Vacuum switch for my road runner

[73runner]

I noticed that even with the raised compression and greater lift cam, my 73 340 was dragging a bit with the A/C on.

So, I took a clue from some modern cars and just installed an adjustable vacuum switch in line with the compressor clutch power lead.



The switch is available with adjustable vacuum actuation from about 5 to 25 psi according to my vacuum gauge.

It works great except that it was engaging the compressor too soon with my initial setting at 5 psi so I adjusted it to 8 psi to come off a little sooner.

The second problem I noticed was that the switch was trying to re-engage the compressor too soon while I was still at high rpm ( over 3000). This was causing slipping of the compressor clutch due to the still high head pressure in the compressor and the higher rpm that was restoring vacuum to the switch.

That's where the car's OSAC valve came in to play. Rather than connecting directly to engine vacuum, I connected it to the OSAC valve which is basically a vacuum restrictor to delay the build up of vacuum to the distributed advance. This worked great at the 10 psi setting of the switch and gives the compressor a chance to engage without slipping at around 1500 rpm when vacuum builds through the valve enough to reactive the the switch.

I forget to turn off the a/c switch when I shut down. So, an added benefit was that this switch automatically allows me to restart the engine without the drain on the electrical and starter of the a/c engaged at immediate start up; hot or cold.

Im planning to make up a mounting bracket to go on the intake manifold to clean up the install and keep the stock look.

I found the switch on ebay. Here's the link if interested.

http://www.ebay.com/itm/Adjustable-...ior-/300985660634?hash=item461424b4da&vxp=mtr

 

[bm02tj]

this should be Inchs not pounds
and controlling your AC is good

 

[idrivemopar]

So, I am curious what your compression and lift are on your 340? I have upgraded my 340 to 9.4:1 compression and cam with 454/480 lift. Also, I am using the bigger 6322 Thermoquad and not the original 6319 thermoquad. The other difference is my distributor is setup for mechanical only advance. I am rebuilding my original AC system for R134a, so when I get it back and on the car will be interesting to see if I get the same issue you are seeing.

 

[73runner]

this should be Inchs not pounds
and controlling your AC is good

You are correct! My bad.... I know better.

But you have to admit, inches of liquid metal in glass tube to creat a vacuum is a funny way to measure anything but ambient air pressure.

 

[73runner]

So, I am curious what your compression and lift are on your 340? I have upgraded my 340 to 9.4:1 compression and cam with 454/480 lift. Also, I am using the bigger 6322 Thermoquad and not the original 6319 thermoquad. The other difference is my distributor is setup for mechanical only advance. I am rebuilding my original AC system for R134a, so when I get it back and on the car will be interesting to see if I get the same issue you are seeing.

Original carb, but changed the metering rods to those off a 6322SA thermoquad for a 440 HP. Original metering rods were running the engine too lean at part throttle with a little pinging and suboptimal response. New metering rods have same WOT dim but upper diameter was approx 3 mil narrower in diameter at mid point and tapper was less abrupt.

Compression 9.2 to one.

Cam: same as yours.

Heads: stock '73 340.

Intake manifold: stock

Exhaust manifold: stock

Distributer: stock with vacuum/mechanical advance

Ignition Timing: pulled like dragster, peeled out from standing start (even with 275's out back through a 355 sure grip) when engine was cold and set at 12 degrees BTC... Pinged all over when warmed up... Went down one degree at a time to settle at 8 degrees advance. Still pulls really well when engine is cool but doesn't ping at all after fully warmed up. No more peeling out though..... One of the reasons I'm looking/working at cold air for carb through the hood on another thread. It really pulls hard after 2600 rpm.... You'll see!

A/C: DuPont dichlorofloromethane baby! That's right ..... R12 freon !!!!! I live in south Florida and it gets hot. I need an a/c system that puts out 34 to 36 degree air. (regular 650 rpm running idle and raised 1200 rpm idle )

At about 30 bucks a can its not much more than R134a. I recycle it and have a local A/C tech that has a separate vacuum/charging machine since he works on old cars. Yes we do loose a little every now and then but keep a lot on hand...

The best I got out of our systems with 134 was 48 to 52 degree air. And that was with twin pusher fans. For it to work well you really need to upgrade the condenser with tape style tubes and a newer expansion valve that is designed for 134. And some pusher fans!

Running vacuum; cold and full warmed up. Ill be checking vacuum on my engine tomorrow. Still tinkering with the vacuum switch.

Is yours automatic or stick?

 

[idrivemopar]

Mine is an auto. I have 3.55 truetrack diff. It will be interesting to see how well the rear hooks up since I will be running 15x10's with 295 50's. I also have TTI headers and stock intake.