Degreeing Cam, Check My Math.....wtf Am I Doing Wrong?

[Chuckman]

so i'm installing a new tfs stage 2 into my fox, and either i'm doing it wrong, or my math is all fubared.

installed it (so i thought) straight up, crank sprocket has 9 keyways, used the 0 one just by eye.

card specs at .050" lift @ lifter are
intake 5 btdc 39 abdc max 107 atdc 224 duration
exhaust 53 bbdc -1 btdc max 117 btdc 232 duration

my first time thru i got:
intake 10 btdc 40 abdc max 105 atdc 230 duration
exhaust 50 bbdc 4 btdc max 120 btdc 234 duration

so looked 2-3 degrees advanced on centerline, and an extra few degrees duration. looking at my sprockets again, it looks like i used the 2A keyway somehow, so i reinstalled it back to the 0 key and tried again, which is where it gets weird.

next readings:
intake 0 btdc 48 abdc max 114 atdc 228 duration
exhaust 50 bbdc -4 btdc max 113 btdc 234 duration

looks like somehow my intake went from being 2 degrees advanced to 7degrees retarded (and lost 2 degrees duration?) and the exhaust went from 3 degrees retarded to 7 degrees advanced.

so is my math just screwed, am i just doing this wrong, or do i just need to move it the other way?

 

[Chuckman]

So i rechecked my setup and either my wheel or pointer moved 5 degrees. Now sitting at 109atdc intake (should be 107) and 116btdc exhaust (should be 117). My timing set only has 2 degree increments, so adjusting to put the intake on center would put the exhaust 3 out, and if i did the exhaust it would put the intake off 4. Therefore I'm hoping close enough Where it is?

 

[RacEoHolic330]

If you're that close, it's probably where it needs to be. What method did you use to determine exact top dead center?

 

[Chuckman]

i used my dial indicator, and had it setup so that when it read .100" both on the up and down stroke, i would check the wheel, and then turned to the average of those readings and reset the pointer to 0. guess when i was going thru last night i just bumped my pointer somehow to give me funky readings and was too tired to realize it.

 

[RacEoHolic330]

That's very possible with how sensitive the dial indicators are.

 

[Gearbanger 101]

The only really accurate way to measure it would be via a piston stop. As long as you can divide by 2, you can't get it wrong.

 

[Chuckman]

The only really accurate way to measure it would be via a piston stop. As long as you can divide by 2, you can't get it wrong.

Its actually really the same difference, as long as you're not finding the peak with the indicator, same process just using an actual measurement instead of bumping against something.

 

[Gearbanger 101]

Its actually really the same difference, as long as you're not finding the peak with the indicator, same process just using an actual measurement instead of bumping against something.

I guess it really all depends when during the assembly you check for TDC. If you do it prior to installing the heads, then sure, go ahead and use that method. Even still though, IMO, there's more room for error with the dial indicator. Inaccurate or worn gauge, accidental movement of the gauge or base, over-travelling the stop point (even ever so slightly) human error (chain stretch), etc. Not to mention if you've got to remove the degree wheel for any reason after the fact, you risk the change of losing the reading.

I prefer taking the reading after the head is installed, using a good ol home made piston stop.

Thread it into the #1 plug hole after determining the beginning of the compression stroke,
Turn it clockwise until it makes contact
Zero out the degree wheel
Turn it counter-clockwise until it stops again
Take your reading
Divide by 2
That's your zero point

Idiot proof and accurate to within 1/2 a degree. Doesn't get any better than that.

 

[Chuckman]

All those "error points"also apply to a piston stop method. except chain stretch, since both methods keep the cam out of the equation until you actually start measuring it. The indicator way also lets the engine always rotate the same direction taking out any possible backlash (if even a couple tenths I'd be surprised). And you dont run the risk of slamming a piston if you're not paying attention and going too fast.