<VV> A/C Condensers & Fans

[Bruce Schug]

On Jun 4, 2004, at 3:20 PM, Rusty Rose wrote:

*SNIP*

> 2. What affect does the system have on the engine at
> various loads?
>
> If you can get 30-35 degrees Delta T over the
> evaporator (like your modern car) and not have any
> adverse affect on the engine (mainly head temp), then
> you have just designed a good performing A/C system
> for your car.
>

*SNIP*

I want to comment on this issue of the A/C condenser warming up the 
inlet air and thus causing the engine to run hotter.

In addition to having the condenser on the firewall with all the engine 
cooling air being drawn through it, I also have a Mazda oil cooler 
mounted behind the condenser in the plenum. First the air comes through 
the oil cooler, then the condenser, before it is drawn into the engine 
shrouding to cool it. (The oil cooler is shrouded so that all the air 
comes through it.)

Now I understand what logic tells us about such a setup. The air will 
be warmer than ambient and thus the engine will run that much warmer. 
Now I'll tell you what my experience has been. I can drive my car with 
the A/C off at highway speeds until the oil temperature (and head 
temperature) saturates. This typically takes about 25 miles. Then I can 
turn the A/C on. What I experience is that there is little or no change 
in head and oil temps!

I have been told that must mean that the cooling system has the ability 
to cool even more before the A/C is turned on, so that when it is, that 
"excess cooling capacity", comes into play and maintains the temps. I 
will add that in cooler weather, I usually run the engine fan at a 
slower speed. It has the ability to do this and still cool the engine 
just fine. When running the A/C it is imperative that the fan is run at 
normal speed. It just will not cool the engine otherwise! This ability 
to cool while being run at a slower speed if the A/C is off does indeed 
sound like there is excess cooling ability when the fan is run at 
normal speed.

The other issue is obviously the affect that the increased air 
temperatures have on horsepower. Someday, I plan to make a housing for 
my K&N air filters and hook up a cold air hose that bypasses the warm 
inlet air. You can never have too much horsepower, you know! But I 
never use the A/C when I autocross anyway!

Pictures of my engine showing the oil cooler and condenser are at: 
http://autoxer.skiblack.com/schug_cooler.html

Bruce

Bruce W, Schug
CORSA South Carolina
Greenville, SC
bwschug@charter.net

CORSA member since 1981

'67 Monza. "67AC140"