911 AC redo
03-06-2015, 04:31 PM
#3
Rennlist Member
I went with a Griffith's full Keuhl kit including two rear-wheel condensers seven years ago with my 88 911 Cab. Very satisfied. Lived in Florida, southern Virginia, Hawaii, and now southern Germany and routinely get vent temps in the low 30s. There are other vendors so research them all and compare. Go old school and give Charlie Griffith a telephone call after reviewing his Mr. Ice Project so you understand what is involved and what can be achieved. Good luck. Feel free to PM for more info.
03-06-2015, 08:01 PM
#5
Rennlist Member
What are your current recommendations on modifications to bring the 3.2 Carerras up to their best performance?
Thanks,
Dave
03-06-2015, 10:11 PM
#6
Drifting
IMO the singular, major, shortcoming of the factory system is the lack of adequate refrigerant cooling in slow moving stop and go traffic. With consistently low engine RPM there simply is not enough airflow passing through the rear engine lid condenser.
There are several possible solutions for this:
1. Add condenser cooling fan(s) to pull airflow down through the rear lid condenser "on demand"
2. Wire the cabin heater blower such that it cycles on automatically.
My initial effort involved using 2 Spal ("pusher") fans under the whale tail in a '78 Targa. The fans activate automatically using a trinary pressure switch mounted in the high side A/C line. Like the 964/993 series, the fans operate at low speed, series connection, unless the trinary switch "calls" for additional cooling. The fans are switched into parallel connection in that case.
Yet another design idea taken from the 964/993 series.
There is a counter argument that this approach would add additional heat loading to/of the engine, but my opinion is that the condenser heat load must flow through the engine one way or another, sooner or later. Also, it seems to me that moving that extra HEAT load OUT during the low engine load time would be more helpful than otherwise.
That car has now spent 2 summers in Fresno with the new owner perfectly pleased with the A/C
The cabin blower use is a new idea and hasn't yet been tried during the summer. That's installed in my '88 Carrera. The engine fan outflow duct is blocked off and the cabin heat blower inlet is open to engine compartment atmosphere.
More cooling airflow for the engine plus extra cooling airflow for the rear lid condenser with the trinary switch activating the cabin heat blower via a connection to the cabin heat control module.
The '88, and back to '83(?) had/has a feature unique to that series.
If the roadspeed is consistently low and the thermostatic switch sensing the engine oil temperature closes (oil temperature is to high) then the cabin heat blower is activated to help cool the engine via moving more airflow over the exhaust manifold.
I simply bridged the 3rd section of the trinary pressure switch into the engine oil temperature switch circuit.
Still experimenting with a TDR, Time Delay Relay, to run the cabin heat blower for up to 20 minutes post ignition off. Stole this idea from The 964/993 series, they both use a TDR to help cool the engine down should the engine temperature rise to high post shutdown.
I remain of the opinion that one of the causative factors resulting in annual loss of refrigerant is that there are times that the refrigerant pressure might well rise high enough to fully permeate the non-barrier hoses.
The most obvious of those would be the post engine shutdown period with the convection and radiant heat from a HOT engine and HOT exhaust manifold HEATING the rear lid condenser. Causing the possibly already high pressure refrigerant to rise to extraordinary levels.
The other is due to the relative poor factory design of the compressor cycling control.
According to my '88's owner manual there is no method for cycling the compressor off even with the R/D filled with LIQUID refrigerant, and this with the engine at idle. One can only imagine how high the refrigerant pressure might rise with increasing RPM.
There are several possible solutions for this:
1. Add condenser cooling fan(s) to pull airflow down through the rear lid condenser "on demand"
2. Wire the cabin heater blower such that it cycles on automatically.
My initial effort involved using 2 Spal ("pusher") fans under the whale tail in a '78 Targa. The fans activate automatically using a trinary pressure switch mounted in the high side A/C line. Like the 964/993 series, the fans operate at low speed, series connection, unless the trinary switch "calls" for additional cooling. The fans are switched into parallel connection in that case.
Yet another design idea taken from the 964/993 series.
There is a counter argument that this approach would add additional heat loading to/of the engine, but my opinion is that the condenser heat load must flow through the engine one way or another, sooner or later. Also, it seems to me that moving that extra HEAT load OUT during the low engine load time would be more helpful than otherwise.
That car has now spent 2 summers in Fresno with the new owner perfectly pleased with the A/C
The cabin blower use is a new idea and hasn't yet been tried during the summer. That's installed in my '88 Carrera. The engine fan outflow duct is blocked off and the cabin heat blower inlet is open to engine compartment atmosphere.
More cooling airflow for the engine plus extra cooling airflow for the rear lid condenser with the trinary switch activating the cabin heat blower via a connection to the cabin heat control module.
The '88, and back to '83(?) had/has a feature unique to that series.
If the roadspeed is consistently low and the thermostatic switch sensing the engine oil temperature closes (oil temperature is to high) then the cabin heat blower is activated to help cool the engine via moving more airflow over the exhaust manifold.
I simply bridged the 3rd section of the trinary pressure switch into the engine oil temperature switch circuit.
Still experimenting with a TDR, Time Delay Relay, to run the cabin heat blower for up to 20 minutes post ignition off. Stole this idea from The 964/993 series, they both use a TDR to help cool the engine down should the engine temperature rise to high post shutdown.
I remain of the opinion that one of the causative factors resulting in annual loss of refrigerant is that there are times that the refrigerant pressure might well rise high enough to fully permeate the non-barrier hoses.
The most obvious of those would be the post engine shutdown period with the convection and radiant heat from a HOT engine and HOT exhaust manifold HEATING the rear lid condenser. Causing the possibly already high pressure refrigerant to rise to extraordinary levels.
The other is due to the relative poor factory design of the compressor cycling control.
According to my '88's owner manual there is no method for cycling the compressor off even with the R/D filled with LIQUID refrigerant, and this with the engine at idle. One can only imagine how high the refrigerant pressure might rise with increasing RPM.
Last edited by wwest; 03-06-2015 at 10:42 PM.
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03-07-2015, 06:31 PM
#8
Rennlist Member
I did some of Mr. West's mods today. I hooked up new 8" fans and attached my engine fan to blow out the bottom of the engine bay. All I need is some hot weather to test.
03-08-2015, 05:58 PM
#10
Rennlist Member
