I have been using the MSD AL6 and it has performed very well. The only downside of this unit is some other units of the same price range have infinatly adjustable rev limiters. While the MSD uses these little resistance pils. The unit comes with 4000,6000,7000 RPM. You can buy others but they only come in sets of ten ( in 100 RPM increments). For street use in a 911 the 6000 that comes with the unit will work great (4000 if you realy want to protect your engine and don't go very fast
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