How are the connecting rods on the flat-six arranged?
 

The crank throws come in pairs and point in opposite directions (see Fig. 5:10) so does that mean for one pair of throws one connecting rod is associated with one side of the motor and the other rod with the opposite side of the motor? I guess it would also be possible for both rods to be associated with just one side of the motor with one piston at TDC when its pair is at BDC.


http://www3.telus.net/public/alpine65/Smith138-139.jpg

Where's Wayne when you need him? :)

The 911 engine is a boxer layout, with one crank throw per cylinder. Pistons in opposite pairs go out and in together. Both reach TDC at the same time. Both reach BDC at the same time. Same for the Type 1-4 VW engines, the Subaru flat fours and sixes, the Ferrari flat-12, the BMW flat twin, and the Citroen flat twin.

Hey, Alfred... You can just look through the 911 section of our site and find pics of the crankshaft.... Heck, there's even one in the 914 (-6) section!

http://www.pelicanparts.com/911/911_Parts/1974-75/1-2.JPG
http://www.pelicanparts.com/914/Parts/914-6/914-6_crank_big.htm

You can see that the three pairs of crank throws are spaced 120 degrees apart.

--DD

There's one bearing per rod, with one main bearing between each crank throw.

The throws are all 120 degrees apart.

Most cranks have counterweights, bot some did not, including the crank in the 914-6.

This makes for a very sturdy engine that can safely spin very fast.

Michel Richard
914-6 2.2E MFI

I did some calculations to try to figure out why the flat-six is "inherently balanced" but the flat-four is only in primary balance. What I came up with is that the accelerations of the pistons in the flat-four always sum to 0 and the same is true for the flat-six but when you look at the sum of the accelerations for the pistons on just one side of the motors, then the flat-six is much more balanced compared to the four. The calculations I did were for motors with stroke of 71mm and connecting rod lengths of 131 mm and these are the results I got.


http://www3.telus.net/public/alpine6...aftDiagram.jpg

http://www3.telus.net/public/alpine6...tEquations.jpg

Sum of piston accelerations for one side of flat-six.

http://www3.telus.net/public/alpine6...leration-1.jpg

Sum of piston accelerations for opposite side of flat-six.

http://www3.telus.net/public/alpine6...leration-2.jpg

Sum of piston accelerations for one side of flat-four.

http://www3.telus.net/public/alpine6...leration-3.jpg

Sum of piston accelerations for opposite side of flat-four.


http://www3.telus.net/public/alpine6...leration-4.jpg

For the flat-four, the maximum "unbalance" is 20 and occurs every 90 degrees of crankshaft rotation but for the six the maximum unbalance is only 0.01 and occurs every 30 degrees of crankshaft rotation.

Hey Alfred whenever I read your techno posts, my brain hurts for 15 minutes.

My big technical reply -

"My cat's name is Mittens"

:cool:

Anyone know the length of the crankshaft and the distances from the end of the crank to the centers of the crankpins (vertical lines marked d1 in the picture below)? If I know these dimensions, I should be able to calculate and graph the torque or "rocking couple" that rocks the motor (with respect to the angle of crankshaft rotation) because of the crankpin (and cylinder) offset.

<center>http://www3.telus.net/public/alpine6...Dimensions.jpg

http://www3.telus.net/public/alpine6...kPinOffSet.jpg</center>

Primary and Secondary Forces and Couples 1

Primary and Secondary Forces and Couples 2

Primary and Secondary Forces and Couples 3

Primary and Secondary Forces and Couples 4