XK8's supposedly have VVT (variable valve timing) but normally aspirated air intake.
While XKR's have static valve timing, but forced supercharger air intake.

What's more problematic, adjustment-necessary and breakage-prone?
The n/a VVT 8, or the supercharged static-valved R?

Is either the vvt or the supercharger more of a maintenance headache?
Is either one smoother than the other?

Would anyone like both in one engine, or neither?

 

Do a search on the forum for VVT and supercharger and see which of the two comes up the most.

While you are at it, compare how many threads there are on ZF gearbox problems (4.0 XK8) versus Mercedes gearbox problems (4.0 XKR). You don't get many reports of problems with the metal thermostat housing on the XKR either, compared to the plastic one on the XK8.

At the end of the day though, if you wanted a super reliable car you would buy a Toyota or a Honda, so the choice is really more about exactly how fast you want to accelerate and the XKR really is a quick car.

 

dibbit is right.

I bought the XKR and XJR that I have BECAUSE of the improved reliability. I have experiences with the ZF 5HP24 used in the 02 and earlier XK8s and all X308 XJ8s in Audis and BMWs. I do not like it. I do however like the 722.6 from Mercedes.

The Eaton blower on the SC cars is a very reliable unit. With time, a rebuild is necessary but not too hard or expensive to do yourself, or at least do the remove and replace and pay a shop to do the rebuild or just buy another unit and swap.

CATS shocks are expensive, but if you need to replace them, you could eliminate the system and put regular shocks on.

Given the improved tstat housing, better transmission, and overall reliability, I prefer the R to the early 8. The later XK8s resolved most of the issues, but realistically, the R will always have better resale and be more sporting (not that the 8 is not fun!).

 

Did the later XKR not get VVT (as the STR did)?

If the XKR has the hose under the SC, that's a pig when it fails....

 

I don't know on the 4.2, but the 4.0L XJ and XK cars I can verify did not.

 

Yep, my hose(s?) failed last year. SC off to replace... And now I'm looking at a knock sensor that needs to be replaced. Guess where it is!

Hard to say the XK8 is more reliable with its weaker trans, it's really a give and a take as was said. The best bet is to just find something in great shape you can fall in love with. I probably would have done a XK8 coupe had I stumbled across it at just the right time like I did with my XKR convertible. But these days with the brand being pushed by the likes of F-Types, the added power and sportiness of the XKR still gets my nod. Though there are days (friggin' heater hoses and hydraulic rams!) that I wish I had the XK8 coupe!

 

An eaton SC is going to need a ebuild after 100k miles or so. So to answer your question the supercharger is less reliable than VVT. However as others have alluded to the supercharged engine comes with a better transmission and better water pump housing.

The ZF 5 speed does not have a good reputation. The Mercedes/Chrysler unit is arguably the best mass produced 5 speed ever built. It was used in everything from exotic sports cars to heavy duty pickup trucks.

 

If you have a look at posts on the forum about the supercharger, you will see people needing rebuilds from less than 100K miles, and others whose s/c are still going strong at 150K or even 250K miles. Most people seem to suggest changing the s/c oil at 100K, if not sooner.

If I was buying an XKR now, I would immediately change the s/c oil irrespective of the mileage and check the s/c for rattling sounds before purchase - it shouldn't rattle or make any kind of knocking sound, cold or warm.

Looking on ebay, rebuilds costs are from £95 to £229 and replacement second hand units from about £400 if they can't be rebuilt, which should obviously be factored in to the purchase price of a car with a rattling supercharger.

 

From the posts I've seen neither the supercharger nor the vvt
seem especially troublesome.
Certainly an s/c oil change every 80-100 miles isn't much.
And I don't see alot of questions about adjusting or repairing the vvt.
Neither seems to be a big problem.

I'll assume they're both fairly adjustment/maintenance/repair free.
Them being similar as far as reliability goes, the supercharger seems
to provide a safe, simple way to boost horsepower substantially,
so I guess I'm not seeing a downside.

Tell me, since a supercharger is just a mechanical blower,
are their electric superchargers?
An electric motor with a fan would seem to be cheap,
reliable and adjustable/controllable.

 

There are, but they are more of a gimmick than actually practical.

Getting electrical energy harvest from a mechanical source of variable velocity is highly inefficient. You lose energy converting from mechanical to electrical, you lose energy further converting it back from electrical to mechanical. One Horsepower is about 700 Watts. On a 12V DC circuit, 10HP to drive the super charger would be about 600 Amps. This would be a constant strain of about double your starter motor, yet for sustained periods, rather than a few seconds.

The conventional 12V alternator and battery system would croak given the loads required, or, you'd have a setup with a smaller load, but no appreciable difference.

The Mechanical(crank) -> Mechanical(drive system) -> Mechanical(supercharger) is far more efficient than Mechanical(crank) -> Mechanical(drive system) Electrical(alternator) -> Chemical(battery charge/discharge) -> Electrical(Supercharger electric motor) -> Mechanical(Supercharger)

There are more advanced sources of electrical energy release and storage, but they are far more expensive than the relatively simple systems used on our road cars. The extra conversions result in a lot of extra heat as waste, the higher the heat, the lower the efficiency. To make such a system work, requires VERY expensive low resistance electronics.

I used to race Electric RC's, and there are some fantastic new technologies out there in recent years, in every aspect, but the cost scaled to a full size car is rather prohibitive compared to the simple mechanical means of power available from the crankshaft. An electric system brings the advantage of allowing more control of the boost pressure independent of engine RPM.

 

Thanks alot Tervuren. Your last sentence was what I was thinking.
I hadn't appreciated all of the energy losses required in the conversion.

I figured electrical would give alot of control, like instead of being fixed to RPM, a non-mechanical supercharger could be set to only come on at say 3500 rpm and only reach full boost at 5500.

Electric fans are generally efficient. What percentage loss would you think?
It might be worth it if you set it to only came on 10% of the time.

 

The existing SC has a bypass so in effect can be set only to come in when wanted. (Which is what happens now.)

It presents a small load all the time when not in use.

 

"Set". Interesting JagV8. How adjustable is it?
Is it done by vacuum maybe?

And how easy is it - cockpit dial or under hood?

 

It's described in JTIS. Not sure why you'd want to mess with how it works...

 

You said one could 'set [it] only to come in when wanted'. How?

Don't know what JTIS is. Jaguar Technical something maybe.

 

Read up on how it works and off you go...

It strikes me as something no-one would want to mess with. You'll need a tuner I think and I doubt anyone already does anything relating to this. Gonna be $$$