Join Date: Jul 2000
Location: Central California
I make really good vacuum in the water box, generally around 18" (I do high gear burnouts), until I start to roll out and the tires begin to recover, at which point I let off (to save the converter sprag). The only reason I had a vacuum guage in the car was to determine how the engine was running, in concert with the tachometer. Back when I raced, most cars were uncorked (mine has a full exhaust system) so I could hardly hear myself think much less hear my engine well.
In your case, I'd do the burnout, which should provide you with decent vacuum, and then, after rolling out and stopping the first time to clean out the engine (if necessary), keep your foot on the brake and proceed forward to pre-stage. I've never had the need to do dry hops, but doing that (if necessary for you) could cause some vacuum loss. By keeping your foot down on the pedal, you keep the same vacuum working for you, rather than losing it when lifting. The way the booster check valve works is to isolate the booster diaphragm chamber from the outside whenever there is less vacuum outside.
As for not being able to hold the car, I'm at a loss. I don't have nearly the braking capacity you have, no power brakes and tires far bigger (no slippage) but I can never recall the brakes not holding at the starting line, even when I'd run the engine up to 4-4.5K. Granted that was with a 289/302 which didn't make the torque of the bigger engines, but it still launched impressively *G* With the W and a 10" converter, I ran it up to around 3.2K against the brakes with no problems whatsoever. I never even thought about that stuff until preparing this post.
I do know, from my brake light racing episodes at the big end, I could lock up the Airhearts with the stock MC I was using. Perhaps my friction materials are softer or something. I know for sure everything is still asbestos; yeah, it's that old...*G*
You might try having the drums machined slightly and having the rotors ground....maybe a change in the friction surface might help.
If you set the line loc, can you slide the front tires? You should be able too, with slicks on the back.
Try a few things and get back to us.