[Daniel Jones]

> The EFI units actually run a lot hotter than carbs. That is how they
> become more efficient in burning the fuel. It is much more lean than
> carbs, therefore hotter.

That's not an inherent characteristic of an independent runner EFI
system. In fact, most closed loop EFI systems use a narrow-band oxygen
sensor to trim the fuel around stoichometric (the mid-point between rich
and lean) during part throttle cruise and go to a pre-defined map when
wide open throttle. A friend runs an independent runner EFI system and
it runs no hotter than the 4 barrel carb that was there before. My guess
is your mixture or spark weren't set correctly. Also, be aware that lean
isn't necessarily hot if the timing is adjusted to compensate.

The advantage of TWM type EFI is due primarily to the independent runner
intake manifolding. On a 90 degree V8, there are adjacent cylinders
that fire consecutively. In a plenum intake, the reversion from one
adjacent firing cylinder fouls the incoming mixture of the next. Also,
flow in the intake runner flows both directions, due to valve overlap.
These two characteristics give a hot-cammed V8 it's characteristic rumbly
idle. With an independent runner intake, the reversion from one cylinder
is isolated from other cylinders. This allows one to run a larger overlap
cam but still retain a decent idle and low/mid range performance. I know
a guy running a Weber look-a-like independent runner EFI system on a 377
cubic inch Cleveland stroker (iron 4V heads with Weber lower intake manifold
and 48mm TWM throttle bodies) in his Pantera. He runs a 288FDP Crower solid
flat tappet oval track cam. Specs on that cam are 254/258 degrees at 0.050",
0.569"/0.580" lift with 105 lobe centers. That's a lot of duration and
tight lobe centers for a street car (he drives his Pantera to and from work
and also competes in a lot of open track events). With a 700DP on a Ford
aluminum dual plane intake manifold, it had a wild idle and wouldn't start
pulling well until 3000 RPM (Crower rates the cam range as 3500 to 7000 RPM
in a 351C). When he installed the independent runner EFI, the first thing
he said was "Where'd my idle go?". The idle quieted down so much that he
wants to install a bigger cam to get some rumble back (it used to set off
car alarms) and it now pulls 5th gear from 1500 RPM. Kirby also noted it's
tough staying off the 7200 RPM rev-limiter in lower gears.

Another advantage is that you can have equal length runners that can
actually be tuned to a particular resonant peak which is difficult to
due with a conventional wet flow 4 barrel carb plenum intake with short
and long runners. Actually, the equal length part isn't that important,
since you'll still have cylinder-to-cylinder variation (even with equal
length headers) due to the uneven firing order of a 90 degree V8 (unless
you run a flat plane crankshaft and 180 degree headers). By its nature,
resonant tuning at one frequency (RPM) tends to rob another. It's not
unusual for stack injection systems to have two different stack heights
to widen the power band. Some builders have also experimented with
different cam profiles or rocker ratios to match the different stack
heights.

> but I am not sure that the intake used for these conversions has long
> enough runners. If you look at the Ford 5.0, GM LS1 or 6, the intake
> runners are three or four times longer than what TWM or Imagine use.

Not a direct comparison as the the Ford and GM intakes are independent
runner. Look to a V8 BMW M5 for an IR EFI set-up. The runner length
is from the valve face to the bellmouth (not the butterfly, resonant
tuning is only viable at near wide open throttle) and is treated as a
Helmholtz resonator. The harmonic you wish to tune to and the RPM
you want to make peak power at determines the best intake tract length
and diameter. In general, the stack EFI systems are on the short side
which raises thr RPM at which it works best. In practice, it's not
terribly important and even a short stack height will make better power
at low and mid RPM.

Dan Jones