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| Joined: | 05-01-2005 |
| Location: | Minneapolis, Minnesota USA |
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I think I just answered my own question- cam timing is always TDC on the compression stroke. I think I measured it up for TDC on the intake stroke. Both valves need to be closed, correct? So that would explain why I am 180 degrees from what Tim suggested: intake at 10 o'clock and exhaust at 2 o'clock, just for an initial setting to bolt down the cam towers and get everything ready.Cam Timing is always spec'd, and timed with the #1 cylinder at TDC. That's your starting point.
The cam lobes need to be where they need to be... they stay put. Then the adjustable pulleys allow you to then rotate the outer rim & teeth as need be to mate up with the timing belt. The timing belt is in a fixed position by the crankshaft, which by definition is not allowed to rotate off of the TDC position during timing... so the belt can't move either. The pulley rim must go to the belt, without disturbing the cam lobe positions.
Anything that affects the distance between the crankshaft centerline and the camshaft centerline affects the amount of slack in the un-tensioned belt loop (tensioner backed all the way off). The crank must stay at TDC (it can't be allowed to move), so the slack in the belt strand between the crank and the exhaust cam pulley must go up and over the pulleys to/ or from the tensioner... rotating the pulleys a little.
If the head and/or cam carriers have ever been milled to flatten a warp, if the block has ever been decked, or just a simple stack-up of production tolerances... anything that alters the "PERFECT" center-to-center distance also alters the amount of slack, and hence, the positions of the cam pulleys at TDC, and the alignment (or lack there of) of the timing dots.
So, the cam lobes need to be where they need to be when at TDC, and the adjustable pulleys only allow the rim to be moved as necessary to follow the belt that might be moving as non-standard slack is pulled out of the timing belt. Make sense.
I previously gave you angular locations for the cam/ pulley keyway locations above the centerline between the two cams. That is where they (and the lobes) need to be when the crank is at TDC. Now you're just screwing around with the pulley rim.
NOTE: Head gasket thickness has a large affect upon the slack in the timing belt loop. Therefore, degreeing the cams is done 'after' the engine is fully assembled and the head is torqued down (gasket it crushed to final dimension). Only then can you accurately 'degree' the cams.
You are trying to pre-set the adjustable pulleys to a ball park setting (close, but not spot on) with the head off the engine. Don't confuse that with actually 'degreeing' the timing as a final procedure.
See the attached JPEG. JHPS only allows a 100kb maximum file size, so that's all the resolution I can give you here.
The intake cam lobe is shown in three positiions. The one pointing down & right is MOP... the intake valve is at it's Maximum Opening Position. The lobe to the upper left is where it would be when the #1 cylinder is at TDC. There's a note in bold red font, "To Red Centerline, 14.75° degrees... that's for a 110° MOP. In the lower right corner of the page, there's a chart of angular dimensions for other MOPs, and 105° MOP is 17.25° degrees above the centerline.
You can either:
1) McGyver a way to establish a 'working' centerline between the two cams, and a protractor or cut pie wedge to indicate 17.25° above the centerline, or...
2) Put a degree wheel on the cam, and a dial indicator perpendicular to the tappet (NOT on the lobe itself). Rotate the cam to find the Maximum Opening Point via the dial indicator, and zero the degree wheel. Now use the attached drawing to map your way (rotating the cam) to the 17.25° dimension above the centerline between the cams.
The details for the exhaust cam lobe are just the mirror image of the intake lobe, so they're not included on the drawing.... just a red centerline for where the keyway should end up.
In either case above, the result will be a rough, ball-park approximation of where the cam should be... ignoring all the factors that can affect the amount of slack in the timing belt prior to tensioning. Now, with the keyway's located, you can move the pulley's rim to align the timing dot with the centerline between the cams.
But be aware that timing with the head on a bench is not allowing for head gasket crushed thickness, or for any past milling of parts. Install the head on the engine, and the end result of this work will be wrong to some small degree. But not by enough to bend a valve as the head is torqued down.
Now the real work begins, and you can do a proper job of degreeing the cam timing with the #1 cylinder at TDC.
Good luck,
Tim Engel
Attachment: 9XX Cam Timing - Pulley & Cam Keyway Orientation - Cyl Head DRAWING.jpg (Downloaded 92 times)