I did a compression test on my 2003 XJR and got the following readings, but experienced an issue when I get to cylinder 8.
fuel pump relays pulled, warm engine, throttle body open

1 - 95psi
2 - 100psi
3 - 145 psi
4 - 145 psi
5 - 120 psi
6 - 95 psi
7 - 95 psi
8 - when i went to go crank the car, rather than the can making a sound of a cranking engine, it just made a constant whirring sound rather than the cranking sound.

Not sure what happened that is causing this, either way not super happy with my figures. Should I chalk it up to a toast engine?

 

Thats not a good sign. I had similar on my bmw race car and it turned out to be a burned exhaust valve so i had zero compression in that cylinder

 

relative comp test with an amp clamp would have saved time

put a camera in #8 find out what happened somebody probably overheated the **** out of it and dropped a valve seat

 

Looks like you either exhausted the battery by the time you came to No. 8, or maybe your starter motor got "tired" from too much cranking. Recharge the battery and try again later.

 

A dying battery would give slow cranking with plugs out or just click. The whirring is the sound of the motor spinning over without building compression