Incorrect as I see it

Posted by jdemaris on December 31, 2008 at 12:32:24 from (67.142.130.20):

In Reply to: Re: Timing Question. posted by John/Ont on December 31, 2008 at 09:19:58:

Your 30 years do not prove, or disprove a thing. Many here have been turning wrenches a lot longer than that - including me. All I can say is - so what? Some people learn much in short periods of time, and some learn little throughout their lifetimes. We all vary, as do our experiences and interest in learning. I say that as a general statement - not targetted personally at you.

Internal combustion engines with conventional pistons moving in cylinders are engineered with ignition timing and valve timing relating to the position of the piston and it's distance from top or bottom of its travel, NOT the crank or flywheel reference marks. Those crank-marks or flywheel marks are shortcuts to indicate that piston placement and nothing more. They are usually reliable, but not always. Some engines are more precise and give timing specs that relate to direct dial-indicator measurments to indicate piston position.

The plain fact is - the only true constant when it comes to TDC, BTDC, etc. is the piston as it relates to it's position in the cylinder. The other reference markts are just that. Marks that refer to something else. When something calls for TDC, is is assumed that when the piston is at TDC, so will be the crank-throw - but not in all cases.

In response to your " Go ahead if you please name me one part of an internal combustion engine that needs to be timed that does not relate back to a crankshaft . . ."

With a multiple cylinder engine - the crank reference mark does NOT work for all cylinders if all is not perfect. All it takes is one bent connecting rod (that is now shorter than the rest) on a four-cylinder engine, a bent crank with one throw out of wack - and your one crank-mark will lie to you about that particular cylinder. If your crank-mark says - let's say - top-dead-center . . . with a bent rod, it will never get to the engineered point of TDC travel is was designed for - even though the crank or flywheel mark says it is. NOT an uncommon event.
If a crank-throw is not where it is supposed to be - the piston travel timing on that one cylinder can differ from the others - and the crank reference mark will NOT tell you that. I'll add that John Deere had a bunch of three cylinder diesels come through that way in the early 1980s. The Deere flywheel timing mark lied about where #2 piston actually was - on several brand new engines.

Note that even crude tractor engines have their spark ignition timing, or fuel time-of-injection, or time of valve opening - expressed as degrees before, at, or after top-dead-center or bottom-dead-center. Center of what? The crankshaft position? No. It relates to piston position.
Granted that there are a few modern tech manuals that assume crank-throw position has to be the same as piston position - but that is rare -and to be technical - it is incorrect.

Hope you never have to work on a Gnome rotary-cylinder engine. The crank is stationary.

I'll also note, that the term that Guido used - "timing" is indeed vague. It has mutiple connotations, even just within the scope of internal engine mechanics.

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