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. 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. 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. 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. 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. 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. 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.

    • 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.
      The great idea of building a bulletproof Racing Engine with a very specific set of cylinder heads, cam, intake, throttle body, fuel injectors, tune, rear end gear ratio etc. designed​ to make a maximum of 450 HP at the engines peak HP efficiency in the powerband I believe would be the best idea and would save NASCAR teams a lot of money in the process​. Fixing repairing and spending the money on replacing everything damaged during the process of every single wreck that happens due to this very specific problem would be a massive amount of money saved​. NASCAR would have to change the rear end gear ratio by certain amount but not a drastic amount like what was talked about when using the 900+ HP unrestricted engine, and just using a (lower numerically) higher gear ratio in the rear end. If NASCAR used 2 different V8 engines of the same size and shape then we could actually watch races at those tracks that used to run restrictor plates, and actually have cars passing each other, and have normal racing characteristics, and capabilities​. We would be able to watch exciting racing again on those tracks, and there would be a lot less wrecks. NASCAR drivers hate restrictor plate racing because without the proper throttle response when you hit the loud pedal the car doesn’t respond when it’s supposed to. That’s defeating one of the most important aspects of a racing engine. This issue really isn’t rocket science and I don’t know why Nascar has not just created two different engines for these cars to be used​ at Super Speedways, and then the other at all other tracks. It has a lot of similarities of the incompetent crazy rule change of reducing NHRA’s tracks length for Top Fuel dragsters, and Top Fuel funny cars to a 1000 ft instead of a drag strips traditional length of 1320 ft or 1/4 mile that has always been used since the inception of NHRA drag racing since the 1950s. NHRA could reduce Top Fuel dragster and Top Fuel funny car engines HP very easily to reduce their mph at the finish line​ in order to make it safe enough for them to have enough shut down area on any of the tracks that they currently race at even the ones with a very short shut down area (New Jersey’s Englishtown Raceway). Reducing the amount of nitromethane from 85%, and 15% alcohol which I am guesstimating to something like 60% nitromethane and 40% alcohol and or reducing the blower gear step up ratio which word make the supercharger on the engine add less power to the engine. Those are two very easy things to do which would only require a different tune for the engine which NHRA crew chief could do very easily. NHRA absolutely ruined the two fastest Pro classes in NHRA drag racing IMO no right-minded Racing Organization would ever think of reducing any race tracks length in order to fix a problem of the car is going too fast at the finish line that’s just insanity IMO. That would be like NASCAR saying we’re going to chop off a third of Daytona Speedway’s​ length because the cars are going too fast and build an entire new Racetrack​ that’s only this many feet long or miles long. Yes NHRA was able to do this without building new race tracks but that’s beside the point. They completely destroyed NHRA drag racing history and tradition of racing 1320 feet or a quarter mile who are the two fastest classes of drag racing on planet Earth which is complete Insanity to me. Because now NHRA has totally screwed themselves because in the last 9 years they have allowed those two pro classes to continue to create more HP over the years that now at 1000 FT the same two classes of race cars are now running the same mph they were running 9 years ago that they considered to be unsafe, and the decision was made to the track for those two classes reduced. So what are they going to do now say Top Fuel dragsters, and Top Fuel funny cars can only race to an 1/8th mile? NHRA lost so many spectators, and fans which lost them millions of dollars by making that stupid rule change which really upset me because I have been drag racing my entire life and watched NHRA drag racing almost my entire life until that rule change. If anyone reading this a drag racing enthusiasts the elapsed time mph a vehicle at a quarter mile is extremely important and the only that matters in this form of Motorsports so that’s why it’s such a crazy roll change. 1000ft drag racing means nothing to me the elapsed time of a 1000 ft drag race means absolutely nothing to me when I visually see the numbers on the scoreboards I have no clue what they mean they don’t relate to anything in drag racing and have not related to anything to drag racing since the 1950s other than the last 9 years. It’s horrifying to think that NHRA employees who were in charge of these decisions are that mentally incompetent. Over 60 years of NHRA E.T., and MPH records have been basically obliterated, and forgotten because of this stupid rule change for the two quickest, and fastest classes of any form of automotive racing in the entire world. I think NHRA should go out of business, and be shut down for good. Or they should return to traditional quarter mile drag racing and just reduce the HP of those two classes of race cars to a lower amount which would make them safe to race and then they wouldn’t be going too fast where the scenario would be would be they would not have enough shutdown area. The death of Scott Kalitta at Englishtown was supposedly the reason for this rule change because of it’s short shutdown area. But this was seen coming long before that, and could have been prevented if the NHRA simply policed the team’s ability to make more HP as technology advanced. It would have been very easy to lower the HP of at that time approximately 8000 HP nitromethane V8 engines to something in the range of 6000 HP in order for it to be safe for these race cars to have enough room to slow down and stop in the shutdown area of any track they race on. These same engines are now making 11000 HP due to the advancement of technology that NHRA has not been policing so now they’re running the same mph approximately 335 mph at 1000 ft which nine years ago was deemed unsafe. When the rule change was implemented to 1000 feet those two classes of race cars​ were running approximately 25 mph slower at that point in the track. NHRA’s incompetency has allowed these vehicles to increase speed once again to the same speed they were running 9 years ago that they deemed unsafe so now it’s time for another rule change LOL it’s almost too hard to believe that NHRA is that incompetent. So now NHRA has a enormous problem to fix that will make their last rule change look even more insane or they may redeem themselves​ using comma sense by going back to the 1/4 mile drag racing for every single class of racing in the NHRA organization with engines making less HP for their 2 fastest​ classes of racecars. Obviously nobody that works at NHRA or is in charge at NHRA has any common sense or intelligence. Reducing 2000hp from a top fuel engine is very easy to do it’s not difficult in any way, and that cannot be disputed. And it would not decrease the fans experience or decrease the racing experience for the drivers in any way. Top Fuel engines didn’t always make 11000 horsepower! And it has always been exciting spectacular racing even back when they were only making 4 or 5 thousand HP until they made this ridiculous rule change of chopping off part of the track and putting the finish line 320 feet closer to the starting line which basically nullifies 60 years of NHRA drag racing records. That’s the most ridiculous thing I’ve ever seen a Racing Organization do in my life. I hope NASCAR can create a fix to restrictor plate racing very soon and at that point they will stop purging spectators, and fans from there very important revenue stream. For them I guess it is rocket science for the people in charge at NASCAR. But in reality building a bulletproof 450 HP racing engine is very easy, and it would operate at peak efficiency, and operability. That doesn’t sound like a hard problem to solve at all. But apparently those in charge at NASCAR aren’t that bright.

      • What a ridiculously long rant when completely ignoring the context of not costing engine builders millions of dollars by designing a new engine type. Not the skinny tires at the front end of a dragster. But like 1cm-1in skinnier than the current tire. Not 1cm wide completely. That would be retarded. WITH COST IN MIND…thats an option.

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