Stock Car Science

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.

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.

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: bleacherreport.com reports that the Milwaukee engine tests came up with the following numbers:

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 “Retrictor Racing Shoe” Good idea being so clear about it being satire, Dennis!