Installing pistons backwards
- Thread starter 63Rancher
- Start date
spikeomatic
Well-known member
then the rods would be backwards(its because of the rods)
The hole in the piston for the connecting rod pin is offset slightly. It has something to do with reducing stresses on the reciprocating parts.
Here is a quick explanation: http://www.motorcycleproject.com/motorc ... ffset.html
Some guys in the Corvair community, when mating the flat six to a VW or Porsche transaxle, are reverse rotating the engine (Corvair engines run counterclockwise). When they do this, the pistons are rotated 180*.
In the later Mustangs with 302s, they got a 20HP increase just by rotating the pistons 180*. However, I don't think they addressed the increased stresses when doing this.
I wouldn't recommend doing this to our engines.
Hope this helps.
Here is a quick explanation: http://www.motorcycleproject.com/motorc ... ffset.html
Some guys in the Corvair community, when mating the flat six to a VW or Porsche transaxle, are reverse rotating the engine (Corvair engines run counterclockwise). When they do this, the pistons are rotated 180*.
In the later Mustangs with 302s, they got a 20HP increase just by rotating the pistons 180*. However, I don't think they addressed the increased stresses when doing this.
I wouldn't recommend doing this to our engines.
Hope this helps.
Offset pins reduce side loads on the cylinder wall. Not all engines are designed like that.
Flipping the piston changes the angle of the connecting rod relative to the piston. It is sometimes possible to gain some increase in torque by doing that, but at the expense of longevity.
Flipping the piston changes the angle of the connecting rod relative to the piston. It is sometimes possible to gain some increase in torque by doing that, but at the expense of longevity.
Howdy All:
Many OEM engines used to use offset wrist pins. The offset was on the order of 1/16" and helped keep the piston loaded against the cylinder wall for less cold start noise. Reversing the pistons moved the offset the other way, which, through non-intuitive geometry, Actually increased the piston stroke slightly - very slightly – and reduced cylinder wall loading, at the expense of slightly more piston slap, particularly when cold.
The problem is, many OEMs quit using pin offset years ago, possibly for bean counter reasons. Replacement pistons may or may not be offset - Badger used to offset everything, whether the OEM did or not, while Sterling didn't. "Performance" pistons commonly have no offset. So you'll need to check your pistons before having them mounted backwards.
Piston pin offset being reversed would result in slightly lower friction on the one side of the engine cylinder wall but that force will now be exerted on the opposite side of the cylinder wall.
The gain from reversing pistons is the result of the change in angularity of the rod to the tangent of the crank pin travel.
Think of it in terms of a boat trailer winch crank. Is the crank easier to push in the 12 o'clock position by placing your arm in the vertical position and pushing straight down? Or is it easier to push by moving your arm 45 degrees away from the previous vertical position opposite of the crank travel and again pushing in the direction of a vector that would travel from your elbow to your fist.
The angularity of the example above is highly exaggerated when compared to the .120 change in pin location, but it helps to make the process more clear. This is also the reason the short rod engines make slightly more power down low. The rod reaches a better angle with that tangent much quicker than the longer rod would.
Any way, I wanted to let you know that I had my 250 built with the pistons reversed. Can I tell the difference? Only on cold starts. I'd not do it again unless I was building a balls to the walls racing engine. For a street engine or daily driver it offers very little advantage and alot of piston slap noise on cold startup.
Adios, David
Many OEM engines used to use offset wrist pins. The offset was on the order of 1/16" and helped keep the piston loaded against the cylinder wall for less cold start noise. Reversing the pistons moved the offset the other way, which, through non-intuitive geometry, Actually increased the piston stroke slightly - very slightly – and reduced cylinder wall loading, at the expense of slightly more piston slap, particularly when cold.
The problem is, many OEMs quit using pin offset years ago, possibly for bean counter reasons. Replacement pistons may or may not be offset - Badger used to offset everything, whether the OEM did or not, while Sterling didn't. "Performance" pistons commonly have no offset. So you'll need to check your pistons before having them mounted backwards.
Piston pin offset being reversed would result in slightly lower friction on the one side of the engine cylinder wall but that force will now be exerted on the opposite side of the cylinder wall.
The gain from reversing pistons is the result of the change in angularity of the rod to the tangent of the crank pin travel.
Think of it in terms of a boat trailer winch crank. Is the crank easier to push in the 12 o'clock position by placing your arm in the vertical position and pushing straight down? Or is it easier to push by moving your arm 45 degrees away from the previous vertical position opposite of the crank travel and again pushing in the direction of a vector that would travel from your elbow to your fist.
The angularity of the example above is highly exaggerated when compared to the .120 change in pin location, but it helps to make the process more clear. This is also the reason the short rod engines make slightly more power down low. The rod reaches a better angle with that tangent much quicker than the longer rod would.
Any way, I wanted to let you know that I had my 250 built with the pistons reversed. Can I tell the difference? Only on cold starts. I'd not do it again unless I was building a balls to the walls racing engine. For a street engine or daily driver it offers very little advantage and alot of piston slap noise on cold startup.
Adios, David
Joe Sherman the famed Ford engine builder uses this technich on his small block ford builds and claims an extra 15 #s of torque at the expense of cold start piston slap.
so If pistons were installed 180* out, you'd get more power? that's cool!
as our engines are inline and the pistons are straight up and down is there a lesser chance for piston wall wear? (compared to bent 8's)
and as mentioned the only reprecution is less longevity and piston cold start slap, i guess the slap goes away after it's @ temp?
would this help boosted applications or be potentially deadly for the engine as it's forced at a more unnatrual angle?
Fasinated,
richard
as our engines are inline and the pistons are straight up and down is there a lesser chance for piston wall wear? (compared to bent 8's)
and as mentioned the only reprecution is less longevity and piston cold start slap, i guess the slap goes away after it's @ temp?
would this help boosted applications or be potentially deadly for the engine as it's forced at a more unnatrual angle?
Fasinated,
richard
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