Cam Timing..Woohoo!

Superman · 03-16-2002, 08:02 PM

Okay, she rotates. Now how much "lift" should I give her? Specs for these 20/21 cams is 1.9-2.2mm at overlap. Stock is 1.4 to 1.7. So, should I go all the way to 2.2 (or close), or should I go closer to 1.9. 2.2 is a lot of overlap for CIS, probably, but these cams are supposed to leave me some driveability. So, should I go with the wives tale (which Bruce Anderson is not sure he believes) that says to advance the timing (2.2) for greatest bottom end. I bet that if I didn't go as advanced as possible, I'd wish I had.

Opinions?

GIBSON · GIBSON's Garage

Super, I've said before that all of these "adjustments" are a trade-off. A bit more low end for a loss of high end power. IMHO, most of us could not tell, without the aid of a dyno, where power gains were or even if there were any.
On cams which are more agressive, I always adjust dead center because your are getting close to valve interferance if you don't. On milder cams you can try advancing but I always aim for the middle of the spec range and figure it's the best compromise.
Just my opinion.

Early_S_Man · 03-16-2002, 08:23 PM

Nominal would be 2.05 mm, but, if you feel like bottom end for the Ax is where it's at, go for 2.2 mm! Those '20/21' cams are still pretty mild by 'S' standards! Any lift value within the specified end 'limits' is perfectly safe!

In fact, lift is set at max at the factory with new chains ... in anticipation that wear during break-in will retard the timing back to the 'nominal' value. Just another reason why cam timing used to be checked at the 1000 mile service at dealerships iin the 'old' days!

MMARSH · MMARSH's Garage

Superman,

Just got done doing 20/21 cams. I too was wondering were to set them. I ended up right in the middle of the range. I have put about 300 miles on the motor, and it seems to pull pretty hard. I think you will be happy.

Superman · 03-16-2002, 10:02 PM

Thanks, guys. Mike, I'm a little irritated that you finished your engine project so much more quickly than I am. Congrats.

In inches, the range I had to work with was .074 to .086, roughly, and I ended up with a matching .084 on each side. That's 2.134mm. Yeah, I guess I went for the bottom end. I'm partial to low end torque and I'm not building a track car. And if Warren's report about the factory habit, then my chains should have room to stretch.

One step closer......

MMARSH · MMARSH's Garage

Superman,

It'll come together, If I had taken a nice Vacation in the middle of my rebuild I would still be working on mine too

I was kinda lucky everything like the machine shop and getting all the parts came together real smoothly. The motor feels strong but I'm having an idle/under 2500 RPM stumble problem that Im trying to figure out. Looks like I should be at the track next weekend though according to plans. When do you think you will be done?

Superman · 03-16-2002, 10:39 PM

I'm still shooting for next Saturday's autocross. I''m going to have to focus.

Superman · 03-19-2002, 08:55 AM

Okay, cam spec is up to 2.2 mm for these 2-/21 grind cams. That's about .086". I was able to adjust both mine to .084" So, this is as advanced as I'm supposed to go, using cams that have more overlap than stock. How close am I coming to valve/piston interference? That is, if valve travel at overlap were a little more than 2.2mm, would the valves and pistons be in danger of touching? (I'm not going to do this, but I am curious)

Hey, valve timing is not that complicated you guys, unless I did it wrong. It seems to be just a matter of adjusting the sprocket so that at TDC overlap (180 degrees from TDC on the compression stroke) the intake valve is open the correct distance. So, you move the engine to that position, then move it back or forth a little until you get the desired 'lift,' pull the pin in the sprocket, turn the crankshaft back to TDC overlap and put the pin back in. Then double check of course. Really pretty easy.

Oldporsche · 03-19-2002, 11:18 AM

The best I've found is to crank in a little advance but have both sides match. This point is generally just a little more than the middle of the range of lift specs for the cam.

Good luck,
David Duffield