At what centerline do I time my camshaft?
It is a Melling SPD-3 I installed wayyyy back in 1986.

Specs from Melling at that time are:

Duration at .050" , Cam Lift , Centerline , Valve Lash

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Intake 198º .270 106º .010" hot
Exhaust 203º .275 112º .020" hot


I checked the centerline with a new MP double roller timing set and it was 105.5º Intake, and 112º Exhaust. Is this where I should install it at?

Dang, I had the specs all typed out at correct spacing but it looses it when I submit it. Why is that?

It sounds like you have the cam installed "straight-up" at this point. You may want to try it there and see how you like it.

If you find that you want more low RPM power, advance the cam 2-3 degrees.
DD

Looks good where it is. This cam appears to have three degrees of advance ground into it.

Mitch

Doug, Wouldn't the cam be installed "straight up" only if the exhaust and intake centerlines were equal?

Mitch, that makes sense to me that it is 3º advanced right where it is. 6º difference divided by 2 = 3º

What about achieving maxumum fuel economy? I was just reading on CompCams website that while advancing the cam will increase low end torque, it will also decrease fuel economy.

At what RPM ranges does this "low end" torque occur? My 1976 Valiant with present 2.76 gears and automatic transmission puts me at about 2500 RPM at 60 MPH which is where I usually drive on the highway.

With the old worn timing set that was in this engine I was getting 22+ MPG quite regularly. I checked cam center line with it and I came up with 107.5º intake centerline.

Mabye I need to retard the cam timing for max fuel efficiency?

You can look at it this way: if the cam is installed 'straight up', then you'll have 3 degrees of advance. Technically, you are correct though.
For an application such as yours, your stock slant will probably improve mpg slightly with +3 cam timing.
2500 sounds a little high. What size tires are you using? The answer you are looking for is hard to quantify. On a dyno, your peak Tq & HP numbers would probably occur perhaps two or three hundred RPM lower with +3. Qualitatively, however, you will have higher manifold vacuum at all lower engine speeds resulting in less throttle opening to give the acceleration/speed you want.

Mitch

So what is it? Straight up or 3º advanced. I'm getting confused now.
Here's a little more info about my "stock" engine. I rebuilt it back in 1986, long before the internet but I was subscribing to The Slant 6 News at the time. Gosh, hard to believe I was only 20 years old back then. I feel so OLD now. Anyway, back to the engine.... The engine was bored .040 over (found a wrist pin out of place and wore big groove in #2 cylinder wall, #2 cylinder had a sleeve put in it), head milled .050", and a little inexperiences port polishing which I'm sure did not make any big difference. Before the rebuild the car (a 1975 4-door Valiant) got 20 MPG regularyly on highway trips. After the rebuild the mileage was WAY down and stayed that way for many years. The machine shop had everything to minimum clearance in the engine. The last two years I drove it (70,000 miles after the rebuild) the fuel mileage finally came up to 22+ on highway and one summer I even got 20 MPG in short trip driving to town and back every day. The car rusted away years ago and now I'm going to put this engine (now with about 80,000 miles on it) into my 1976 Valiant - same exact car as the 1975 except this one has factory Air Conditioning. This 1976 Valiant mostly gets 18 MPG and occasionaly better but average fuel economy is below what my 1975 was. I want to swap in this engine hoping I will be back to 20 MPG+ again.

P185/75R14 tires run about 840 revolutions per mile figuring 24" rolling diameter. 840 x 2.76 gears = 2318 drive shaft RPM. Then add 10% for torque converter slipage and I get 2550 engine RPM. Is this correct?
Oh by the way, planned "upgrades" will be a 2V SuperSix setup from a Volaré and a lock-up torque converter. I will leave the 1V carb on this engine for now so I can verify any fuel economy change by swaping the engine from my rusty 1975 Valiant into my 1976 Valiant.

No confusion. Let's make this simple. If the cam, as I suspect, has three degrees of advance built-in, then if you install the cam using a multi-keyway timing gear, the 'straight-up' setting will give you +3 degrees. Still confused? Install the cam using your MP timing set and that's just about where you want to be.

Using a methematical model based on your tire size, I get the RPM at about 2457, so you were close. I have 2.76s and with a 904 lock-up and 215/75x15 tires, 60 mph gives me about 2010 RPM!

Mitch

I agree with the statement that this cam seems to have some advance already "ground" into it.

Since you are more concerned with low RPM performance / MPG you may want to run some "cranking compression" tests at different cam advance settings to see if you can get an increase in cylinder pressure.
I use a manual trans bell housing and flywheel so you can mount the starter, then run a compression check at the different cam advance settings. I usually see the most cylinder pressure (in psi) with 2-4 degrees of cam advance.
DD