top dead center (TDC)


1 reply to this topic
  • speddie3

Posted October 12, 2005 - 06:54 AM

#1

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!!!

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • grayracer513

Posted October 12, 2005 - 07:06 AM

#2

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.





Related Content

Forums
Photo

2016 YZ450 by CaptainKnobby


Dirt Bike   Dirt Bike Technical Forums   Suspension
  • Hot  59 replies
Forums
Photo

Chubby dad, looking at bikes , First trip to the orange/black forum ! by Slow_ride


Dirt Bike   Make / Model Specific   KTM   250-530 EXC/MXC/SXC/XC-W/XCR-W (4-Strokes)
  • Hot  28 replies
Forums
Photo

Michigan Motocross Tires by 288yz450


Dirt Bike   Dirt Bike Regional Discussion   North
  • 1 reply
Forums
Photo

yz250x vs 450 4t comparisons (preferably desert riding) needed by JakeNorthrupYZ450F


Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 9 replies
Forums
Photo

Megabomb Fitment by 288yz450


Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply
 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.