On-center? Off-center? Please explain...

[Roc87]

Could someone explain to me what on center and off center mean? I have no clue to anything regarding stage II engines, but want to learn TIA

[49-blues]

I believe it means that a on center block would have the rods centered on the journals of the crank-which makes it stronger verses off center journals which puts more stress on the rotating assembly.

[ijames]

Buick started with a V8, where each crank throw has two rods side by side, with the cylinder banks positioned so each rod is in the center of each bore. I'm talking front to back (bumper to firewall), since of course the rods flap back and forth in the side to side direction. Now cut off two cylinders to make a V6. The problem here is that losing those two cylinders leaves gaps in the firing timing, so now the engine will fire-fire-pause-fire instead of fire-fire-fire-fire, so it runs rougher. This was the odd fire V6 from the 70's. To smooth it out Buick "split" each crank throw into two throws so now as you rotate the crank the engine goes fire-fire-fire-fire (don't you wish you could hear my sound effects :-)?). To do this they needed room between the rods to stop one throw and offset and start the next throw so they slid one rod forward and one backward, along the length of the crank (again, in the bumper to firewall direction). However, they did not change the block so now the rods are not centered in the bores (still in the bumper to firewall direction). This is the off-center setup, which the 4.1 and 3.8 V6's in the 80's used (okay, I'm not positive about the exact year of the switch but all of the injected turbo motors from 85-89 and the 4.1 V6's are off center and can use the same crankshaft). The problem is that when the cylinder fires the piston tries to rock in the bore (picture trying to balance a dinner plate on a pencil with the pencil a little off center). This causes extra friction and stress and at some point limits the max hp the engine can make and live. To fix this in production engines Buick slid one bank of cylinders backward and the other forward the same amount they moved the rods, so now the rods are once again centered up. This started with the 3800 fwd engines in 88, and these are called on-center motors. The cranks are the same as far as all the bearing journals and throws, but since the cylinders slid "apart" the block is a little longer, the intake manifold had to get a little longer with the ports shifted to line up with the heads, and the front cover had to change and the exhaust crossover has to change a little. Okay, that's for the production stuff so the motors will last longer at 200 hp and get a tiny bit better gas mileage from the friction reduction. For max hp, Buick was making the stage II blocks, and somewhere in the mid-80's they changed these from off center to on center so they could live longer at ridiculous power levels :-). A long time ago I read on the Internet that an off-center stage II was good for maybe 1000 hp and the on-center for 1500+, but just like the stock 109 blocks have now been pushed to the 1000-1200 flywheel hp zone, off-center stage blocks have been pushed way past that.

[Roc87]

So, I'm assuming most guys who will build a stage motor would prefer to have an on center, depending on HP goals. Are most stage motors 274ci?

[DonWG]

Originally Posted by roc87

So, I'm assuming most guys who will build a stage motor would prefer to have an on center, depending on HP goals. Are most stage motors 274ci?

Are you planning something?

[Roc87]

Well, I've always been more of a guy known to hang out at the drags than at the car shows.....

[BlownV6]

James, THat was a great explanation!!!!!! well done mike

[ijames]

Originally Posted by BlownV6

James, THat was a great explanation!!!!!! well done mike

Thanks, Ooher. Usually when I type that much no one reads past the first sentence :-).

[DonWG]

Originally Posted by ijames

Thanks, Ooher. Usually when I type that much no one reads past the first sentence :-).

Don't be silly. Good stuff.

[10secvega]

Thanks that was good info.

["More Boost"]

yea, what he said.........

[DonWG]

Huh?