[Diy_efi] TBI, heat riser, crossover passage

[Fehn, Ron G]

i have   two one barrel TBI's on custom plenum on 84 vette.  called
xram.   the TB's are 2.00 inch each. this plenum is sitting on a single
plane weiand manifold.  90# injectors.  i am in cold climate milwaukee
wis.  yes it is a problem.  in summer at 70 deg the issue goes away
soon.  on cold start the intake gets xtra fuel as coolant temp low.
when closed loop comes on and 70 deg or warmer no issue as car is coming
up on temp under hood.  no ext crossover in manifold. plenum is pie
plate shaped and sitting an 1 1/2 inch above top lip of manifold.  when
morn temps are 40-45 it is a big problem. stumbles as it gets a blast of
ice cold intake air.  non heat soaked plenum/manifold/TB.   also AE
needed to be doubled as lack of atomization on hard accell.  that is
partly due to plenum one inch thick only so AE hits plenum floor and
condenses.  i am keeping manifold and adding adapter and adding 454 TB
this spring. that shopuld help as i lose the plenum.  route of a/f is
direct into manifold.  now it moves zig zag to arrive at top of intake
opening.  get the picture?=20

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]
On Behalf Of Michael J Flaherty
Sent: Wednesday, March 03, 2004 2:32 PM
To: diy_efi at diy-efi.org
Subject: [Diy_efi] TBI, heat riser, crossover passage


Hi all,

I've been wondering about the importance of maintaining an exhaust
heated
intake with TBI and a cast iron manifold.

EGR aside, is this as necessary for cold weather driveability as it was
with
carburetors ?  I'm hopeful that I can do away with the heat riser on 2
EFI
retrofits that I've been thinking about.  Obviously the shape of the
intake
won't have changed (sharp turns at the bottom going both ways), which
would
still tend to cause the fuel to want to puddle at the bottom of the cold
manifold, causing lean stumble until such time as the intake is warmed
by
either the crossover exhaust gases or heat radiating from the block
below
(which takes at least 5 times as long to warm the intake as a
functioning
heat riser/crossover, especially in the coldest weather).

Is the superior atomization (smaller, lighter fuel droplets) that is/are
achieved with injection enough to counterbalance this tendency to puddle
?
In other words, all other things being equal, will the gasoline make it
around the corner ?  Even at an idle or accelerating from a stop with a
cold
engine and a tall gear at -10 Fahrenheit ?

I'd like to be able to achieve this (reasonable cold weather
driveabilty)
without resorting to prolonged start-up enrichment and the bad for the
engine things that go along with it.

I do a lot of short trip city driving, which makes this a fairly major
issue
for me.  With carbs, I've had to take great pains to keep crossover
passages
clear and heat risers and thermostatic air cleaners freed up and
functioning.  This is becoming impossible as you can no longer get parts
for
this stuff.  I'm pretty sure that I bought the last small block Chrysler
heat riser kit in the Americas, for example.  FWIW, I had to order it
from
Thunder Bay, Ontario...

Thanks in Advance,

Mike

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