How much variation in factory cam timing adjustment?

[mike100]

Would there be any reason to keep the exhaust lobe center at 114 degrees as per the original early engine specs or maybe split the difference and put it at 112 deg?

(I'm leaning to use the 114/110 since it is well known to add area under the curve).

[Dynomite]

Originally Posted by mike100

Would there be any reason to keep the exhaust lobe center at 114 degrees as per the original early engine specs or maybe split the difference and put it at 112 deg?

(I'm leaning to use the 114/110 since it is well known to add area under the curve).

Marc Haibeck suggests to have a broad torque and power range (3000 to 7000 rpm) use Intake 114 ATDC and Exhaust 110 BTDC. 114/114 is nice when trying to tame idle roughness out of a cammed engine.

I notice the factory cam timing specs may be different on a 90-92 versus a 93? I see on one diagram that the 90-92 are set at I114 and E114. I assumed that is factory engine with the pinned settings.

I also notice that the single flats on a 91' camshaft/vernier plate have a play of up to 6 degrees (crankshaft degees). I think 93's have double flats with the play cut in half or 3 degrees (crankshaft degrees).

1. Timing Chain Effective Lengths.

The crankshaft rotates clockwise looking at front of engine. The Timing Chain Tensioners are located on the bottom (passenger side) chain before Camshaft Sprockets and on the top (drivers side) chain before Camshaft Sprockets. The chain between and after the Camshaft Sprockets returning to the timing chain Idler Sprocket is normally relatively tight. That section of the Timing Chain remains relatively tight as long as the crankshaft is rotated in a normal direction and as long as cams do not push back (lifters pushing the camshafts clockwise). Timing chain Effective Lengths are related directly to timing chain tightness and Timing Chain paths as directed by the Timing Chain Guides (That path changes with Timing Chain Guide wear).


a. Chain Tension different with engine running as compared to the Camshaft Timing Process.
If the Timing Chains are tighter during engine running with hydraulic lifters pressurized the Exhaust camshafts may be advanced a degree and the Intake camshafts may be retarded by a degree. For Exhaust camshafts at 110 deg BTDC may actually advance to 111 deg BTDC. The Intake Camshafts at 114 deg ATDC may actually retard to 113 deg ATDC.

b. Fixed Timing Chain Guide wear.
As the Fixed Timing Chain Guides wear the chain returning to the Idler Sprocket between the Camshaft Sprockets and Idler Sprocket decreases in effective length Advancing the Intake Camshafts and retarding the Exhaust camshafts. The Exhaust Camshaft may retard from 110 deg BTDC to 109 deg BTDC. The Intake Camshafts may Advance from 114 deg ATDC to 115 deg ATDC.


Tech Info - LT5 Timing Diagrams



See Item #4 Camshaft Timing

What you want is equal lifter depression before and after max lifter depression noting the crankshaft rotation degrees (degree wheel set up on front of crankshaft pully) before and after max lift set up on (No. 6 cylinder passsenger side and No. 1 cylinder drivers side). For Intake example you might see equal lifter depression at 104 deg ATDC and 124 deg ATDC and splitting the difference would be 114 deg ATDC. You do the equal lifter depression "trick" because it is almost impossible to see max lifter depression using a dial gauge within say 5 deg either way as the cam lobe rotates over the flat.

Marc Haibeck suggests positioning the cam at .010" before top of lift and .010" after top of lift point where .001" change in lift is about 1 degree of cam rotation.

If you want to make sure of the TDC marks on the Harmonic Balancer, you can install a dial gauge to determine when piston #1 is TDC to determine TDC easy enough engine in car and can then mark harmonic balancer and use degree tab bracket easy enough engine in car. The Tab has marks 25 deg Before and 5 deg After TDC (A total of 30 deg). You can use the marks on the Timing Tab to mark your Harmonic Balancer. Mark 110 deg BTDC by rotating the Harmonic Balancer 3 complete Tabs (90 deg) + 20 deg Counter Clockwise and mark. Mark 114 deg ATDC by rotating the Harmonic Balancer Clockwise 3 complete Tabs (90 deg) + 24 deg and mark.

See Item #2 Camshaft Timing