Correct vacuum for 360?

[Turbo301]

I don't have the fancy exhaust sniffer that the manual calls for to set idle/mixture, so I did it the old fashioned way of vacuum monitoring. The best I could get was 18 in-Hg in neutral at ~ 980 rpm. This then drops to 14 in-Hg and 750 rpm in drive. It's very smoot in neutral, but still shakes quite a bit in drive (it did before, too, when rpm was around 730). It's not obtrusive or anything. It drives the same as it did before I got to tinkering, so I guess that's something LOL.

I just wanted to confirm if those numbers sounded typical for a stock 360 with 106,000 miles .

 

[CBODY67]

WHY the high rpms? Idle speed in "N" or "P" should be more like 700 or less. UNLESS it has a hot rod cam that needs that much idle speed to idle "clean".

I have never seen any "factory specs" for intake manifold vac level at hot base idle. Reason is that such vacuum can vary with altitude. Plus variables in base ignition timing and such. Generally, no matter the engine, anything approaching 18" Hg at a 600-700 rpm idle (near sea level elevation) is great. Higher altitudes generally reduce the vac reading 1"/1000ft.

Personally, I like to use a dwell tach to set idle mixture with. Much more accurate, to me, than a vac gauge.

Just some thoughts and experiences,
CBODY67

 

[Turbo301]

WHY the high rpms? Idle speed in "N" or "P" should be more like 700 or less. UNLESS it has a hot rod cam that needs that much idle speed to idle "clean".

I have never seen any "factory specs" for intake manifold vac level at hot base idle. Reason is that such vacuum can vary with altitude. Plus variables in base ignition timing and such. Generally, no matter the engine, anything approaching 18" Hg at a 600-700 rpm idle (near sea level elevation) is great. Higher altitudes generally reduce the vac reading 1"/1000ft.

Personally, I like to use a dwell tach to set idle mixture with. Much more accurate, to me, than a vac gauge.

Just some thoughts and experiences,
CBODY67

If I set it to 700 rpm in neutral, it'd probably die in drive. I've never had a carburetted car that didn't idle higher in neutral than it did under load, in gear, and that's across Chrysler, Ford and GM.

My altitude is 823 ft above sea level.

 

[darth_linux]

Slightly off topic but the idle speed spec for my ‘66 383 is 500rpm in Drive, which I found shocking at first, but after careful adjustment of the mixture and timing it idles perfectly at 500 in drive and goes up to about 750-850 in neutral. Not sure what the vacuum is specifically but I know it’s good otherwise I couldn’t get such a low idle speed. I’m at 1800 ft. Above sea level.

 

[Turbo301]

Slightly off topic but the idle speed spec for my ‘66 383 is 500rpm in Drive, which I found shocking at first, but after careful adjustment of the mixture and timing it idles perfectly at 500 in drive and goes up to about 750-850 in neutral. Not sure what the vacuum is specifically but I know it’s good otherwise I couldn’t get such a low idle speed. I’m at 1800 ft. Above sea level.

I've never had luck with my classics idling so low ^_^

 

[CBODY67]

It IS common for a lower idle speed under load, in gear, than in "N" or "P" on all carbureted cars. Look in the old repair manuals for 1950s cars and idle speeds were more in the 400rpm range than higher. They also had smaller carburetors back then, as to air flow, too. BTAIM

On modern EFI cars, there is an Idle Air Control motor which controls the idle speed as the throttle valve in the throttle body is completely closed. ALL of the idle air goes around it, modulated by the Idle Air Control motor. It adjusts things according to computer control to keep the idle speed within a particular value. In gear, out of gear, a/c on or not . . . all the same idle speed.

With the advent of California Emissions, cars with that engine calibration were generally idled a bit faster, to get possibly better fuel atomization by the carburetor, for a bit cleaner emissions out of the exhaust pipe. Not so it might not die in gear so easily.

The cam specs on the 360 2bbls are very close to what was in the 1966 383s and prior Chrysler 383s, 361, and 350 motors all the way back to 1958. So, it should have no issues idling at 600rpm and lower in gear. Smoothly and reliably. With the ignition base timing set at those lower speeds, too.

At a lower idle speed, you MIGHT also get a higher vac reading, if that matters.

Just some observations and experiences,
CBODY67

 

[Boydsdodge]

I start with timing, find what the engine wants with the vacuum gauge, then adjust idle mix and curb idle speed. take for test runs, find my total timing at 2500 and check if it is still climbing with speed, often stock distributors are not done adding timing past 4000 rpm. Unfortunately or fortunately it often leads to a distributor recurve for best results.

 

[furious70]

I would think it should be closer to 18" in drive at 800rpm or so.