Details, details. When it comes to squeezing the utmost performance out of a given combination of parts, it's often little things that count the most. This month's installment in our Project SuperNova series proves that, as we dig deeper into the science of chassis setup.
Last month we bolted a set of slapper bars onto our '71 Chevy Nova and were stumped when they didn't prove to be a magic solution to the car's traction problems. We were already running slicks (Mickey Thompson 28x9.0-15 ET Drags) and 4.10 gears, but the car's 60-foot times were no better than before we installed them. We scratched our heads and called Competition Engineering for some advice; we even sent before and after photos showing how the car looked when it launched. Our ensuing conversation with Competition Engineering's Mike Kugler revealed some interesting background on how a leaf-spring rear suspension works, and how it reacts to slapper-style traction bars.
In any stock leaf-spring car, the front half of the spring basically acts like a control arm to locate the rearend housing and control its motion. The rear half of the spring from the spring perch back to the shackle, controls ride height and ride quality, which can be altered by changing the number of leaves in the spring pack and the free arch of the spring. Back in the days when Stockers and Super Stockers ruled the dragstrips, Mopar was famous for its Super Stock springs, which combined a relatively short front half with a stiff spring pack that essentially acted like a built-in traction bar and kept the rearend from wrapping up under hard acceleration.
Monoleaf springs like the ones on our Nova do a good job of providing ride height and quality. In fact, after 30-plus years of service, the springs on SuperNova had sagged less than 11/48 inch when we checked the free arch. But although monoleaf springs are popular in drag racing, they work best when combined with a traction device to prevent the rear axle from twisting or wrapping up under acceleration. The cheapest and simplest solution is a set of slapper bars, like the Competition Engineering traction bars we installed last month. A slapper bar prevents the rearend from rotating too far upward, which allows the pinion yoke and driveshaft to overextend and unload the tires. The result is that the body of the car should lift and separate from the tires, planting them harder to the ground and increasing traction.
Suffice it to say, after we bolted on the traction bars, SuperNova was lifting and separating, but not enough. We had the pinion angle checked and found it to be at zero degrees, which meant that the yoke and driveshaft were actually rotating into a positive orientation under launch, which is bad, as the pinion has a natural tendency to climb the ring gear under power, and this unloads the rear tires. To counteract this, pinion angle in a drag car running a leaf-spring suspension generally needs to be set to a negative angle when the car is at rest. How much pinion angle is necessary can only be determined through on-track testing, but Competition Engineering suggests starting at 5 degrees negative angle for a slapper-bar setup like ours.
We took SuperNova to a local shop, Hotrods to Hell, where owner Steve McClenon set up our Dana 60 on his chassis jig with 5 degrees of negative pinion angle. We also had him remove the stock-type Camaro/Nova spring perches and install conventional perches with U-bolts, which makes adding or subtracting pinion shims to tune the pinion angle much easier.
Last month we promised that we were also going to make some changes to the front suspension. To aid in weight transfer, we planned to add a set of Moroso Trick drag-racing springs and 90/10 front shocks that work together to shift the car's weight onto the rear tires under acceleration. We received the parts but ran out of time to install them prior to our scheduled date at the dragstrip. But even without them, we were amazed with the results. SuperNova's 60-foot times, the best indicator of a car's traction, improved nearly a full tenth of a second, from 1.78 to 1.66 seconds. With the chassis working to provide maximum traction to the tires, SuperNova ripped off an 11.77 at 113 mph and entered the realm of a truly fast street car. Awesome!
Project RecapSo far we've spent a grand total of just over $13,000 on SuperNova, which includes every nickel and dime from the initial purchase price of the car ($1,200) to the engine buildup, a rearend swap, a new transmission, all new brakes and suspension, wheels and tires, and lots more. Along the way, we've taken it from a 15-second tire burner to a seriously fast 11-second ride, knocking off a total of 4.1 seconds in elapsed time and picking up 17.5 mph in the quarter-mile.
|Part ||Mfr. ||Price |
|Trick springs ||Moroso ||$69.95 |
|90/10 shocks ||Comp. Eng. ||75.90 |
|Reset pinion angle || ||150.00 |
|Subtotal || ||295.85 |
|Previous installment totals || ||$12,965.56 |
|Grand total || ||$13,261.41 |
On The WebIf you've missed any of the previous installments, you can catch up with the progress of Car Craft's Project SuperNova on our Web site, www.carcraft.com.
|Dragstrip Performance |
| ||60-ft ||11/48 e.t. ||11/48 mph ||11/44 e.t. ||11/44 mph |
|Baseline w/540hp 383 |
| and stock Powerglide ||2.60 ||10.57 ||No data ||15.87 ||95.87 |
|TH350 w/3,000-stall ||2.54 ||9.08 ||87.30 ||13.49 ||111.98 |
|Dana 60 w/4.10 gears/slicks ||1.76 ||7.66 ||91.93 ||-No data*- |
|Traction bars ||1.78 ||7.68 ||91.81 ||-No data*- |
|Optimize pinion angle ||1.66 ||7.46 ||92.21 ||11.77 ||113.37 |
|*Testing for these installments was conducted at an eighth-mile track |
Coming Next MonthWe've got lots more in store for SuperNova over the coming months, but we're taking February off. Our next installment will involve brutal wrenching and wicked dragstrip flogging, which will take more than a month to finish.
80 Carter Dr
Hotrods To Hell
100 East Prospect Ave