Hi AussiePhil, sorry it took me a while to reply - I haven’t checked this thread in a while!
So it’s still not 100% solved but I've taken some steps
I’ve bought and installed an aeroflow 140 amp alternator. I’ve been chatting to their tech support and they have been great. They provided me with this which is very helpful.
https://i.imgur.com/d1YlyI9.png and here’s the manual for the alt
https://aeroflowperformance.com/pub/media/Instructions_PDF/AF4273-1140.pdfThe manual states to calculate pulley on the inside diameter (where the belt sits) not the outside diameter. I’ve spoken to aeroflow tech support and they insist on the 44mm number (as stated in the manual) for the calculation. The alternator came with a 68mm OD, 44mm ID pulley (and the manual is wrong stating an 81mm OD- they are fixing it). Pair that with my March underdrive crank pulley at 140mm OD and I’m calling it 117mm ID, I have a 2.65 ratio.
I am guessing I am idling at 600rpm (I like it low ish) but I am not sure so I have a nice moon eyes tacho that I’m going to install. At 600, the alt would be spinning at 1590rpm which is too low at idle. My target is 2000rpm to give me 60a to be safe with the elec fan running at idle, but 1800 for 42a might be enough.
That powermaster pulley you mention Phil is interesting. Is it this one
https://www.ebay.com.au/itm/255692844342?mkcid=16&mkevt=1&mkrid=705-154756-20017-0&ssspo=V0KU3YCwQTa&sssrc=2349624&ssuid=v5w5lP4lS-O&var=&widget_ver=artemis&media=COPY ?
At 54 OD and my crank pulley at 140 OD I’d be on 2.59 but it would have to be better than that as the aeroflow is a 68mm OD. Aeroflow insisting to calculate on the 44mm number is making it hard for an apples to apples comparison so I’ll prob get that power master pulley and install it as it’s quite a bit smaller
so:
1) next step is to put in phat pos and neg ground cable (existing cable looks small ish) to the alternator.
2) hook up tacho to calculate my true alt rpms
3) power master alt pulley
4) if all that doesn’t work, get a larger crank pulley
The crank pulley is interesting. I have a 4 bolt and I can’t seem to find anything aftermarket (billet alu) that is larger than 5.75 inch/146mm in a 3 groove (I’ll be putting on aircon at some point). Had a look at March and cfv and no 6inch plus sizes. 5.75 might do the job, just won’t know until I get the tachometer set up.