Need wisdom - Page 4 - PeachParts Mercedes-Benz Forum

whunter · #46 11-07-2005, 06:03 PM

Quote:

Originally Posted by Brian Carlton

The exhaust valve opens at 25° BBDC on the power stroke. At this piston angle, there is not much pressure left in the cylinder. If the valve is tight, I might guess that the valve would open at somewhere around 30° BBDC. Sorry Roy, but I don't see how five degrees early on the crankshaft would be significant in terms of reducing start capability.

Naturally, if you set the valve lash to zero (nominal is .014"), then I might agree that the difference between the 25°BBDC specification and whatever this result would be might be of significance.

Note that the intake valve would be far more critical on a late close due to reduced compression, and, I would agree that this valve will affect the capability to start.

To be continued:
When I am not so pressed for time.
I will try to get data from Daimler Chrysler, if possible.
I still contend that premature opening of any valve is bad for COLD starting.

t walgamuth · #47 11-07-2005, 10:40 PM

to absorb the information regarding this discussion.

so the lift is what about .5" or so? and the clearance is .05" or so? so if it is running completely tight then the lift is .55", right? and whunter is saying that this will cause the valve to stay open a little longer and there for allow some of the air to escape and aviod being compressed. brian is estimating that it will open 5 degrees later. that is 5 degrees out of 90 degrees that the crankwill turn while compressing the air. and it is five degrees that will come at the most bottom of the stroke while the primary motion of the bottom of the rod is horizontal.

i am going to have to go with brian on this one. i cant see that it would make much difference. yes, a little but not much. the question is would it be enough to be noticalbe? the first time after a valve adjustment? seems more likely that those noticing immediate results are those with zero clearance and valves that are leaking.

this is one of those subtle arguments that will be pretty hard to prove i think. the kind we all like.

tom w

Brian Carlton · #48 11-07-2005, 10:48 PM

Quote:

Originally Posted by t walgamuth

to absorb the information regarding this discussion.

so the lift is what about .5" or so? and the clearance is .05" or so? so if it is running completely tight then the lift is .55", right?

The lift is about .50"

But the clearance is .004" on the intake and .014" on the exhaust.

If it is running completely tight, the lift is .504" on the intake and .514" on the exhaust.

If you lose clearance on the exhaust and it drops down to .005", the difference of .009" cannot have more than a 5 degree effect on crank angle (I believe it's far less than 5 degrees).