Turbo 4.8-Liter LS Breaks Stock Bottom End 1/4-Mile Record

In March 2019, while at TX2K, Jack Roberts put the LS racing world on notice with his 1987 LS-powered Ford Mustang with a 7.913 at 170.49 mph in the 1/4-mile. With the help of Lil’ John Motosports (LJMS), the fox-body was able to accomplish this feat with a stock bottom end (SBE) 4.8-liter LS with over 140,000 miles on the clock. According to Jack, the heads at the time were unported, and they didn’t remove a single rod or main cap. You can read more about it here. As impressive as this pass was, evidently, Jack and his Mustang aren’t done setting records.

On December 1, 2019, Jack loaded up the Mustang and headed down to Texas to make some passes at the Texas Motorplex. With some newly added changes, the car managed to run a sub-five second 1/8-mile pass and deepen the 1/4-mile record. Jack’s timeslip shows a 1.15 60-foot time with a 4.952 1/8-mile time at 140.83 mph. The 1/4-mile time was 7.808 at 168.11 mph. There are a couple of notable things to take away from the elapsed times. To our knowledge, this is the first stock bottom end 4.8-liter LS in the 4-second range in the 1/8-mile. Secondly, even though the Mustang reset the 1/4-mile record too, it was down on mph. So what changed since the cars last outing?

 

We reached out to Jack to see what adjustments were made since we last caught the car at the track. Jack said, “We figured out the scale I used was inaccurate, and the Mustang actually weighs 2,945-pounds. After the 7.91 pass, I changed the torque converter to a 20-2 PTC 9.5 from the 18-0 and switched the rear end up. We went with a 35-spline Strange Engineering spool, axles, 9-inch ends, 4.10 gears, and also narrowed it a little bit. In order to stretch the fuel pumps, a 50/50 blend of VP Racing Fuels C16 and e85 were used. Finally, we had to make a shock adjustment. Other than that, we just turned up the boost!”

When asked about the mph difference between the passes, Jack said, “I blew a flex joint on the hot side at about 1000-foot, so that’s why boost dropped, and it didn’t go over 170 mph. We may have snuck into the 7.70s if it had held, but I can’t really complain about beating the record by a tenth.”

Here is an image of the blow flex joint that kept the Mustang from running even faster than it did.

Jack would like to thank LJMS for helping him pick this combo and also T&E promotions for the excellent track preparation. Now that Jack reached his goals, he will be making some more changes this winter to make the little 4.8 work harder and push the boundaries.

About the author

Brian Havins

A gearhead for life, Brian is obsessed with all things fast. Banging gears, turning wrenches, and praying while spraying are just a few of his favorite things.
Read My Articles

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