vac adv., man vac, camshaft, questions

[Rick C.]

A month or so ago, the Western NY Chapter held a great tech session when Gene Manno demonstrated his sun distributor machine. Recently, I thought I about checking my '71 LT1 distributor on it, but I have questions and one question led to another, etc.

Question #1: If the actual manifold vacuum is incorrect for the engine from which the distributor came, won't that then make the set up for the vacuum advance done on the sun machine inaccurate?

Question #1 came about because I suspected my engine has low manifold vacuum. Question #2: Would it be correct to say that not all new engines would have the same manifold vacuum because of the difference in camshafts used in each engine? Question #3: I have 8" of vacuum at the carburetor base and 10" at the power brake fitting on the manifold behind the carburetor. The vacuum reading is steady. Isn't that a little low?

I then was wondering (Question $4): could my valve lash adjustment be the reason or part of the reason for the low vacuum. Which led me to question the cam that is in the engine. I bought the cam from the Corvette parts supplier that I was using at the time (1995) and the spec sheet they provided is below. ( I can't seem to get it any clearer - lash is listed as 30H for intake and exhaust) Question #5: Can it be dertimined from the information given what this cam really is? Is it actually the 30-30 cam used in the '64-65 375 HP engines, etc? Question #6: How does this effect the performance of my LT1 vs. a stock camshaft?

I have been setting the valve lash per the service manual. Question #7: I will set them per Hinkley/Williams. When I do so, should I see an increase in manifold vacuum?

Thanks in advance.

Rick

[Duke W.]

Re: vac adv., man vac, camshaft, questions

Your have the wrong cam in your LT-1. The CS-118R is the 3849346 "30-30" cam not the 3972178 "LT-1" cam.

The combination of the 30-30 cam with the '71 LT-1's low compression is a very poor configuration - less low end torque than the LT-1 cam, but not significantly more top end power.

Because the 30-30 has more overlap it produces less vacuum at the same idle speed as the LT-1 cam - about 10" at 900 versus 12" @ 900.

For pre-emission or emission controlled engines converted from ported to full time manifold vacuum advance, use the "two-inch rule" to select a proper VAC. Do some research in the archives. Most pre-emission OE engines follow this rule.

Your OE VAC/ported advance was selected for emissions compliance, not fuel efficiency, and it is ill-suited to full time manifold vacuum.

If you set the lash according to the referenced paper it will produce less low end torque and more top end power, but is easier on the valve train for the reasons discussed in the paper.

My recommendation is to install a proper LT-1 cam - Speed Pro CS-1145R.

Duke