Manifold Vacuum
 

Now that I have my '68 running (again), I'm starting to work out all the small bugs:

One thing that is concerning me is manifold vacuum. The reading that I am getting seems a little on the low side. (My gauge shows a reading of about 9. ) I realize that there are a meriad of factors that contribute to the vacuum readings. But I believe I have taken those into consideration:

1. There are no apparent leaks. The brake booster is new and working fine. RS system is new and working fine, transmission modulator is new and working fine. All unused ports are plugged. The only hissing noise I hear is from the throat of the carb. No whistling.

2. I have a cam which has 450/460 specs.

3. I have a 1" spacer on top of an aluminum Holley intake. Brand new correct gaskets on both sides of the spaces.

4. My initial timing is 14 degrees.

5. I am using a new Holley 457S carb.

6. It looks like a stock distributor with a VA but using Pertronix Ignitor II instead of points.

7. 327 Engine which is freshly rebuilt.


Is this vacuum reading not a little on the low side?

If there is a vac leak at the base of the intake, how would I be able to tell?

Vic

Manifold vacuum is different from venturi vacuum (the carburetor), and isn't affected by your carb or intake manifold.
A couple of factors will alter manifold vacuum:
1) The duration of the cam, more duration will lower your vacuum.
2) Severe engine wear (worn rings for example), causing a poor pumping action in the cylinders.

Thanks Chuck,

Since this is a very recent, professionally rebuilt motor, I doubt (hope) it is not your #2 reason.

But we did put in new cam. Below are the specs. Would it be responsible for this type of behavior?

ES1095M- .450/.461 lift, 224/224 duration , 60 degrees overlap


BTW, engine was bored out 40 thou over standard.


Vic

Hi Vic,
That does seem a bit low.
Here are the specs on my cam, and I'm getting just under 10 lbs of manifold vacuum at idle.

12-678-4 - XTREME Energy

Where are you taking your manifold vacuum reading from?

Also, how recent is this engine? Is it possible the rings have not sealed yet?