The 2004 XJR is listed @ 390bhp and 399lb/ft. torque and the 2008 is listed @ 420bhp AND 413lb/ft. torque. Same engine...does anyone know what exactly was changed to achieve those larger numbers in 2008 vs. the 2004 XJR?

Adam

 

No responses on the specifics? I can offer a generic answer. Same engine? Probably not. Outside of the engine itself, all that can be done to get this much increase is different induction system or exhaust. And it's not likely for those two to have as great an impact as 30 HP. The engines boast the same cid but they could have differences. The cylinder heads are likely to be different castings, redesigned with more efficient combustion chambers and better flowing ports. They could have larger valves. These 2 things contribute the most to performance increases. Improved combustion efficiency can often allow higher static compression or dynamic compression (often done by shortening camshaft durations to advance the intake closing event). The advances in engine management alone allow for some of this above technology to be employed on later engines. Examples: engines can run with more ignition timing due to knock sensor feedback, fuel and ignition timing are computer mapped for optimum power & efficiency in different RPM & load ranges.

 

Appreciate the response, but couldn't they have simply changed the supercharger pulley making it spin faster providing a further presurization of the intake charge? We're talking only a 7% increase in HP...Just wondering if it's all that simple.

 

Atweis, the mode by which they can increase the power is possible via many different ways. If you are looking to get a few more ponies out of your car, then the easiest/cheapest way is going with a slightly smaller pulley which will generate a slight bit more boost. To gain the 30 hp, you will need to raise the boost by about 2 pounds, that is it. The big issue is how much boost the engine can handle total. That number I don't know.

Like was mentioned, the heads are normally the biggest limiter and some simple work on them can result in some decent gains. You can take your current heads and through a process of porting and polishing the heads, a gain of say 50 hp is easily acheivable. Granted, the cost of tearing the engine apart, paying someone to port/polish the heads can make you think twice. You would probably be looking at about $2000 for a job like that.

Chris "Thermo" Coleman

 

The Xkr and xf s/c are 420hp, the xj is 400.
The only difference is 2 intakes for increased airflow with 2 mass airflow meters. The software is different and the boost is slightly increased.