The last time I saw it was in Hot Rod Magazine. To know what's really going on, you probably need a calculus equation that exceeds the size of this page (or Sheldon). I'm satisfied with the OEM's engineering. By the way, A/c uses a thermostat too - it's the Low Pressure Switch (exception is the variable stroke compressors now being used) and fan on pressure/temp.

 

You're misunderstanding something. At "around town speeds", (below 30 mph) stop and go, or when stopped, the cooling system relies on fans. There isn't enough airflow over the radiator. (I know I'm stating the obvious here)

The factory fans don't come on until ~230, so temps will get up to ~230, fans on, temp goes down. Fans shut off at ~217...temp goes back up. Are we good so far?

Turn on the AC. Yes, you are absolutely correct that the AC condenser increases the temp of the air that the radiator "sees". however, the BENEFITS of that air flow over the radiator vs. no air flow is far more than the detriment of slightly hotter air.

Result? Cooler running than with no fans (or airflow) at all (until 230).


Today, I went to SLC. Weather says it's 96 down there today. I had my AC on high. I got off the highway and drove through stop lights for about 4 miles. Stopped for my errands and left the car running, AC on, (just for this thread!). Came out, got in the car and drove it through the same 4 mile, stop lights back to the highway, and right before getting on the on ramp, I took this picture:



Where do you think the temp would be had I not had the AC on? I'll throw it out there that it would have been between 217 and 230. The benefit of the airflow provided by the fans FAR outweighs the slight detriment of the warmer air, created by the condenser.

 

That is exactly my experience as well.
This is just how it works with a "stock" car.

Once you start modifying the engine you might have different results, as those engines tend to run hotter in the first place.

 

Interesting.

On L98's, the [publicly-available] AYPY bins show the same fan turn-on temp regardless of A/C on/off. It's 225. Also, the fans shut off at 11mph. So, L98's don't really get any fan-aided cooling beyond idle/parking-lot crawl. Plus, they don't operate any differently with or w/o the air conditioner running.

I realize LT-1's have reverse cooling. It's interesting that this aspect works differently on the two.

I went looking (again) for links about the miminum distance between condenser and radiator. I couldn't find one. I think I reduced the OEM config by about half. A local mechanic says any malfunction/loss of freon would reduce the heat load to the condenser. Mine is finally getting a LITTLE "iffy" after 25 problem-free years but I think it's' finally time to add a pound of freon. More importantly, I need to figure out why A/C operation seems to heat up operating temps more than it should.

Interestingly, that same mechanic thinks I should move the units closer -- until they touch AND close off any end-gap that would allow "mixing" air. I'm starting to disagree with his thinking.

Because my compression is up from stock (more in the LT range), I'd like to keep my running temps lower. Really, I'm looking for a 215 ceiling. (Keep in mind I don't have reverse cooling!)

 

Not true. My fans come on when the A/C is on. Try turning the key on without starting the engine. Turn the A/C on. You'll hear the fans.

 

Cumbercr is right. The ECM may indeed not command the fans untill 225, but there is a circuit/wiring that over rides that as part of the A/C system. The AC condenser can not work with out airflow over it! Can't. So one of three things needs to be in place;
1. A mechanical style fan that is always drawing some air
2. an electric fan that is activated by the AC sytem
3. Sufficient vehicle speed to provide air flow over the radiator.

I don't think that 11 mph is sufficient. Not even close. I'd double check your 11 mph figure. If you drive your 'vette up a hill (or even on flat ground) at 12 mph (so the fans will never come on according to your claim) the car WILL over heat.


Lastly, Reverse flow cooling has nothing to do with the fundamental properties of cooling systems, that we're discussing in this thread. The benefit of reverse flow cooling is cooler HEAD temps (for a given average operating temp) and more uniform cylinder bore temps. These benefits (real or not) have nothing to do with how the cooling system functions, over all. My '83 Trans Am with an electric fan functioned the same as I've been describing this whole time.

 

May sound simple and common knowledge but occasionally blasting the fins of he condenser out makes a night and day difference on a higher mileage car just as it would the radiator. Didnt change temps much but the fans run less for sure
Just did mine at 150k and was instantly rewarded with cooler ac..that was a few days after having it all gone through and charged up also.

 

 

I really don't know what I'm looking at there. I see that there is "11" written in a "MPH" dialogue box. I don't know who typed that in there, or what that really is referring to. What I DO know is this;
1. The car will not keep itself cool at 11 mph w/no fans, so if your particular car is set such that the fans do not run above 11 mph, and you drive you car on a hot day at 12 mph....it will over heat.
2. The AC condensor can not work w/o airflow over it, and 11 mph is not sufficient speed to get meaningful air flow over the condenser.

Run the test that cumbercr suggested.

 

I will -- though it won't prove what happens above 11 mph. (And, I'm not sure I can hear if the fan is running at 12mph.)

What you see isn't what I "typed". The lower box is from the stock AYPY bin. The upper box shows I accepted -- and used -- that same value for my tune. I did not alter ANY A/C parms. I did lower fan-turn-on after changing to a lower-temp stat. Mine comes on at 188 versus 225.

 

It would be interesting to quickly wire in a test light to your fan power wire...then drive and see if it illuminates above 11 mph. I bet it does. I know the car won't stay cool below around 20 mph w/no fan.

 

Highway Driving (Steady state >50 mph):
Coolant Temp: 194F
Oil Temp: 204F

City Driving (Steady state <40 mph):
Coolant Temp: <200F
Oil Temp: 210 - 215F

Stop and Go:
Coolant Temp
<205F with AC ON
210 - 220F with AC OFF
Oil Temp >210, but I've never seen it >230F

If I get on it (rapid acceleration from a stop to highway speeds), then my coolant and oil temps both climb, but coolant quickly recovers down to the steady state temp. Oil temp takes significantly longer to recover.

This driving season I had a problem with my thermostat in that the coolant temp was running about 206F at highway speeds, but would drop down to under 200F when coming to a stop or steady state at under 45mph. Since this was new behavior for the car, I replaced the thermostat and the problem went away (confirming my theory that the old one was only partially opening).

This is on a stock LT1 with the exception of 4.10:1 gearing.

Steven

 

The stock bin on my 85 enables the fans below 35 MPH.

 

And it makes a LOT more sense, then 11 mph. 11 mph simply will not work. If GM put cars out the door w/fans shutting down above 11, they would have had overheated Corvettes all over the country! 11 mph won't work.

 

These are the temps that I typically see also. A/C in stop and go results in lower temp then with A/C off.

 

Would be nice to know what that parm is for then.

FWIW. The line ABOVE the one I highlighted specifies 35MPH as the temp for using the fan duty cycle table (where all entries are 100%).

 

My literal interpretation of that line means:
IF MPH > 35, THEN Fan Duty Cycle = 100%

Where Fan Duty Cycle is the amount of time that the fan is ON (meaning a 100% duty cycle results in the fan constantly ON).

If so, then it doesn't make any sense at all. When the car speed is > 35 MPH, there should be sufficient air flow through both the condenser and the radiator to eliminate the need for the fans.

I respect that these are the values you found in the file - but the data doesn't support the science.

When the car is NOT moving, you use the fans to force air through the condenser and the radiator. Even if the air coming off the condenser is hot (say 150F), it's still cooler than the water temperature (~200F). A delta-T of 50F should be sufficient to bring on cooling to the radiator such that coolant circulating back to the engine will act to bring down the coolant temp in the engine.

Steven

 

I think its' the opposite. IOW < 35mph.

I understand your point about delta "T". If condensor surface temps really don't exceed 150, then I agree temps still shouldn't rise.

Can a malfunctioning A/C unit exceed 150...or even 200-degrees? (at the condensor) I should add that it also blows cold air. It was measured closer to 60-deg than 45 though.

 

But the line above where you highlighted reads "Minimum MPH for Using Fan 1 Duty Cycle". If that value is set to 35, then that seems to mean that the MINIMUM speed that the car is going BEFORE Fan 1 duty cycle = 100% would be 35 MPH (thus making all values BELOW 35 MPH either subject to some other condition, or making Fan1 go to an OFF state).

Additionally, if you verify that your fans are NOT running when the engine is idling, and the car is sitting still, and the AC is ON, then that would support my reading of the condition.

The way I see those two lines is that your setup calls for Fan1 to turn ON at 11 MPH, and reach 100% duty cycle by the time the vehicle speed is 35 MPH or greater.

And that seems bass ackwards to me.

With regard to your AC temperatures, I had one of our AC guys check mine last summer, and with the car idling, the air temp coming out of the vents was in the low 40's (if I recall correctly). That was on a pretty warm summer day, because I remember that my shirt was soaked in sweat by the time we got through playing with the car.

Steven

 

I agree the wording is misleading but I think it's the opposite of what it sounds. My fans are working -- as desired -- at idle. I have not checked them at low-speed but will assume they are working as intended.

Really, my biggest issue is at idle anyway. Everyone I've asked has pointed to the need for more air flow at idle. No one (manufacturer) really has a clear answer on how much MORE flow I need -- though I've gotten some clues. I have to decide if I need: One bigger fan? Dual Fans and a bigger alternator? I'd rather avoid the later -- if possible. That's when I started wondering about the condenser's (closer) proximity after putting in a physically larger radiator. I figured getting a better understanding of that issue might lessen the level of fan upgrade needed.

FWIW: 45-degrees is what my local shop said A/C should blow.