nicely put flintstone

I wouldn't like to see more than 270 psi on the high side with a 100 degrees F ambient temperature. The 315 psi pressure can indicate several problems: insufficient condenser cooling, refrigerant overcharge, air in the system, expansion valve issues, etc. Do you have a low side pressure under the same conditions? Are the low side fitting/hose just after the evaporator covered with frost or moisture or are they hot to the touch?

Sorry Captain/Sir.
I needed to cool my jets, 16 hour days are now boring.
I keep thinking it is my turn to volley but no one wants to play. But we will share our toys.

Becks in hand:
So you are saavy enough to be cautious and you want to know why your head pressures are above normal for R134a on a 97 F day!

THE FINAL ANSWER IS AIR!
a) Air in the system (inadequate evacuation, not enough hours on the pump or the system iced up during evacuation and there is residual moisture), or
b) Not enough air across the condensers (ie. deck lid down, front condenser motor works), or
c) Fresh air ventilation system is open (all slide knobs s/b left)
d) Heater box(es) are open (heat coming in the cockpit, cables dysfunctional).
e) Left thermostat knob not fully clockwise.

Or. ASSUMPTIONS:
f) Verified equipment: thermometer is accurate within a given range of xx-x F of +/- x F.
g) Vacuum pump pulls xxx microns.
h) Service gauges are accurate (needles are true).
i) Charge hose set purged.
j) That your refrigerant scale is working, or your metering device is true, or you know you used 12 ounce cans and not 16 ounce cans.
k) Some years have "automatic temperature sensors", either located in the dash or in the headliner between the visors, if your vehicle has this feature you have tested them.
l) You noted in the first post that the "Windows down". Do you mean up ? Or are you attempting to chill out the neighborhood. ?
<i> and you have checked all these above items</i>


On the "average" the replacement charge of R134a refrigerant ranges from 70-90% in weight of the R12 OEM amount (according to typical automotive retrofit guides; in like systems: reference MAC's guide publication #44201, or "the good ole" Snap-On Technical Training Systems manual ACT 2798).

Assuming an OEM amount of 47 oz. that would give you a range of 33 to 42 ounces. Why the vast range stated by the guides? All depends upon what heat loads they used the day they tested their charges. Did one fella determine the perfect charge on an 80F ambient day and another on a 100F ambient day... ?

Porsche's shop manual suggests, with R12, @77f, deck lid down, 2k rpms, you could have a high side range of 115-205 psi. (double or nothing!). Compare that to a more finite or average industry guideline for R12 of 184 psi @77 F, or extend the temperature out to 97F and you might have 232 psi with R12.

Jim has pointed out that a high side of 270psi @100F would be near the end. I agree. Fudge in the R134a curve and 270 psi @97F does not sound too far off for a stock 911.

Now if you wish you could drive the head pressures right up there to 375psi, blow compressor nose seal or fry some ribs, but that gets to be expensive.... so keep the head pressures down and extend the life of the compressor.

If you have reviewed the AIR and ASSUMPTIONS, have you considered exploring what your evaporator core temperature is under your testing conditions; replacing the thermometer in place of the thermostat's thermal expansion (aluminum tube) tube (in your evaporator); or simply, did you have frost or sweating on the evaporator outlet pipe?