Controlling the AC compressor


If you are using a Sanden-type AC compressor as is provided in the kit, there is a risk that the compressor may not survive periods- even brief ones- of high RPM use. Factory Five blew up three AC compressors on their LS7-equipped GTM before the cause was fully understood.

The Problem

The problem is that the standard AC compressors don't tolerate very high RPM. They are designed for "normal" cars that don't spin over 6000 RPM or so. Regular, or even occasional runs to redline with the AC on will usually blow the seals on the compressor, usually requiring a new or rebuilt unit. This is expensive, in addition to aggravating, but can be avoided with a little work (and money...).

Here's a link to a PDF that discusses a wide range of compressors, and their max intended speeds.

The Sanden 508-style compressor that is provided with the kit is designed for a "maximum 6000 sustained RPM" as stated on the Vintage Air compressor page (no link is provided because it changes every year, but search for the compressor page in their current catalog).  If you never run your engine over that, you probably don't have to worry. If you do, consider the solutions listed below.

Most SLC builders don't have a problem with blowing compressors because they don't run their AC when running the car at sustained high RPM.  If you do blow a compressor though, this page will help you to understand why, and how to prevent it later.

For clarity, don't depend on a trinary switch to solve the problem. It doesn't know anything about RPM, which is what kills the seals in the compressor.

Some Solutions

The easy solution is just to remember to turn off the AC when you might see high RPM. But sometimes you might miss a shift and run to the rev limiter, or just forget that the AC is on when you want to wind it out, so there is a risk associated with this approach. The consequences of failure are pretty expensive and can be prevented.

In addition, just turning off the AC (which has the effect of decoupling the clutch to the compressor, so the pulley on the compressor just freewheels instead of driving the compressor internally), can still put strain on the belt.

A better solution is to use an RPM window switch. These are available from Jegs, Summitt and many others. Essentially, these switches can turn on or off a circuit based on engine RPM. They can turn a load on at a low RPM, and off at a higher RPM, allowing the load (e.g., the AC compressor) to be on during a "window" of RPM from the low turn-on point to the high turn-off point.

These are easy to wire into the AC clutch circuit. To do so, find the wire that the InfinityWire system sends to the AC clutch, and instead of sending it to the compressor, wire the window switch (with a relay) into the circuit, following the instructions with the switch. You'll need the switch, a relay and some extra wiring.

PM Will Campbell on the GT40s site for a detailed wiring diagram if you need one.