Are OBD2 plug-in performance chips worth it or are they b.s.?
Anybody here use one? Which one?

 

ALL of those are completely fake. A OBD2 interface cannot do anything worthwhile to a vehicle without already knowing all its parameters, and then it would have to modify those. Not what a small little dongle is capable of.
Some of those CAN modify the speed of the electronic throttle so you may THINK you have more power, as more throttle is applied much sooner to give the illusion of more power. Most of the time those are also 100% fake.
ESPECIALLY on these Jaguars. Very little can be done without a lot of work already done to make that possible. There are some tunes, and some of those work. It's a full-on replacement of the existing ECU map, not just a little tweak of one parameter.
You would also need (not 'need', but highly recommended) to do some modifications to (usually) exhaust and/or pulleys.
Nothing's cheap and easy for these things.

 

So funny, I was just reading an article yesterday about these fake things (probably old info, but I just got to it). Taking it apart, they find only the power and ground pins connected to anything, and all it does is power a flashing LED. Since it doesn’t shut off, it just drains the battery.

 

If you bought an XK and now want more power, I suggest that you save your money and sell/trade it for an XKR. You'll save time, money, and especially aggravation.

Unless you track your car, the 4.2L XK is quick enough, the 4.2L XKR is more than enough, the 5.0L XK is almost a 4.2L XKR, and the 5.0L XKR is overkill and scary quick. My point is that things will go sideways faster than you think. The electronic nannies can't always protect you from yourself.

I have both SC engines, the 5.0L SC in my 2013 XJ and the 4.2L SC in my 2009 XKR. The 5.0L has more low-end torque than you can safely use on city streets and, I admit, it's a hoot on highway entrance ramps and passing slowpokes. Don't forget that traffic cameras and license plate readers are everywhere. 1984 is now and Big Brother is watching!

Of course, I've thought about getting a tune and pulley on both cars. For me, it's not worth the cost. Advancing age brings sanity and awareness of slowing reflexes and vulnerability.

No worries, because I sleep well thanks to my Umbrella Policy!

 

As a guy who used to drag race quite a lot, I find the 5.0 XKR decent for the street but not even close enough for a good stoplight getaway just for FUN. Anything that takes longer than 1-1/2 seconds to get the first 60 feet is not quick enough. That'd be about 2.2 second 0-60 time. 1.35 seconds almost equals out for 60-foot 0-60.

 

Exactly my point. Sure, it's a lot of fun, but that quick can get you sideways in a heartbeat. It's not just about your own life, it's also about the lives of the bystanders.

 

I believe the OP needs to clarify what performance increase he/she is hoping to attain.

 

Snake oil.

 

This advice chimes completely with my own thoughts. Look up "XK Breaker" on Ebay, or chase a few down on Copart. Almost always they are nice cars turned into breakers having had the back wing and rear wheel pushed in from having the rear let go in circumstances where a kerb or other hard place was there to bend the aluminium. Too much power for the friction on offer. I have two 4.2 atmospheric engined Jaguars, neither have limited slip diffs (although they of course have the various software surrogates) and both are nippy enough for my use on public roads. Enjoy!