top dead center (TDC)

1 reply to this topic
  • speddie3

Posted October 12, 2005 - 06:54 AM


So now I have a question. When everyone refers to starting a bike with compression release they always mention top dead center. Now when you think about it, isn't the crank/rod/piston actually at bottom dead center. Compression is made from the piston pushing up on a chamber of air with no escape, in order for it to be pushin up it has to be at the bottom. Is my understand of this term (tdc) wrong?? Am I missing something???

I think top dead center is really bottom dead center!!!

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  • grayracer513

Posted October 12, 2005 - 07:06 AM


Compression resistance in the kickstarter is felt as the piston approaches TDC. In a manual decomp engine, this happens just after the intake valve closes, about 60-70 degrees before TDC, depending on the cam grind.

The idea is to move the engine past TDC so that as you kick it through, starting from part way down the power stroke, you will have a little more than one full revolution to build inertia in the engine to help it run all the way through the next compression stroke and start.

Auto decomp works by relieving the first half of the compression stroke, allowing the exhaust valve to reseat about 30-40 degrees BTDC, which reduces the cramking compression from nearly 200 psi to something more like 100-120, enough to start the engine, but not so much that an e-starter can't turn it.

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