I think that with vehicles that they produce today that are more complicated in assembly and function than earlier vehicles "do it yourselfers" have a hard time screwing with stuff that requires much more precision and sequence. Take a 4.6L engine for example. You have to be able to line up the chains (DOHC engines are a nightmare, i've never done one and don't really plan on it either), and it won't seal if you don't torque it in sequence, with new torque to yield bolts, and make sure that you reconnect every sensor and move the 100 other things that are in the way something may be done wrong that will require re-dissambly. Hell, if you don't follow sequence you can warp a head by taking the bolts out. Aluminum may be lighter, but it sure sucks when it comes time to have to re-seal an engine because of the many precautions you have to take. Another thing to consider is that newer vehicles also have alot of power and better matched parts out of the factory and tried and true methods of increasing power aren't really applicable anymore. You can't adjust timing, you have to remove the engine basically to put on headers and using the wrong sealants can affect bolt torque and sealing as well as destroy, for example, oxygen sensors (silicone, etc.).
Basically cars ARE harder to work on and as a result are also harder to tinker with. This, at least, is my opinion.
-I think this is also why kids make those "rice grinders" so stupid is because all they can really do is bolt on intake, exhaust and stickers.
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'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT