NHRA Pro Mod, one of the most diverse classes (in terms of engine combination) in heads-up drag racing, now has yet another power adder in its rules set. Screw supercharged entries limited to 526 cu in and weighing 2740 lb are now permitted, which means there are now six engine combinations permitted in the series, alongside the existing permitted combinations – two variations of nitrous-assisted engines, Roots supercharged, centrifugal supercharged and twin turbocharged entries.

To add context, these are the combinations permitted in 2022 along with engine sizes and weights:

• Screw supercharged, 526 cu in; 2740 lb
• Nitrous oxide, 910 cu in; 2540 lb
• Nitrous oxide, 960 cu in; 2570 lb
• Roots supercharged, 526 cu in; 2640 lb
• ProCharger supercharged, 526 cu in; 2735 lb
• Twin turbocharged entries, 526 cu in; 2615 lb

Additionally, the NHRA allow a 50 lb weight break for certain ‘nostalgia’ style body types which are deemed to be at an aerodynamic disadvantage.

Todd Tutterow running the PSI 210C supercharger, fresh off his win at the World Street Nationals, is gunning for another win at the Snowbirds (Courtesy of Todd Tutterow)

Huge power potential

It is no secret that the screw-type superchargers make prodigious amounts of horsepower, and will outperform a Roots-style one in terms of power output in this application. That is because the screw supercharger design being more efficient. Efficiency in this case is the amount of boost pressure added to the engine versus the amount of horsepower required to drive the supercharger.

A front-running 14-71 Roots-style blower is able to displace more than 540 cu in of air per revolution (of the supercharger). The screw supercharger permitted for the 2022 season is the PSI ‘C’ rotor, the larger variant of the two that PSI produces, and it can displace 476 cu in/rev.

As an air pump, the Roots supercharger will move more air per revolution. However, the maximum overdrive for it is set at 14.5%, while the screw supercharger will be allowed to run at 75% overdrive.

The balance of performance becomes even more complex when you consider the fact that the screw-type supercharger is actually a compressor, whereas the Roots supercharger is an air displacer only. With the Roots supercharger, the air-fuel mixture is sucked into the top of it, then carried in the cavities of the rotors, which rotate in opposing directions, around the outside of the case and out of the bottom. The air-fuel mixture ‘stacks up’ in the intake manifold plenum chamber, creating a positive ‘boost’ pressure.

The screw supercharger however consists of male and female rotors (with a ‘screw’ shaped helix) that mesh while also rotating in opposite directions. As the rotors mesh, the air-fuel mix is compressed; furthermore, the volume between the rotors and the housing decreases at the bottom of the supercharger casing.

This compression element, combined with the stacking up of volume within the intake manifold gives higher boost pressures. Also, the rotors do not make contact with each other under normal operation. Therefore, the screw supercharger can make boost with much less parasitic drag and heat generation.

Dyno testing has shown that the screw blower requires much less energy to generate the same amount of boost when compared to the Roots blower on a given engine combination. Mike Janis Jr, of Mike Janis Superchargers, has huge experience with both Roots and screw-type blowers, and is the crew chief for the MJR Pro Mod Camaro (whose engine was featured in RET 131, May 2021). He says, “A screw blower takes about 150 bhp for the engine to turn the rotors, while a Roots blower takes upwards of 500-600 bhp to do the same. On the dyno, with overdrive ratios adjusted to provide similar boost levels, the screw blower will net 350-450 extra bhp over the Roots blower”.  

 

Balance of performance

This shows that the screw supercharged combination is clearly capable of generating more power than the Roots combination. That is not necessarily a problem: history has shown the twin turbocharged combination has been capable of more horsepower and yet has been ‘tamed’ through various rule changes.

The NHRA is under no illusion about the potential of the screw supercharged combination, and has put in place some measures to try to maintain parity. The most obvious measure is the minimum weight, which is 100 lb more than the Roots cars, although it can be argued that it is extremely difficult to get a Roots supercharged car down to minimum weight with a larger driver and all the necessary equipment required to run. That 100 lb of weight will usually account for around 5-7 hundredths of a second at this performance level.

The second measure could make this combination untenable for many competitors. The final-drive gear ratio is set at 3.90:1, compared with 4.56:1 for the Roots supercharged cars (top gear must be 1:1 for all combinations, and there is a minimum tyre circumference that effectively makes a 34.5 in tyre the smallest diameter permitted).

This limit on gear ratio will have a huge impact on the air-fuel mixture that the screw supercharger can displace, bearing in mind that it is well-suited to high rotor speeds. As an exercise, if we want to find the rpm drop for a given speed and tyre size with the final-drive ratio change, we find that if the engine rpm for that speed (and the 4.56 final-drive ratio) was 10,000 rpm then with the 3.90 gear ratio it will drop to 8555 rpm.

 

Motivation for change

It would appear that the motive for including screw superchargers is to attract racers who compete in other race series with screw supercharged cars, therefore negating the need to invest in changing their combination. Car counts have been a little low in 2021 at times, but the mandated gear ratio rule means these racers will have to invest time and money in changing their combination to suit this rules package.

Torque convertor selection will probably be the initial area in need of development to suit the relatively low peak rpm. Camshaft profile design will be important to bring the window of peak power and torque down to a lower rpm. It may be that some racers choose to experiment with different bore and stroke combinations to make the most of the lower rpm range. As time goes on, it is feasible that NHRA will make adjustments to the screw supercharged combination, which again from a financial standpoint could make this route less attractive than the more established power adders in the class.

 

Expert opinions

So what to the industry experts think of this move? Anthony Lum, crew chief on the Elite Motorsports/Modern Racing entry driven by Lyle Barnett, and someone with experience of all power adders, says, “I think NHRA letting the screw combo in is a good idea from the standpoint of giving those racers another place to race, although with the 75% overdrive, 3.90 mandatory rear gear and the 2740 lb minimum weight, it will be difficult to be competitive for a while.

“With this given rule set, it would be a drastic change for most of the screw racers’ current set-ups, which always means spending lots of money, and which will most probably be a limiting factor in any added participation. I also believe that the NHRA Pro Mod Technical Department already has more than enough combos to police, so this will make it more difficult.”

When asked about the current balance of performance within the Pro Mod class, Lum says, “Based on my nitrous experience, I think both nitrous combinations [903 and 959 cu in] have the potential to run at the front in just about any conditions we see on the tour.

“As for the ProCharger and Roots cars, I believe the parity is extremely tight with the current rules, but the ProCharger combo does seem to be a lot more sensitive to weather conditions than the Roots. With regard to the turbo combo, I believe that after running this combo successfully over the past couple races that its rule package makes it competitive in the right track conditions, although they are extremely hard to race at times owing to the inconsistencies associated with the combo.”

Janis Jr says, “Allowing screw blowers in NHRA Pro Mod was a pretty big shock to most of us, especially after we had a meeting in Indy with all of the drivers, and we all voted against it yet somehow it still got passed.  I get it, they are trying to attract cars from different venues.  The way the rules are written now – 75%, 2740 lb, 3.90 rear gear – will make it hard to run out of the gates, no doubt. They left themselves a lot of room for adjustment though.”

It has been said that true parity between power adders in Pro Mod is really an illusion, owing to changing atmospheric conditions, track elevations and track conditions that suit one combination over another. Will the screw supercharged option bring more cars to the series at different points in the season, or could we see teams take the lead of the Elite/Modern Racing team and switch power adders at different points in the season (they switched from a ProCharger combination to a twin turbo combination late in 2021)? 2022 is shaping up to be a very interesting season.