[jeff5may]

I guess this means you're not using the OEM power supply or onboard relays to control the fan or compressor any more. Make sure to fuse everything correctly.

As to the r600 (butane) issue, it has a vapor pressure a little less than r134 or r12. At evaporator saturation temps below freezing water, your suction pressure is going to be near 0 psig. At seawater freezing temp, evaporator pressure is below atmosphere. I doubt your compressor could move enough mass to keep its motor cool.

Since your compressor is made for r410a, it needs to see a suction pressure in that range to keep cool within its operating envelope. Discharge pressure is not so much an issue as far as the compressor is concerned. All it really cares about is temperature.

What I would do is install some piercing valves on the stock unit and run it like they built it. Slap some gauges on it and log suction and discharge pressure for both sides. Choke the airflow on each side separately and observe pressure changes. Run the bad boy at all speeds and measure away. This will give you a detailed snapshot of the operating envelope it was designed to survive within.

After a decent approximation is made, you can blend propane and butane together to get a mixture that will satisfy your suction pressure range while giving you good discharge and condensation temperature. Raw high-side pressure will be lower than that of r410a, which in my mind is a good thing. Less pressure means less stress on everything.