Is the Tapered Spacer Rule Fair?

Combustion is the chemical reaction that converts gasoline into motion.  If you remember back to balancing equations in high school chemistry, that often tedious exercise is entirely due to the fact that chemical reactions are just plain picky.

The octane molecule – one of the common molecules in gasoline – combines with oxygen according to the following chemical reaction.

2 C _{2}H _{18}+25O _2 \longrightarrow 18 H_{2}O+16CO_{2}+\text{energy}

In English, this says that exactly two octane molecules combine with exactly twenty-five oxygen molecules to make eighteen water molecules and sixteen carbon dioxide molecules, plus the energy that is used to make the car move.  Each chemical reaction contributes a tiny bit of energy.

Power is how fast you make energy.  If you want to increase the power an engine outputs, you have a couple options.  One way is to make energy more quickly.   NASCAR engines run at 9,000 rpm (revolutions per minute).  A normal passenger car runs around 3,000 rpm.  The NASCAR engine is making energy three times faster than a passenger car engine just because it is running faster.

Another possibility for getting more power is to have more chemical reactions inside the cylinder, which means putting more air-fuel mixture into the engine through the carburetor for each cycle of the engine.  As long as you maintain the appropriate ratio of air to fuel, the more of the mixture you get into the cylinder, the more energy you’ll get out of the cylinder.

This is where the restrictor plates and tapered spacers come in.  If you know how to make an engine put out more horsepower, you also know how to make it put out less. Mandating lower engine speeds is more complicated than decreasing how much air/fuel mixture enters the engine manifold.  Restrictor plates and tapered spacers are both positioned between the carburetor (where the air and fuel mix) and the intake manifold (which directs the mixture to the different cylinders.

A restrictor plate is a thin plate is four holes punched into it.  The four holes correspond to the four barrels of the Holley carburetor that all cars are required to run.  The carburetor venturis are about 1.6” in diameter.  A restrictor plate with a hole of about an inch diameter cuts engine horsepower from about 850 hp to roughly 450 hp because less fuel/air mixture gets into the engine. Restrictor plates are used at Daytona and Talladega only, where the tracks are so long and banked that the cars can easily enter the regime where they become airborne.

There is no debating the fact that Toyotas(and specifically Joe Gibbs Racing Toyotas) have been dominant in the Nationwide series this year, having won 14 of 21 races.  NASCAR tested ten Nationwide engines after the Chicago race. NASCAR Now reported the maximum horsepower results for the top five on their Thursday show. Kudos to ESPN for specifically noting that they were reporting the max horsepower. I am, however, just a wee bit dubious about reporting the results to a tenth of a horsepower, having seen an actual engine dyno last week. The best engine dynos have a reproducibility of about 2 hp, so I would have stuck with just the numbers to the left of the decimal point.

CarWinsMax Power (hp)Difference from Top Engine (hp)

99 (Toyota)




18 (Toyota)




17 (Ford)




16 (Ford)




20 (Toyota)




This is an extremely interesting table; however, it would be much more interesting if we had the entire dyno data of horsepower as a function of engine speed.

As a result of this measurement, NASCAR came out with a new rule for the Nationwide series.

At all Events, unless otherwise specified, all engines with a cylinder bore spacing less than 4.470 inches must compete using a tapered spacer with four (4) 1.125-inch diameter holes. At all Events, unless otherwise specified, all engines with a cylinder bore spacing of 4.470 inches or more must compete using a tapered spacer with four (4) 1.100-inch diameter holes.

A tapered spacer is different from a restrictor plate.  A restrictor plate is thin, while a tapered spacer (shown below) is about an inch thick.  Tapered spacers have four tubes running through the spacer. Each tube has a hole at the top and a smaller-sized hole at the bottom (which is why they are called ‘tapered’).

Tapered spacers have been used in all Nationwide cars since the beginning of this season. The large end of the spacer mates with the carburetor and the narrow end points toward the intake manifold.  The tapered spacer gradually narrows the path through which the air must travel to reach the engine.  Try breathing through a scuba tube and then pinching the tube down. A smaller hole decreases the flow, which decreases how much air/fuel mixture is getting into the engine and thus how much horsepower the engine can generate.

The rule doesn’t mention Toyota: It only talks about the cylinder bore spacing (the distance between cylinder centers.). In my figure, x = 1.125” for engines with a cylinder bore spacing less that 4.470 inches (i.e. Ford, Chevy and Toyota) and x= 1.100” for engines with a cylinder bore spacing more than 4.470 inches (i.e. Toyota). 

Toyota estimates that the tapered spacer is going to cost them 16 horsepower.  Of course, that again is a maximum horsepower number, so we don’t really know how that is going to affect the cars.

A couple of the techie boards have been debating whether there is a difference between a tapered spacer and a restrictor plate.  There is, and you can see it graphically in this video on YouTube. The video compares the time it takes to empty a water jug (air and water are both fluids) through a hole cut in the bottom (like a restrictor plate) to the time it takes to empty the same water jug through its sloped top (which is more like a nozzle).  It’s a great video, and shows that a restrictor plate has a much bigger effect.  For those of you inclined to learn more, check out aerodynamics sites that compare orifices (restrictor plates) and nozzles (tapered spacers).

Changing a restrictor plate by 1/64″ (0.0156″) diameter changes the power by about 12-13 hp.  With the tapered spacer, a change in diameter of 0.025” is expected to produce a change of about 16 hp. The amount of airflow is only one impact. Teams spend a lot of money modifying the insides of manifolds to manage how the air reaches the engine. Passing the air through a restrictor plate or spacer not only changes how much air passes into the engine, but also how turbulent the air is when it reaches the manifold. A tapered spacer is much gentler in the way it changes the airflow.

Is the new rule fair?  Asking engine experts doesn’t provide a clear answer. Those working for Toyota say no and those working for the other manufacturers say yes. Those who have worked for more than one manufacturer have mostly admitted that they would change their answer depending on who they were working for.

From a purely technical point of view, we can’t tell because we don’t have the dyno curves.  We have the maximum horsepower values, but if you read my earlier blog on this topic, you know where the maximum horsepower occurs and the speed at which the engines spend most of their time on a track are important. We simply don’t have that information.  It’s unlikely that the spacer simply lowers the power vs. speed curve by a uniform amount for all speeds.

I sort of understand why NASCAR doesn’t make the dyno curves public: they would spend the next eight weeks defending every little feature, and we’d hear owners and drivers making overly generalized and often misleading statements. We have enough of that already.   From a scientific point of view, I’d love to see those graphs.  If you just look at the table with the maximum horsepower numbers and how they correlate (or don’t) with the number of wins, imposing the narrower spacer on all the Toyotas doesn’t seem fair.  So either this is a bad decision, or there is something in the dyno data that we don’t know about, but that justifies the decision.

The argument has been made that Toyota had an advantage because they didn’t have a production engine on which to base their competition engine, so they got to design an engine specifically for racing.  I’ve also heard the argument that they have the newest engine and that the newest engine will always have an advantage.  I think that the new rule shows that all it takes is a rule change and being new may not be an advantage all of a sudden. Perhaps NASCAR needs to establish a window, say once every three years, for everyone to submit a new engine, so that everyone gets a new engine at the same time.

I’ve also heard Toyota remind us that they had to design a new engine this year because NASCAR disallowed the one they used last year. I can appreciate the frustration on the part of engine designers watching a significant amount of work go right out the window.  The Nationwide only Toyota teams that are struggling are likely to be disproportionately affected by this change as well. They now not only have less horsepower than they had when they were struggling, they also have less engineering resources to figure out modifications to compensate. 

Regardless of whether the decision is ‘fair’ or not, we likely won’t have a chance to evaluate the impact of the change for a couple weeks. This week’s Nationwide race is at a smaller track and next week’s is at Montreal, which is a road course. One wonder if the timing of the decision was such to give the Toyota teams a couple of weeks to figure out how the new change is really going to affect them before they head to Michigan where the tapered spacer is likely to come into play.

Regardless of fair, it’s what we’ve got and we won’t be able to make a firm conclusion for at least a couple of weeks.

NOTE ADDED: reports that the Milwaukee engine tests came up with the following numbers:


Max Power

Difference from top engine

20 (Toyota)



40 (Dodge)



88 (Chevy)



60 (Ford)



Remember that the 60 won at Milwaukee. In comparing to the first table, realize that this table is the data for the top engine for each manufacturer, not the top five engines. The difference here is a little more pronounced between the top engine for each manufacturer. You’ll notice that in the report of the top 5 from the Chicago tests, there are no Chevys or Dodges mentioned.

For the next big issue, check out Dennis Michelsen’s column about the new “Restrictor Racing Shoe” Good idea being so clear about it being satire, Dennis!


  1. Here are some things I don’t understand about NASCAR testing…perhaps you can enlighten me….

    1) Post race testing doesn’t make sense since you are testing engines after they have been abused! NASCAR quotes 666HP for a typical Nationwide engine in their official techie info. Any engine ages as parts wear and that results in lower horsepower with engines at the short tracks that I am more familiar with in my work. But the same rules should hold true for NASCAR engines…right? So aren’t they testing which engine fell off the least? A shortr track engine specialist I talked to said that post race the variability between engines would be greater than freshly built engines.

    2)No matter how EVEN you try to build engines there is always variability. Roush-Yates and Hendrick have said they aim to have engines within 5HP. Wouldn’t you have to factor that difference into the statistics?


    PS: In short track racing they joke about the “Asphalt Dyno” and I ran some interesting stats. David Reutimann and Jason Leffler are the only two drivers in Nationwide that were with their current teams and driving a Toyota both last season and this season. The combined stats of those two drivers show a drop off in wins, poles, and laps led. Top five percentage is up 4% and top tens up 10%. Reutimann you could argue should improve in his second full year anyway.

  2. You have the best blogs I have read regarding NASCAR. I am a Physics Major and I love reading this content. The more science the better.

  3. Restricting engine outputs to level the
    field is nothing new. In the late sixties and early seventies they did this to the Dodges due to the hemi engines winning everything.Toyota complaining about attracting sponsors is ironic considering they’ve been winning everything. How do they think the other makes will attract sponsors when toyota is winning everything? ALso, Nascar won’t let chevy run its ro7 engine to close the gap with the toyotas so how was that fair! Chevys engine is ten years old and Nascar won’t let them run their new engine even though dyno figures show the toyotas with more power.

  4. Hi Dennis: Post-race testing isn’t ideal, but they can’t really pull engines from cars before the race without advance notice. If they gave the teams advance notice, I’m guessing there might be a bit of sandbagging.
    You are 100% right that the engine is going to experience wear and tear during the race and that this is going to affect each engine differently. If we look at where the
    As for absolute numbers, there are some corrections to the power for temperature and pressure and the absolute numbers can vary depending on whether and how you make those corrections. But I think you’re right that fall off is an important variable that will affect these results.
    Finally, the “asphalt dyno” is the important one. Don’t forget that there are many other factors to winning beyond pure engine power. Driver talent, aerodynamics, frictional losses between the engine and the wheels, and getting the suspension set-up right are all important as well.

  5. Diandra,
    It might be blog worthy from a technical standpoint to compare the differences of the Nationwide and Cup engines as a whole. (the most glaring difference is that the CUP engines don’t use these spacer I believe) We already can see/hear the tin foil hat theories of how this NationWide ruling should apply to CUP as well.

    To Larry, and the RO7 Chevy motors comment. If and when NASCAR approves the NW ro7 motor, the Chevy teams will also have to use the same spacer configuration as the Toyota’s. So they may want to reconsider a new engine package as well.

  6. Hi Phil:
    So if I were a Chevy engineer, I’d be working on a modified R07 with a 4.4699999″ cylinder bore distance, right?
    You are right that the Cup cars do not run a tapered spacer. They are allowed to run a spacer, but specifically without tapers. Tapered spacers are used in other racing series to boost horsepower by improving the flow of the air/fuel mix into the manifold. The difference with the tapered spacers being run here is that the holes are much smaller than you would make if you were trying to enhance horsepower.

  7. nationwide commentators commented tonight that the tapered spacer cut back on wheel spinning on the restarts. clearly, the #18 took off like a bullet on the restarts but was he the only toyota to be successful on restarts like that? and could the tapered spacer have that effect?

  8. Hi Red: The tapered space cuts back horsepower, which would decrease the chances of getting wheelspin on restarts; however (and this is the case with much of the ‘Toyota advantage’, the quality of the Gibbs drivers is a factor. The problem is that if you take very talented drivers and give them just a little more than the other drivers, you’ve got a lopsided series. Just goes to show that having an advantage technology-wise doesn’t translante into an advantage on the track unless you have a driver who knows how to capitalize on it. DLP

  9. I can’t believe the NNS cars only produce 650 HP. Everybody “ABC and FOX” always said 800 HP? I guess you have to believe what the DYNO said in the tables in this article.

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