Last Updated: 02/25/2023 @ 09:38 am

Credit: boredmder

.org member Curt has come up with a unique way of mounting his 4th gen IACV with the swap, as well as routing hoses. Here is his idea…

Lots of discussion and controversy about where to route the air from the 4th gen. IACV to the UIM, and I don’t know if there’s any one answer. But here’s mine and the logic behind it.

First, the what. I used the 4th gen. IACV with a Stephen Max adapter mounted hanging from the end of the VIAS. I kept the 3/8″ brass nipple that Steve had pre-installed, since I was confident in his experience and knowledge (and to also mention his intelligence in thinking these sorts of things through). The output hose from the IACV was routed underneath the UIM runners, exiting from under the #1 cylinder runner by the chain cover end of the engine, then looping around back to where the plastic nipple is on the end of the UIM that applies vacuum to the brake booster. I T’d the IACV hose into that line just above the plastic nipple.

Now, the why. Here’s the logic of why I tried (and have been completely satisfied with) this setup. An engine is, in simple terms, and air pump. These 3 liter engines at idle try to pump almost one cubic foot of air per second. With the TB butterfly closed, the engine is straining to suck in air from wherever it can get it.

On startup it initially gets only a tiny amount from the brake booster as all that’s really happening is creating a vacuum within the brake booster. And even if the engine sucked all of the air out of the booster (in effect creating a perfect vacuum), that would only be a fraction of one cubic foot. Secondly, once there’s a vacuum created in the brake booster there’s a check valve in the vacuum line between the UIM and the brake booster that’s intended to keep the booster from losing the vacuum.

Now let’s assume that you’re cruising down the road with little vacuum in the UIM (in other words, heavy throttle) and for some reason the brake booster has been having its vacuum bleed off. If you were to try to brake with your left foot as you kept the right foot on the accelerator (not recommended), the brakes wouldn’t likely have the boost assist necessary and stopping would be a problem. But it used to be that most people learned to drive using their left foot only for the clutch so that if sometimes they were driving a MT car and sometimes an AT car, they wouldn’t get their feet confused in a panic situation.

So, properly using you right foot to stomp the brakes, even if the brake booster had been real short of vacuum when your foot was on the accelerator, as soon as you lifted it to apply the brakes, the butterfly plate in the TB would have closed and in an instant there would have been good vacuum in the brake booster unless there was sort of a problem beyond the check valve at the booster (and having the IACV plumbed into the booster line wouldn’t affect that kind of a problem anyway). Simple logic persuaded my to try this setup and I’ve had no regrets.


Comments are closed.