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Old 08-19-2011, 06:04 AM   #1
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A/C Drag

I bought my "99 Boxster base about 1.5 monts ago. It has 68,000. mi. Runs great, no major problems except the stupid gas gauge. (but I digress) Anyway I was was wondering if anyone else notices major strain on the engine when you turn on the the A/C. My '944's were pathetic once you turned on the A/C, and I see that the boxster, while not pathetic, has more strain than my BMW.
Anyone care to comment?

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Old 08-19-2011, 06:19 AM   #2
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Yes. All vehicles, regardless of the HP output, feel the strain of AC to varying degrees.
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Old 08-19-2011, 06:26 AM   #3
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Yes I know this as well. To define the question a little further... does anyone find the drag on the boxster engine a liitle more dissapointing than it should be?
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Old 08-19-2011, 06:34 AM   #4
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Yes I know this as well. To define the question a little further... does anyone find the drag on the boxster engine a liitle more dissapointing than it should be?
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Old 08-19-2011, 07:04 AM   #5
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Define “disappointing”………… Just exactly what were you expecting? The AC drag on any car is substantial, as a percentage of the total HP available; it is relatively large number on the Boxster. Try driving a 90 HP compact with air, when it switches on, you’d think you hit the brakes.
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Old 08-19-2011, 07:04 AM   #6
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The smaller and lighter the car, the more you tend to feel the A/C kick in. I used to have a 1.6L Datsun pickup with A/C and it practically stopped the truck when the compressor engaged.
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Old 08-19-2011, 07:19 AM   #7
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lol. I know. I had a '79 VW Rabbit w/ a/c. Wasn't passing anybody on hot days. IDK, like I said earlier, you would think that 2.5 engine, 200hp, modern technology, it woldn't be so noticable. My '04 325i isn't bad at all. Maybe I'm just spoiled.
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Old 08-19-2011, 09:31 AM   #8
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The typical parasitic draw from a car AC compressor is 10-15 crank HP.

Add to this, an additional 1-2 HP is drawn from crank HP by the alternator (which basically 'free wheels' unless a load is put on it) to power the blower, and you're getting dangerously close to a 10% reduction in usuable output on the 2.5L motor.

If that 2.5L motor is well used, and no longer putting out the 218 ponies it originally came with (or you're not driving at peak HP on the rev curve), the total reduction of usable output could easily approach 15%.

Those are numbers you're likely to feel.

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Old 08-19-2011, 09:43 AM   #9
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I personally don't feel the difference.
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Old 08-19-2011, 11:27 AM   #10
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Quote:
Originally Posted by Lil bastard
The typical parasitic draw from a car AC compressor is 10-15 crank HP.

Add to this, an additional 1-2 HP is drawn from crank HP by the alternator (which basically 'free wheels' unless a load is put on it) to power the blower, and you're getting dangerously close to a 10% reduction in usuable output on the 2.5L motor.

If that 2.5L motor is well used, and no longer putting out the 218 ponies it originally came with (or you're not driving at peak HP on the rev curve), the total reduction of usable output could easily approach 15%.

Those are numbers you're likely to feel.

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2.5 engine is factory rated @ 201 HP. I would think Boxster AC compressor load is more like 5HP.
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Old 08-19-2011, 06:47 PM   #11
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Quote:
Originally Posted by BYprodriver
2.5 engine is factory rated @ 201 HP. I would think Boxster AC compressor load is more like 5HP.
Thanks for the correction, but that makes the draw even more significant.

I seriously doubt the AC compressor load is only 5 HP.

The concept of compressing freon (or any other gas) is well understood, there is no 'magic' to it. The power required is directly related to the amount of heat (energy) transferred in the cooling process. This is actually much less than the total parasitic draw - somewhere in the neighborhood of 1-4 HP (dependent upon the compressor's capacity).

The biggest factor in the total draw are the inefficiencies involved in transferring the power from the crankshaft to the compressor - a belt and pulley system + the magnetic clutch on the pulley adds additional load by the alternator (small, but measureable). Lots of energy is lost in the transfer as the belt flexes and throws off heat. This is where most of the 10-15HP is lost.

V belt and pulleys systems are pretty inefficient at transferring power, significant power is lost in the transfer. Their advantage is simplicity, cost and adaptability.

The Corvette C6.R does claim a draw of only 1.5 HP using a Sanden compressor. But, this uses a totally different direct-drive system, drawing power from the driveshaft 'directly' - (also mounted on the rear of the motor). It is more costly, and less reliable in terms of length of service life (as many racing components are) which is why most manufacturers stick with the tried and true pulley/belt system - the non 'R' C6 uses belt/pulley.

Most manufacturers use virtually the same rotary compressor from Sanden - the world leader in automotive AC components. Even those labled as Frigidaire, FrigiMax or Bosch are either re-labled, or manufactured under license from Sanden.

The cases may differ depending upon application, but the rest is virtually identical. In the case of the Boxster, the compressor is a Sanden model #1152. So while I have not personally measured the draw in a lab run, I'm sticking with the industry accepted 10-15 HP average.

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Last edited by Lil bastard; 08-19-2011 at 07:11 PM.
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Old 08-19-2011, 10:36 PM   #12
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I agree with the OP, when the AC is on I really notice the difference in power but the interior cools down quickly and completely so I'm happy to trade off a bunch of power for a continuous stream of ice cold air on a hot summer day!
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Old 08-19-2011, 11:26 PM   #13
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No noticeable difference. Not even climbing in the mountains. Maybe it's an auto vs. Manual thing. Mine is manual and no problem. Also may be where you're driving in the rpm range. I'm almost never running below 3,000 rpm.

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