Idle Vacuum - Noticed Something
TECH Apprentice
Joined: Nov 2016
Posts: 371
Likes: 18
From: switzerland
as the experts are all here, i have this question: why does vacuum go up from idle when you get off the brake and the car starts moving without a change in rpm (a4, stock converter, 650 rpm)?
TECH Senior Member
Joined: Jun 2016
Posts: 22,291
Likes: 3,616
From: Central Cal.
Because when you get off the brake, the load lessens as the car rolls. The idle is regulated, but with less load, the vacuum increases. With foot on the brake, the engine is working against a static load. As the car gets rolling, that static load decreases more than the dynamic load increases.
Thread Starter
Joined: Aug 2004
Posts: 11,258
Likes: 146
From: Pace, FL
Brake is a load on the car as well with vacuum assisted brake booster.
Btw, the stage 4 cam was the truck cam. Not the Stage 4 BTR. Didn't realize Brian had a Stage 4 truck. Thought he just did 3 stages.
224/230 111... so 55-60kPa is right. Lowered idle to 800 and bumped timing up a lot for when it goes into drive. Works well now. Lopes good too. Still recommending a 3000 stall to really do well with that cam.
Btw, the stage 4 cam was the truck cam. Not the Stage 4 BTR. Didn't realize Brian had a Stage 4 truck. Thought he just did 3 stages.
224/230 111... so 55-60kPa is right. Lowered idle to 800 and bumped timing up a lot for when it goes into drive. Works well now. Lopes good too. Still recommending a 3000 stall to really do well with that cam.
