Class Structure: Determining Your NHRA or IHRA Class

Class Structure: Determining Your NHRA or IHRA Class

Part 1: Figuring Out NHRA Stock Eliminator

Want to go class racing? Stock looks pretty good, doesn’t it? But hang on – how on earth do you figure out what fits what? From an outsider’s perspective, it might look like you pick your favorite car, blueprint the engine and go from there. But wait a minute. It’s not quite that simple. You see, figuring out what class the car actually fits (and fits well, which is really important) is critical, and it should be your first step toward competing in Stock Eliminator.

First things first, however; let’s start at the top with an important excerpt from the NHRA Rule Book: “Stock Cars: Reserved for 1955 or newer model-year factory- production automobiles and some sports cars. Classified per NHRA performance rating as listed in the Official NHRA Stock Car Classification Guide. Only those cars listed in the guide are eligible for competition. All cars in Stock classes must be factory production assembled, showroom available, and in the hands of the general public. A minimum 500 units of a particular body style must be produced. OEM may apply for inclusion of any special production runs into the Official NHRA Stock Car Classification Guide. Special run must include a minimum of 50 units of an already accepted body style, need not be showroom available. Applications evaluated on an individual basis. Acceptance will not imply precedent.”

What that means is, cars like the Cobra Jet Mustang, Drag Pack Challenger and COPO Camaro get in by the “Special run” wording. It also means that cars like fourth gen SLP Firebird Firehawks with LT4s are not legal – even though they were generally available and actually produced by a GM vendor. Bottom line here is, if the car is not in the NHRA Stock Car Classification Guide, you cannot run it. The NHRA Stock Car Classification Guide is posted online, and it allows you to download a PDF with classification info for all accepted cars. The link is as follows:

All cars within Stock Eliminator are classified by using the factory shipping weight, divided by either advertised factory advertised horsepower or the NHRA factored (re-rated) horsepower. Shipping weights are provided by the Original Equpment Manufacturer. Ditto with Advertised Horsepower.

Class Structure: Determining Your NHRA or IHRA Class
One of the most popular traditional cars in Stock Eliminator is the 1969 Camaro. There’s a multitude of engine and transmission combinations available, and as a result, it can fit a staggering number of classes.

Let’s pick a fast class 1969 Camaro as an example. It carries an L72 427 (factory rated at 425 HP; with an automatic transmission, NHRA also rates it at 425 HP. Using a NHRA legal replacement cylinder head (for example, a Chevy 401 casting aluminum head), the horsepower rating changes. We’ll get to that below). The shipping weight is 3367 pounds. 3367 pounds divided by 425 HP equals 7.92235 pounds. That’s class “break.” If you check the Rule Book, A/Stock Automatic has a weight break of 8.00-8.49 pounds, while AA/Stock Automatic has a weight break of 7.50-7.99 pounds. The Camaro can run either of these two classes by removing weight. It can also run in B/SA by adding weight (we’ll get to adding or subtracting weight later too).

Essentially, there are two distinct groups of cars within Stock Eliminator: The traditional cars (anything from 5-6-7 Chevys to Hemi Cudas to 5.0 Mustangs ) and Factory Stock race cars. The FS cars are COPO Camaros, Drag Pack Challengers and Cobra Jet Mustangs. Each class has its own index. This was set up so that the modern high performance factory racecars could not make older combinations (such as our Camaro example) obsolete overnight.

Since many of the traditional cars racing in Stock Eliminator are getting long in the tooth, critical parts are often hard to track down (have you ever tried to find 1969 Camaro parts at the local junk yard?). NHRA has recogized this, and the rules allow for replacement and superseded parts (excerpt from the Rule Book):


Superseded and/or Replacement Parts

“Superseded parts are restricted to cylinder heads, intake manifold, crankshaft, cylinder block, and transmission only when manufacturer lists such parts in its published parts book, notifies NHRA of said change and change is accepted by NHRA.”

Class Structure: Determining Your NHRA or IHRA Class
Later model cars, such this LS-powered Firebird, are also very popular. When the first LT1 powered F bodies appeared in 1997, they were way down in G and H/Stock. Today, the LS cars compete right at the top. The reason for this is because the engine horsepower has been regularly “factored” by the NHRA.

Our sample 1969 Camaro L72 engine is allowed either a GM “401” replacement aluminum cylinder head or an Edelbrock 60547 cylinder head. With either head installed on the engine, the horsepower factor increases. In this case, the factored horsepower increases to 435. This in turn means the car must weigh more with the improved cylinder head. To “fit” A/SA, the Camaro must now cross the weigh scales at a minimum of 3480 pounds plus driver. For AA/SA it must weigh a minimum of 3262.5 pounds plus driver, and for B/SA it must weigh a minimum of 3697.5 pounds plus driver.

Class Structure: Determining Your NHRA or IHRA Class
You’ll note this Nova competes with a stick (E/S on the windshield; not E/SA). Today, stick shift cars may have different horsepower factors than their automatic counterparts. You have to watch for this in the Classification Guide.

This is a good time to point out that in the old days of class racing, cars were not allowed major weight additions or deletions. That meant it was in your best interest to nail down a combination that fit at the “top” of a given class (for example, an A/SA car that had a performance factor of 8.01 pounds). Otherwise, you’d be at a disadvantage to another combination.

But how do you get to the minimum weight? Remember when we mentioned adding or subtracting weight? Here’s NHRA’s take on it (from the Rule Book):



“All cars will be permitted to run on the class minimum weight, but may be no heavier than the minimum for the next heavier class. Car can move up (lighter) one class or down (heavier) one class. If weight is added, it must be properly attached; see BALLAST. Class and classification weight are determined without driver weight. Once classification weight is calculated, 170 pounds is added for driver to arrive at total weight. All cars are weighed with driver.”

What this means is you can add or subtract weight to fit a given class. This is important because there might be a car out there that proves to be incredibly fast in A/SA. If you don’t want to risk a heads-up run in eliminations (no breakout rule is in effect for heads up races), you might want to move up or down one class. Additionally, the track in question might not be conducive to “hooking.” A heavier car might have an advantage at such a facility.

Back to the driver weight: The 170 pound driver weight can be misleading. This does not mean you have to go to Jenny Craig if you’re “big boned.” What it means is, the total car weight at the scale must be classification weight plus 170. So in the case of our ficticious A/SA Camaro with the superseded aluminum heads, it has to weigh 3480 + 170 = 3650 pounds in order to fit the minimum in the class (if the car is less than minimum, your run gets thrown out – you automatically lose). If you weigh 220, then the car weighs 3430. The total is still 3650.

Class Structure: Determining Your NHRA or IHRA Class
Different body styles can have different horsepower factors, even though the model years and engines might be the same. Again, keep this in mind when searching for a good “combination” inside the Classification Guide.

With all of the above, you’re set to figure out where your car can run within Stock Eliminator. We may have left the best for last, however: Long time Class Racer Dwight Southerland has a website dedicated to determining class using proprietary software. Dwight also includes a clearing house of information with regard to horsepower adjustments (which obviously can have an effect upon car weight and class fit) and other new NHRA rules, regs and policies that affect class racers. Here’s the link to Dwight’s “Class Racer Info” website (be sure to bookmark it if you’re serious about class racing):

Next time around, we’ll look at what it really takes to race Stock Eliminator. Class Racing might seem easy to the uninitiated, but we can assure you there’s much more to it than meets the eye. Watch for it.

Class Structure: Determining Your NHRA or IHRA Class
Photo courtesy Chevrolet.
Cars such as the COPO Camaro race in Factory Stock classes. As noted in the text, FS classes were established so that traditional stock cars weren’t immediately made obsolete by new technology.
  • Robert Harrison

    The following is an e-mail I sent to NHRA. It fell on deaf ears. ————————- The classification guide needs to say ” NHRA does not want any 66 Novas with 283 automatics in Super Stock”. That engine is factored at 233 hp. (original 220 hp). That same identical engine (64-66) in a Chevelle is 217 hp. What IS it with that ? SIXTEEN HP DIFFERENCE ? 16 hp x12 lb/hp =192 pounds ! That’s 2 tenths of a second difference, and THAT is ridiculous ! Just because it’s a Nova doesn’t make it 2 tenths faster. I know you are going to say that the numbers are fed into the computer and it crunches the numbers— BUT the results are NOT fair ! That AHFS has serious flaws and needs fixed. You are penalizing the people who build good engines, spend a lot of money, and just happen to race a Nova ! The way it is now, a $10,000 283 in a Chevelle will run (qualify) as good as a $35,000 engine in a Nova. The OTHER ridiculous, unfair disadvantage to us is the 305 Camaro (235 hp) in the same class as us continually dodging the horsepower hit by running a SMALL restrictor plate. That engine should be rated about 260 hp. This is another serious flaw in the system that needs fixed. If NHRA cannot get these horsepower ratings in line, then I am going to quit running NHRA. I noticed there was NOT a full quota in super stock at the US Nationals, and I have never seen that before. Maybe others are tired of the system ! I guess you might call this “whining”, but this is way overdue to be said, and I am tired of being factored unfairly by NHRA. Please respond. Your Friend– Bob Harrison

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