CrazyAl

The EFFICIENCY of the system is determined by the heat flow out of the condenser....but the total cooling output is determined by that AND the pressure.

For example, if you have a 5 kw compressor and your condenser is 60% efficient then you're getting 3 kw output.

On the other hand, if you have a 4 kw compressor an a 75% condenser, you get the same number.

That means if you diminished output due to UDPs and you then got a better condenser, you might be able to gain back your cooling loss. But that is not really practical. We have no control over the efficiency of the condenser. But we do have control over the compresor output. No UDPs=full output. UDPs= diminished output, above and beyond the compressor efficiency.

tedjac06GT

OK... I've read through this thread and can see both sides of the discussion. I have a question to toss into the equation. If you have stock gears (in my case, 3.31 I think), the engine turns at X rpms for a given speed. If I install 3.73 or 3.90 gears, then my engine will be at X rpms + Y rpms (Y being the additional rpms necessary to reach same speed with new rear gears). Won't this additional engine rpm at any given speed reduce the negative effect that the UDP have on the accessories. I'm assuming that we are talking only a few hundred rpms at most... but the engine will ALWAYS be spinning more RPM at any given speed. This would seem to "help" the situation.. at least marginally. What d'yall think?

Ted

cekim

No, because the engine spinning at higher RPM will add load to the accessories (additional electricity for spark,fuel, etc... and additional cooling for the water pump).

The A/C and PS pump will indeed "compensate", but
a. the A/C is a non-issue WRT to performance... turn it off ~0 HP
b. the PS pump is not generaly troublesome with underdrive pulleys...

CrazyAl

Remember that the cooling needs of the engine and the electrical demands are linked to ENGINE rpm. What happens afterwards (driveline gearing) is irrelevant. Actually, it might make things slightly worse, becasue with gears you are decreasing the relationship between forward speed and RPM. That means that for a given engine RPM you are pushing less air through the radiator and AC condenser.

...in other words, yes, the engine is spinning faster at a given RPM. But the engine's demands are increased proportionally. And becasue of the UDPs, the engine's demands will always be more than the accessory output (at least if we consider the OEM configuration to be "correct").

MBDiagMan

Just because the engine is spinning faster does NOT mean that it is generating more heat. The heat generated is determined by the work performed. The work performed at a given speed is virtually the same regardless of gear ratio within reasonable limits.

Yes the a/c compressors pressure IS related to the effectiveness of the a/c system, but the heat exchange capacity of the condensor in the practical sense is a MUCH greater factor. Slowing down the compressor a few RPM's will not be a significant change. If your a/c is not coolinig properly, start by checking pressures and post the low and high side pressures along with the ambient temperature and then we can troubleshoot from there.

CrazyAl

I agree that the efficency of the AC condensor is the biggest factor as to the operating performance of an A/C system. But, that isn't particularly relevant to this discussion because not only does the condenser have nothing to do with UDPs, but you also cannot (reasonably) change or mod the condenser. We are stuck with what the factory gave us.

If we have two identical A/C systems: both in perfect condition, properly charged, etc, EXCEPT that one has a slower-spinning compressor (UDPs) and the other is bone stock, then the slower (UDP-equipped system) is going to have worse performance than the other one. Now then, it may not be a huge difference, but there will be A difference.


That statement is not true. A certain minimum amount of fuel must be burned simply to keep the motor running at a given RPM. This is related to the engine displacement and the RPM. Running at a higher RPM is definatley going to use more gas than running at a lower RPM at the same load level. That's the same reason why a large displacement engine uses more gas than a smaller engine, even during identical driving. For example, a V-8 Mustang will use more gas than a V-6 Mustang, even if both are driven at the same rates of acceleration around the same track at the same speed. The difference may not be huge, but it is there, and it has been documented many times. One good example is EPA numbers for identical cars that are available with different gearing options. More gas burned = more waste heat.

Now, under some conditions a heavly loaded low-RPM motor may burn more fuel than a lightly-loaded motor turning higher RPM, but that situation is rare enough that I don't think it will matter much. Most heating issues are going to come during long-term cruising type situations, and in those scenarios the increased RPM will become the dominant variable.

This is explained clearly in Design and Simulation of Four-Stroke Engines by Blair; this was my textbook in IC engines back in college....damn good book, I still have it on my shelf.

MBDiagMan

Crazy Al,

You are taking me TOO literally. I understand that there IS a difference in these things. I am saying that the difference both with the a/c issue AND the work to move the car issue are insignificant differences.

There are ZILLIONS of variables when you start talking about such issues and theory does not ALWAYS work out in practice.

SOOooo...., continue on perfessor!

Have a great day,