Big Hit
695 hp at 6,100
667 lb-ft at 5,300That's closing on 700 hp from 370 inches with a cast crank. Nitrous is sick. At this level, we were creeping up on some safety issues. Our personal experience says that you can run up 175 hp with hypereutectic pistons if you use premium gas and a knock sensor to retard the timing. Brul uses what is called the brake mean effective pressure (BMEP) to eyeball the point where pump fuel is going to break down and race fuel is required. BMEP is a dyno calculation that multiplies torque times 150.8 divided by cubic inches, and the magic number that Brul has discovered through years of dyno testing is 225. If you go beyond that, you need to use race gas. For the record, we spiked at 271.
Thurmond also used a straightedge to periodically check for valve-cover clearance. Just the tricks, ma'am. | 
Nozzles
The are several crazy designs, but the basic idea behind the nozzle is to spray a stream of low-pressure fuel in front of a high-pressure blast of nitrous. The shearing action atomizes the fuel and gets it ready for combustion. |
Bottle Pressure
In an ideal setting, nitrous runs should use 1,100 pounds of bottle pressure. If you are testing for a race, Thermos recommends using about 950 pounds since pressure will likely drop off going down the racetrack, causing a different result. | |
With a tank full of Rockett Brand 114-octane racing fuel, the same plugs, and a total of 28 degrees of advance, we hit the button again. For nitrous runs over 200 hp, Brul likes to hit the button at 4,000 rpm at WOT, right where the tunnel-ram starts making power. We installed #25 nitrous and #28 fuel and jetted the secondary side of the 650TRs to 78. This might seem like a lot of extra fuel, but we wanted to be safe. Since we are not gaining horsepower with a bunch of rpm, the only way to break stuff is by detonating or leaning out the mixture and turning the tops of the pistons into molten aluminum. Both Brul and Thermos think the safety margin is between 10.5 and 11.5 A/F ratio. At the very top, we even dipped into the upper 9s. This is on the too-fat side, but again, we were using a cast crank and detonation would snap it for sure.
After the pull, we had some choices to make. Should we try for the 700hp number? The temptation, especially on the dyno, is to take away a little bit of fuel jet and bring the A/F ratio into the 10s at the top. This would likely find the lost 5 hp but would also be risky. Instead, we should have added perhaps one jet to both the nitrous and the fuel jets and stepped the entire curve up. In reality, the big hit is something that is suited for race gas and a car with a 'cage since it will likely be deep in the 10s at the dragstrip. The 175 shot will be plenty for a pump-gas test-and-tune or a cruise to the local digs, so we left it there.
Safety
Somewhere out there, a traffic safety school instructor is frothing at the mouth. For his piece of mind, we offer some safety tips.
Consider a separate fuel system with a pressure-operated kill switch when you are running the 500hp hurricane the Pro Shot was designed for. If you lose fuel pressure at the top end, the nitrous solenoids will click off and likely save you a lot of cash.
Make sure your charging system is in proper working order. Don't skimp on the alternator and battery; get the good stuff because the solenoids become erratic when system voltage is low.
Run it fat like we did and you won't melt anything. If you pull a plug and see a little pepper on the porcelain, you are lean or detonating, and you are going to break parts. And finally, always use nitrous at an approved racing venue in an appropriate manner. We always do