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relative piston positions

708 views 6 replies 3 participants last post by  zpat 
#1 ·
1988 ZX n/a 5 speed (not that it matters for this question).


Am getting ready to pull right cylinder head, looking for exhaust leak. For my own info, I performed
compression test and started doing leak down test. That prompted me to think I should know
exactly how to put any given cylinder at TDC. So I made up a little table to help me visualize it. (Yeah,
I know I could just watch the distributor rotor).


I'm assuming the crankshaft goes 180 degrees to get a piston from TDC to BDC. i.e. each of the 4 strokes is completed in 180 (crankshaft) degrees. Two complete rotations of the crankshaft get a piston
back to exactly the same state. Also tells me 720 degrees / 4 strokes = 180 degrees per stroke.


Secondly, I read and observed that the distributor rotates at 1/2 the rate of the crankshaft. And
since there are 6 cylinders and the distributor rotates 360 degrees for a full rotation, it must be
360 / 6 = 60 degrees between each distributor contact. And that the firing order is 1,2,3,4,5,6.


See the attached JPG image. The table seems to work out perfectly. Noting that each piston is at TDC (beginning the power stroke) 120 crankshaft degrees after the previous piston. Also true going from piston #6 to piston #1 .


The problem is ... in my Haynes manual, on page 68 (in the lower left corner of the page), it says
that cylinder 1 and cylinder 6 are at TDC at the same time. Obviously, if #1 is beginning the
compression stroke, #6 must be beginning the intake stroke.


According to my diagram, at 0 degrees (#1 at TDC) , #6 is NOT at TDC. It is nearing the end of the
power stroke.


The Haynes manual sounded pretty definite. So I figured I must be wrong and confused. Is my table completely wrong ???
 

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#4 ·
Not sure how you put your chart together, but you're equating a compression stroke in one cylinder with an intake stroke in another and that seems backward. One action is pushing (compression) and one is pulling (intake).

Anyway, without getting in to all of the math, I went out and found a picture. The VG30 is a typical evenly balanced 6 cylinder, with paired cylinders on alternating cycles. One's on the exhaust stroke when another is on the compression stroke. Found a picture showing two pistons at TDC.

http://www.vondrachek.org/automotive/datsuns/71_2dr_blue/
 
#5 ·
pdx280, thanks for response. I guess I didn't see the image you referred to in that link.


for the table, I just based it on the firing order 1,2,3,4,5,6 and the fact that the distributor contacts
are 60 degrees apart. the downward motions of pistons are paired on opposite sides of engine:


#1 PWR -- #4 INT
#2 PWR -- #5 INT
#3 PWR -- #6 INT
#4 PWR -- #1 INT
#5 PWR -- #2 INT
#6 PWR -- #3 INT


if I understand, you were trying to point out that a downward piston motion is required on both
sides at the same time to balance the torque force. even though one side is pushing (PWR) and
the other is pulling (INT).


would this pairing do it, even though the 1-4 and the 2-5 are not directly across from each other. ??


I searched and searched internet for such a diagram, but didn't find one. even thought I might
find it in FSM, but didn't see it.
 
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