Installing pistons backwards
Posted By: trauschu Installing pistons backwards - 10/08/04 12:20 AM
Has anyone heard of installing pistons backwards in a poncho 400 rebuild, so that the arrow on the dish points to the back, instead of the front of the block?
What is this supposed to accomplish, if anything?
Posted By: P. M. I. USA Re: Installing pistons backwards - 10/08/04 02:34 AM
Check the location of the oil spurt hole in the rod. The hole points to the opposite cylinder bank.
Posted By: TOHCan Re: Installing pistons backwards - 10/09/04 08:46 AM
This changes the direction of the pin offset in the piston. The manufacturers use pin offset to lessen piston slap noise. By installing the piston, the other way 'round, the connecting rod and the crankshaft throw form a different angle at and around TDC that makes the engine act like it has a longer rod. This has to do with the location of the piston at each degree of crank movement. One of the more important effects is the change in dwell time around TDC, and another is the rate of acceleration change around BDC. Most of this to say that, most noticeably on an engine that is induction limited, when reversing the piston pin offset by installing the pistons backwards, the general effect is an improvement in output and efficiency on either side of the torque peak.
I have assembled several street engines and a few race engines over the years this way and have been hard pressed to detect any additional noise when cold or hot. I have, however, detected an increase in output and fuel economy. The longest I have driven one of these engines was around 50,000 kms before I sold the vehicle and found no adverse effects.
A fair percentage of race pistons are machined with no offset to reduce the number of different pistons needed (by half). Most "street" pistons have a .060" offset.
Circle Track magazine did a dyno test on this a few years ago, using pistons that had .100" offset, I think. The results were illuminating, and confirmed what I had been feeling from my test engines. Every time I do an engine the "normal" way to recheck this effect, I regret not reversing the pistons!
P.S. If the rod has a squirt hole, it has to be facing the correct direction, so the piston must be intentionally installed backwards on the Rod.
You said from this article:
P.S. If the rod has a squirt hole, it has to be facing the correct direction, so the piston must be intentionally installed backwards on the Rod.
Mike wants to know if this is why my Piston number two with the only correct orientation is installed so, maybe?
Installing pistons backwards
https://firstgenfirebird.org/forums/...32&type=thread
Stan
Mikes reply:
What if all 7 of my pistons have the oil hole on the Rod, forcing them to be installed facing rear and the number two Piston had to be assembled correctly (factory like) because of lack of the oil hole