David Vizard goes into alot of depth about this subject. He says the bigger
the pressure drop inside the crankcase the better (well almost). This will
actually gain power as the crank and rods do not have so much air to
"slosh" around in the crankcase. he also says that you get a good vacuum at
idle to drop the pressure by that at part and full throttle the vacuum is
not enough so he suggests using the exhaust system as a vaccum sourse at
wider throttles. This only works with a very non restrictive exhaust system.
About having large tubes with little restriction as leaning out the mixture
too much. If you have an airtight engine (or almost) the inlet vaccum
cannot pull in so much air as too lean out the mixture so the car will
hardly run, at least I think. As long as you have an airtight block
the inlet vac should only pull a slight volume of air (mostly toxic vapours)
from the crank and maintain a vaccum.
I have not actually worked much with these so the above is my untested
opinion. (I've just had a paper of mine slamed so I'm going to give out
my opinion only very gingerly, BTW it was not a legitimate slam and I'm
writing a rebutle now!)
Cheers Dennis
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