does your AC pull engine down?
Thread Starter
Racer
Joined: Aug 2014
Posts: 287
Likes: 9
From: Toledo
does your AC pull engine down?
I am wondering if the amount of "drag" my AC puts on my engine is normal. I know its hard to explain or even measure but when I turn the AC on in my car I can definitely tell when the compressor cycles. Its so drastic that I am wondering if the compressor is going out. Would this be a symptom to a dying compressor? Air is cold and not noisy. I can be driving 55 and feel it. Can you feel it in your car?
1995 Lt1
1995 Lt1
Last edited by chadk; Jun 14, 2015 at 10:34 PM. Reason: included year
Team Owner
Joined: Aug 2004
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From: Park City Utah
IDK what year your car is, but my '92/LT1 does this quite noticeably. I had thought that my compressor was also going bad, but started a thread on it HERE, and it seemed like the general consensus was that LT1 cars do this, and there's not much we can do about it. (?)
Race Director
Joined: Oct 2008
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From: Lansing MI
High side ac pressures can easily be 250 psi depending on outside temp. It can take as much as 8hp to properly pressurize the ac system. Pretty much everyone who is paying attention will feel the compressor kick on. Don't worry about it if its not noisy and cools down properly, also the belt shouldn't chirp the instant it kicks on.
Team Owner
Joined: Aug 2004
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From: Park City Utah
Race Director
Joined: Oct 2008
Posts: 17,982
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From: Lansing MI
It just seems like it since it takes very little hp to maintain a steady speed on the highway. Lets say it takes 80hp to maintain 70 mph, then subtract 8 and you lost 10%. That's surely noticeable.
If someone really feels like there is something wrong then start by verifying the system is not over charged, and if its not I really wouldn't worry about it unless the belt chirps or the clutch makes noise at the moment the compressor engages.
If someone really feels like there is something wrong then start by verifying the system is not over charged, and if its not I really wouldn't worry about it unless the belt chirps or the clutch makes noise at the moment the compressor engages.
Melting Slicks
Joined: Jun 1999
Posts: 2,931
Likes: 127
From: Ashland Alabama
My 86 coupe was noticeable when it cycled and so is my 97 lt1 fbody.
The 383 in the 87 convertible I cant even tell its on other than cold air coming out of the vents. But the 87 motor is stronger than the other 2 cars.
Idle wise the 86 dipped when it came on and then recovered while the 97 lt1 fbody doesn't change that much and the 87 doesn't change idle with the compressor engaging.
The 383 in the 87 convertible I cant even tell its on other than cold air coming out of the vents. But the 87 motor is stronger than the other 2 cars.
Idle wise the 86 dipped when it came on and then recovered while the 97 lt1 fbody doesn't change that much and the 87 doesn't change idle with the compressor engaging.
Team Owner
Joined: Aug 2004
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From: Park City Utah
It just seems like it since it takes very little hp to maintain a steady speed on the highway. Lets say it takes 80hp to maintain 70 mph, then subtract 8 and you lost 10%. That's surely noticeable.
If someone really feels like there is something wrong then start by verifying the system is not over charged, and if its not I really wouldn't worry about it unless the belt chirps or the clutch makes noise at the moment the compressor engages.
If someone really feels like there is something wrong then start by verifying the system is not over charged, and if its not I really wouldn't worry about it unless the belt chirps or the clutch makes noise at the moment the compressor engages.
If I'm climbing Parley's Canyon (SLC>Park City -3000' rise over 13 miles), my car will just about maintain my desired speed in 6th gear. There is a little power in reserve to accelerate with. With the AC on, there is NO WAY the car will make it in 6th gear. The AC kills the power. My truck, same speeds same hill, AC on or off, no discernible difference.
And
I can't help but compare it to my '96 Silverado; same engine (basically), and in the Silverado, you can barely tell when teh compressor engages. When you're driving, you can't tell at all. If you pay deliberate attention at idle, you can hear a change in tone when it engages, but the engine doesn't change speed. NOTHING like the 'Vette at all.
Safety Car
Joined: Aug 2009
Posts: 3,745
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From: Melbourne, Victoria, Australia
Hi
Yes i can feel the a/c cycle on when cruising on a highway, it is barely noticable but as a driver you do notice. Its nothing to worry about, as someone has already mentioned it draws quite a bit of power to start.
Yes i can feel the a/c cycle on when cruising on a highway, it is barely noticable but as a driver you do notice. Its nothing to worry about, as someone has already mentioned it draws quite a bit of power to start.
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Race Director
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From: Lansing MI
Team Owner
Joined: Aug 2004
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From: Park City Utah
I just charged the 'Vette yesterday. In ~85* temps the high side was and the low side was ~40 lbs, high side was ~200.
Race Director
Joined: Oct 2008
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From: Lansing MI
Why? Same motor, same manufacturer....I think it's a pretty valid ting to compare, and pretty curious the difference. The load on both engines should be about the same...shouldn't it?
I just charged the 'Vette yesterday. In ~85* temps the high side was and the low side was ~40 lbs, high side was ~200.
I just charged the 'Vette yesterday. In ~85* temps the high side was and the low side was ~40 lbs, high side was ~200.
As far as the comparison goes, the engines have very little in common and very few parts are interchangeable. The fuel system is completely different as well. The load from the ac compressor is approximately the same.
Team Owner
Joined: Aug 2004
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From: Park City Utah
Well those numbers are pretty normal. I don't think you have anything to worry about, even if it is irritating.
As far as the comparison goes, the engines have very little in common and very few parts are interchangeable. The fuel system is completely different as well. The load from the ac compressor is approximately the same.
As far as the comparison goes, the engines have very little in common and very few parts are interchangeable. The fuel system is completely different as well. The load from the ac compressor is approximately the same.
The motors in the 'Vette and the truck are functionally the same. SBC 350's w/MPFI. What is different about the truck motor, that would nearly completely mitigate the effects of AC compared to the 'Vette's motor? Nothing.
Race Director
Joined: Oct 2008
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From: Lansing MI
I agree that it's normal (numbers and function). That is why it's so weird that the effect is so profound...and you've noticed that I'm not the only one who feels that way.
The motors in the 'Vette and the truck are functionally the same. SBC 350's w/MPFI. What is different about the truck motor, that would nearly completely mitigate the effects of AC compared to the 'Vette's motor? Nothing.
The motors in the 'Vette and the truck are functionally the same. SBC 350's w/MPFI. What is different about the truck motor, that would nearly completely mitigate the effects of AC compared to the 'Vette's motor? Nothing.
Melting Slicks
Joined: May 2013
Posts: 2,692
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From: Sacramento California
I can feel it for sure when the compressor cycles/kicks in. It's not a problem when cruising steady speeds. Driving city streets or slow highway traffic, it can sometimes be difficult to drive smoothly due to the change in power/throttle response when the compressor cycles... I typically drive with a light foot and my car has the 6-speed, so little jerks on the engine will pronounce the tiny bit of slack in the drivetrain.
Team Owner
Joined: Dec 2012
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From: Outside the Quick Stop N.J.
That's not true. I don't know if you bought your truck new but if you did then check the brochure that came with it. One of the big selling points at that time was the new throttle body that has one big opening and one big throttle plate. This change was made with the express purpose of minimizing throttle sensitivity at low engine speeds and at small throttle openings. The reasoning behind this change was that it made it easier to back up trailers. Chevrolet and GMC actually mentioned this in the sales brochure. The entire engine was designed with these sorts of character traits. Everything from the throttle body to the rotational inertia of the reciprocating assembly was designed to enhance this characteristic. Guess what ?? ..... That's the exact sort of engine that wouldn't even hiccup when you turn on the ac. Guess what else ?? they did a darn good job with that engine, and it works great for its intended purpose.
While they both displace the same ci, the cam specs are different, as is the intake/exhaust flow, and maybe even the fuel map, and don't forget about the gearing in the trans/rear. If you put the two side by side on a dyno they would tell a very different story. Just as the truck can't keep up with the Vette in a race, the Vette can't haul/tow what the truck can.
Melting Slicks
Joined: Jun 1999
Posts: 2,931
Likes: 127
From: Ashland Alabama
I agree that it's normal (numbers and function). That is why it's so weird that the effect is so profound...and you've noticed that I'm not the only one who feels that way.
The motors in the 'Vette and the truck are functionally the same. SBC 350's w/MPFI. What is different about the truck motor, that would nearly completely mitigate the effects of AC compared to the 'Vette's motor? Nothing.
The motors in the 'Vette and the truck are functionally the same. SBC 350's w/MPFI. What is different about the truck motor, that would nearly completely mitigate the effects of AC compared to the 'Vette's motor? Nothing.
Following this analogy if the op's has an automatic then it would have very tall gears and that would make the a/c effect much more noticeable. Would need them to respond to know for sure.