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Diesels are very noisy in the air intake,

I don't know where you are getting that from, if set up right it shouldn't be much more noise than a gas engine. I wouldn't be using the word 'noisy'. The exhaust should be more 'noisy'.

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a new industrial unit (about $5000us).

Ouch, thats gotta hurt. I can get second hand 4JB1-T's non intercooled over here for $2500 (~$US1500), and adding an intercooler is no problem. Hell you could buy a whole 4JB1-T MU for $US5000 over here, that and the 3.1L MU's too.

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Even large hills are not a problem in 5th once you are doing 100km/hr.
Yeah, they eat hills for breakfast, but if someone is going slow and you drop the revs and lose the turbo boost it's hard to get it back. <img src="/forums/images/graemlins/frown.gif" alt="" />

Try some off road stuff and see how it holds the revs. I've climed up river banks wondering why it would only hold the revs and not accelerate... it was in third. <img src="/forums/images/graemlins/rolleyes.gif" alt="" />