Project Paw-Paw: Part 11
After being under the knife for three months, the moment of truth finally arrived. Was all the work we’d put into our ’97 F-350 going to pay off? With the truck’s fluids topped off, all the odds and ends wrapped up and the 7.3L started for the first time in nearly 100 days, it was time to find out. Following a few spirited driving miles (namely to purge all air from the high-pressure oil system), we were penciled in for a date with the chassis dyno. We didn’t know if Paw-Paw would be able to belt out 600hp or not, but judging by the lack of traction present under 40 mph and the 50-psi of boost we were seeing in overdrive, we knew it would be close. On judgment day, the old Ford laid down a respectable 567 hp.
Once free from the rollers, it was time to put that horsepower number to good use. This meant making a trip to the nearest quarter-mile drag strip that was still open—a four-hour trek. With my Nitto Ridge Grappler tires aired down a bit and the transfer case locked in 4-Hi, we staged Paw-Paw under boost and left the line hard. At the end of the track, we were pleasantly surprised with the time slip that was waiting for us. We’d just put a 23-year-old, former farm truck through the quarter-mile in 13.3 seconds. When subsequent passes produced similar results, we aired back up and drove the truck 250 miles home.
With 567rwhp, 1,086 lb-ft of torque and low 13-second potential in our arsenal, Paw-Paw is a real beast now. But can it still tow? Tune in for the finale of our series (Part 12) to find out!
Load Cell Chassis Dyno
In our experience with diesels, it pays to dyno them on a load-cell dyno and with an operator that’s used to dynoing diesels. During the process of trying to simulate what happens on the street—where the engine is forced to accelerate a horribly-aerodynamic, 4-ton brick—accurate horsepower and torque numbers are only possible if boost pressure is maximized (or close to maximized). In our case, we turned to the folks at Maverick Diesel: a shop that specializes in dyno tuning all makes of diesel-powered pickups. The portable Mustang dyno they use, a joint venture that’s shared with nearby shop, Tri-County Diesel, employs a large eddy current brake, which is used to place each truck under considerable load.
567rwhp & 1,086 LB-FT
Looking at the numbers under the MAX column (above), you can see where horsepower topped out at 567rwhp (a 408hp increase over stock) and torque swelled to 1,086 lb-ft (up 805 lb-ft). To load the engine even harder than what the eddy current brake was already providing, the runs were made with the transmission in overdrive. Note the 556rwhp and 1,031 lb-ft numbers. Those were made during the first dyno pull on the same tune, which we deemed a bit of a warm up pass. Being that diesels love load, a third run might’ve yielded even more power. However, as with any engine diesels can also become heat-soaked and start to lose power, too. Knowing that the real horsepower tale would be told by what happened at the drag strip (and specifically the trap speed achieved), we decided to unstrap the truck from the rollers.
When viewing the power curve, you can see that 567rwhp checked in around 2,800 rpm and torque swelled to 1,086 lb-ft at roughly the same engine speed. This is intentionally done in our PCM tuning in an effort to keep the factory short-block (specifically the rods) from seeing excessive cylinder pressure (i.e. torque). By limiting low-rpm torque and then pouring on the fuel up top, a solid horsepower number is still on the table, with a minimal (yet completely necessary) sacrifice made in low-end torque.
A Happy HEUI System
Taking advantage of the AeroForce Technology Interceptor gauge we installed in Part 10, we monitored injection control pressure (ICP) throughout the course of both dyno runs. Hoping to see the SRP1.1 high-pressure oil pump maintain 3,000 psi, we were content with ICP holding steady at 2,880 psi. On the first pull, we also kept an eye of the duty cycle of the injection pressure regulator (IPR). Its 38-percent reading was ideal. For the second dyno run, we switched from IPR duty cycle to boost on the Interceptor and saw 45-46 psi displayed with the engine at full song.
Airing Down & Selecting 4-Hi
While we knew racing in four-wheel drive would remove most traction issues, we also knew we would be racing on a cold (and lightly-prepped) track on a cloudy day—not the best scenario for obtaining the quickest possible elapsed time. To widen our foot print, we dropped the cold air pressure in our Nitto Ridge Grapplers to roughly 30 psi. We had zero problems finding traction at the drag strip, though a stickier, warmer track would’ve been highly welcomed.
Getting the truck’s weight down to where we wanted it to be on race day proved trickier than expected—primarily because we had a four-hour drive to the track. Despite ditching most of the truck’s tools, its bed liner, the spare tire and showing up on a half tank of fuel, Paw-Paw still tipped the scales at 7,400 pounds (driver included). With the tailgate removed, less fuel onboard and no tools whatsoever, we could’ve easily scrapped another hundred pounds.
Hurry Up and Wait…
In order to work off of a Sportsman tree (our preference), we signed up for Test ‘n Tune once we made it to the track. After passing tech inspection we were assigned as car #21, hopped in the staging lanes and began patiently waiting for T&T vehicles to be given the chance to go down the track. Because NHRA guidelines stipulate that an approved helmet be worn by drivers in vehicles going 13.99 or faster, we also made sure to bring a SNELL 2015 rated helmet.
13.3 at 102 MPH
Up against the brakes, we brought the S468 up to 8 psi of boost when staging the truck. Then, leaving on the last yellow we were able to cut a sub-2-second 60-foot on our way to a 13.34 at 102 mph. With more heat in the track, a couple more psi of boost built up at the line and maybe even a few more pounds stripped off of the truck, we think a low 1.8-second or a high 1.7-second 60-foot is possible. This pass, along with all others made that day, was made in tune number 7 on the Gearhead-tuned Hydra Chip we installed in Part 9. Before we had a chance to see if tune 6 could produce a quicker elapsed time or faster trap speed, the sky opened up and our cold day at the track became a wet one, too.
Backing Up Our Dyno Numbers
Armed with the evidence our time slip provided, we plugged the truck’s elapsed time, trap speed and weight into Wallace Racing’s highly reputable online horsepower calculator. According to E.T., Paw-Paw is making 558rwhp, but according to MPH—a much more accurate indicator of horsepower—it’s making 561rwhp. This horsepower number is within 1-percent of the 567rwhp we made on the dyno at Maverick Diesel, indicating that we’d strapped the truck to one of the most accurate chassis dynos around.
One of the most rewarding aspects of the build was the reaction people had to it at the drag strip. After having seen Paw-Paw go down the track, we got to meet a handful of bystanders and other drivers who stopped by (in both the pits and staging lanes) to pick our brain about the truck’s setup. Each one mentioned the fact that they never would’ve expected to see it run 13’s, which justified the entire purpose of the build. Paw-Paw is definitely a sleeper.
If you’re just tuning in, trust us, we didn’t put this truck together overnight. For a look back at the extent of the build, follow the links listed below.
Part 1: Why we chose the 7.3L Power Stroke
Part 2: Valve spring, pushrod and head stud upgrades
Part 3: 350/200 hybrid injectors
Part 4: Larger displacement high-pressure oil pump
Part 5: Electric fuel supply system
Part 6: T4 turbo kit and intercooler
Part 7: Billet S468 turbocharger
Part 8: Built E4OD transmission
Part 9: Custom PCM tuning
Part 10: Gauges and traction bars