Timing Belt Oddity

[barnwerks]

Tom was great to remind us that the belt tension is specified to be measured with the "drive" run of the belt slackened.
I would like to question if the theory put forth of cylinder compression, or other crankshaft loads, having an effect on the belt tension is correct. I theorize that the belt can only "know" the loads along the belt path. This would include loads such as friction from the tensioner / idler and waterpump bearings, pumping resistance from the waterpump impeller but mostly the frictional resistance from the camshaft turning and opening valves against spring pressure.
I suspect the OP's observation of the belt teeth climbing with the cyclical belt load increase caused by valve lift resistance is related to insufficient belt tension.

[Tom]

Tom was great to remind us that the belt tension is specified to be measured with the "drive" run of the belt slackened.
I would like to question if the theory put forth of cylinder compression, or other crankshaft loads, having an effect on the belt tension is correct. I theorize that the belt can only "know" the loads along the belt path. This would include loads such as friction from the tensioner / idler and waterpump bearings, pumping resistance from the waterpump impeller but mostly the frictional resistance from the camshaft turning and opening valves against spring pressure.
I suspect the OP's observation of the belt teeth climbing with the cyclical belt load increase caused by valve lift resistance is related to insufficient belt tension.

Fair point and good to question that. I may need to experiment with and without plugs next time. I'm generally of the mindset to do it just like the manual says so that all tensions/slack are where the factory contemplated when the belt is measured. I definitely see your point about compression though, since the crank is driving the belt and the amount of force needed to drive the crank seems irrelevant, but it changes the dynamic so much I'd just want to confirm that in the real world.

[Rick Thomson]

This is a belated "Thank you" reply to all who helped resolve the issue described in this post.

ReidMcT: Explanation of tension variability due to varying valve spring tension.

t36: Confirmation of Reid's and Tom's comments on varying belt tension.

barnwerks: Confirmation of insufficient belt tension as root cause.

Tom: Invaluable lesson on timing belt tension effects, proper tensioning process, and the indispensable magic tensioning tool. Thoroughly enjoyed the discussion.

Just saying "thank you" to you all seems less than appropriate, given that each of you took the time to share your thoughts, experiences, and wisdom to help resolve my problem. Believe me, it is very appreciated.

Happy New Year to you all!

[ReidMcT]

Just saying "thank you" to you all seems less than appropriate, given that each of you took the time to share your thoughts, experiences, and wisdom to help resolve my problem. Believe me, it is very appreciated.

Happy New Year to you all!

So, what was the resolution?

[Rick Thomson]

Just saying "thank you" to you all seems less than appropriate, given that each of you took the time to share your thoughts, experiences, and wisdom to help resolve my problem. Believe me, it is very appreciated.

Happy New Year to you all!

So, what was the resolution?

It boiled down to a lack of belt tension. I did not have access to the Porsche belt-tension tool, so I used a method that seemed appropriate but was not. Tom's adjustment tool solved the issue.