The flywheel on my Plus 8 is lightened but recommending it for an otherwise standard engine is another matter as my engine is a road-going, race spec engine built by John Eales.
Personally, I'd ask John Eales or JE Engineering for their advice. You may release couple of gee-gees but you may find some drawbacks. In your favour is the Morgan's light weight in comparison with a Range Rover so the loss of flywheel mass may not be a real issue.
Wow, yours looks like a right beast! What are the engine details? I will be replacing my engine next year, and would love to hear what you think of yours. I definitely want more umpf, but don't want to over do it. I wont be racing mine, but certainly taking on some track days, and I'm an experienced driver (started Karting when I was 8), but even so, is 280-300bhp over kill?
