She's a blow-through set up (like the Normans), so you end up with a lot of volume full of air/fuel mix... add a burp through a passing valve (or high valve overlap) and you bang the blower. The bigger the plenum (... or add an intercooler) and they get worse (Eldred used the word "nuclear"
).
I would not recommend a "blow off valve" for this. The are generally made to stop surge in centrifugal turbos when the throttle plate shuts... and not for relieving the sudden, hot and heavy pressure from detonating air/fuel. Not likely the BOV would last long. It would then start to pass - letting loose an air/fuel mix into the engine bay on the sly. Whilst you can plumb a BOV back into the inlet manifold, it is useless in this case... it just transfers that explosive pressure back into the carb/mixer, blowing of the air filters. Many BOV's are also set up to sense vacuum before they open (throttle plate shut, engine sucking madly on it, tapping downstream, high vacuum, open BOV, vent that compressor that is spinning madly up against the shut throttle plate). What you would be looking for is a BOV that opens under high (...explosive) pressure rather than vacuum - finding one could be hard.
You could use an industrial relief valve ... but they don't look right. I've also seen aircraft cabin pressure control valves used, though finding one is like rocking-horse poo.
A better (and dead simple) option is a relief valve plate - either make your own, or buy one to weld/bolt on from Weiand (part number 7155 for the smaller one or a larger size available as part number 7158), The Blower Shop (part number 2589) or Aussie Speed ("universal back fire valve"). I've got dimensions for the typical home made triangular Norman one if you want to make one.
I've got a fairly good write up on this with photos, dimensions, spring settings etc. I'm aiming to post it around next weekend in Harv's Norman Supercharger Thread... if you need the info earlier, PM me an email address and I'll PDF it up and mail it back.
Cheers,
Harv (deputy apprentice Norman supercharger fiddler).