Dan, is there a way to correlate (loosely) 1" H2O delta plenum pressure to delta CHT in flight? Seems I've seen some info on that before but can't remember where.
A standard Lycoming cooling chart offers some insight. Here I've plotted the effect of one inch less deltaP, 5 vs 6 at 5000 feet. At 75% power and 60F OAT, pretty common at 5000 feet in the summer, one inch deltaP is the difference between bumping max CHT and running somewhere below it. The numbers are appropriate for a cruise climb around 135 KTAS.
Do remember that the cooling charts are empirical, and assume "standard baffles", an undefined factor. Our goal as builders is to better this performance.
Would one assume correctly that lower plenum pressure remains the same if one covers (2) mag blast tubes, but upper plenum pressure increases by roughly 1.5" H2O?
Reducing a leak means the lower plenum pressure would become slightly less, the degree depending on the area of the exit, and the ratio (leak flow/cyl-OC flow).
Upper plenum pressure would increase, roughly by 1.5" if Looper's number can be applied literally. Thinking about it this AM, I'll recant slightly and venture an opinion that blast tube removal would
not equate to 1.5", because (1) corrugated plastic tube is a very draggy duct, and (2) the inside diameter is smaller than 1", i.e. an area of 0.785. A quick look at the net says ID is around 13/16, so area would be about 0.5 sq inches per tube, and corrugation would make it flow less than a simple hole of 0.5" area.
I'll venture 1/2" H2O loss, an opinion. Measurement trumps opinion, so someone with blast tubes should do a with and without. I'm not cutting holes in my rear baffle and installing tubes just to quantify a leak I think is useless anyway.
Also, this post seems to be saying a Slick mag will not necessarily benefit from a blast tube. Is this a correct interpretation?
My opinion, yes, but if you want 'em, go for it. I'm sayin' it ain't no free lunch.