WillDC7

My AC is working fine, but when its turned on and the car is at idle, it emits a loud moan. It never does it when the engine is under load. The sound seems to be coming from the compressor, and it's loud enough that it's annoying inside the car. Being clueless about air conditioners I have no idea what this might mean. Is this just a nuisance that needs a good dose of lubricant, or am I looking at the impending death of my air compressor?

Also, another *completely* unrelated question that I've been pondering long enough to drive me mad. Is there anyone who's good with air flow dynamics and such who could answer an intake question for me. I understand very well the role that runner size and length plays in determing where the horsepower and torque of an engine comes out. What I've thought about for hours and I'm having a hard time grasping is why. Can someone give me a physical explanation of this phenomenon. I'm not understanding why, if the engine has adequate airflow, the length of the intake runners should have any effect on the torque curve.

Thanks in advance for the help.

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86 Trans Am WS6: The muscle car lives

JP84Z430HP

A/C compressor:
I'm guessing you have dual electric fans. When you're not moving, the one electric fan doesn't really pull enough air through the condenser, and the High side pressure gets pretty high. Mine was around 290 psi. which makes the compressor work much harder.
Where I found the sound to be actually coming from is the belt that goes around the power steering. The one that also goes around the A/C is somewhat tighter, and it makes the other one "flop around" when the compressor is running.

Runner length:
I'm not sure of the physics behind it, but it has to do with the velocity of the air in the runners, and the air "bouncing off the sides of the runners" causing turbulence. I'm not very clear on this, either.


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Working on:
'84 Z28 LG4 305
Added dual elec fans.
145 MPH IROC Speedo
Building 430 HP 350
using primarily GMPP parts.
Block is in shop!

Eggplant Jeff

Runner length... I think it has something to do with the fact that air does not flow constantly down the runner. The cylinder opens and shuts several times a second. With a short runner, the air stops moving (or slows down a lot) when the intake valve closes, then has to accelerate again when the valve opens again. With a long runner, the air at the beginning of the runner is still moving, it hasn't had time for the pressure to build up enough to counteract the momentum, so when the valve opens again it will fill quicker.

I'm no expert but that's just what I've inferred from what I've heard about the subject.

Krom

here is a simple version, there are equations to determine the lenght you want to use for a certian rpm, but I wont get into that here (I'm at work and dont have them here any way) the runner lenght tuning has to do with the shock wave that is created when the intake valve closes, this wave travels up the runner till it hits the end of the runner, it is then reflected back down the runner, if the intake valve is open when it gets there it acts like a supercharger, forcing more air into the combustion chamber. longer runners take more time to get the wave back to the valve, so they are effictive at a lower rpm than short runners

Scott 88 GTA

Krom, where did you find equations to determine the proper runner length? I've been looking and I've never been able to find them. Could you post or e-mail them to me? Thanks

five7kid

iTrader: (14)

Krom has it right. Shock wave theory. Has to do with the speed of sound in the fluid, which is affected by density. Injected engines respond to tube length tuning because the air is "dry", i.e., no fuel mixed in. Carb engines don't respond as well because the density is constantly changing with mixture ratio changes. That's why tunnel rams can make awesome power, but you only see them on the track (speaking only of the sane, of course).

"Velocity stacks" on injected drag engines of the 60's employed this ram effect: Shorter the stacks, higher the RPM the power comes on; longer stacks, lower RPM. Popular Hot Rodding had an article on this within the last year, very well done.

Similar thing happens in headers. The fluid is hot, therefore low density, low speed of sound, longer tubes for the same RPMs compared to intake. Street tubes are longer than full race, etc.

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82 Berlinetta, orig V-6 car. Rescued w/86 LG4/TH700R with all harnesses, sensors, ECM, etc. 3.08 open, cat-back from '90 305HO, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress.
57 Bel Air, my 1st car. Currently 396 .030 over, Weiand Action+, Edelbrock 1901 Q-Jet, Jacobs Omnipack, 1-3/4" headers, TH400 w/TCI Sat Night Special conv & shift kit, 3.08 10-bolt, AMSOIL syn lubes bumper-to-bumper. Best 15.1 @ 5800' Bandimere. Daily driver while Camaro was being put together.

87kevroc

I have the problem you described with your A/C on my 87 IROC. I think I know what's causing the sound, but don't yet have a solution. When the compressor comes on, its vibration is transferred to the power steering pump because they're bracketed together. The hose running from the pump to the steering gearbox ocillates like hell and causes the sound you described.

There are a couple of Technical Service Bulletins from GM on this problem, I found the titles on alldata.com. But the contents of the TSB's cost, and I haven't bought them yet. Anybody else familiar with this problem?

Robert Desmarais

87kevroc, I already obtained the info on the TSB's. According to it the solution was to add another bracket, and a special damper on the compressor. Notice I said was, because the parts are now discontinued. The only way I found to fix the problem is to convert to the 88-92 serpentine brackets, or go aftermarket and install a Sanden compressor. I personally dislike the GM R4 compressor. I believe it to be a loud parasitic piece of ****! One other possible solution is to use the brackets that came on some of the TPI C4 vettes and use the Nippendenso compressor, but this from what I know is very expensive.

Robert

WillDC7

I was just trying to make sense of airflow, I didnt even think about a shockwave, although, in retrospect, that makes perfect sense. If you do find those equations, and you would be kind enough to email them to me at will@cdc.net, they would be greatly appreciated.

For the people who's AC is doing the same thing as mine. Has yours done this the entire time you had the car, or did it recently start. I bought my car with 90k miles on it, and I've owned it for 10 months and put 10,000 miles on it. It's only started making this sound within the last 1000 miles. Is this similar to everyone else's experience? It seems like to me that that sort of vibration would have shown up before 100k miles? Any comments/speculation?


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86 Trans Am WS6: The muscle car lives

Hummer

My AC does the same thing. When it starts making that noise, the belt vibrates like nuts. I've tried tightening the belt, but I guess that's not the problem. I thought the compressor might be on it's last legs. Explain to me how this bracket is supposed to work. I can't stand to leave my AC running anymore. It causes my car pain!!! LOL!

mbarland

My compressor does the same, the belt on the PS pump vibrates (tightning does nothing), but the one on the PS pump an AC compressor works fine.

Wondering if anybody has more info since this was last seen...

S92RS

iTrader: (1)

I'm looking for a solution to this one too. My car makes the same exact noise and it's annoying. Mine is an 84 with v-belts and has a mechanical clutch fan. The compressor is tight and the belt is tight also. Mine is a remanufactured unit and is about 6 months old.

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- the 92 RS is gone..
- 1984 Z28 with 89 bumpers and ground effects.
- 355 with ported World S/R Torquer Heads, 214*/224* .471/.496 lift cam, Crane 1.6 Full Roller Rockers, 2500 non-lockup stall converter, Edelbrock Performer carb and intake, Hedman headers with custom 3" Flowmaster exhaust, Lakewood LCAs, Powertrax Locker with 3.23 gears.
- http://s84z28.fbody.com

Vader

Those same fluid dynamics formulae can be applied to exhaust tube diamter, length, and design. This is precisely what a truly "tuned" header set does for an engine - enhance flow at a givent RPM (pulse cycle) range.

Later,
Vader

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"Some Body...Someone"

c55

test

zz4mula

I had the same moanin g noise at idle only. What solved my problem was a loose power steering pump. Apparently, the nut that tightens to the backside stud of the pump worked itself loose. Look there. Hope this solves your problem.

Ward

iTrader: (1)

First, check the bracket that holds the compressor. mine made noise because it was a little loose. Then, check all the rest of the brackets. Also, check where the A/C hoses lay across the fan shroud, because at idle, mine vibrate on the fan shroud and make noise...

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1983 Pontiac Firebird SE
LG4-Based Chevy 400
700R4 Tansmission
Modified 4 Barrel Q-Jet Carb
Accel Super Stock 8mm Wires
Edelbrock Pro-Flo Air Cleaner
3.23 Posi Rear
14X7" Cragar SS/T Wheels
35X4 CD/AM/FM Head Unit
100WX2 Amp
2 Pioneer 12" Subs in Custom Box

suh_wheat_86305

iTrader: (1)

Hey JP84Z430HP,
I like your comment on dual fans... did you do something to lower the High Side Pressure and did that help the moaning?

I just bought a 86 TA, 305 TPI... in very nice condition, well maintained. The compressor is quite new, but moans so bad... it's obnoxious!

What about installing another type of compressor... a better one. Not too exotic/expensive
Thanks!

>>>>JP84Z430HP wrote:
I'm guessing you have dual electric fans. When you're not moving, the one electric fan doesn't really pull enough air through the condenser, and the High side pressure gets pretty high. Mine was around 290 psi. which makes the compressor work much harder.
Where I found the sound to be actually coming from is the belt that goes around the power steering. The one that also goes around the A/C is somewhat tighter, and it makes the other one "flop around" when the compressor is running.
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Vader

Your '86 should have had an R-4 compressor. There is nothing wrong with that type, since it is a more modern design and was used at least through 2002 on many models (and may still be in use). One thing to check for is the presence and condition of all the braces, including the rear brace between the compressor and exhaust manifold bolt.