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True TDC

Old 12-10-2018, 04:14 AM
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True TDC

Just as a general discussion, I would like to cover True TDC and its finer points.

If we use the piston stop method and rotate the engine forwards then back and average the distance in between, we arrive at what we call True TDC. If we use the dial indicator method we will likely end up pretty close, but at a different spot on the degree wheel. This is due to Crank Overlap or Piston Dwell. If you set a motor at TDC and rock the crank back and forth you would find that there is quite a bit of movement of the crank before the piston moves up or down. I believe this is where crank angle becomes important, that spot in between BTDC and ATDC. As a rule of thumb we split the difference and call it good unless there is a spec called out by the manufacturer, but how should we determine where the sweet spot is, the beginning of the dwell period, the middle or the end? When we start trying to time the cams as well, or in our case 4 cams, we are now multiplying the effect of dwell time and creating an even larger gap of precision. Is anyone aware of an article or other publication where this discussed at length? So far I am unaware of any that dig deep.

Or am I off my rocker?

AV
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Old 12-10-2018, 05:34 AM
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Using piston stop, find contact on both directions on the degree wheel, then find the middle from them.
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Old 12-10-2018, 07:13 AM
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Originally Posted by ACE VenTRa View Post
Just as a general discussion, I would like to cover True TDC and its finer points.

If we use the piston stop method and rotate the engine forwards then back and average the distance in between, we arrive at what we call True TDC. If we use the dial indicator method we will likely end up pretty close, but at a different spot on the degree wheel. This is due to Crank Overlap or Piston Dwell. If you set a motor at TDC and rock the crank back and forth you would find that there is quite a bit of movement of the crank before the piston moves up or down. I believe this is where crank angle becomes important, that spot in between BTDC and ATDC. As a rule of thumb we split the difference and call it good unless there is a spec called out by the manufacturer, but how should we determine where the sweet spot is, the beginning of the dwell period, the middle or the end? When we start trying to time the cams as well, or in our case 4 cams, we are now multiplying the effect of dwell time and creating an even larger gap of precision. Is anyone aware of an article or other publication where this discussed at length? So far I am unaware of any that dig deep.

Or am I off my rocker?

AV
Yes and no.

You can (and I do) the exact same procedure with a dial indicator as happens with a piston stop.
You approach TDC in both rotation directions and find the point the indicator tops out, True TDC will be the point mid way, just like the piston stop and degree wheel.

One of the reasons you do not get "true" TDC from just a single rotation check is the clearances in crank, rod and piston bearings/ bushings
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Old 12-14-2018, 04:20 PM
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All true stuff, but within the known tolerances of dwell, between BTDC and ATDC, there is quite a bit of judgement to make. Why do we only use the middle; why not a a few thou in one direction or the other? There is certainly enough room for error in that regard, on my Triumph it's an eternity of duration at dwell, relatively speaking.

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