Offset wrist pins
10-28-2003, 11:27 AM
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Offset wrist pins
I was recently reading some engine building material written by Smokey Yunick where he was talking about offsetting the piston wrist pins so that combustion pressure will start to do meaningful work sooner after TDC. The idea here is that normally if the piston is parked right at TDC, the cylinder pressure isn't going to be able to force the piston down. It isn't until the crank has rotated several degrees that the connecting rod is at an angle where the combustion pressure in that cylinder can do work. By offsetting the pin, the combustion pressure can do work even when the piston is close to TDC (if not right at it). My question is, has anyone experimented with this? What kind of torque/horsepower gains can be seen, and does it increase side loading on the piston skirts and cylinder walls? Are off the shelf pistons available with the pin offset?
10-28-2003, 02:00 PM
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Re: Offset wrist pins
Originally posted by Brett H. - 89GTA
I was recently reading some engine building material written by Smokey Yunick where he was talking about offsetting the piston wrist pins so that combustion pressure will start to do meaningful work sooner after TDC. The idea here is that normally if the piston is parked right at TDC, the cylinder pressure isn't going to be able to force the piston down. It isn't until the crank has rotated several degrees that the connecting rod is at an angle where the combustion pressure in that cylinder can do work. By offsetting the pin, the combustion pressure can do work even when the piston is close to TDC (if not right at it). My question is, has anyone experimented with this? What kind of torque/horsepower gains can be seen, and does it increase side loading on the piston skirts and cylinder walls? Are off the shelf pistons available with the pin offset?
I was recently reading some engine building material written by Smokey Yunick where he was talking about offsetting the piston wrist pins so that combustion pressure will start to do meaningful work sooner after TDC. The idea here is that normally if the piston is parked right at TDC, the cylinder pressure isn't going to be able to force the piston down. It isn't until the crank has rotated several degrees that the connecting rod is at an angle where the combustion pressure in that cylinder can do work. By offsetting the pin, the combustion pressure can do work even when the piston is close to TDC (if not right at it). My question is, has anyone experimented with this? What kind of torque/horsepower gains can be seen, and does it increase side loading on the piston skirts and cylinder walls? Are off the shelf pistons available with the pin offset?
The downside, from the article I read some time back, is cylinder wall wear is accelerated.
You might want to check JE's or TRW's, etc. website to see if there is a Tech article on this.
Jake
10-28-2003, 02:55 PM
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All or almost all factory pistons (and many aftermarket ones)already have a small amount of offset to them. Problem is, it's in the wrong direction for making more power. It's offset the other way to reduce piston slap and bore wear. Ever wonder why pistons have a notch that's supposed to point towards the front of the engine? That's why.
Offsetting it in the opposite direction increases slap and wear but basically "tricks" the motor into thinking it's got longer rods than it actually does, with the power making benefits that would imply.
"low buck" guys sometimes put the pistons in a motor with the notch facing the REAR of the engine so the offset is in the "power making direction" and then they cross their fingers. Sometimes works OK, sometimes not. I've never cared to try it.
Offsetting it in the opposite direction increases slap and wear but basically "tricks" the motor into thinking it's got longer rods than it actually does, with the power making benefits that would imply.
"low buck" guys sometimes put the pistons in a motor with the notch facing the REAR of the engine so the offset is in the "power making direction" and then they cross their fingers. Sometimes works OK, sometimes not. I've never cared to try it.
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