TTA PERFORMANCE LLC
Specializing in 1980-1981 Turbocharged 4.9 (301) Pontiac Trans Ams and Firebird Formulas.
Power Enrichment Vacuum Regulator (PEVR)
What is a PEVR?
PEVR stands for Power Enrichment Vacuum Regulator. It was used on GM’s turbocharged Pontiac 4.9 liter V8 in 1980 Trans Am/Formula (also 1981 Canadian/Export cars) and on Buick’s turbocharged 3.8 liter V6, prior to fuel injection and Computer Command Control (CCC).
What is it’s purpose?
The purpose of the PEVR is to allow the primary metering rod circuit of the carburetor to provide the necessary fuel enrichment and air/fuel ratio during the various operations of the engine.
How does it work?
The PEVR is a simple yet complex device. First we need to understand why it is needed. These early carbureted turbocharged engines are of a “draw through” design. This means the carburetor is in front of the turbochargers inlet. The turbo is actually drawing in the air and fuel mixture from the carburetor, compressing it and forcing it down the intake manifold.
Let us look at the function of a non-turbo, Quadrajet carburetor’s primary metering rods. The primary fuel metering circuit is controlled by engine vacuum at the base of the carburetor. There is an opening exposed to the inside of the intake manifold. When the engine has high manifold vacuum (idle), the primary metering rods are pulled down into the metering jets against spring pressure to close off any fuel. When accelerating or at Wide Open Throttle (WOT) manifold vacuum starts to decrease, due to engine load and the spring pressure pushes the metering rods up out of the jets to allow for fuel flow.
The draw through turbo carburetor is in a different environment from the non-turbo. Technically, the base of the carburetor is always under a vacuum due to the inlet of the turbo always demanding air. This would not allow the metering rods of a conventional carburetor to work properly when accelerating. So the GM engineers invented the PEVR to help. Instead of an opening on the bottom of the carb, an external vacuum hose port was placed at the front of the carburetor base, going to the primary metering circuit. The PEVR regulates vacuum going to this port to control the metering rods similar to a non-turbo application.
The PEVR is screwed directly into the intake manifold. The base of the PEVR sees manifold vacuum and boost. The top of the PEVR has two vacuum hose ports. The center port at the top has a constant vacuum source to it (from another port on the carburetor); the side top port connects to that special external port at the base of the carb. When manifold vacuum is high at the base of the PEVR, it allows full vacuum flow on the top side to go to the metering rods pulling them down into the jets. As the engine accelerates, manifold vacuum decreases at the base of the PEVR which restricts the amount of vacuum to the metering rods allowing more fuel. The PEVR continues restricting vacuum at the metering rods even under small amounts of boost. At 5 psi of boost and above, all vacuum has been blocked to the metering circuit allowing for full fuel enrichment. This is the key element of the PEVR and why it is so important. It allows some vacuum to enact on the metering circuit even when the engine is starting to make boost. Without this feature, the engine would see too much fuel too early and cause an over rich fuel mixture. If the PEVR diaphragm fails, you end up having a lean condition and a vacuum leak causing poor performance and possibly detonation.
How to "quick test" your PEVR.
Test #1: Remove the two top vacuum hoses and start the vehicle. If any vacuum is felt on any (or both) of the two top vacuum ports of the PEVR, the diaphragm is ruptured internally and the PEVR has failed. If no vacuum is felt, move on to test #2.
Test #2: With the engine off, attach a hand held vacuum pump to the side port of the PEVR. Plug the center port and pump vacuum into the PEVR. If it holds vacuum, it should be good. If it does not hold vacuum, the PEVR has failed.
I have a failed PEVR now what??
If your PEVR has failed, you can send it to us to be rebuilt with a new diaphragm. I can also repair any broken vacuum ports on the top of the PEVR.
Price is $70 to rebuild your PEVR and that includes shipping back to you (lower 48 states only, international shipping available).
Contact us at: firstname.lastname@example.org