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Please Explain SBF Engine Balance To Me

Those flywheel bolts will need some sealer on them or you'll have a helluva oil leak!

I can say, that my SFI aluminum flywheel will only go on one way. No way it'll go any other way, the bolt holes will only align in one specific location.
 
I'll see if I can find a pic of the flywheel. I've had it apart soooo many times there's bound to be at least one pic!

I think it's riveted on.
 
"bravenrace" said:
Okay, now I understand about the 28oz and 50oz imbalance (I think), and I fully understand internal engine balance, but I'm having a problem that has led me to question some things.
I converted from an automatic to a T-5 this past winter. My '65 currently has the original 289 in it, so I bought a 28oz flywheel. I made no changes to the engine at all.
Now I have a vibration that increases with RPM, regardless as to whether or not the clutch is engaged. All the conversion parts are brand new, with the exception of the T-5
bellhousing.
Jim
Click to expand...

The flywheel bolts will line up only one way to get them all in.
If it was installed wrong, at least one has screwed up threads or hosed the crank.
Did you use the correct flywheel bolts for the install? Auto flexplate bolts are differant.
When you installed the bell, Did you check for it being centered to the crank?
The vibration could be coming from it being misaligned.
 
can anyone explain what 28 oz imbalance means? is there 28oz of extra weight at some location in the flywheel (or flexplate) and an additional 28 oz of weight in the damper? is the mass not really 28 oz but located at some position on the flywheel to make it appear to be 28 oz when spinning? just something i've never understood.
 
"B67FSTB" said:
I too did make this conversion.
When I bought my flywheel , it was delivered with both weights. 28 and 50 oz so I had to bolt the correct weight to the flywheel. :confu
Is yours bolted to the flywheel too ??? :confu :confu
Click to expand...

This. ^ Make sure you have the correct weight bolted to the back of the flywheel.
 
No, not your post. Actually, when I first posted I was hoping someone could explain in detail what the imbalance really is. IOW, exactly how are the rotating assembly, flywheel and balancer balanced, and why 28oz? Why not zero? These are the only engines I've encountered that are done this way, and I have to admit that I don't fully understand the theory.
 
IIRC the external balancing that these type of engines needs , is due to the build up of the chrankshaft.
I think the chrankshaft has an imbalance of 28 oz due to it construction.
Why they constructed the chrankshaft that way , i don't know ( anymore ).
Later engine ( mid 80's ?? ) have the 50 oz imbalance chrankshaft.
It all comes down to what chrankshaft you have.
Check your flexplate ( used with the auto trans ) and see what weight that thing has . IMHO
 
Thanks. I sold the flexplate already, but am certain that it is 28oz., as it is the original 289 in the car and they didn't change the weight until the 80's.
I believe you are on the right track with the imbalance theory. I think it's something to do with not being able to take enough weight out of the crank to balance the reciprocating assembly, so they offset that by counterweighting the flywheel the same amount....or not? :shrug
 
As a total stab in the dark, check to make sure the plug wires are connected & in the proper order. If one or two are loose or crossed it'll cause a heck of a vibration.
 
Found this someplace about all the Ford engine designs....

"Small block reciprocating assemblies have some balance. The compact lightweight crankshaft itself does not carry enough counterweights to achieve zero balance. Extra counterweighting is cast into the flywheel and harmonic balancer bring the entire assembly into neutral balance. Early engines use 28.2 oz.-in. external balance, while 1982 and later engines use 50 oz.-in. flywheel and 34 oz.-in. damper external balance. All 351W engines use the early style 28.2 oz.-in. external balance."
 
"AzPete" said:
Found this someplace about all the Ford engine designs....

"Small block reciprocating assemblies have some balance. The compact lightweight crankshaft itself does not carry enough counterweights to achieve zero balance. Extra counterweighting is cast into the flywheel and harmonic balancer bring the entire assembly into neutral balance. Early engines use 28.2 oz.-in. external balance, while 1982 and later engines use 50 oz.-in. flywheel and 34 oz.-in. damper external balance. All 351W engines use the early style 28.2 oz.-in. external balance."
Click to expand...

yea, i've read the reasons why ford did it and what's compatible with what and that a neutral balance is much better. i'm just trying to understand if there is really an extra 28 oz in both the damper and the flywheel. as an example, if i removed the weight added to create the 28 oz imbalance from a flywheel, would it litterally be 28 oz lighter?

i've been told that it's better to stay with a cast crank instead of the forged crank i have because the cast crank is lighter and strong enough. however, when i look at the weights from Scat comparing a cast crank to a forged crank, they are around 4 lbs difference. with a cast crank, there is less mass, but that mass is occupied in the flywheel and damper. 28oz X 2 = 3.5 lbs which ends up being about the same total weight.
 
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