Mechanical Valve timing - 2.8L in 86XJ

Anyone know the mechanical sequence (in degrees) for a 2.8L GM with externally balanced crank?

GM doesnt seem to have the specs. in their archives (?????????). The local Stealership has no idea of what I'm talking about - not surprised.

Am looking for at what degree of crankshaft rotation the Intake and Exhaust valves of #1 begins to open and what is the degree at full closed. I have the dwell, the lift, ....but not the degrees (vs. TDC) to open/close.

Note to Mike Romain ---- yup, I guess you were right that the mechanical timing was off. All the damn 'indicators' lined up correctly ... but the engine is still running quite retarded. It HAS to be the mechanical valve timing.

Other question: if I find that the mechanical timing is indeed off (probably due to a 'stack-up' of tolerances. How does one readjust? A long time ago, I remember watching someone use a 'special bushing set' on the vibration damper, which allowed advancing or retarding the mechanical timing. Seems that Alzheimers is beginning to catch up as I cant quite remember .............

Thanks for any input.

Reply to
Rich Hampel
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Reply to
L.W.(ßill) Hughes III

Thanks for the info

Its not as gutless as it seems and for the purpose that I want ..... cruise at 65-70mph and with good fuel economy. You just need to 'finesse' it and make sure that all the eco-nazi crap is working perfectly. Most other 2.8s that Ive had in this XJ deliver about 22-23 mpg at cruise.

Thanks again

Reply to
Rich Hampel

Almost all of GM's modern engines utilize a wide lobe center angle for emissions purposes and a broader torque curve. Generally the peak lift occurs on the intake at about 110-112 degrees after top dead center. It is a common "trick" to advance the camshaft to boost low end torque slightly, and to lower the torque peak RPM of the engine. Be careful about using valve opening and closing events for cam timing, as most specs vary depending on who wrote them. Advertised duration as it is known, is an almost useless number in the real world.

I am assuming that you have a degree wheel to check the cam timing. If so, do it the right way and find out the intake lobe centerline. This is done by finding true TDC first. Put the wheel on the balancer or crank and put a dial indicator on the piston. Bring the engine to apparent TDC and zero the indicator and the pointer on the wheel. Now turn the engine in its normal rotation direction (always do this to take up the slack in the bearings and timing components) until the piston is .100 down in the cylinder. Take a reading on the wheel. Now go the other way past TDC and past .100 down in the cylinder. Now bring the piston up until the indicator reads .100 (again, do it this way to take up all the slack). Take another wheel reading. If they are the same, then you have true TDC. If not, then adjust the pointer by half the difference of the 2 readings. Repeat the .100 down check until you have the same readings.

Once the wheel is set, you can put the indicator on the #1 intake lifter. Be sure that the lifter plunger does not move during the measurements. I usually use a solid lifter to avoid this, but it can be done with a hydraulic if you're careful. Turn the engine past TDC until the lobe reaches peak lift. Zero the indicator. Now turn the engine until the indicator moves .050 down and take a reading. Go back past peak lift and .050 down (again, go past this point and come back to it if you are turning the engine counter clockwise). Take a reading at this point. Add these 2 number up and divide by 2. This will be your intake lobe centerline. The number should end up somewhere in the range of 106 to 110 degrees.

HTH

Chris

Reply to
c

Reply to
Rich Hampel

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