How the Timing Belt Tensioner Actually Moves

[Maddog]

So I have always been curious how much the timing belt tensioner moves while going through the rpms. I built an engine test stand for a different project and decided I would try to measure how much the tensioner moves. I always assumed that the tensioner plunger moved inward as the rpms increased. I thought this was because the belt would get forced outward more and more as rpms climbed. I wouldn't say I ever analyzed the system though.

Anyway I took a video today of a dial indicator that I placed against the lever arm for the tensioner pulley. Its hard to get in that area in general and I couldnt get an exact measurement relative to the plunger. However you could assume the movement is more than the dial indicator shows. The video is boring and unedited but the first part of it was with the engine starting at 1000 rpms. Every ~10 seconds I increased the rpms by 1000 up to 5000.



I was surprised to see that the tensioner plunger actually moves outward as the rpms increase. I thought maybe the magnetic base of the dial indicator was moving around but when I let it go back to idle, the plunger actually slowly moves back inward. In the second part of the video I increase rpms to 5500 and slowly bring it back to idle. Again you can see the tensioner does the same thing. Its very repeatable which makes me think the measurements are fairly accurate.

I don't know exactly why the system is working this way but my assumption is the belt is stretching slightly and the placement of the crankshaft pulley and tensioner pulley prevent the belt from moving outwardly in that area. From the tensioner pulley to the first cam gear I can see the belt expand outward as you would normally expect. From idle to 5500 rpms the dial indicator shows movements of about 40 thousandths. The plunger itself will actually move more than that.

Thoughts about related subjects:
I wonder if a kevlar belt makes the tensioner act the same way? They claim they dont stretch... Anyone want to send me one to try this out on?
If you are using a solid tensioner, are you actually loosing some performance as the cam timing would be off a little because it can't expand outward?
I was always worried the washer mod for the tensioner would potentially cause the tensioner pulley bolt to loosen up because the plunger couldn't move inward and the belt would be trying to push it that way. It appears this isn't the case.

Thanks
Dan