Throttle Body Injection Question

[ski4evr]

I couldn't find any sources online for this. How does the fuel flow through the throttle body when the car is idling? On carbs, the fuel comes in below the throttle plate, correct? Is there an equivalent idle circuit on a throttle body injection? The injector is mounted over the throttle plate, so it appears to me that for idling, the throttle body is similar to a carb? I would think not, though - since throttle bodies presumably have better idling characteristics.

ski

 

[weeds]

The Idle Air Control (IAC) valve modulates idle speed. Effectively, it a bypass valve around the throttle plate. The injector on a TBI still injects the fuel required, but it goes through the IAC.

 

[ski4evr]

Thanks weeds

I talked with Holley about a multiple TBI setup and they referred to the IAC. Only one TBI would be used for throttle postion sensing and IAC. Seems resonable if you have 1 TBI over the middle of our log heads (1 TBI or 3 TBI set up). But, if someone went to a 2 TBI setup, direct mount, I don't think the IAC would be that effective since the one controlling carb is not flowing equally to the whole head (it's off center).

 

[Anonymous]

The IAC on a TBI only works when the engine is cold, it's primary purpose is to let additional air bleed thru to speed up the idle. Once the engine is warmed up, it is not needed. Basically a controlled vaccum leak.

On the Chevy Crossfire manifold, one of the TBIs had a TPS sensor for throttle position, both had the IACs connected. You also need to balance the fuel pressure between the two throttle bodies. Once again, on the Crossfire setup, a fuel pressure regulator was removed from one of the TBIs and the TBIs were connected in series.

On the OE MPFI setups, the IACs have evolved to continuesly vary the throttle plates to produce a consistent idle speed.