REFLECTOR: Overhead NACAs

[Sid Knox via Reflector]

> For those with the overhead NACA vents for engine cooling, does the duct 
> connect directly to the plenum or is there a flexible joint for vibration? 
> Thanks,
> John

John, I have not seen an answer to your query so I will partially answer 
...partial because a photo would make this response more meaningful, and I 
am not currently able to make a photo but if necessary I can in a few weeks.
Yes, you need a gap (mine is about 1/4 inch) for two reasons, one for, as 
you suggested, allowance for the engine to move in its mounts, and two, so 
you can remove the plenum if required for major engine work.

On each of the ducts, mine has a "garter" fabricated from about a three inch 
wide strip of tire inner-tube wrapped around the joint with about a two inch 
overlap (sealed with contact cement) of the end.  The plenum side of the 
garter is pop-rivited to the plenum.  The duct side uses a (large) nylon 
tie-wrap for a seal.  This top NACA mod was done (by the previous 
builder/owner) about 16 years ago and the rubber looks to be as fresh as 
new.

You must have access to the top of both banks of the engine for spark plug 
removal and you *do not* want to have to remove the plenum every time you 
examine the plugs!  To properly make the plunum a tight fit around the 
engine requires fitting and sealing the edges (where the plenum interfacs 
with the engine) with silicone (or equivalent) and this, if done well, 
requires much time and effort, so once the plenum is installed, you will not 
be anxious to remove/re-install it every time you check plugs.  On mine, 
over each cylinder bank are removable panels to allow spark plug access and 
access to the injectors.  Also, there is a removable panel over the fuel 
divider ("spider") to allow access to it.  This word description is where a 
photo is valuable.

$0.02

Sid Knox
Oklahoma

Velocity  173 RG  N199RS
KR2         N24TC
W7QJQ



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