Anyone achieving a sub 14 second run on a stock SC400 would have to be serious and that would almost certainly mean having the timeslips to back up the claim. I have seen more than one WELL MEANING person just not remember correctly. This is why "eye witness" testimony at a trial is highly unreliable even if it has a strong impact on a jury. Much more likely is the car ran 15.89s and he "remembers" it as 13.89s.
Anyway, I would say yes... it is doable. My recipe:
Definition; stock SC400 has nothing on the car that is not factory except tires and maintenance items.
Stock SC400 with high enough mileage for the differential to be worn and begin working as an LSD. I have seen this on several old Lexus vehicles including LS400s.
Thoroughly clean and properly tune up the engine to assure peak power.
Remove anything and everything that is not necessary.
A/C and heating system, everything including in car parts.
Stock exhaust after the Y pipe
Modify the stock air filter box for cold air
Dash
All seats except drivers
all interior parts EVERYTHING not required to run the 1/4 mile
Everything and I mean EVERYTHING not required to run the 1/4 mile
Everything under the hood... again I mean EVERYTHING not required to run the 1/4 mile
All sound deadening materials... again, gut the car, remove everything and anything anywhere not required or HELPFUL in the 1/4 mile run. Some things are helpful like headlight housings and other aerodynamic parts. Remove rear spoiler.
Cut the springs to lower the car as low as possible.
Doing the above could remove as much as 500lbs from the car.
You now have a stripped out bare metal inside lightened SC400.
Install the skinniest, lightest front tires that will SAFELY work on the stock wheels inflated to the highest safe pressure. Install drag radials on the rear.
Give the keys to someone that can really drive in the 1/4 (preferably a skinny someone)
Do the above and I feel confident that a sub 14 is possible. Just not practical or enjoyable.
