66 AC idle

[Paul K.]

Hello, I have a 66 327/350 with AC, all restored to factory specs. When I turn the AC turned on, (curb idle set at 800 rpm) I get about 3-400 rpm drop, How do I compensate for this?, I have researched quite a bit, but I cannot find the solution.
Do I need a carburetor with a Hot idle Compensator ? Any help is welcome.

Paul klemetson

[Don H.]

Re: 66 AC idle

Paul, I had a '64 300 HP with AC & currently have a '65 300 HP with AC. I have always bumped up the idle in the summer to compensate for the AC "drag". Good luck, Don H.

[Leif A.]

Re: 66 AC idle

Paul, I had a '64 300 HP with AC & currently have a '65 300 HP with AC. I have always bumped up the idle in the summer to compensate for the AC "drag". Good luck, Don H.

I agree. I bump mine from 750 to 950 in the summer.

[Paul K.]

Re: 66 AC idle

Thanks for your advice, I thought there would be a bracket and solenoid to bump up the idle when the AC is turned on, like the 69 vette.

[Larry M.]

Re: 66 AC idle

Thanks for your advice, I thought there would be a bracket and solenoid to bump up the idle when the AC is turned on, like the 69 vette.

You can add one........

Larry

[Duke W.]

Re: 66 AC idle

Normal L-79 idle behavior without AC is 750 @ 14-15". The compressor load which reduces engine speed and manifold vacuum requires that you go through the idle speed/mixture adjustment procedure with the compressor engaged. Select the lowest idle speed with the compressor engaged that is stable and you find acceptable, which might be around 700.

Achieving a stable idle requires fixed total idle advance. The OE 236 16 8" VAC is more aggressive than necessary on a non-AC L-79, and a 12" VAC passes the Two-Inch Rule, which is why I recommend a 12" VAC (modern B26) in place of the OE 236 16 (modern B28) on L-79s without AC. You need to verify the installed VAC ID and whether it meets spec.

Of course, you'll need a vacuum pump/gage. Set idle speed as above with the AC compressor engaged and apply the Two-Inch Rule to determine if the currently installed VAC meets spec and passes the Two-Inch Rule or buy a new functionally correct VAC - one that passes the Two-Inch Rule idling with the compressor engaged.

With the compressor off, idle speed will be higher, and you have to accept that.

The reason why Chevrolet used the 8" 236 16 VAC on L-79 may have been to achieve stable idle advance/idle speed with AC. The same applies to the base engines. Earlier 250/300 HP engines have 15" VACs, but beginning in '66 it was 12". The lowest base engine idle vacuum is likely with AC and Powerglide, but I don't have any data on that config. No AC and a manual trans yield a smooth, stable idle, 500 @ 18-19" so a 15" VAC easily passes the Two-Inch Rule, but the additional loads of the converter and compressor will lower vacuum at the same idle speed, so a 15" may not pass the Two-Inch Rule

Duke

[Paul K.]

Re: 66 AC idle

Duke, thank you very much for the advice,

Paul Klemetson

[William F.]

Re: 66 AC idle

What ill effect on L79 engine does using the more aggressive B28 vac can have? Detonation at low speed, etc.? My 67 has ac; should I use the B28 then to get a better idle with compressor on?

[James W.]

[Frank D.]

Re: 66 AC idle

You have to set the idle higher even with aftermarket air - but its less -- about 100 RPM..

The idle step-up solenoids I've seen (and used) were for Holleys mostly and pretty crummy.
The solenoid couldn't push the linkage off curb idle by itself so you wound up blipping the throttle at stop lights so the solenoid plunger would extend and quicken the idle - hokey IMO.