I'm sorry. The Black Dog kit is what Motorcars LTD calls their coilover conversion.
And I have to agree about high speed highway ride. I just took the SV8 to OK and the speeds out there are 75 to 85. It's a dream ride then. The most comfortable car I have. So, it would depend on cost and parts availability as to whether or not I would repair an air suspension fault. But, the seats in the 09 are a different design than the 04 and much more comfortable. That and the suspension make the SV8 the most comfortable car.
But, the XJR has made multiple cross country trips on coil overs and really is the go to car for reliability and its "driver" condition.
https://motorcarsltd-parts.blogspot....lt-spring.html

 

Thanks for the link. Looks like you can't order online and have to call, I'll do that when I'm ready. You said its 2600 for their kit these days, wonder why its so much more than everyone else?

I drive about an hour to and from work each day (100 miles round trip) and the Jag is by far my favorite car to take to work. Usually do about 85-95mph and its just a rock at that speed, so composed and comfortable. On days I don't drive it I'm actually a bit sad when I leave work and realized its not there.

 

Yes, their main site is down. Found that second link with some info.
They claim to have used Jaguar engineers for the design of their set ups. I actually thought they were in the UK, not Texas. Hard to say why so much more than the others.
They do ride well, very well in fact. Had to splice their module in behind the rear seat, but no biggy.
When I bought my XJR, I had that same long round trip daily drive. Why I got the car. And I loved it. Still do. I'm retired, so don't drive every day, but when I do that's usually the one I take. I save the SV8 for the special trips.
I'd say if you haven't bought this car, don't let the coil overs scare you away if it's an otherwise good car. Try it as is and if you don't like it, you have options.

 

Oh sorry, I didn't start the thread and not looking at a car with a coil conversion, I have an 06 XJ8L on original air. I guess I just hijacked this tread, apologies.

 

Oops. I didn't notice either. Hope the originator is following along. I can't tell when I'm replying to a PM or a thread. Not qualified I guess.

 

What you're describing there is active suspension, which this system most definitely is not. It's nowhere near fast enough to move air into or out of the struts to accommodate instantaneous changes in strut extension. If that was what it was doing, the compressor would never stop running.

 

There being a leak in my memory system, I need to regularly top it up by rereading DonB's article (via the Sticky's)
Air Suspension & ECATS System Summary: Components & Operation - Jaguar Forums - Jaguar Enthusiasts Forum

As Far As I Know, my car is 15 years old (that's unarguable then ), still works and has never had any air suspension work done.
Unfortunately I had to spend ($US2500) on rear suspension bushes. Just when you thought Air Susp was the only problem
(obviously not DIY, though the parts would have been much of it)

I comfort myself that it's a $100,000 car...dirty worn-out jeans...oh never mind...still happy it's the best Jag I have owned and people still look at it and think I'm rich or something..

 

After re-reading the Computer Active Technology Suspension section of the 04 XJ Model Technical Guide and personal experience, I'd have to respectfully disagree. The movement I'm talking about is within the range of the reservoir to compensate for and the suspension does exactly that, switch from soft to firm to follow "undulations" and keep the tires in contact with the pavement. The compressor would come on only to re-charge the reservoir if it couldn't keep up. Doesn't happen all the time, but it is noticeable occasionally on long freeway trips.

 

Switching from soft to firm is CATS, not the air system. That's a change in shock valving, not a change in air pressure.

That's literally the piece you read and quoted... Computer Active Technology Suspension, i.e. CATS.

And only the OEM Bilstein struts can retain the CATS operation, as far as I know. The lack of CATS function with coilovers, and with aftermarket air struts, is why you need that electronic module on 2006 and newer to get rid of the CATS dash error.

CATS is part of the ride experience you're describing, not necessarily air vs steel springs. It's the dynamic shock valving change. Even keeping air with aftermarket air struts will not have the same ride as the OEM with CATS.