Engine builders please

seijirou · Mar 1, 2006

How much of a big deal is it to have a balanced rotating assembly, how much slop can I have without the motor really throwing itself apart?

I'm building a rather frankenstein combo because I have extreemly limited means. Have the whole thing balanced is going to cost quite a bit of change. I guess I just want to know how important it really is. The stroke will be 2.87 and the engine will probably never see 7k rpms.

Also if anybody has any idea what the "typical" weight of various stock ford stuff is. I'll be using a factory 289 crank, factory late model 302 rods, and factory 351C pistons.

Is this going to put me close or am I going to be way off.

If this is a dumb question, give me a dunce hat and call me dumb. I'm bare-butt new to engine building.

Vipersix · Mar 2, 2006

A balanced rotating assembly is very important. The centripetal force created by a crankshaft imbalance will depend upon the amount of imbalance and distance from the axis of rotation. A crankshaft with only 2 ounce-inches of imbalance at 2,000 rpm will be subjected to a force of 14.2 lbs. At 4,000 rpm, the force grows to 56.8 lbs., double the speed again to 8,000 rpm and the force becomes 227.2 lbs.

This may not sound like much when you consider the torque loads placed upon the crankshaft by the forces of combustion, but centripetal imbalance is not torque twisting the crank. It is a sideways deflection force that tries to bend the crank with every revolution. Depending on the magnitude of the force, the back and forth flexing can eventually pound out the main bearings or induce stress cracks that can cause the crank to snap.

If you have a decent set of pistons, they should already be within about a gram of each other. I personally like to see that figure much smaller but since you are asking about the economic route, we'll go a bit higher.

The rods will give you boatloads of trouble and are a great reason why I would recommend a professional to do the job. A special support is used so that the big ends of all the rods can be weighed and compared, then the little ends. If you simply weigh the entire rod by itself you could end up with one end being significantly heavier than the same end of a different rod. I would recommend down to at most 1 gram difference on the rods. Even the way you grind the extra metal off the rods to balance them is important. If you grind parallel strips you could cause stress cracks along the natural tension lines.

By far the most difficult portion is the crankshaft. The countermass opposite rod journals on the crankshaft are balanced for a certain weight. While it is certainly possible to have a bit more leeway in weight tolerance than between rods and pistons, the effects of having an imbalanced assembly are more pronounced in the crank.

Most shops charge $150 to $225 to balance a V8. I HIGHLY recommend it.

Black1987Stang · Mar 2, 2006

I agree with Vipersix, especially with a mix match combo i would deff do it.

Vipersix...u got some nice mods, got anypics of ur stang? i see ur in TN, im in tn too for the timebeing.

Vipersix · Mar 2, 2006

No good pics at the moment. I've got a few of the initial engine build but I had a problem with the block just before I installed it in the car so I had to tear it apart. Currently everything is wrapped up with the hopes of not becoming rusted while I finish my deployment here in Iraq.

When I get back I'll buy a new block (the Dart one listed) and re-build the stupid thing for the third time. I'll be sure to take many detailed pictures of it as I have a high speed digital camera now.

seijirou · Mar 2, 2006

$150-$225? Man I need to find the machine shops that you guys do.

Michael Yount · Mar 2, 2006

Would be a good idea to take them the flywheel, clutch ass'y and harmonic balancer in addition to the crank, rods, pistons too...

Rick 91GT · Mar 2, 2006

Vipersix said:
A balanced rotating assembly is very important. The centripetal force created by a crankshaft imbalance will depend upon the amount of imbalance and distance from the axis of rotation. A crankshaft with only 2 ounce-inches of imbalance at 2,000 rpm will be subjected to a force of 14.2 lbs. At 4,000 rpm, the force grows to 56.8 lbs., double the speed again to 8,000 rpm and the force becomes 227.2 lbs.

This may not sound like much when you consider the torque loads placed upon the crankshaft by the forces of combustion, but centripetal imbalance is not torque twisting the crank. It is a sideways deflection force that tries to bend the crank with every revolution. Depending on the magnitude of the force, the back and forth flexing can eventually pound out the main bearings or induce stress cracks that can cause the crank to snap.

If you have a decent set of pistons, they should already be within about a gram of each other. I personally like to see that figure much smaller but since you are asking about the economic route, we'll go a bit higher.

The rods will give you boatloads of trouble and are a great reason why I would recommend a professional to do the job. A special support is used so that the big ends of all the rods can be weighed and compared, then the little ends. If you simply weigh the entire rod by itself you could end up with one end being significantly heavier than the same end of a different rod. I would recommend down to at most 1 gram difference on the rods. Even the way you grind the extra metal off the rods to balance them is important. If you grind parallel strips you could cause stress cracks along the natural tension lines.

By far the most difficult portion is the crankshaft. The countermass opposite rod journals on the crankshaft are balanced for a certain weight. While it is certainly possible to have a bit more leeway in weight tolerance than between rods and pistons, the effects of having an imbalanced assembly are more pronounced in the crank.

Most shops charge $150 to $225 to balance a V8. I HIGHLY recommend it.

very nice

I pay anywhere from $175-$225 for balancing a rotating assembly, really depends on what it is... If your seeing prices above that look around a little bit more. Like Mike said, take your flywheel, flywheel bolts, balancer, balancer bolt, 1 set of rod bearings and your crank, pistons, 1 set of rings and rods....You should need to take your clutch assembly for your application.

Engine builders please

seijirou

Vipersix

Black1987Stang

Active Member

Vipersix

seijirou

Michael Yount

Mustang Master

Rick 91GT

Mustang Master

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Engine builders please

seijirou

Vipersix

Black1987Stang

Active Member

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seijirou

Michael Yount

Mustang Master

Rick 91GT

Mustang Master

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