94' G50: No Acceleration When AC On

[sdkhalsa]

When my AC is on the car does not accelerate decently from a stop or low speed. My thought is that there is a cut-out on the AC linked to the vacuum or TPS which is not doing it's job. I have manually selected 1st gear to see if there was a downshift problem, having heard that some of the early 90 Q's start in second gear. This did not help the situation. Aside from the above AC situation, the car runs great. Any ideas?

[elwesso]

The Q does accelerate noticeably slower with the AC on.. figure about 20HP loss assuming its on full duty cycle.....

The only AC cut off is above 4000 RPM or something along that line.

[sdkhalsa]

I would expect a certain loss of power, but you can mash down on the accelerator, i.e. to get into traffic, and the car does not move for like three seconds.

[Q45tech]

Hp loss is not the usual way to describe decreased ACCELERATION.

20 HP at 2,200 rpm = 5252/2200= 2.387 x 20 hp= ~48 lb/ft............20+% of the total accelerative force............hope I'm being clear the lower the engine rpm the more the effect of the compressor [however the parasitic drain in horsepower is not constant. Specing a drain must also provide rpm.

A 24,000 btu/hr system would only need 2 hp [if 100% efficient] more like 5-7 HP as installed [depending on inside fan speed.Back in the 60's ac compressors had 12-15 Hp losses.

The engine ecu cuts compressor on load and rpm which works fine on 2nd [above 60 mph] -4th gear acceleration because, however 1st gear is problematic until 35 mph.

I was assummed that those who wished to drag would understand this and manually shut off compressor...........to avoid the lag while ecu decides what you are trying to do.

Robert can reprogram ecu to shut off compressor at any rpm and load.

From another forum concise and useful:"Air resistance is:Drag = 0.5 x Cd x A x rho x V^2where:Cd = coefficient of dragA = frontal arearho = density of airV = velocity

Horsepower required to overcome drag is:P = V x Drag

So, using my car as an example:A = width x height = 73.6 x 47.7 = 3500 sq in = 24.4 sq ftV = 75mph = 110 ft/secrho = 0.00237 slugs/cu ft

Drag = 0.5 x .31 x 24.4 x 0.00237 x 110^2 = 108.4 slug-ft/sec2 = 108.4 lbf

P = 21.3 hp (which still leaves me with some 320 odd horsepower to accelerate with )

So if the A/C adds 5hp, then power required increases by ~23%. Assuming the V6 is making 200hp @ 6000rpm, it's probably only making 65 or so horsepower in 5th gear at 75mph (~2500rpm) even at WOT. Given that 21 hp is already used to maintain speed, then your available horsepower goes from 44hp to 39hp. ....................................

.............If I'd used metric, I would have had to convert hp to Watts and back, and MPH to m/s, instead of just in to ft, and mph to fps.

Argh! I can't believe I neglected rolling resistance!

RR = Crr x W x V

using a Coefficent of Rolling Resistance of .02 (wide, sticky tires), and a weight of 3300lbs (my car with me in it).

RR = .02 x 3300 x 110 = 7260 lb-ft/sec = 13.2hp.

And the power required to maintain speed is 34.5hp.

(Note that this is power at the drive wheels, which is about 15% (for a stick) or 20% (for an auto) less than the 'rated' horsepower at the crank)

Editing the last paragraph:So if the A/C adds 5hp, then power required increases by ~13%. Assuming the V6 is making 200hp @ 6000rpm, it's probably only making 65 or so horsepower in 5th gear at 75mph (~2500rpm) even at WOT. Given that 34.5 hp is already used to maintain speed, then your available horsepower goes from 30.5hp to 25.5hp. "

[Q45tech]

A 90Q has 28.1 square feet [27.6 if lower 1"] and a cd of 0.30............that would INCREASE THE DRAG and thus the power requirements by 13.6% or 12% [if lowered] for the previous calculations [sample car in off forum post]

Wider than oem [215] ties would increase frontal area by tiny amounts as would their rolling resistance.

Correcting for Q weight and all the above say give or take that the 90Q needs 40+- a few HP to move 75 mph.

[bullittandy]

Am I dumb? I don't understand how these 2 previous posts answer the question. Maybe a clarification is necessary?

[wamQ45]

Just for grins....check the brake booster hose.....I had this problem years ago because the hose was cracked. My Q45 was very slugish due to a huge vaccum leak. I replaced the hose and all was well.

[Q45tech]

Gave you the methods to calculate the actual decrease in acceleration AC vs AC off at any speed [75 used as an example]............wind resistance and rolling resistance all you need is to translate the engine torque curve and the Ac compressor HP drag curve and net out the difference in torque.

After all the other parasitic losses will be the same........only variance is AC compressor and alternator [fan draw].

You need to tell us the exact diffence in zero to 35 mph you are concerned with. It shouldn't be more than 0.3 seconds in 1st gear. 2nd gear is not problematic since a WOT start should down shift to 1st.

What do your drag strip graphs show. Stop watch test repeated enough times to be accurate say 20-25 samples!

Tip in sudden acceleration may be reduced depending upon coolant temp prior to start of acceleration. Subject to contriversay above 194.9 or 199.9F will start the ecu overheat protection. Common for old coolant systems to operate 10-15F too high compared to brand new..........in summer a significant power reducer.

In summer depending on fuel quality the knock sensor may retard timing even more - the extra load from compressor might be enough to cause this also tipping you over edge.

Use a Consult to data log the ignition advance vs rpm vs coolant temp AC on and off afew times to see whats happening......also MAF voltage to make sure underhood/ambient ingestion doesn't change.