Adverse effects depend on what system you underdrive, and how much. e.g. alternator will drop power ( you might lose charge when you run lots of gear at once), and water might increase your temp, meaning you need better radiator etc.
Usually you can get a fairly happy medium tho (the stock systems have a little extra margin built in).
Not too sure on power gains however.
They may take a few days to respond, but barry batten is helpfull.
I can't recall the pulley price (round $140-150+postage)
you will need another drive belt also.
I picked my belt up from B.S.C. motion tech (bearings/belts specialist)as it was cheaper to do this than buy one from battens(I also get trade price),just ask for a part number for the belt from Battens, they were more than happy to help me out.
A small improvement in the seat of the pants, main benafit is you wont be thrashing your power steering pump etc.
Voltage can drop a bit if your idling with an amp pumping out rockabilly...(or swing!)
My water temps were fine, but I have thermos with an adjustable thermostat switch,and a 180deg coolant thermostat.
I had to cut my crank pulley bolt off with a grinder/cutting disc as the B!^ch of a thing wouldnt budge,apart from that it is an easy mod to do
I also run the Batten Performance one and am very happy with it. The pulley was $165 and the belt was $80 but that may have changed by now. You can't really get a better $$$ for kw mod than the underdrive pullies.
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