Doug's A/C System, R&D/Build Thread Options

[dlee6204]

I thought I would document my A/C adventure in the hopes that it would be a good reference for anyone else considering A/C. I’ll first touch base on the stock system and on custom options and then start building my own system to start testing on. I’ve working on a few systems before but I'm sure there is someone that knows more than me so don't be shy sharing or adding anything. (IMG:style_emoticons/default/smile.gif)


To begin things off here is one of the better overviews of an A/C system. Follow the link and you can see a breakdown of the whole system.


http://www.movacs.com/How%20it%20works/index.html





Anytime I refer to the stock system I will be referring to the dealer installed VPC system mostly because the only A/C cars I had came with this system and I have plenty of parts around.

I also borrowed some photos off this site so if I used any of yours... Thanks. (IMG:style_emoticons/default/smile.gif)


Compressor:

The compressor that came on dealer installed A/C systems was a York Piston style compressor that had a capacity of 6 cubic inches/Rev. From what I could find it used about 7-10 horsepower from the engine. You can still use this compressor however there are better, more efficient options out there. When choosing a new compressor it is a good idea to find one with a similar capacity. In this case going bigger does not mean better. An A/C system works as a whole unit and if you dramatically change one aspect of it, you will likely see negatives effects elsewhere. Choosing a compressor that’s too big will overwork the system, yielding no little to no benefit, and create a not-as-efficient system. During my search I looked mostly at Sanden style compressors so that's what I'll be referring to in my build. Using a modern compressor like the Sanden will only use about 3-4 hp from the engine. I have heard quite a few people use the Sanden 508 compressor with good results. It has an approximate displacement of 8 cubic inches/rev, which IMO is slightly more than I'd like to make a jump to. I'm not saying it wouldn't work, it obviously works, however I think I will get slightly better results and pressures using the Sanden 507 (approx. 7 cubic inches/rev). The SD507 is also slightly smaller in size than the 508.

Here is a picture of the York Compressor


A side by side with the SD507





And here is a SD508

[dlee6204]

Condenser:
IMO this is where the biggest improvement can be made. Whichever option you go with make sure you get plenty of air movement across the condenser. The way I see it there are three options with mounting the condenser....

-Cut a hole in the front for the original condenser box. If doing this you might consider replacing the old tube&Fin condenser with modern parallel flow condenser. Parallel flow condensers are about 25-35% more efficient than the old style. So a modern condenser would compensate for the inefficiency of using r134. (R134 requires about 30% more condensing area that R12). So in theory it would give you just as cold temperatures for the same size condenser... in theory. I have to be honest though… I think the original condenser will be sufficient for enough for cool temps. It won't take much to cool the cockpit of the 914. Its something I’ll be testing out. The original squirrel cage fan also leaves something to be desired so you might consider upgrading your fan.

Original Condenser Box


Tube&Fin Condenser



Helpful diagram showing different condensers.




-Install a condenser on the engine lid. If I were to do this I would install a parallel flow condenser with fans and let that be it. The biggest downside to this is introducing even hotter air into the engine bay. Clay Perrine has a similar setup and he said that he hasn't had any issues so that shows that it could be possible. Another thing to note about this setup is that the engine bay is going to get crowded specially if someone has a top-mounted compressor with stock EFI. The top-mounted compressor might even interfere with the condenser. You will also need to mount the receiver drier in the vicinity as well.


-Install one in a fender-well. I have heard that some 911's had a condenser in the fender. I haven't looked into this option very much because I think it would be the most difficult to pull off specially when you consider that lack of room under 914 fenders and how big a condenser you would need.




Evaporator:
There are ample aftermarket evaporator units out there. You simply have to find one that would fit, most however are going to be under-dash units. For my build I am going to use the stock VPC unit. IF using a used unit, make sure to replace the expansion valve. It is usually located at the inlet of the evaporator. If you have a water-cooled car, you might also consider a dual-purpose unit that handles both A/C and heat.

VPC Evap.


If an under-dash unit doesn’t strike your fancy, one of the best solutions but most difficult would be to install an evaporator unit in place of the fresh-air fan under the cowl. It would have to be a completely custom setup however you would still be able to use the stock heating system in conjunction with the dash air controls. This is something that I might consider later down the road.