TOPIC: Chevy 350 TBI w/IAC Restriction

Here we have a GM TBI throttle body from a '94 Chevy 350. This was sent in by Marty in Pennsylvania, a very cool guy. Marty had cut down the short vertical air dams around the upper TB bores, and he uses a 1" spacer w/ open center under the TBI. I told Marty he could A/B test it with and without the spacer, since it's not so hard to remove and re-install these TBI's. Thinking about it, the open center in Marty's spacer could cause reduction in Groove effectiveness. The stock Chevy intake manifold is dual-plane, each cylinder in firing order draws air thru only one side of TBI. The spacer is to increase power by allowing each cylinder to draw air from both TBI bores, also adding plenum volume to the intake manifold, to increase torque. But- each cylinder will draw air through BOTH Grooves, I think this will decrease overall Groove effect, by spreading airflow through both TB bores, airflow velocity will decrease. The Groove waveform from the "secondary" throttle bore will be drawn thru the spacer gap to the "primary" bore, and the waveforms will collide. A spacer without the center air gap might be better, and if anyone has input on this please jump in!

I did the usual epoxy fill-in of the Idle Air Control (IAC) passage and built up the center between bores to re-direct and restrict IAC airflow. Then cut the narrowed air trenches in the epoxy to complete the IAC mod. Grooved w/ Large bit. I put a piece of folded paper against upstream edge of throttle blade, to move the Grooves a bit downstream. Often with these TBI's, the IAC restriction causes some reduction in idle RPM, so the idle speed screw needs turned in. That will open the throttle some at idle position, so I moved the Grooves downstream to account for that. If an engine sees reduced hot idle RPM, when IAC airflow is restricted like here, then that validates what Ron said about how some IAC's never fully close. Marty said that hot idle RPM was down to about 600 in gear (auto trans) affecting alternator charging, I recommended he adjust idle speed screw up to 750 in gear.

You will see in 2 pics I've included how I did the PCV re-route on another TBI Groove job. This of course will help boost manifold vacuum, I recommended to Marty he copy what I did here.

I'll post any feedback from Marty on this Groove job, and invited him to post here.


Marty took the excellent pic of underside of TBI w/ spacer, very interesting to see this ,along w/ the spacer on top of manifold, a nice clean engine! thanks Marty!

Tracy G