IMSA: Final Rolex 24 Prototype BoP adjustments announced

IMSA: Final Rolex 24 Prototype BoP adjustments announced


IMSA: Final Rolex 24 Prototype BoP adjustments announced


Lead pic: LAT photo; all others Marshall Pruett

With the help of Scot Elkins, IMSA’s VP of competition and technical regulations, RACER chronicled the expected Balance of Performance changes for all four TUDOR United SportsCar Championship classes after the Jan. 3-5 Roar Before The 24.


Elkins followed up on Friday by issuing the first of two final BoP releases that will govern next weekend’s Rolex 24 At Daytona season opener, starting with the Prototype and PC classes.


Among the P2s and DPs in the Prototype class, the Daytona-only BoP has indeed been altered to increase power for the P2s and decrease power for the DPs to bring their relative top speeds and lap times closer together. Compared to the state they ran in at the Roar, look for the P2s to trade fuel economy for the extra power, and as one would expect with the DPs, they’ll regain a bit of fuel mileage by splitting the difference between 2013’s 560hp and 2014’s 610hp power levels.


Beginning at the top of the list, Elkins locked in the restrictor size and boost pressure for the 2.8-liter twin-turbo V6s used in HPD’s ARX-03b P2 chassis, increasing the airflow to the turbos and the amount of boost, settling on two 31.0mm restrictors, up from 2×30.8, and 29.7psi, up from 28.2psi.


“We were targeting a 5 percent restrictor increase and typically that’s about a millimeter, which translates somewhere between 15 to 20 more horsepower,” said Elkins.


Nissan’s 4.5-liter V8 engine, carried in chassis from Morgan and ORECA, have had a clerical error fixed in Friday’s BoP, correcting the previous inlet restrictor size of 40.9mm to the actual 40.1mm they use. Mazda’s 2.2-liter SkyActiv diesel P2 Lola received no changes, and will race with a single 50.0mm restrictor and 65psi of boost.


The 900kg (1984.1 pound) weight limit for all P2 cars at Daytona complies with ACO regulations.




Among the DP cars, Chevy’s 5.5-liter V8, the power source for its fleet of Corvette DPs, has seen its pair of 35.4mm restrictors decreased to 34.5mm. The 3.5-liter Ford EcoBoost twin-turbo V6 DPs have undergone a similar power reduction, moving from twin 34.5mm restrictors and 25.8psi of boost at the Roar to 33.1mm and 24.5psi for the Rolex 24.


As Elkins shares, the restrictor and boost modifications have been done to take away roughly the same amount of power the P2’s have been granted at Daytona.


“It’s in the same realm, 15 to 20 horsepower, depending on the motor; it’s a 5 percent reduction on the restrictor so it should be about the same,” he said.


The final DP engine in the matrix, Steve Dinan’s 4.5-liter BMW V8, has had a restrictor confirmed after options were tested at the Roar, settling on a single 76.4mm plate.


All DPs must weigh a minimum of 1039kgs, or 2290.6 pounds.


Don Panoz’s 2.3-liter Mazda-based Elan 4-cylinder turbo, which powers the DeltaWing, has had its boost set at 29psi. An amendment to its minimum weight has also been made after the DWC13 chassis was weighed during the Roar.



“We had 525kgs (1157.4 pounds) for it initially, and then we checked the car at the Roar, we found that it was running actually at 515kgs (1135.4 pounds), which is its new official minimum,” Elkins added.


IMSA also tweaked the aerodynamic spec for the PC class at Daytona after data generated at the Roar revealed the top speed of the 6.2-liter Chevy V8-powered ORECA FLM09s would place undue stress on the drivetrain for the 24-hour race. The new Rolex 24 spec calls for a higher minimum angle for the rear wing’s upper element, which will reduce the cars’ top speed.


“The PC cars were getting into the rev limiter quite heavily, especially in the draft, so we did a test at the Roar to increase the ratio on the sixth gear,” said Elkins. “We did that, tried it and it wasn’t quite enough. So we’re kind of aero limiting the car by saying they can’t be any lower than hole six on the rear wing to help that rev limiter situation.”


Elkins’ team addressed the other performance disparity between the Prototype cars by slightly adjusting fuel capacity for the P2 runners and tailoring the refueling hose flow restrictor sizes to better balance how long P2s and DPs can run on a full tank and how long each type of car will take to fill during pit stops.


For the P2s, Honda’s turbo and Nissan’s naturally-aspirated engine will be fed E10 fuel through 32mm restrictors, a sizeable increase from what they used in the ALMS. Mazda’s diesel, the first of its kind in P2 competition, will drink from a 28mm restrictor.


All of the DP tanks will receive fuel through a slightly larger 33mm restrictor, while the DeltaWing, with its small 48-liter (12.6-gallon) fuel cell, will sip fuel from the 23.495mm restrictor placed in the refueling hose.


“Looking at the P2 cars from last year, they used a 28mm restrictor for fueling,” Elkins noted. “The DP cars used an almost-28mm restrictor. So what we’ve done in trying to balance the tire change and fueling times, we’ve increased the restrictor size to speed up the fueling process – because we’re doing tires and fuel at the same time now – so we tried to speed up the fueling process so that if you come in for a stop the fueling is, ideally, and we’ll see if we get this right when we start getting some stops, the fueling isn’t the longest thing that happens, so you have some time for strategy to not change tires and just take fuel.”


The last series of Rolex 24 BoP changes involve fuel capacity modifications for the P2 cars. P2s and DPs are fitted with 75-liter (19.8-gallon) fuel cells, and with a bit of settling and stretching, 76 liters (20 gallons – or something close to that figure can be achieved, which is what IMSA has set for the DPs at Daytona.


All of the P2 cars, barring the Mazda, will be required to reduce their capacity to 72 liters (.8 gallons) through the use of dense plastic balls placed inside the fuel bladder. Mazda’s pair of diesels will have 70 liters (18.4 gallons) to work with on each stint.


“The difference between the two kinds of prototypes here is obviously about economy in terms of how the DPs use fuel versus the P2 cars,” Elkins remarked. “The DPs are limited by their tank size and 76 liters is the max it can do; it’s a 20 gallon tank. The P2 cars are able to go a little bit farther than the DPs, so that’s why we made the reduction there. We reduced the fuel tank capacity and reduced the fuel restrictor to try to balance that flow and bring them closer to what the DPs are giving up.”


By pulling back on DP power, turning up the wick on the P2s and reducing their fuel capacity, Elkins hopes to see the Prototype class intermingling on a regular basis during the Rolex 24. So what was maybe a three lap difference on the P2 side is now a lap and a half difference to the DPs,” he said. “We’ve done all the calculations we can in the office and now we need to see how it all measures up once practice starts.”


Click here for the final GTD and GTLM BoP tables.



Tune in for the 52nd Rolex 24 At Daytona at the following times:


Saturday, Jan. 25

2-4 p.m. ET on FOX (Live)
4-9 p.m. ET on FOX Sports 2 (Live)
Overnight (Jan. 25-26)
9 p.m. – 7 a.m. ET on (includes live images, in-car cameras and announcers)
Sunday, Jan. 26
7 a.m. – 3 p.m. on FOX Sports 1 (Live)


Tune in for the Daytona season opener at the following time:

Friday, Jan. 24

6 p.m. on FOX Sports 2 (Same Day)