Some useful debate about exhaust manifolds here Phil -
http://club.triumph.org.uk/cgi-bin/f...?m-1339015610/
Chris Witor still do a range of cams for the iron six, would be worth giving them a call to discuss what you want from the engine -
http://www.chriswitor.com/proddetail.php?prod=CW3021EX
3.27:1 diff is becoming rare now Phil. I love mine on the road - easy 60mph in second without over revving and more relaxed cruising - but if you highly tune your engine, for race use, the lack of low down torque might just leave you bogged down on the line or even stalling while everyone else storms away. It would also put a lot of extra strain on the clutch.
A 3.27 with o/d on 175/80/14's and a red line of 7,000rpm would give you a theoretical top speed of 199mph, which, if you can pull it off would be, ahem, interesting. Based on the rather buffety experience of running my spyder at around the legal limit on the motorway for long periods, you might want to swap your flat cap for a space suit if you go for that option.
A 3.89:1 diff with overdrive would give you the same theoretical top speed as a 3.27 without o/d, which is about 143mph at 6,000rpm or 167 at 7,000, while giving you suitable gearing to get off the line. Even your old Courier 4.11 diff would give you a potential 158mph at 7,000rpm in o/d top.
It's up to you Phil, but it might be worth bunging in what you have in the workshop for now, then work out which diff will best match the ratios to the power band once you know the characteristics of the engine you end up with. After all, it only takes a few hours to change the diff.
Here's the speed/gear calculator to check out what the different options would give you -
http://www.mintylamb.co.uk/gearspeed/
Good luck Phil, I'll drop in on a dry day next week and see what you're up to
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