It really depends what you want to do to your car. You can start with a manual XR6 and replace the doorskins & seating with luxo gear (keeping in mind that fitting electric windows, Ghia cluster, climate control, etc may be difficult electrically), or you can start with a Ghia and slot in a T5 box, pedal box & clutch, etc. There's negligable difference between the motors, and the non-XR motors respond better to general modifications (for turboing or decent NA mods)
I just recall reading from an old Jim Mock article that non-XR heads are better for porting & shaving, due to the XR heads already having some work done to them. Think of the non-XR heads as being a "blank slate" on which better working can be achieved.
Having said that, XR heads tend to respond better to your basic N/A mods I am lead to believe, such as exhaust, extractors, XH snorkel, etc. (if I'm wrong, someone please correct me!)
As for turboing a car, you don't want a high(er) compression head! Boost from a turbo will "give" your car extra compression, and more compression = you need a higher octane-rated fuel to avoid detonation. Since a non-XR head has a lower compression rating than its XR counterpart, it will perform better if that particular motor has a turbo hanging off the side. I think every 2PSI that a turbo pumps into the motor = 1 point of compression. And once you go above (I think) 10:1 compression ratio, I think you need 95RON (premium) fuel for your motor.
So, if your car normally runs on 9:1 compression, having an 8PSI turbo will bump up the compression to 13:1 (8PSI = 4 points --> 9 + 4 = 13). So you'll probably need 98RON fuel for this compression ratio. This is why higher-boosting turbos need intercoolers, water injection, timing retardation, etc, to control/minimise detonation.
Your XR head might have a 10:1 compression ratio, so slapping that same 8PSI turbo on will give you 14:1. You might need 100RON fuel for this, being either 98RON + octane booster, or some form of avgas, etc etc. (or better intercooling)
Makes the car a bit more of a bitch to run.
Oh, and then there's toughening up the bottom end, such as the pistons, so that they're happy with that compression that they're dealing with. Holed piston = ouch!
The AutoGuide.com network consists of the largest network of enthusiast-owned enthusiast-operated automotive communities.
AutoGuide.com provides the latest car reviews, auto show coverage, new car prices, and automotive news. The AutoGuide network operates more than 100 automotive forums where our users consult peers for shopping information and advice, and share opinions as a community.