Owner of 2010 Jaguar XJL supercharged and I finally just bought the VAP tune (arrives this Monday). I understand my car’s speed is “limited” to 155mph from the factory. Does the VAP tune alter this limit? Thanks in advance!

 

Nedoerr, I know people have gotten the limiter atleast raised up to 186 mph. But, before just cutting it off completely, I would be looking into why Jaguar limits the speed to 186 mph. Part of it is tires. But, there are physical limitations in the design of the car that also dictate things. I will use the Dodge Challenger as a prime example. The 707hp Scat-Pack Challenger has a top speed of 186 mph, but, you get the 1000 hp Demon 170 Challenger and it is speed limited to 155 mph. Obviously with more power, you can get to a higher top speed, so, why is the Demon limited to something less? Dodge explained that it had to do with the braking system is different between the two cars and also that the Scat Pack is dimensionally wider, providing more stability at speed.

A 510 hp XJ is probably drag limited at about 220-230 mph. The car (atleast when I look at my 2016 XJ) is geared for almost 300 mph (my car turns 1500 RPM at 70 mph, so, theoretically, it will turn 6000 RPM at 280 mph in the same gear). I know the brakes factor into the speed equation (hence why the NA V8's and V6's have the smaller brake setup as compared to the supercharged V8's). Kinda like I thought I had heard that the diesel variants are computer limited in speed to like 125 mph (not sure on the basis for this).

Just some tidbits to ponder before you start getting too wild. Would be curious if there is data out there explaining why said numbers were picked. I know the 155mph was picked because that equates to 300 kph and that most of Europe has some sort of law relating to vehicles that have the potential to go over that magical 300 kph speed.

 

Thermo: Small corrections: 155mph = 250kmh (1mph = 1.6kmh) The common limit came from "gentlemen agreement" of European car manufacturers about 40 years ago, to keep speedwar away from German Autobahns. (no speed limits exept near citys) Sure an hypercars etc are free from that... (btw: Diesel is limited to same 250kmh. It stop like a wall there)

The brake force calculation is extremely interesting. Back in times when i raced, we kept brakedisks as small (light) as possible for speed and grip, but we need carefully calculate how much braking energy in second the disks can cope. Vehicle mass is same, but braking from 150 to 50 takes more time than 100 to 50, so faster you go the longer time you stand over brakepedal.
Engine power in fact have nothing to do the disk/brake size, its mass and speed.

Edit: Coming back to question how fast x351 are up to. Tons of factors. For sure the 250kmh are safe, propably 280kmh still, but i believe standard aero is not desingned much more. At least need front splitter to give more steady airflow and back might need underneath trunk diffusor to keep it calm. You don´t want to experience any fishtailing at speed. (there migh be skidmarks somewhere else than road)

 

Yes VAP can set it where you want it. Mine is at 200 mph. I have not hit the limit....yet.
I have seen 180 mph on the speedo. Car was surprising quiet and stable. No drama at all.
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Thanks, I just sent in my stock file today and requested the speed limiter to be boosted to 200mph as well.

I suspect the performance boost will be particularly noticeable on my 2010 XJ as the stock tune only generated 470bhp and 424 ft lbs torque. With the tune it is should increase to 565bhp and 517 ft lbs torque. Exciting days ahead!

 

Be prepared to have a permanent smile on your face!
While these cars are strong when stock there is no comparison to a tune and pulley setup.
Until I got use to it full throttle was a bit breathtaking. Just make SURE the car is pointed straight when doing that.
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