Vaccum Advance At Idle

[Jeff C.]

There has been a lot of discussion on using the correct vaccum can for your engine (ie B28 for 340 HP 63) but how many degrees of vaccum advance should you see at idle?
Jeff

[Jim T.]

Re: Vaccum Advance At Idle

That all depends on what year and what engine your Corvette came with.
For example information from my 1970 Chassis Service Manual it has three pages of information on vacuum advance (in crank degrees). My 70 base engine 350/300 Corvette vacuum advance shows 0 degrees at 8" of vacuum and 20 degrees at 17" of vacuum. An 350/370 LT-1 shows 0 degrees at 7" of vacuum and 12 degrees @ 12 inches of vacuum.
The centrifical on my base engine goes from 0-30 degrees @5100 rpm. The LT-1 goes from 0-20 degrees @ 4600 rpm.
The BTDC timing is 4 degrees for my base and the LT-1 is 14 degrees.
From what I remember my 70 350/300 would idle in park or neutral at about 21-22 inches of vacuum and in gear with the turbo 400 about 17-18 inches of vacuum with the A/C off. Driving down the highway would usually see 17-18 inches.

[Duke W.]

Re: Vaccum Advance At Idle

There has been a lot of discussion on using the correct vaccum can for your engine (ie B28 for 340 HP 63) but how many degrees of vaccum advance should you see at idle?
Jeff

It's real simple. The VAC should provide full vacuum advance at idle. In fact, full vacuum advance should be achieved at no less than 2" less than idle vacuum to keep the VAC locked at full advance during idle. This is the Two Inch Rule.

A 327/340 pulls about 12" at 900 idle speed, so the VAC must be fully deployed at 10" or less. The B28 is 16 deg. @ 8". The next one up - B20 - is 16 deg. @ 12", which does not meet the two inch rule. The second one up - B22 - is 16 deg. @ 15". One of these three will meet the needs of all OE non-exhaust emission controlled engines or exhaust emission controlled engines converted to full time vacuum advance as follows.

B22 - all base SB or optional SB with base engine camshaft as long as a manual trans. is fitted.

B20 - all big block, all SB with auto trans, L-79, L-46; L-82w/ manual trans

B28 - all SB with mechanical lifter camshafts, L-82 with auto trans

Total idle timing is the sum of initial timing plus plus full vacuum advance plus a few degrees of centrifugal for engines whose centrifugal starts below idle speed. Total idle timing should be in the range of 20-34 degrees, and the greater the valve overlap the closer it should be to the upper end of this range.

Exhaust emission controlled engines with ported vacuum advance don't have to meet the two-inch rule. Their spark advance maps are set up for emission control, but this extracts a price in performance and fuel economy.

Most non- exhaust emission controlled OE engines with OE VACs meet the two-inch rule, but there are exceptions and one is the '63 327/340. This was the first year GM used VACs on SHP engines and they just didn't get it right, and the FI engine had ported vacuum advance. They fixed the situation in '64 with full time vacuum advance on the FI engines with 8" VACs on FI and SHP engines.

Duke