I'm adjusting the Spark Advance table in TunerPro RT to maximize the vacuum at idle to ensure my new brake booster has sufficient vacuum.
I've set the base timing on the car to 6 degrees BTDC, updated the Spark Reference Angle-Base Timing to match, and updated the spark advance table between 400 - 1200 RPM's to 16, 18, 20 and 22 degrees.
Latest update measured timing at the harmonic balancer @26 degrees BTDC.
Car starts and idles fine but I cannot get anymore vacuum beyond 13-14 inHg, regardless of the advance table set between 16-22 degrees.
I've backed out the idle screw on the throttle body so throttle plates should be closed.
I've plugged the vacuum lines coming into the car at the Fan/AC controls to avoid any vacuum leaks.
I might try blocking off the port to the brake booster and Supercharger by pass valve.
I can also smoke the intake to see if any leaks are present.
Have I missed any other options to diagnose why I cannot get more vacuum at idle?

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You need to set your total timing to 34-36 degrees, which would probably be 12 to 16 initial...

 

Is the cam stock?

 

Camshaft is a GM Hot Cam" (typically PN 24502586 or similar LT4-based kits), hydraulic roller camshaft for 1987-1995 L98/TPI.

 

Thats why youre not likely to get more vacuum. That cam has a lot more overlap than a stock cam, resulting in lower vacuum being produced at idle speeds.

What youre getting sounds about right for that cam. 218 intake duration @.050 on a 112 LSA.

For comparison... my old setup was a 350 with a cam that had 212° intake duration @.050 on a 112 LSA. I could get about 16 inHg at idle in park running about 25° spark advance.

My current setup is a 383 with a cam that has 215° intake duration @.050 on a 114 LSA. This one gets me about 18 inHg at idle in park with the same ~25° spark advance.

You might want to consider putting a vacuum reservoir on the brake system. During the times when you are producing high vacuum... elevated RPM with very low throttle, the reservoir will store that vacuum... your booster can then access that vaccum when needed. The engine then replenishes the vacuum as you hit those high vacuum conditions again.

 

I ran smoke through the rear end and vacuum hose from the brake booster. Found out my exhaust is shot with multiple seam leaks which becomes yet another project. Smoking the intake revealed very small leaks around the throttle body shafts. This is a new BBK 58MM so assume I pushed passed the o-ring's when pressurizing the intake. Next, I increased the timing advance from 22-28 degrees BTDC, in 2-degree increments. No noticeable increase in the vacuum, stayed at 13-14 inHg. If my RPM's are at 1K, vacuum is 15 inHg. Increasing RPM's above 1K increases vacuum to 16-18 inHg. I decided to increase the RPM's at idle from 750 to 800 RPM's which increased the vacuum at idle to 14-15 inHg. I adjusted the timing looking for highest volume, settled on 26 degrees BTDC with 15 inHg. Has anyone had success with adding a vacuum cannister before the brake booster to increase vacuum for the brakes during deceleration?

 

As has been stated above, vacuum is primarily a function of valve timing, not ignition timing. Ignition timing would have to be extremely askew to affect idle vacuum significantly. A ±1" Hg is usually about all you could manage by massaging ignition timing, and even that would be at the extremes. Increasing idle RPM could help a bit IF the setup will tolerate it.

As for the vacuum reservoir, the system already has that. It's the big black disc behind the master cylinder. The older Bendix style single chamber boosters were slightly worse, and the larger Morraine units were slightly better, Verifying the integrity of the check valve at end of the booster hose is important to permit best brake assist, but even at that you get about two good applications with assist before the reservoir is at a point where the level of effective brake boost falls off. Use your brake sparingly, and don't pump the pedal, since every release motion dumps some vacuum. Adding more reservoir beyond that is a bit of a band-aid, but could help if you have extra room somewhere to store more of nothing.

Changing to a two-chamber booster can help a little, since more assist is produced at a lower vacuum level. Additional options are an electric vacuum pump, an old-school mechanical fuel pump with a vacuum pump built in (good luck finding/fitting one of those), or switching to Hydro-Boost brakes slaved off the power steering pump. Those are golden in setups with superchargers.

 

More than 20° initial spark advance is way too much. Find out what your total timing is (at 2500 to 3000 RPM) and keep it between 32° and 36°.
What is the altitude at your location? What is the vacuum reading at 1,000 RPM?
A 218° duration @ .050" lift and 112° LSA should produce plenty of vacuum for power brakes...

 

Welllll...that depends. Typically yes. You did ask about elevation and that is for SURE a factor. So is compression and cam (which we know the cam).

 

Elevation is 200 feet above Sea level. Compression is 9.25:1.

 

Vacuum @ 1000 rpms was ~15-16 inHg.

 

Copy. In that case, TL is correct. 20 degrees initial would pretty likely cause hard starting, among other things. I'd think ~12 initial might be good?

 

I found an issue with the brake booster, recently purchased and installed Duralast remanufactured 54-71245. I pulled a vacuum to 20 inHg and let it sit for 10 minutes. Vacuum held but when I depressed and held the peddle down once, all of the vacuum bled out to zero which is a sign of a bad internal seal. I did not hear any hissing. Test stated the vacuum should have reduced to 15 inHg and hold until I pumped the brakes again. I did this test twice with the same result. I decided to buy another brake booster from A-premium APPBB0142 which claims to be a new unit. Will install that before proceeding.