Drag racers have long favored the straight axle as the best source of traction when the tree goes green. Imagine my surprise after asking Graham Behan, Lingenfelter Performance Engineering’s chief engineer, what they did to the IRS on their ZR1 Vette project to achieve a 1.5-second 60-foot time… and the answer was “Nothing”.

After picking my jaw up off the floor, we proceeded to get into some of the finer details about this particular Lingenfelter project.

We have reported previously about this car being the first 2009 ZR1 ‘Vette into the 9-second club, but along the way a significant part of the story was the 1.5-second short time largely unrecognized when the tale was mainly about the car’s 9.81-second, 145.7-mph finish at the big end.

The ZR1 project was all about seeing what could be done using stock GM components. Behan was kind enough to detail a number of the iterations for us. In the first go-round, the LS9 motor had a simple series of bolt-on additions, but it was the M/T drag radials that pushed the car to a 10.55 @ 134 mph level. In stock form, the ZR1 had been churning out 1.75-second short times. Now, those were reduced to around 1.624 seconds.

An interesting, but not ultimately helpful diversion came when the team tried to fit a set of 3.91 gears to the car. The only way to make this work was to modify and install a C6’s Z06 differential, but that particular mod broke on the first pass. In search of ever better mojo, the group installed a Tremec 6060 manual transmission from a Z06 model. The key here was a change in first gear ratio. The taller 2.66 first gear more than offset the change in rear end ratio from the previous attempt, when compared to the car’s original 2.29 first gear.

Then, Behan shared a small secret with me. During the previous runs, all testing had been done at a strip in Muncie, Indiana. A summer event for Lingenfelter customers resulted in an invitation to run at Maryland International Raceway (MIR). The team took advantage of that and the differences were staggering. The project car easily popped a 9.813 ET at 145.75 mph, largely due to the reduction in both ambient temperatures and a significant improvement in density altitude, 2800 ft in Indiana compared to -900 ft at MIR. According to Behan, the previous version of the project would likely have broken into the nines without further mods, but the recent changes had sealed the deal.

Along the way, Behan spilled the beans about getting great short times out of IRS cars. Back as far as 2002, a Lingenfelter customer’s C5 twin-turbo had turned in results of 8.95 @ 153.5 mph, with a 1.3-second short time. More recently, another Corvette customer’s Viper hit the same milestone. In both cases, the cars had been fitted with Penske dual-adjustable rear shocks.

These let you adjust both the compression and rebound performance. When the compression phase is set to almost zero resistance, it lets the car squat in the fastest way possible. Setting the rebound to the maximum setting assures that the improved weight transfer stays in place as long as possible. The results, as we have previously reported, are simply spectacular.