# Eliminating Restrictor Plates?

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.

1. Paul03 says:

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.) I’m missing something. We don’t want to limit the hp to 450. Keep the same hp rpm and reduce the ratio to where 9000 RPM=200 mph.

2. Marc says:

Doesn’t this just lower the max speed, determined by RMP and gear ratios, and not the actual potential HP the engine makes? And thus the engine still will be accelerating at 9k+ RPM since they havent hit an aero limit or limit. Expect a lot of engines blowing up as engine builders push the RPM limit of these engines. Also expect the field to split as one engine builder has a higher rev limit than someone else and say Hendrick engines run away from the field.

Also does anyone know the RPM where NASCAR engines valve float and if there is valve/cylinder head tolerence?

How about narrowing the tires to the point the cars can’t hold the speed flat out? Anyway…a restrictor plate is the cheapest/easiest way to get the speeds down.

3. Jack says:

With the advent of fuel injection, can’t they just electronically limit the top speed and do away with the plate entirely? This would let the drivers have the acceleration and throttle response they want. I know this ios done on some cars and motorcycles, so why won’t this work for NASCAR?

4. Dennis McGough says:

OK Moody sent me here, I read it, but like Paul I still don’t get it. It’s a simple ratio. Change the ratio(s) so that the engine has to turn more revolutions to create the same number of revolutions to the tires = lower top end speed.

I want the same amount of HP. I want them to have acceleration again. I want the driver to have to worry about being able to get the power to the ground to accelerate and make that pass – which currently takes him, his newest best friend behind him pushing, and maybe a few others in the freight train.

5. Sorry about the delay – I’ve been in Idaho. Here’s where I think I’m not being clear enough:
9300 rpm is the peak in the HP curve – you’d like the engine to be in the peak range as much as possible. If you 9300 rpm to equal to 200 mph via gearing, there is still no reason a driver can’t run at a higher rpm – if they step on the gas and run at 10,0000 rpm, they would be well over 200 mph. The problem is that you have to limit the maximum speed possible to 200 mph. The maximum speed (for a given track) is determined by the horsepower (assuming fixed banking, turn radius, etc.) of the the engine. The only safe way of keeping the cars on the track is lowering the horsepower the engine is putting out.
Let me know if that helps or if I should write something longer. Again, sorry for the delay in answering – I was at a conference all week and didn’t have reliable Internet! DLP

6. Dennis McGough says:

I guarantee if they never shift into 4th gear, they will not run 200MPH on any track given current engines. As your curve shows, the power drops off after a certain point. That’s essentially what we’re talking about here – lowering the “4th” gear ratio into something more like “3.5th” gear.

Now, can they build the engines to turn more RPM? Sure. But it seems to me that modifying the power curve of an engine that currently peaks at 9K-10K RPM to move the peak out to 12K-13K is a seriously expensive endeavor and will take some period of time. It won’t be instantaneous. During that time, as speeds rise, adjust the max ratio downwards to keep speeds under the desired limit.

7. Marc says:

The only way NASCAR is going to keep the speeds down by using the engines as the limiter is by using a restrictor place. Everything else will be too expensive. Changing rear end ratios so that 9300 RPM = 200 mph still means the engines are making the power to run faster. And the engine builders will push it so that they will run at higher RPMs if the engine RPM will be the limit. Which means spending a ton of money in new areas of engine development.(Sacrificing HP for RPM). Spending godly amounts of money is something NASCAR has done a lot to try and limit. Car of Tomorrow as an example giving everyone the same bodies.

No engine builder wants to have their engines running at 9300 RPM all day in pack racing. They cringe at Michigan the way it is now with hanging RPMs, let alone the entire race.

If the goal is to keep speeds down without pack racing going the engine route will be expensive. So we’re left with aero drag but the cars would have be gigantic boxes to limit 850 HP under 200 mph.

But skinny tires…let them go 220 down the straights and make them slow down at the corners like any other track.

• Briane Howland says:

Putting skinny tires on a 3500 pound race car that travels 220 mph (with restrictor plates) down the straightaway into a corner on Super Speedway track would destroy the cars ability to have sufficient traction going through every corner, stability, handling capability, ability to apply HP to the ground, tire durability, overall safety, and IMO would be the most dangerous scenario of any kind trying to fix this issue. The difference in the amount of mph between 280 mph (using unrestricted engines on super Speedways) and the mph that would be a safe mph in order to race through the corner would be such a massive difference it would be absolute carnage in every corner of racing. Anybody talking about putting skinny tires on a 900+ HP 3500 pound NASCAR racecar isn’t using any common sense at all.