Basic question please

[Sal Lubrano]

What makes the motor an on-center and off-center. What is the difference between them. Also, why would there be two types of 3.8 blocks. I understand stages because they were increasing strength but what was the reasoning for different configurations? Thanks for the help with such a beginner question.

 

[WarWagon]

Cylinder bores on-center of crankshaft connecting rod journals.

 

[forcefed3.8's]

The on center cylinder banks were shifted to make the piston centered on the rod journal. On an off center motor, the connecting rod isn't in the middle of the piston pin. On center has less stress on the piston and piston pin.

 

[Sal Lubrano]

I see now.... isn't a stock 3.8 is an off-center? Why would GN build a motor where the conn rod did not sit center of the piston?

 

[WarWagon]

Because this engine started out as a 215 v8.

 

[Sal Lubrano]

Thank you for the info. I looked up some history on the 215 and I was able to find a lot of history how the 231 came about. I was unable to find anything about why the rods were uncentered. Why would anyone make such a design? If you want the piston straight up you put the force in the center. Putting the force off to the side will push the piston on a slight angle causing uneven wear in the cylinder bore. There had to be a reason it was designed like this or am I looking in to this too much?

 

[charlief1]

Economy. They wanted a quick and cheap alternative for gas mileage in the early 60's. They increased the bore slightly on the 215 and just cut 2 cylinders off so it could be in production faster. If they'd gone through more R&D it would've taken longer and cost more.

 

[WarWagon]

Word up on that Charlie. Sal plus GM then had to rebuy the engine tooling and equipment back from jeep when they sold it, so time was of the essence they wanted it in production now.

 

[gnxtc2]

Thank you for the info. I looked up some history on the 215 and I was able to find a lot of history how the 231 came about. I was unable to find anything about why the rods were uncentered. Why would anyone make such a design? If you want the piston straight up you put the force in the center. Putting the force off to the side will push the piston on a slight angle causing uneven wear in the cylinder bore. There had to be a reason it was designed like this or am I looking in to this too much?

Research odd fire vs even fire.

The odd fire motors had crank similiar to a SBC, 2 rods shared a journal. The odd fire motors vibrated big time. To smooth out the motor, Buick split the rod journals. This made it known as an even fire motor and made the motor run smoother. When the rod journal was split, it put the pin off-center on the piston pin. Hence what we have to today, off-center motor.

When Buick started with the racing program, the knew about the pin location. Instead of making a new crank, the shifted the cylinder bore locations in the casting to line up with the rod journal.

The odd fire cranks are stronger. I think some still use them in racing but that is a whole different topic.

Billy T.
gnxtc2@aol.com

 

[Sal Lubrano]

Now I understand. Thank you to everyone who posted up.

 

[WarWagon]

Welcome! Yay! :smile:

 

[twodafloor]

these cars give me an ice cream headache!