My 88 has a carb'd 350 which was installed and built by the previous owner twice removed (I bought the car from a guy who bought it from a guy...). I've had it about 3 years, but I have never actually seen the camshaft. Pulling the cam is labor intensive, and I want to know my own cam profile and just happened to have the intake off sooo....

I put timing tape on the harmonic balancer and adjusted the rocker arms on #1 so that there is no slop but the pushrods rotate freely. I also removed the spark plugs. I wrote down the crank position when each pushrod went tight and got the following numbers:

EVC: 24 ATDC
EVO: 80 BBDC
IVO: 24 BTDC
IVC: 80 ABDC
hydraulic flat tappet lifters, 1.6:1 rocker arms

for a profile of 284/284. Looking at a crane tech article and at cams in catalogs, it takes about 35 degrees of crank rotation from the start of valve lift to 0.050, and similarly 35 degrees at the end for the valve to close from 0.050, giving me something close to 220/220 @ 0.050. The lifter rise is almost exactly 5/16" for a valve lift in the .480 to .500 range, and I calculate the LSA to be 118 (284-24*2)/2.

How it performs: The car has trouble maintaining brake boost vacuum. I replaced the vacuum lines (looked to be from 1988) and check valve, which helped a bit and there is the obligatory check valve in the vacuum system between carb and everything else. In a normal stop the boost is fine...it's an annoyance in parking lots and such.

I get only about 6"Hg vacuum at the manifold at 800rpm and as such, I still have a weak boost and pulsating brake pedal at idle. The idle is lopey/rough below 1000, and the engine smooths out above 1500 rpm. It really comes alive around 2500, and screams through 7000rpm, above which I'm afraid of breaking something, plus the tach pegs...but my God I love that sound.

A few questions:
1) do the measurements and calculated profile sound reasonable?

2) Is the vacuum measurement reasonable for this cam profile?

3) The car has a t-5 manual transmission with a stock (3.08) rear end, zoom clutch p/n 30009. I have a couple complications and I wonder what to do (reasonably economically) about them. First, the engine is weak below 1500 rpm so clutch slip on any grade has to occur in the 2000-2500 rpm range. If the rpms sag below 1500, I'd better push the clutch back in or get laughed at. Here in West Virginia, you are on a grade more often than not, and on a moderate to severe grade, I find myself either spinning the tires or smelling hot clutch. This concerns me since one of the first things I did was put the aforementioned clutch on as the one before was warped. I turned the flywheel at the same time. There are no symptoms of warpage with the new clutch yet...maybe my clutching technique needs work, or perhaps I should put a clutch in that is more tolerant of high-rpm slippage.

 

To get more accurate results, use a degree wheel and dial indicator on the lifter, pushrod or valve retainer but factor in rocker ratio if you do it on the valve. If its hydraulic cam then the lifters will likely bleed down before giving accurate lift numbers so i like to measure at the lifter or pushrod

Sounds like its some sort of street strip type cam, likely 280's duration advertised but if lazy lobes from old tech grinds it could be higher. Probably 230 or so deg at .050 and a tight 108-110 lsa if its that rough in a 350. 6" Hg is very low so its a big cam. Then again the tune has good influence on vacuum. Proper air fuel and timing advance will raise vacuum. Poor tune like overly rich and not enough timing will lower vacuum

Get it to idle at 1000 rpm and give it a good 26-30 deg timing at idle and vacuum should go up some

 

Based on the LSA you've calculated, I'd say possibly not.

That's good advice. As Orr states, tune aside, the poor idle may be from a tighter LSA (than your measurements indicate) and the overlap that results. A 118 LSA is unlikely (in a Gen 1 SBC) and that value wouldn't provide enough overlap to cause of problem with vacuum.

 

You're not going to find an off-the-shelf SBC cam that's got a 118* LSA. Typical performance cams are in the 108-112* LSA range. Even a "stock" cam will typically be in the 112-116* LSA range. 118* is off the top end of the chart.

For an easier way to find LSA just find the point of max lift for both the intake and exhaust valves. The number of degrees between them divided by 2 is your LSA. So if the intake is at 110* ATDC and exhaust is at 114* BTDC the LSA is 112* (110 + 114) /2 = 112* LSA.

Trying to get an exact measurement of where the lobe just starts to open the valve and just closes the valve is VERY difficult to do. Finding the .050 points is much easier if you have a degree wheel and dial indicator setup. Finding the max lift points for intake and exhaust is the easiest of all.