wrist pin offset

[belize1334]

I was looking at my Wiseco stroker pistons and I noticed that the wrist pin offset is backwards from what I expected. My understanding of offset is the following. If you offset in such a way that the rod is more vertical while the piston is rising, then at TDC the rod has a slight angle so that it can exert torque on the crank and you ultimately make more power. If you offset the other way then there is less side loading during the burn and you get quieter operation and less piston slap. Now, according to Wiseco, they offset the same way as OEM to ensure quiet operation. But on these pistons the offset is toward the front, which would be for more power and more slap. Apparently, Mitsubishi was more interested in a motor that felt spunky than in one that had quiet operation.

Anyway, why do I care? Well, I'm building a stroker and I've been thinking alot about the unfavorable rod angle and the increased chances of piston slap due to the shorter skirts... and it occured to me. If I flip these babies around, then the offset will be more conducive to quiet operation and during the burn portion of the cycle the rod will be more vertical so there should be less side loading. Of course there are issues though. The valve recesses would have to be machined on the intake side, and the cam timing might need to be adjusted due to the altered effective TDC. But I put the question to you. What do you think about flipping pistons around to change the wrist pin offset for reduced piston side loading?

 

[belize1334]

I've done a little more research and it seems like all OEM applications offset the piston to the major thrust side so that as the crank moves through tdc the lower skirt tilts across and contacts the other side wall first. Switching this around gives you better rod geometry, marginally more power, and causes the top of the piston to rock across before the skirt. So the question is, why is it important for the bottom of the piston to contact the cylinder wall first. Either way the rocking motion is less violent than the simple slap that you get with non-offset pistons. What's the down side?

 

[prove_it]

I'm sure having the bottom skirt touch first helps reduce the stress on the top ring land. I think anyways. I think offset is used more by oem to create quiet engines since the average consumer would freak out if they heard piston slap.

 

[belize1334]

Yeah my digging always reveals the same end of the road statement of "offset must be to the major-thrust side for quietest operation", but with no explanation as to why you get less slap that way than with offset to the other side. But the big-block guys all agree that offset to the minor-thrust side produces more power and has a more favorable rod angle with less thrust load and less friction. Maybe I'll just call Wiseco and see if one of their techs feels like talking...

 

[prove_it]

lol, let me know what you find out. IIRC my wiseco pistons have no offset. Not sure, I still get some slappage though. Defiantly gets old on a daily driver, especially trying to explain that it's normal to people all the time.

 

[belize1334]

The offset isn't big. Maybe 1mm. I talked to a lady at Wiseco today who wasn't able to adequately explain anything. She referred me to someone more knowledgeable but I was only able to leave a message. We'll see.