IM vacuum reading at idle - what are the variables?

[DeWitt Harrison <de@aztek-eng.com>]

In the process of investigating the control of A/C air distribution
in my type 44 it was found that at 800 rpm, my IM pressure was
about -5 in-hg while that of a similar car was about -10 in-hg at the
same rpm. (All but one of the air control flaps are moved by
vacuum motors and one of mine was balking.)

The reference car was stock with the same MC motor. Mine has
a freer exhaust but a stock cat. Mine has a higher duration cam
but stock air box, filter, IC, throttle body and manifold. My cylinder
head has been worked some and has intake valves that are a couple
mm larger. My ECU is a mildly tweaked unit from IA.

Given that the same amount of air is being displaced  - same rpm x
displacement - and the intake path is essentially identical, would it
not be reasonable to predict the IM pressures should be about the
same? I would think most of the drop should be dropped across the
throttle body at idle anyway. And I would expect the throttle butterfly
to be cracked about the same amount too since both engines should be
doing the same amount of work at idle. (But maybe that logic
doesn't apply in an injected engine.)

Could the differences in breathing between the two motors account
for this pressure difference or should I worry that my setup is
wacky or leaky  in some way even though it apparently responds
normally to mixture and idle speed adjustment?

DeWitt Harrison
Boulder, CO
88 5kcstq