timing chain elongation curious results - Page 4 - PeachParts Mercedes-Benz Forum

Carpenterman · #46 11-25-2011, 12:28 PM

I think another thing to remember in this discussion is that the 2mm lift technique is done with zero valve clearance. So when running normally, the valve is actually opening later.

leathermang · #47 11-25-2011, 12:36 PM

Quote:

Originally Posted by Carpenterman

I think another thing to remember in this discussion is that the 2mm lift technique is done with zero valve clearance. So when running normally, the valve is actually opening later.

Yes , and that is another reason that on these really old cars with huge mileage on them and lots of mechanics ' laying on hands' that base checks for things like the position of the balancer compared to the crank/piston tdc, the crank degree pointer, and the other basics which one might inherit on an well worn car are affirmed and or corrected if needed before one test throws the can into the crusher bin....

Stretch · #48 11-25-2011, 12:36 PM

Quote:

Originally Posted by Brian Carlton

Correct me if I am mistaken, but it's probably likely that the intake valve starts to open just before TDC on the exhaust stroke. This "overlap", if you will, compensates for the fact that the air mass needs some time to accelerate and travel.

However, it certainly doesn't start opening at an estimated 35BTDC based upon his data.

after further reflection, there is no possibility of the exhaust valve being mistaken for the intake valve in this issue, simply because the exhaust valve starts to open slightly before bottom dead center and there would be no indication whatsoever on the damper.

The intake valve opens once in a four stroke cycle - the exhaust valve opens once in a four stroke cycle. The FSM specifies that the intake valve should open after TDC at an amount depending on your cam code and closes before BDC

It also specifies that the exhaust valve should open before BDC and close before TDC.

A valve opening before TDC on an OM61X engine is just scary - the pistons protrude from the block by 1.6mm; if it wasn't for that lovely head gasket thickness they would be kissing the head on every revolution. If fochs' measurement is correct the only thing that is saving him (if there isn't already some damage done) is the valve shaped recesses in the piston crowns...

Stretch · #49 11-25-2011, 12:37 PM

Quote:

Originally Posted by Carpenterman

I think another thing to remember in this discussion is that the 2mm lift technique is done with zero valve clearance. So when running normally, the valve is actually opening later.

Good point

Stretch · #50 11-25-2011, 02:13 PM

...I'd forgotten that I had done so many measurements.

OM617 (non turbo) cam profile specs, piston height specs etc

In this post I measured the distance the valve can travel from the piston surface at TDC to its position when seated.

For my engine (and each engine will be different because of the depth of the valve seat) I measured these drops

Inlet (cylinder #1) 3.18mm
Exhaust (cylinder #1) 2.88mm

(EDIT - there was a bit that I put in here that was wrong! I've now removed it)

So roughly speaking there is a gap of 3mm between valve and the recesses in the piston crown - assuming my valves did actually fit in the recesses! This is a pretty good assumption as the pistons can protrude between 0.5 and 0.9 mm from the block and the head gasket in an uncompressed state is what 2.2mm (I think!) etc etc etc

Anyway if we grip on to the concept of a 3mm clearance gap between valve and piston crown at TDC then putting that into the graph I posted showing the cam profile of a non turbo cam in the same thread...

...we've got a lift at the cam of about 3mm between tooth "6" and tooth "7" which is equivalent to a camshaft rotation of about 50 degrees.

Now we know that the camshaft spins at half the speed of the crankshaft - because it is a 4 stroke engine - so when we've got 3mm lift at the cam, which is different from at the valve, we've got an equivalent rotation at the crank is 100 degrees.

Also at the cam a lift that is equivalent to the the valve clearance on the inlet valve => 0.15mm comes in with a camshaft rotation of one camshaft cog tooth or 9 degrees. Translated to a crank rotation that's 18 degrees.

Now the problem is that I've been a bit slack and not yet measured the actual lift of the valve at each point of camshaft cog rotation - so we can't say for sure whether we get more lift at the valve than at the surface of the cam or not.

Does anyone know if the rise and fall of the cam profile is magnified by the rocker arm assembly or not?

Brian Carlton · #51 11-25-2011, 02:27 PM

Quote:

Originally Posted by Army

The intake valve opens once in a four stroke cycle - the exhaust valve opens once in a four stroke cycle. The FSM specifies that the intake valve should open after TDC at an amount depending on your cam code and closes before BDC

It also specifies that the exhaust valve should open before BDC and close before TDC.

A valve opening before TDC on an OM61X engine is just scary - the pistons protrude from the block by 1.6mm; if it wasn't for that lovely head gasket thickness they would be kissing the head on every revolution. If fochs' measurement is correct the only thing that is saving him (if there isn't already some damage done) is the valve shaped recesses in the piston crowns...

Yep, clearly, on a diesel, there cannot be any overlap. Effectively, you cannot begin to open the valve until the piston is at TDC.

I'm sure they are not opening before TDC on his engine, as well.

leathermang · #52 11-25-2011, 02:29 PM

Army, we need to find you a job where your talent and compulsions can be put to use making big money and getting something more exciting accomplished... like that new probe to Mars... your focus is too small and limited...inside these old engines.... we need to expand/explode your possibilities for greatness.
I can understand it when you were having cabin fever last winter.....but....

fochs · #53 11-25-2011, 03:01 PM

I did not read the correct scale on my dial. I was reading 10x too small. My chain elongation is 5 degrees ATDC. The value read was 16 degrees ATDC. Nominal value for my cam is 11 degrees ATDC. I have fitted a woodruff key to correct for the elongation and am now working on setting ip timing with drip tube method. I apologize for my error, I have not used a dial indicator before.

Stretch · #54 11-25-2011, 03:02 PM

Quote:

Originally Posted by fochs

I did not read the correct scale on my dial. I was reading 10x too small. My chain elongation is 5 degrees ATDC. The value read was 16 degrees ATDC. Nominal value for my cam is 11 degrees ATDC. I have fitted a woodruff key to correct for the elongation and am now working on setting ip timing with drip tube method. I apologize for my error, I have not used a dial indicator before.

Well done for owning up!

I'm glad it is all sorted out now - you had us (well me for one) all thinking...

vstech · #55 11-25-2011, 09:37 PM

Quote:

Originally Posted by leathermang

Army , I was just referring to you saying "degrees of "elongation" (crappy expression sorry - but you know what I mean)"....because it more accurately describes the physics than ' stretching' which so many used to call it.... so I am glad you use it.
At some level on tech discussions it does help to have standardized nomenclature to help get to the actual problems as efficiently as we hope to...
If it helps... visualize the chain laid out straight on a table.. so it is linear in its increased length... does that help ? LOL

I read on here one time that a member had his chain replaced and the mechanic said he laid the old one against a new one, and it was a full link longer... I of course said the mechanic was lying to him as the overall length of the chain should not have chainged, and was TOTALLY impossible for it to be an entire link longer...
I understand chain stretch to be the wear in the rollers on the pins wearing and the overall total wear is off a few degrees over the entire chain to make the gears out of synch.
please correct me if I'm wrong...

leathermang · #56 11-25-2011, 09:44 PM

Lets give this problem to Army.... how long are these chains ? Like 8 feet long ? How many links do they have .... LOL.... they are pretty long... people leave them in the engines for decades and several hundred thousand miles because the working distance is half that..( cam to crank) and the chain tensioner operates well... so it only means if true for total stretch that half a link has to be compensated for ....and we have offset keys for the cam, a moveable injection pump... They are so overbuilt...and the double roller design so good....
I can believe what the mechanic said....

tangofox007 · #57 11-26-2011, 12:22 AM

Quote:

Originally Posted by vstech

I of course said the mechanic was lying to him as the overall length of the chain should not have chainged...

So...chain elongation, in your estimation, has nothing to do with the length of the chain? In other words, the chain elongates, but it does not get longer?

leathermang · #58 11-26-2011, 12:33 AM

Quote:

Originally Posted by tangofox007

So...chain elongation, in your estimation, has nothing to do with the length of the chain? In other words, the chain elongates, but it does not get longer?

He was young when he said that.....he is older than that now...

" Yes, my guard stood hard when abstract threats
Too noble to neglect
Deceived me into thinking
I had something to protect
Good and bad, I define these terms
Quite clear, no doubt, somehow.
Ah, but I was so much older then,
I'm younger than that now."
----The Byrds

Stretch · #59 11-26-2011, 04:23 AM

Quote:

Originally Posted by leathermang

Lets give this problem to Army...

I'll make some measurements of the chain at some point this winter.

I then need to measure / work out the pitch circle diameter of the bottom sprocket on the crankshaft.

Brian Carlton · #60 11-26-2011, 09:58 AM

Quote:

Originally Posted by vstech

I understand chain stretch to be the wear in the rollers on the pins wearing and the overall total wear is off a few degrees over the entire chain to make the gears out of synch.
please correct me if I'm wrong...

When the pins and rollers wear, they introduce clearance............say .002" at every pin.

Add up the number of pins and you might find .150" of clearance.

Set a new chain next to an old chain and the old chain is .150" longer, although the individual parts that make up the chain have not "stretched".