Has anyone on here ACTUALLY tuned their 5.0L XK ?
I'd really like to hear some feed back, I still really want to do mine
but I can't get my LTFT to balance. must have a leak somewhere

Yes, see my post on the first page of this thread. If you follow my instructions I will send you a pdf of the dyno results.

I will be having mine tuned ASAP, and I will have before and after dyno results. I'll probably go ahead and start another post when it happens.

Along with Dyno results before and after would love to also see before/after real life testing on the 1/4 mile that would be brilliant .

I know that you don't know me or my background. I'm very much a completist, and a bit of a numbers nerd. I can assure you that it will get a few 1/4 mile runs as well as dyno action.

I get really frustrated online when I read about tunes, tuning boxes, modifications and so on because so much of it is based on "it feels faster". I spent a few years of my life at the drag strip every weekend and it becomes obvious to anyone other than a casual observer that most mods either don't affect things at all, or slow the cars down. People don't seem to understand that moving your air intake to a big can right behind your radiator is actually a HOT air intake, or that hollowing out cats and slapping them back on just makes a big area where the exhaust cools and condenses and gets harder to move, or that slapping on enormous wheels can reduce your gear ratio, or that letting air out of your tires just increases your rolling resistance. I have a methodical approach to things and would experiment with the results verified by numbers, not the buttometer. By the time I am done I should have a wide volume of data.

Here. This is a a video of me tweaking my Mark VIII into a high 12s car. It started at a 15.5. I finally got it down to consistent 12.3s and could dive into the very very high 11s on a great day. But this might give you an idea of my method:

Who's tune will you use?

A very methodical and wise approach to tuning and leaving nothing to chance, I like your style .

I was initially talking to Paramount Performance who I believe is based in the UK. I'm not 100 percent sure they understood that I was looking for a tune for a NA XK and not an XKR, since the XKR will return more power per $ when tuned and apparently not many people tune the NA Jaguars. I also talked to a William Waldock at RSC Tuning in Columbus, OH. He assured me that he could get 412 HP and appropriate torque out of a tune and I had for the most part decided to go that route until I ran into this thread, and started talking to AlexJag. At this time I think I am going to go with AlexJag because he is active in the community and has a reputation to consider. I've had a phone call with him and at this point we are mostly just waiting for the stars to align properly so we can go ahead with it.

I would do more research on RSC before considering them. A former well respected member here had a lot of experience with them.

A quick check on Wikipedia and how does one get 280hp , a tall diff ratio and only a 4 SPD transmission into the 11s that's truly impressive

( As my YouTube isn't working at the moment i can't see whether you have added forced induction or not )

Or... just watched an interesting television show where they decided the DB9 needed a supercharger, adjustable suspension, carbon brakes, 100 cell cats... just like in the mustang forums, it is difficult to get the gains and efficiency without a supercharger. And the Superchargers we have is rather mild mannered as compared to some others.

280HP quad valve 4.6L motor, transmission was a 4R70W three speed with overdrive, original rear end ratio was 3.08:1. However... the 93 and 94 models had motors that were hand built by two man teams in Italy in the same plant that makes Ferrari motors: Teksid. The block was easily capable of handling 1500 horsepower. The weakness of the engine lay in the powder cast rods which would fail at 600HP or so. The pistons were also hypereutectic and were prone to cracking because of engine knock / detonation. So you had to watch the fuel delivery closely and make sure you were always at least slightly on the rich side. The transmission and diff could easily take 800HP from the factory, and the half shafts were good to about 600-650.

I had built a custom progressive nitrous system that started at 0 and typically went to 150 in the first three seconds. I would sometimes shorten that but I typically ran it at the full 3. I also had a custom progressive water/methanol injection system that would go from 0 psi to 200 psi in about the first 1/2 second. I ran the water/meth for the same reason that the forced induction guys did: To keep the fuel octane high enough to remove the chance of detonation as much as possible. I believe on 87 octane pump gas I was running an effective 110 octane after the water/meth. Plus there are cushioning elements dealing with the explosion to steam and additional torque from the water/meth alone not to mention methanol is fuel. I had the whole system wired to a wide band set of 02 sensors so when I got on the line and turned the system on, there were two green lights that would light on my dash. That let me know that everything was agreeing and I was good to go. At launch the first light would go from green to blue, that let me know the water was flowing at full pressure, and then the other light would go red, showing the nitrous was full up. If you watch the video from inside my car you can see the system activate. If anything went wrong (primarily with the A/F ratio and fuel pressure) the whole thing would shut down and I'd be back on regular power. The car was tuned to expect the water/meth and on the dyno it was producing about 330 HP pre-nitrous. Total engine horsepower was at about the 480-500 mark which was just inside the margin of safety for several of the components. To push that system any more would require forged pistons, forged rods, different half shafts, etc.

Oh, I forgot, I also ran 3.73:1 in the diff, most people would run 4.10:1 but I knew that the 4R70W didn't deliver transmission fluid to the rear bearing under pressure, and with the transmission constantly revolving at that rate on the highway it would spin-dry that rear bearing.

Short of that I did NOTHING to lighten the car aside from taking out the spare. I drove it to the track and drove it home. In 500+ passes never had a breakdown.

Edited to add: I had a set of 3" exhaust cutouts immediately after the exhaust manifold that I would open up on the track primarily so that I could hear my car from inside my car when I was next to those crazy loud mustangs and Camaros. It actually ran slightly better on the full factory exhaust, but I wanted to be able to hear the engine while racing.

I had a 1998 Continental with that same engine, although front wheel drive. I surprised many a ricer at the stoplights with it, but got rid of it when I bought my 2000 XK8.

The connie motor was close, but it was drilled only to accept the FWD transaxle so you couldn't transplant it into a RWD car. Still had that insane variable geometry intake on top of it if I recall correctly. The XK8 was quite a different car after driving that monster connie, I am imagining?

I had a 2001 XK8 and while I love my current XK, honestly I think the sheet metal on the XK8 was sexier.

Of course it is easier to get power out of forced induction car but don't underestimate the conservative tuning on na cars for whatever reasons, sometimes it's emissions or they are trying to keep power down do to a higher specd vehicle in same sales spectrum.
A long time ago I owned a 1992 Lexus sc400. The early versions dynoed around 175rwh. After I had an exhaust fitted, modified the intake and tuned excessively rich afr ratio to a more acceptable number. End results were a gain of 35 rwh and around 45 rwtrq 208whp and 232rwtrq. As
​​​​to na XK , they tend to respond very well especially at low to mid RPMs range. Badicedog maybe can chime in on before and after of his na XK that I tuned.

I also prefer naturally aspirated engines - dealing with heat in blown engines adds A LOT of complexity if you want to retain longevity. Many people who tune supercharged engines don't appreciate that they also need to attend cooling side of things or the car is only good for a couple passes under full power. I mostly do track, so if it can't take 2 hours at full throttle on 100F day, I am not interested.

SEAICE. I replied to your pm , not sure if you got it let me know pls