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Valve timing

Submitted by antony_ryan on Wed, 10/07/2013
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Hi all, I need more help please. In my Norton books there is a valve timing diagram for a 1938/1939 engine which mine is,the only trouble is I can't make head or tail of it! The book says that coming up to TDC the inlet valve will start to open and just after TDC the exhaust valve closes (going the right direction of the engine) if I read that right I get that, but when I look at the diagram it looks to me that the exhaust closes 25/30 degrees before TDC and the inlet opens the same after TDC. Which is right? Help please Tony.

Submitted by dave_graham1 on Wed, 10/07/2013

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I'm not sure what book or diagram you are refering to but the Inlet valve opens before TDC and exhaust closes after.

Submitted by antony_ryan on Thu, 11/07/2013

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Thanks for that Dave, the book is Book of Norton on page 100 on one copy and 91 on another, I just need someone to explain to me how to read the diagram! I know I'm thick. Tony.

Submitted by david_cooper on Fri, 12/07/2013

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Sorry if you already know all this but sometimes it's useful to write it all down...someone might find it handy.

In the 'suck-squeeze-bang-blow' cycle the inlet starts to open 27 to 30 deg before to dead centre on the 'blow' stroke while the exhaust is still open and the exhaust finally closes 27 to 30 deg after top dead centre on the 'suck' cycle. So for neary 60 degrees both are open. Valve operlap like this always seems to me to be improbably wrong since it risks mixing inlet and exhaust gases but it's all to do with dynamics of gas flow. So at TDC the two cams will be symmetrically balanced: inlet open a bit and exhaust nearly closed.

Some modern engines (Toyota Prius) have no overlap so as to improve fuel consumption and reduce noxious emmisions and the engine makes slightly less power for its size.

Good luck! I can't even get the bottom sprocket off to inspect mine.

 


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