I have the opportunity to get a complete high mileage 2001 supercharged engine delivered to my house for $600. My problem is that the supercharged engines had a compression ratio of 9 to 1, and the normal 99 XK8 has a compression ratio of 10.8 to 1. If I do this nightmare, complete with the larger injectors, and all the correct pulleys would I need a blow off valve on the plenum? Would I need to change ECMs? I think I would save a lot of grief, by using the original ECM. The engine looks basically the same, except for the supercharger plenums, and that massive compression ratio difference. I am sure the bottom end would take it, but I am worried if the naturally asperated XK8 trans would hold up, if I drove like a human being. I can't really do burn outs with wire wheels anyway. What codes could I possibly be looking at, if any? I saw the fishman's steps, but I think I would go another way, with a smaller crank pulley, and maybe the blow off valve if it is needed. If I talk to some of my IMSA friends, they may have a spare adjustable blow off around. Is all this grief worth it? There is no way in Hell I can afford an XKR.....Thanks for your ideas.....Mike

 

You wouldn't want a blow off valve that vents to the outside, that would confuse the heck out of the ECM.

There already is a vacuum actuated bypass valve that connects the output side of the supercharger to the inlet side so it isn't constantly making pressure at light throttle.

It's basically a flapper valve and you could probably modify it such that it leaked when closed and limited the boost.

Might be easier just to do the math and reconfigure the pulley ratio like you mentioned. Then the boost could never be more than you intended.