TPI Runner Question

[David Papworth]

The small tube on the driver's side runner set is for
the cold start injector, if present.
The small tube on the passenger side is for EGR.

 The design of the earlier runners was rather clever -- one
casting could be made in pairs, and reversed
to form either a right or left hand runner set, with the
afformentioned small tube switching roles in the process.

The cold start injector was removed in 1989/1990.

In both cases the tubes connect a point source on the
lower manifold to the upper plenum to then get distributed to all the
runners.
Such a tube could not work for idle air -- the
IAC has to get bypass air to the UPPER manifold,
not the lower manifold, or the engine would idle
on just one cylinder.

Regardless, there is no place on the 1990+ plenums for
a "left hand tube" to connect. The distribution passage for
the cold start valve was removed completely, or so it
shows in the "TPI Swapper's Guide".

The IAC does work differently between 85/89 and 1990
on. The throttle body is different, without the internal
passage to route the idle air past the throttle blade.
.
My e-bay TPI system is 1986 -- and the TPI swapper's
book is not clear about how the late IAC works,
 so I don't know how the later IAC is supposed to connect
-- but I doubt it uses the small tube in the runner set.

----- Original Message -----
From: <WEG1192 at aol.com>
To: <gmecm at diy-efi.org>
Sent: Tuesday, December 05, 2000 9:19 PM
Subject: Re: TPI Runner Question


> I recently bought (bid rather) on a set of 1992 speed density TPI stock
> runners. Only after I won the bid did I notice that the small tube on the
> driver's side runners was not there. My understanding is that this tube
> carries idle air from the upper plenum passage to the lower manifold.
Without
> this tube, how does idle air get to the lower manifold? Is the upper
plenum
> on speed density systems designed differently internally? Is the throttle
> body different? As a fix, has anyone drilled holes in the upper plenum fro
m
> the idle air passage into the upper plenum intake tract?
> --------------------------------------------------------------------------
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