Warning: Long article but worth it..! There are a lot of information about how we upgrade our old engine… some says.. : drop in the 2.5 cam, milled the head by 0.02 inch, ported and polish etc etc.. all is good advise but I read this discussion (or debate) with great interest and decided to put it here in case we need reference lets start (I deleted conversation that are irrelevant), and unfortunately, I can not see the poster names unless it stated at the end of the post.
Someone: I think that these engines have a specified max rev limit of 4600. Though going over 4000 for anything short of a few seconds is probably not a good idea. My tachometer has the occasional misbeat but in no way does it wander like yours does. If its something to do with the ignition and not a bad connection it’ll show up on a vacuum guage as well, they’re cheap, easy to fit and very useful.
Roland (ACR): We set the Power Plus engine red line at 5200RPM this will give plenty of safety margin as they will run to 5700 on an intermittent basis with no problems. There is little to be gained by running much over 4800 as the power falls off rapidly over this point. However if you enjoy using the power e.g. racing rush hour traffic, off road competition etc it is useful to be able to run past maximum power so that the engine drops into a more favourable part of the power band on changing up a gear as the spacing between gear ratios on the 4 speed box is large. Regarding oil temperature the red line is around 250 however Mobil 1 will work safely well beyond that . We would keep the 250 as the upper maximum it is very unlikely even desert temperatures will push the temperature this high.