Well, let me start off with the engine. It is a forged internals 383 stroker with AFR 195 heads, Comp Cams 12-433-8 cam(.520/.540 236*/242* 110 LSA) RPM Air Gap manifold, Hooker Super Competition long tube headers, and a 750CFM manual choke edelbrock carb, full MSD ignition with a base timing of 18* and 36* total at 3000 RPM. Anyway, when the engine was first fired and up running, and I was driving the car around, my vacuum at idle was around 10-11 in Hg(rock steady I might add). Now supposedly, that is a little bit higher than what that cam is supposed to make(COMP says around 7-8 in Hg). Now that I have about 1000 miles on the engine, I am now only making between 7-8 in Hg, which is normal for this engine/cam(still rock steady). BUT, why did it drop to that if it started out higher? If anything, I would have thought that it would have increased after a while with the piston rings seating and everything. I did some checks, and nothing in my valvetrain came loose, and nothing else on the engine has been changed in the last 500 miles. Could the lifters have pumped up and are keeping the valves open a little longer than they should?? I am just trying to figure out if this is normal or not. I don't have any vacuum leaks that I can tell. Has anyone else ran into this before? Any thoughts/suggestions/opinions are welcome on this one. Thanks!!

 

Only 2 things I can think of that would explain why the vacuum was that "good" before the engine got broke-in. That's a lot of cam even for a 383. Recheck the timing and get a read on the timing chain and distributor gear mating in.
Check the idle stop on the carb if the carb was new, the throttle settling in to a little lower rpm. The vacuum will vary a lot with just a little higher idle rpm.

 

Supervisor42- Thanks for the reply. Here are some things that I came up with. I had to change out my distributor gear. Both MSD and COMP said that the distributor gears were compatible with the camshaft gears but they were wrong. Only after about 200 miles on the engine, I had accelerated distributor gear wear. At that point, I changed out to a bronze distributor gear.

I didn't think that the cam was really all that big. Heck the stock camshaft in my LS1 98 Trans Am is over the .500 for lift on both the intake and exhaust. As of right now, the engine idles at about 900-950 RPM. I have tried to lower the idle down to about 750-800 RPM, but the engine doesn't like it and wants to stall out. Probably within the next few weeks I will be taking the car to have it dyno tuned. Maybe they will be able to come up with something that I overlooked or missed.

 

Sorry Paul, i'm in a rush here, thought i'd throw in my thoughts before I ran out the door though;
Your idle is fairly high, but you NEED more timing. 18* isn't enough. Try 24* ish. Then you'll be able to back down your idle speed and you'll still have more vacuum.
That means you'll need to limit your mech travel though.