This is my 900 CFM TBI throttle body that I converted to multi-port use (by removing the injector pod) and ported for better airflow. Four barrel, progressive linkage throttle bodies are excellent because they provide a good linear increase in airflow. I also wanted to improve the airflow so it would be comparable to the 1000 CFM MPI throttle bodies. They already have the same size throttle blades/bores but I knew the airflow could be improved.

Through correspondence with Doug F. @ Holley, I learned the TBI units could flow 80-100 CFM more air, if the bore dividers (walls) were raised to the top and the ridges (midway) ported smooth. Evidently, Holley engineers have flow tested and proven this (it's effective even with the injector pod installed). As you can see, the original TBI bores are machined flat & low, which hinders the airflow. So I completely disassembled the TBI unit and mig welded (4043 aluminum .035" wire) material up to the top. Then I ported the bores with a carbide bur inserted in a drill press. Afterwards, I finished porting the bores with a wire wheel. I ported the IAC passage too. The result shown below. (The last two pictures are for reference only.)

TIP: Holley did testing with a 1" open spacer under their 900 CFM TBI unit, and it actually improved the air/fuel distribution and made 10-15 more horsepower above 4800 RPM.

At that time, I reassembled the TBI unit & injector pod, and it ran great on my engine. (I replaced the 1¾" throttle blades with Holley #26-97.) Since then, I converted my Edelbrock Performer RPM 'Air-Gap' dual-plane intake manifold to multi-port EFI. I purchased all the required components, and my throttle body was ready for the conversion.






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Update:
Soon afterwards, I decided to experiment with a 1:1 ratio secondary throttle linkage. I'm glad I did. The 1:1 ratio
linkage is supposed to have better part throttle air distribution and feel more responsive. The 1:1 ratio linkage is
an improvement! I think large cubic inch engines can tolerate the immediate additional airflow without problems
and may actually need it. Also, I think heavy weight vehicles dampen the effect to a point where progressive
linkages feel unresponsive and lazy until you near ½ throttle.

After determining that I liked the feel, I custom made my own adjustable link and installed it. I machined a pin to
fit the smaller hole in the primary lever, then threaded it for the #8-32 bolt I bent to fit the secondary lever. The
throttle body levers are not modified in any way. The link was made to fit the throttle body as is. I think Holley
20-122 may be easily adapted for this use, because a longer piece of threaded rod could lengthen the linkage
(if necessary):
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I'm currently using a straight decline (from left to right) "AE Correction vs TPS" curve. The TBI/MPFI four barrel
throttle body (linear airflow) applications with a 1:1 secondary linkage, use a straight decline (from left to right)
AE Correction vs TPS curve. The TBI/MPFI four barrel throttle body (linear airflow) applications with a progressive
secondary linkage, use an S-shaped AE Correction vs TPS curve (adding at left & subtracting at right). The TBI
injected applications with a progressive secondary throttle linkage, use a center spiked AE Correction vs TPS
curve where the secondaries begin to open (and secondary injectors begin spraying). The mono blade throttle
bodies use a ramp (quarter pipe) shaped AE Correction vs TPS curve (see LSx base calibrations). Also, I settled
on an "AE vs TPS RoC Blanking" value of 4 (large mono-blade throttle bodies may need it lower), and an "AE vs
MAP RoC Blanking" value of 5. Ensure there's no TPS/MAP RoC signal activity at idle, due to noise/kPa fluctuation.

On an opposite note, Acceleration Enrichment can't be tuned until the Base Fuel Table is well tuned.
If you have a momentary lean spike, on light throttle acceleration, check
your AE vs TPS/MAP RoC Blanking values (in Fuel Modifiers/Fuel Control):
If they're 15 & 7, those are the old values from the V1 ECU firmware.
Change the AE vs TPS RoC Blanking value to 4-6.
Change the AE vs MAP RoC Blanking value to 5-7.
Some users reported the need for an even lower TPS RoC Blanking value.
Ensure there's no TPS/MAP RoC signal activity at idle, due to noise/kPa fluctuation.
http://forums.holley.com/showthread....1379#post61379 (AE Tuning Information)
http://forums.holley.com/showthread....7925#post47925 (Additional AE Tuning Information For Stubborn Engines)

Here are three pictures of the finished product:



Notes:
The 700 cfm TBI throttle body has 1½" bores, and actually flows about 650 cfm with the injector pod.
The 900 cfm TBI throttle body has 1¾" bores, and actually flows about 850 cfm with the injector pod.
The 950 cfm TBI throttle body has 1¾" bores, and actually flows 950 cfm because there's no injector pod.
Holley TBI units are CFM rated the same way as four-barrel carburetors - at a depression of 1.5 in/Hg.
(Two-barrel carburetors are rated at a depression of 3.0 in/Hg.)

Fuel injected engines don't suffer from oversize throttle bodies like carbureted engines do,
because the fuel can always be properly metered to the amount of air entering the engine.
Fuel injected engines don't rely on engine breathing (venturi effect) for proper fuel metering.

FYI:
Be careful when shopping for a throttle body. Some manufactures advertise skewed (higher) CFM ratings.
Basically, if it sounds too good to be true, it probably is. The following quote is a good example:
Quote Originally Posted by Doug F. @ Holley
I'd be a little leery of that design... FWIW, Accel sells a "1200 CFM" throttle body. Holley's is a "1000 CFM".
Accel must use some odd-ball depression because when I flowed theirs at 4BBL depression (1.5"), it flowed below 900 I believe.
It has smaller plates so... Point is, four 1.75" plates (Holley) with no boosters is pretty big.