OK, I was goofing around with the engine today. Something was making a noise and it acted like it was my S/C. We I took the belt off and spun it by hand, it seemed ok. So, I hooked a boost guage up just to the left of the FPR. It showed vacuum. I assume this is normal for idle.

How high in the rpms do you have to go to get boost? Take into account, I'm still on a stock pcm with the xp cam and 3.8 pulley.

If I'm correct, boost is activated more by engine load than rpm. If I slowly accelerate I can go as high as I want with rpms and never get into boost. But if there is any decent amount of load on the engine then I can get boost at will.

So vacuum at idle is perfectly normal, correct?

I've only had a running S/C engine for about a week, and drove it for the first time yesterday and today. Those were only quick, 1 mile trips at that.

Yeah, vacuum at idle is normal...

Agree 100% with what Slick has said.
With normal driving you should show a vacuum. Shove your foot in it and you should see boost.

One thing I've noticed is I can do three things with my engine while driving and want to accelerate: I can get the RPMs really high with no boost, I can boost it and keep the RPMs at 3500 or lower, or I can WOT and have both.

I like boosting it without the high RPMs because you can still get good acceleration (if you're just passing someone, for example) but without really chugging gas.

Yeah you should be sitting around -9psi or -18inHg at idle. Anything higher and you might have a vac leak.

Rook, some vac lines will only show vacuum, and some will show boost and vacuum (which is what you'll want to connect your vac/boost gauge to).

I'm no M90 expert, but I think that due to its positive-displacement characteristics, its very capable of producing decently high (nearly full, but not quite) boost at low RPMs. One of the reasons M90 cars have this "instant torque", a benefit over the centrifugal supercharger which cannot produce full boost at low rpms.
To keep out of boost at low engine loads, the bypass valve (on the rear right side when you're looking at it?) recirculates boost (somebody recently pointed out thats why M90's use extremely little hp for cruising).

Notice you can put your car in 1st and slowly press the gas, get to high RPMs, and still stay far out of boost.

So full vac at idle... moving around in vac at normal driving, and when you stomp it should jump way into boost right away, increasing slightly as RPMs go up.

Once you get the tuner, you can setup a display showing boost. Mine idles around -9 to -10.5 at times. It starts to go positive with the touch of the accelerator. I believe you can check out the parameters, maybe TP or something like that, where you know when it will hit positive boost. At WOT I hit 10.5-11 at times.

With an XP cam you will only see 12-14 in Hg at idle, not 18-20 like with a stock engine. Big cam = loss of manifold vacuum

Well, I was on a boosted line (FPR running from lower intake port), so I know that. I was also running a vacuum greater than 10 in/hg, but wasn't paying attention. Won't I also have slightly less vacuum at idle due to the cam?

Anyway, it's nice to know. I'm trying to keep from driving it due to the huge amount of noise/drone without an exhaust and the lack of tuning. I did slightly get into it to pass a tractor during a test run and got over 60mph at a decent pace. Unfortunately, I didn't have the boost gauge at the time, and the car is too loud to even hear the supercharger.

I would think it would be the opposite... More air flow with no demand would create more vacuum on the intake side... No?

There's a reason big cammed V8's need vacuum pumps to retain power brakes. Cams reduce vacuum.

Yes, I was aware of that.... They also make vacuum reservoirs..... I never thought of that. Thanks for enlightening me.

crimpton, the reason big cams reduce manifold pressure is that they have a greater degree of overlap where the intake and exhaust valves are both open.

This is to help fill/evacuate the cylinder of combustion mixture/exhaust gasses. With this process, the intake portion of the stroke doesn't pull as much of a vacuum from the manifold.

Cool. Makes sense. This wouldn't apply to all cams though, just those with lots of overlap (N/A primarily).