Engine Rules Changes for 2015

There’s a lot of talk about all the rules changes for 2015. The limiting of the horsepower has been a hot topic of discussion, with people suggesting that NASCAR is basically mandating spec engines.  Here’s a couple of things to think about in terms of engines as we get closer to Daytona.

“750 hp Engine” Doesn’t Tell the Whole Story

When someone says they have a 900-horsepower engine, the only thing that tells you is that the maximum power it outputs is 900 hp.  Importantly, power output changes with revolutions per minute (rpm), as shown graphically below.


The power curves for these two cars have the same maximum horsepower, but the range over which they have that horsepower is different.  Let’s say the gearing is such that you’re running most of the race in the 8000-rpm range. Car 1 would have an advantage because (in that rpm range), it has higher horsepower. Car 2’s curve, however, is broader, which means it has a higher horsepower over a broader range.

Tapered Spacers

The big impetus for engine rule changes is NASCAR’s desire to lower speeds (which, it is theorized, will improve racing by lessening the effect of aerodynamics making it hard to pass when cars get close to each other).

There are lots of ways to decrease engine horsepower, but remember that teams have put untold amounts of money into designing and refining the current engines. Designing entirely new engines is a major undertaking, and a risk to mandate without pretty high confidence that lower horsepower will indeed help the quality of racing.

For 2015 NASCAR has gone with the simplest solution: a 1.170 tapered spacer that they expect will reduce power by about 125 hp.

Combustion is the chemical reaction whereby fuel mixes with oxygen and releases energy. It’s very similar to another chemical process called respiration, which is how your body converts food to energy.  In this case, you mix two fuel molecules (C8H18 is octane, one of the hydrocarbons in fuel) and 25 oxygen molecules.



Remember your chemistry teaching talking about stoichiometry? Stoichiometry is the ratio of molecules, because they only combine in particular ways. If you’ve got four octane molecules, you need 50 oxygen molecules, etc. Combustion demands that exact ratio. The amount of fuel you put in a cylinder depends on how much air you can get in.

That’s how tapered spacers and restrictor plates work – they limit how much air gets into the cylinder, which limits how much fuel you can put into the cylinder, which in turn limits how much energy is produced.


A couple people have asked if this is going to have the the same effect as a restrictor plate. No! Fluids (and air is a fluid) travels differently through an orifice (the technical word for a hole) and through a nozzle (which is what the tapered space is).  Don’t believe me?  Here’s proof:

The tapered spacer does change how the air goes into the cylinder and that is something the teams are studying using Computational Fluid Dynamics — and figuring out how to use to their advantage.


The Rules Don’t Say “Your Engine Must Be 770 hp”

NASCAR engine rules address the physical properties of the engine – things like cylinder height and bore, compression ratio, and which materials can be used.  This has always been the case. They control things like rpms and horsepower indirectly via things like gear rules.

So teams actually have a fair number of variables with which to experiment.  The big emphasis is energy efficiency. There are two major energy transformations in the engines: combustion, which converts the potential energy in the fuel to the linear motion of the pistons, and then the linear motion of the pistons is converted to rotational motion.

The conversion processes aren’t perfect. If the air-fuel mixture isn’t right, you don’t get all the energy out of the fuel. This is addressed by specific geometry issues (which controls how the air gets into the cylinder), and EFI mapping.

The other main culprit in energy losses is friction.  In a conventional road car, only about 14% of the energy you put into it actually gets to the wheels. The rest is lost (or used by the air conditioning, power windows, radio, etc.).  Most of the energy losses are in the engine. In a conventional car, 60-70% of the energy loss is in the engine.

Surprisingly, NASCAR (and most race car) engines are more efficient than passenger car engines, due in large part to the use of advanced coatings on engine parts. A typical valve has at least three different coatings – A hard coating on the tip to protect against valve lash wear, low-friction coatings on the stem, and hard coatings on the dome to protect against valve seat recession.

MetTech_ValveFailureThese coatings are often very thin – a hair’s width or two or three. Materials used include Titanium Nitride (that’s what those gold-colored drill bits are coated with) and diamond-like carbon (DLC). DLC has a much smaller coefficient of friction against steel than steel (0.7) or titanium nitride (0.3). DLC has a coefficient of friction on steel of 0.2.

There are companies in the Charlotte area that offer detailed failure analysis of engine parts, like the valve below. They use scanning electron microscopy to examine the parts. The culprit is often the coatings coming off.

Remember back in 2008, when Hendrick Motorsports had a baffling sweep of engine failures? The culprit was the coatings on the cam and/or lifters. They delaminated (i.e. came off) and the small flakes got into other parts of the engine. The clearances between moving parts in a race engine are much smaller and the tiny flakes of coating jammed up the engines, leading to failure.

Teams keep such detailed records of which parts go in which cars that Hendrik was able to track down the batch of parts that failed and ensure they weren’t being used in any other cars.

So if these coatings make engines more efficient, why aren’t they in all our cars?  The usual answer – cost. Coated parts are more expensive and people don’t want to pay extra money for a small improvement in performance/duration. Race teams, on the other hand will spend whatever they can to shave a few more seconds off their time.

So there it is. We’re still nowhere near spec engines, even with the new rules.  I suspect at some point, there will be an engine design initiative, but NASCAR has been fairly respectful for not throwing a zillion changes at the teams at one time.

And don’t forget, we won’t even see the tapered spacer until after Daytona because Daytona and Talladega are still using restrictor plates.


  1. Great explanation! I always wondered how those worked together. Plus the tapered spacer vs. restrictor plate made me fell better. Thanks.

  2. Excellent article.

    Back in the ’70’s, tech articles like this were very common because many if not most racing/NASCAR fans were gear-heads. Not the same today. Fans today (NASCAR) seem more defined by their interest in what color dress Danica was seen in two nights ago, or Busch’s latest tiff.

    But there are those that still think auto-racing should be about autos. And we thank you for articles like this one ….

    Keep up the good work …

    • Thanks Steve – glad to know someone is reading. It’s depressing that even some of the television coverage skims over or gets techy things wrong.

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