what would be the best choose.

 

Where they are both gorgeous cars there really is no question in actual performance the XKR is a the performer of the two.

A different computer activated susupension, 80 more hp with the supercharger (xk8 I think is 290hp and the XKR is 370hp) as an example.

We are fairly new owners of our XKR (2000 blk/blk convertible) and it came down to the one we bought or a red XK8 convertible (my wife really always wanted a red Jag conv.) but we drove both and it was like driving a totally different car and the XKR was the one hands down for both of us after just 2 minutes of driving.

Another deciding factor for our choice was that the XK8 also had about 10,000 more miles on it (the XKR we bought had 42,000mi) and was only $1,000 difference in price.

If we by another XK model down the road it will no doubt be the XKR over the XK8 again.

Just my thoughts.

Cheers,

Mark

 

I own an XK8...is it me...or is this a trick question?

 

trick question Jag69. The OP obviously wants the XKR.

R is always faster and better handling.

 

It may not be that simple that the "R" is faster. It may be slower in terms of potential - or rather potential bang for the buck. I assume you're looking at buying a used one. If so, if you buy an 8 you'll have an extra $10K to $25K in your pocket to play/modify with. So it could be when is faster - a completely OEM stock "R" or a really built up "8" for the same (or less) $$? For that savings you could very possibly put a MUCH better supercharger on it, larger intercooler, superior exhaust, upgraded springs and struts, water/alcohol injection - and if an early 5 speed a Q LSD with 3.5 or 3.7 gears still allowing a top speed of 150+

If a twin screw supercharger could be put on an XK8 - go for the 8 IF you REALLY are a motorhead who would do it - but DO YOUR HOMEWORK to make sure the components would work. (basically NO ONE seriously modifies XK8/Rs - likely due to lack of info and components.) Then spend the huge difference you saved for a twin screw and good exhaust.
This ASSUMES a twin screw will go on. Ask AVOS his opinion about that in this forum.

If a twin screw could be put on an XK8 it is the better bargain - as an "R" cost $10k or more dollars than a "8". A twin screw would set you back about $6-9K - and make more far more power.

I've also learned that when Jaguar went to the 6 speed automatic they apparently eliminated any ability to get away from 3.03 rear gears - a real disaster for me... (it's like trying to run the 100 yard dash or play football on stilts - sure, your steps are longer but...)

It appears Jaguar was determined to keep their cars off of drag strips and to insure that among modified cars Jaguar would always be the slowest in acceleration? Mostly you get what you get with little modifications possible, but since you could bump it up to around 550 it'd probably be quick enough for ya.

IF - A BIG IF - a twin screw supercharger (Whipple or Kenne Bell) will go on an XK8 it is very possible that for around $10-15K you could get an excellent XK8 early model (back when they had 5 speed autos) and then have about $15K in your pocket saved over the price of a used "R" - to customize and build it up with.

It appears that the newer the model year, the more you are prevented from modifying it for performance (power) due to a control-freak computer system put in by Jaguar that will decide how you should be constrained for your own good.

The COUPES are MUCH lighter weight than the convertibles and from what I read they put tighter suspension in the coupe, which also has better areodynamics overall.

If you're buying a new XK8/R, then go for the R in a coupe. The '10s have better 3.30 rear gears and 520 hp (if the computer would ever let you use it) and then with minor mods you could probably add 20-30 horsepower. I suspect that with the '10 a remapping of the ECU would yield more power if the programmer knew how to get around all the go-slow stuff Jag programmed it.

Unless you're a real motorhead who really is going to head for serious mods, obviously go for an "R." If a used one, do your research FIRST! The model years are different in a variety of ways. Earlier models have some advantages, later ones have others. Had I done super detailed homework I would not have gone for an 05 - instead probably an 02ish.

 

fine... in stock form.

Remember, the Rs got a different transmission also, so even if you did up the power on an '8' version, you may be doing damage to the ZF tranny. I mean, certainly there was a reason for the beefier mercedes transmission with the increase in HP.

 

The supercharged R might be faster on straight roads by my XK8 is capable of a great deal more than I am on the twisty mountain roads. It is really limited more by traction and centrifugal force than the engine/transmission around here. I had the opportunity to buy again after the first one was totaled, in an intersection, and chose the '8' again.

 

The others have hit the high points--these powertrains aren't easily modified for more power, so don't expect to drastically improve the performance without major investment.

It's up to you if the nonsupercharged cars are fast enough for you--

xk8 -- 0-60 in the mid 6's, 1/4 mile in the mid 14's
xkr -- 0-60 in the low 5's, 1/4 mile in the mid 13's

TTYL
David

 

good summary david. I don't need the Rs extra HP, at least not 99% of the time. The regular old 8 is fast enough for me, but I wasn't raised on muscle or been slowly climing the performance curve on automobiles... 78 camaro, acura 4-door, isuzu rodeo SUV, lexus g400, jeep SUV, now the xk8 - by far, my fastest car of all of them

 

You cannot simply add a supercharger to a normally aspirated motor and run any kind of boost without bottom end modifications. So the idea of modifying an XK8 to make it faster for less money will not work. I doubt you could ad more than 5 or 6 lbs of boost with out engine damage, and the stock boost on an "R" is double that.

 

remember higher compression plus boost = more low end.

If you want speed buy a crotch rocket and be done with it.

 

High compression plus boost = more low end torque yes. But compression plus boost @ higher RPM's = more pressure= more heat = piston failure. The pistons on any motor that uses forced aspiration are designed to handle this, and always have a lower compression ratio. N/A motors not so much.

 

In the early 90's some friends of mine bought a used 28' Cigarette boat with a Chevy 454 CI engine. It pushed the boat into the low 60's. They wanted to go faster. They put a supercharger on it. It went faster, up into the low 70's, for about a month, then the bottom of the engine blew up. They told me that they did not know what happened. I told them that the added boost pushed the air/fuel mixture past the worn piston rings into the crankcase and, BOOM! Luckily, it did not sink the boat, but that was the end of it for them. Of course, there could have been other factors at work as well, such as a main bearing giving out, and so forth, but from what they told me that the mechanic said it was the explosion from the air/fuel mixture in the crankcase that most likely precipitated the whole thing. I grew up going to the drags strip, and there were a lot of dragsters with superchargers, and it was not uncommon to see the bottom of the engine blow up from time to time.

 

WE had a left over motor from a scrap 3000gt we had lying around for parts. so we slapped an extra set of turbos on it at 10.1 compression. it ran great, and pulled like a monster at 9lbs of boost. For about a week! Honestly we were surprised it lasted that long.

 

Plenty of kits around, that you mount on cars with no bottom end changes. But since you have good compression, you keep the engine response and you still get plenty of hp from the added 50% boost.

A friend of mine mounted a compressor kit on his 3.0 Z4 BMW, and gained a 100 hp on it. No bottom end changes or any other mechanical change, and moderate boost.

I'm sure this can be done on a XK8 as well.

I am not sure however how you would solve the engine management problems. It's not easy mounting an aftermarket EFI on these cars is it?

 

You could ad on a piggyback module to alter the fuel management as most after market kits come with that. My point is that for the diiference in cost between an "8" and an "R" you cannot reproduce the power gain reliably just by bolting on a supercharger. Plus as previously stated, you will have a weaker transmission and different suspension, that also was not set up to take the extra power. Look it all boils down to this. If you want the added power buy an "R". If this is not an issue for you then an "8" might be the better choice for you.

 

I'm not entirely sure what the price difference between a XK8 and an XKR is in the US. In Norway, the price difference is almost 20 000$. In that respect I would think that mounting a compressor on a standard XK8 would be a good performance idea. You will wreck the transmission quicker though.

Regarding piston damage. Most modern cars have knock control. If you make sure you have good mixture and the knock control works, you should not be able to hole a piston on a modern car.

Like I said earlier, a friend of mine did this on his normally aspirated BMW, and it has worked reliably for years.

 

These car are "used" now 1997-2006. The price difference deminishes with every year. with that being said, you can hardly use the "my friend did this to another car so it works" argument. I have been in the automotive industry for 30 years, 20 as a master technician, I also hold a smog license. You are correct that modern cars have "knock" sensors, they retard the timing and richen the mixture if its detected, this cools the combustion charge. In this manner it can make up for lower quality fuels being used. The object is to, in a cost effective way, reduce "nox" a harmful emmision caused by excessive heat made in the combustion chamber. If the vehicle is meant for premium and you run regular, the vehicle will run but at a reduced power level. This just makes my point, what do you think would happen then if there is boost? All I am trying to communicate is, the cost will out weigh the gain. I wouldnt even argue the point if there was not a simple solution made by the factory already.

 

Kinda sorta on the same point, does anyone make turn-key performance engines that you can drop into an XK8? Of course I know the transmission and final drive would have to be altered, but are there such engine suppliers out there?

You could always order a rebuilt XKR engine and start from there too, but I'm not sure that would be cheap.

 

Allthough I do agree that buying an XKR is a much better option, I still think that mounting compressor or turbo with low boost on a normally aspirated engine is doable. In Norway at least, big engines are expensive to import, so a lot of compressor kits for NA engines is sold here. It's not just my friend, ES tuning in Norway do this to several hundred different cars here every year.

Personally I wouldn't do this to my Jag, I still don't see why you cannot do it on a jag if it is doable on NA BMW Z4's or on Alfa's for instance.

I think a compressor conversion for the Z4's, including a total remap of the ECU will cost around 10 000$ which is about half of the price difference between an XKR and XK8 here.

Here for instance:
http://www.autodeltashop.com/product...roducts_id=583
Autodelta in UK sells bolt on compressor kits for the Alfa Romeo TS engines.