Holley, Teams Offer Insight Into New Pro Stock EFI Rules

Following the NHRA’s release of EFI requirements for Pro Stock engines yesterday (August 13), intake manifold design will be a highly coveted expertise in the next few months.

“The biggest effect [with change from carburetors to electronic fuel injection] is not the EFI,” says Robin Lawrence, EFI sales manager for Holley–the company that will supply mandated fuel-system components to the teams. “It’s going to be the intake manifold design.”

“I’m sure over time, teams will have several different shapes and designs while trying to figure this all out,” adds Nick Ferri, engine builder for Elite Motorsports, home of current NHRA Pro Stock world champion Erica Enders.

Here are the two main required components from Holley for 2016 NHRA Pro Stock engines: an oval throttle body with 25 square inches of opening and a NHRA-unique HP electronic control unit or ECU.

Under the new rules that go into effect for the 2016 season, Pro Stock engine builders will have to use the following Holley parts to develop the fuel and spark system for their engines: throttle body, ECU, fuel injectors, crank and cam sensors and Smart Coils. If the teams choose to use an O2 sensor for closed-loop operation, there is a spec component from Holley on the list. Additional parts such as wiring harnesses, fuel rails, connectors and optional sensors will be available from Holley but not required and teams can build or source their own as needed.

Crew members for recent 2-time winner Chris McGaha work on the engine between rounds. In addition to mandating EFI for 2016, the NHRA also told teams to stop hiding their cars in the pits and allow fans more access to the technology. Photo courtesy of Chris McGaha Facebook page.

The rules clarification answered many questions lingering since the sanctioning body announced last month that Pro Stock will convert to EFI as part of the division’s most sweeping changes since 1982 when the 500ci rule was introduced. Only eight injectors will be allowed and they must be mounted externally on the intake manifold runners. Also, the throttle body must be mounted forward facing and fit under a stock-style hood. The iconic but rather noticeable hood scoop on current Pro Stock vehicles will be eliminated. Air intake will be drawn from an opening in the front grille. GM and Mopar must submit the location and dimensions of the proposed openings for their body styles to NHRA for approval before competition begins next season.

The most distinctive ingredient in this new EFI recipe is the Holley throttle body. The rules set down a maximum throttle-plate area of 25 square inches. Holley achieved this opening with an oval shape, approximately 7 inches wide and 4.5 inches tall.

“We developed this throttle body specifically for Pro Stock based on discussions and requirements from the teams, manufacturers and NHRA,” says Lawrence, adding that official flow numbers will be announced later. “It’ll flow enough for a Pro Stock engine in excess of 11,500 rpm, plus a little bit more.”

The iconic hood scoops will be gone in 2016 and teams will have to draw fresh air from an opening in the grille. Both GM and Mopar have to submit proposed opening location and dimensions for approval before competition starts up next year.

Do the math

Will 25 square inches be enough for the engine builders?

“I don’t think the size will be an issue at all. If you took a carburetor that we run now and do the math, it actually works out pretty close, in my opinion,” explains Ferri. “There are carbs out there that range from 2.300 to 2.500 in throttle bore size. Times eight and you end up with anywhere from 33.24 to 39.27 square inches.”

Intake manifold design will get interesting with the new rules, especially working under the 10,500 rpm rev limit and throttle-body location requirement. There's a lot of experience with big Pro Mod engines, but most of those are boosted. Engine builders are predicting a variety of designs to ensure equal air distribution to all cylinders. Images courtesy of Steve Morris Racing Engines

Ferri goes to say the carburetors obviously produce restrictions in the form of the verturi area, which could be 2.0 to 2.1 inches in diameter.

Tuners can play with everything but the rev limiter.–Robin Lawrence, Holley

“The area of those range from 25.13 to 27.71 square inches, “ continues Ferri. “Now you have a pretty good size booster in the way—which I’m not even going to try and figure out how much area that takes up—but it’s a lot. So I feel that the 25 square inches that they are allowing us will be more than enough to start with. Plus, because of the mounting position and we probably won’t be able to get as much air into the engine without the hood scoop. That then makes the size even less relevant.”

Insiders say the 10,500 rpm rev limit will do little to reduce the cost of replacing valve springs so often in Pro Stock engines. In fact, the rev limit will require extensive re-gearing of the axle and transmission. Photo courtesy of Chris McGaha Facebook page

Holley says the spec fuel injectors are Bosch units rated at 160 pounds per hour at 43 psi line pressure, and 220 pounds at 90 psi. NHRA has imposed a 90 psi limit on the fuel system and will require a spec dry break coupler from Earl’s Performance that the tech officials can access easily to help verify compliance with the rule.

“The injectors will all be flow-matched within one percent,” says Lawrence.

“The flow rate should not be a problem,” says noted Pro Mod EFI tuner Shane Tecklenburg. “Pro stock engines only require about 1500 horsepower worth of fuel, which on gasoline would be around 700 pounds per hour total. Divide by eight and you see the minimum per-cylinder requirement, assuming the system is synchronized sequential.”

“I honestly don’t know enough abut EFI or injectors to really say if they are to big and or small for our application,” adds Ferri. “All I know is that we have all been dealt the same hand to work with.”

There’s not much room up front for inlet air ducting when the teams are required to draw fresh air from the grille opening.

Tuners have flexibility

Controlling those injectors will be a modified version of the Holley HP electronic control unit or ECU. We’ve seen another version of the HP mandated as a spec controller in the ARCA series but with many more restrictions and controls than NHRA will require.

“Tuners can play with everything but the rev limiter,” says Lawrence, adding that options for building a speed-density or Alpha N fuel map will be available.

The NHRA will allow close-loop functionality if the engine builder desires, but Lawrence doesn’t see many teams pursuing that option.

“Pro Stock engines accelerate extremely fast,” says Lawrence. “I wouldn’t use it until fifth gear.”

Nick Ferri builds engines for reigning Pro Stock champion Erica Enders.

Holding back development of any sophisticated closed-loop operation is the HP’s limitation to only one Lambda sensor. Most teams already run a single Lambda sensor to feed information to the Racepak data logger. During dyno testing, of course, teams could use Lambda sensors for each cylinder to help develop fuel maps.

The HP is also capable of supporting knock sensors, but Lawrence says that development time could probably be better spent on other tuning choices.

“Any time you take timing out you’ll slow down,” he says. “I doubt if a lot of people will be using that option.”

The 10,500 rpm engine speed will be hard-limited with the HP’s firmware but teams will have the ability to set soft limits at any desired RPM. The rev limit remains the most controversial mandate to come out of the rules revision.

“I thought maybe the 10,500 rpm rule might get raised, but after seeing that it didn’t, I wasn’t surprised,” says Ferri.

The Holley HP ECU is also mandated for the new ARCA spec engine.

Insiders say most Pro Stock teams are currently running over 11,000 rpm with some over 11,500 rpm. The most obvious adjustment with a 10,500 rpm limit will be re-gearing the rear axle ratio and possibly the transmission ratios. On the engine, the most diversified changes will be seen with the intake manifold as the teams adjust the power curve downward.

“Physics tell us the runners will get longer,” says Ferri. “Again, with possible lower efficiency of the engine due to the lack of air, the RPM deal and where these engines peak at might take care of themselves. Time will only tell on all these issues.”

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About the author

Mike Magda

Mike Magda is a veteran automotive writer with credits in publications such as Racecar Engineering, Hot Rod, Engine Technology International, Motor Trend, Automobile, Automotive Testing Technology and Professional Motorsport World.
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