The VX already has low gearing - 4:33 ratio. I'm not so sure lower gearing - especially something as low as 5:38 - would be of that much benefit. 1/8 mile - probably, but not for the 1/4 mile. It may be of benefit - but probably not as much as lower gearing does in many other applications. To take adantage of a set of 5:38's - the engine must produce alot of hp at high rpm.
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I think one of the reasons is that the VX engine appears to produce much of its power down low (at lower rpm) ..... unlike many other dohc engines. The power curve appears to be 'flat' - I suspect the maximum power achieved by the Isuzu 3.5 is at a relativley low rpm ........
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This may also be one of the reason that many feel the VX is faster than the published figures .........
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The camshaft profile may be one (primary) reason. Variable valve timing (ala Honda, Toyota, Nissan ?) would probably prove to be very beneifical for the 3.5. Still - I would bet the engineers were more than happy with the performance of the 3.5 given the intended application.
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Again - this is just a theory - I have never been able to locate a chart with the horsepower and torque figures.
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..... and speaking of gearing, I would prefer 4:10 gearing - over the OEM 4:33. The gearing is a tad low for my use. I never realized how fast I drive on the highway until I started driving the VX. I would not be surprised if the Trooper - especially the US spec Trooper - has higher gearing.