestricted engine that makes its maximum horsepower around 9300 rpm.
In order to cut the horsepower back to what you’d need to run safely on a plate track, you would need to restrict the engine to run at about 450 hp – which would mean the engine would have to rotate at about 4500 rpm.
Looking at the curve above, it’s evident that the engine is designed to run at its peak horsepower. What dictates that curve? Cylinder displacement, engine configuration, head configuration, etc. But mostly NASCAR determines the curve because of the rear end gear rules.
NASCAR engines run up to about 10,000 rpm (revolutions per minute). Rpm is a unit that measures how fast something rotates. It’s like miles per hour, but miles per hour corresponds to a linear motion rather than a rotational motion. The minute hand on your clock, for example, makes one revolution every hour. The seconds hand makes one revolution per minute.
The circumference of a typical tires is around 88 inches. Every time the tire rotates once, the car moves 88 inches, so 1 tire rpm = 88 inches per minute. You can convert this into miles per hour. Since I chose a nice round number like 88 inches for the tire circumference, it works out to a really simple equivalence: 1 tire rpm = 1/12th of a mile per hour. This means that if you want to go 200 mph, the tires have to rotate at 2400 revolutions per minute.
The power curve above shows that the engine makes the most horsepower at 9300 rpm. This produces a problem: the engine is driving the car at 9300 rpm and the wheels are rotating at 2400 rpm. That’s why you have a transmission and a rear-end gear, as illustrated at right. The diagram shows the gear ratios for a Borg Warner MM6 manual transmission and a GU6 3.42 rear-end gear, as might be found in a Corvette. Note that NASCAR cars are not allowed to use any gear that increase the rotation rate between the engine and the wheels. No 5th or 6th gear, either. 1:1 is the best you are allowed — which means that the rotation rate coming out of the transmission is the same as the engine rotation rate.
At maximum speed, the transmission is using a 1:1 gear, so the only reduction occurs at the rear end gear. A 4:1 gear means that one gear makes four rotations for every one rotation the other gear makes. If the engine is rotating at 10,000 rpm, and it passes through a 4:1 rear-end gear, you have 2500 rpm at the tires (which is 208 mph).
The whole point of this discussion is to keep the cars at lower speeds so they stay on the ground. Let’s say you want to limit the cars to 190 mph – that requires the wheel rotate at 2280 rpm. We don’t want more than 450 hp, so the rear-end gear has to take the rotation rate from 4500 rpm to 2280 rpm, which means 4500/2280 = 1.97, so you need essentially a 2:1 rear-end gear. (Just for comparison, a typical rear-end gear is 3.3-3.9, depending on the track.)
So it is possible to gear the car down so that it simply doesn’t produce as much horsepower. It is a better solution than what we currently do?
Restrictor plates work by reducing the air coming into the engine, which means you can give the engine less gas and thus you produce less horsepower. Gearing down would reducing the horsepower by making the engine run in a much-less efficient range.
One consequence of a lower rpm is that you would have really back problems with knocking. Knocking happens when the air-fuel mixture auto-ignites (ignites before the spark plug fires). Knocking is much more likely at low engine speeds because the the combustion happens so much more slowly than in a fast-running engine.
Another consequence is that my engine design friends tell me that they can probably tweak an engine, within the rules, to produce more horsepower at 4500 rpm such that NASCAR would have to further change the gear and run the engines at 3500-4000 rpm, which exacerbates the knocking problem.
I wondered whether taking the engine speed down might increase throttle response, but none of the experts I spoke with thought that it would. The problem, they say, is that as long as cars are running at full out power, aerodynamics will dominate plate tracks. You’d have to decrease downforce and increase drag to really make a difference.
Finally, there’s an aesthetics issue. The sound of an engine changes with its frequency. If you went to Indy and we blindfolded you and asked you tell us whether the car on the track was a NASCAR racecar or an Indy car, it would be easy: Indy (and F1) cars sound like mosquitoes. They run at about twice the speed of a NASCAR engine. If you forced a NASCAR engine to race at 3500-4500 rpm, the car would sound like it was in pain – you’d get a low moan instead of the engine sound associated with 9000 rpm that we’ve all come to know and love.
If you were paying attention, you ought to be wondering why you couldn’t just run at very HIGH rpm – the power curve goes down on each side of the peak, so you could have the engine run at 13,000-14,000 rpm and output 450 horsepower. The problem on that side is that NASCAR initiated the gear rule so that teams wouldn’t have to deal with the incredible stress on engine parts that have to run at those very high speeds. High-speed engines would significantly increase the cost to teams – it would be cheaper in the end to just let them build a dedicated open (not restricted) plate track engine.
In conclusion, yes – gearing down would work in theory, but it would introduce its own unique problems that would offset the advantage.