Hi Guys,

Does anyone know exactly what compression should be on 1998 - 2003 Jaguar XJR (Supercharged Motor)?

I'm well aware that these motors may have slightly lower compression due to the added boost from the supercharger. However, upon reading online, people are saying to accept a reading anywhere between 125psi to 150 psi or 150 to about 175 psi, roughly around there.

To me that is a very general board of numbers and I'm looking to get an exact or close to an exact number, etc.

I'm also aware that a naturally aspirated motor like the 98-03 Jaguar XJ8 should have higher compression or in general, naturally aspirated motors typically have higher compression.

Please if someone who has actual knowledge, much more in depth information or insight on this matter, any information would be greatly appreciated!

 

an exact number is not worth chasing. The reason people put a range because generally available tools have loose tolerances, you must get a perfect seal with the spark plug hole when tightening by hand, the engine must be at full operating temperature, battery and starter must be in good shape, and dynamic compression varies from static compression.

As a rule of thumb, you can use the static compression (8.9) times 14.7 psi (1 atmosphere) to get a minimum number, which in this case is roughly 130psi.

The key result when doing a compression test is consistency among all cylinders.

For a better test of engine wear, look into a leakdown test.

 

That is an excellent description Quadcammer. Did you use to race cars?

 

I did and do. I do endurance racing in an E36 bmw and have a track boxster.

 

Since Jaguar didn’t post actual compression numbers, within 15% of each of the cylinders was how I was taught. If I did a compression/leak down test on my daughter’s 98’ XJR with 282k miles, I’d be astonished if it was within 25% of each other. 😆 Albeit I had a valve job done along with new valve springs and mls head gaskets around 230k miles. The car can still boogie with its original engine!

 

That is very cool. I had an E21 until recently. It was not terribly fast however
it was nimble and VERY dependable. I had to make space for the wife's new car
and I was very sad to let it go.