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Flywheel imbalance
Can anybody explain why a particular imbalance is specified when matching a flywheel to an engine? What does 50 in/oz. mean?
Cheers, Glen |
It's an inherited disease of the Ford engine.....
For a factory crank it is "balanced" either with a 28 or a 50 oz external balance. This means that both the rear flywheel and the front balancer has the appropriate weight at the correct spot in the rotating, outer part of the radius. If you use the wrong weight- or the weight is at a wrong place, the engine will not run smooth- there will be very, very unpleasant harmonics coming from the crankshaft. However.... You can get aftermarket cranks that have 0, or internal balancing where the mass of the crank throws and pistons/ rods themselves make the assembly balanced. In such a case the flywheel and balancer need to have 0 balance as well. |
So do I take it that the best setup is a 'dynamically' or zero-balanced crank with zero balanced flywheel/clutch and zero-balanced front balancer? Are there any plusses for the "inherited-disease" way of doing it? I'm assuming that way would be cheaper.
Do you know what inches per ounce (in/oz) means? Cheers, Glen |
I'd guess the in are radial. That is weight (oz) in regarding to distance from rotating centre. Have never given it much thought- all I know is that the crank gotta match the fleewheel which gotta match the balancer. Then the whole assembly need balancing after a rebuild for performance...
Read up on physics would be a starting point. I'm sure you'll find some clues if you do a googlesearch as well. Most high- end builds, and practically all racing builds, rely on a internal 0 balance crank. It has alot of + sides, but also some -'s. However, I see no problem using a external balanced crank in a "hot" build... Regards RS |
When there isn't much room in the crankcase to get the weight where you need it, it is cheaper to external balance the engine. The factory is mass producing with cost in mind. I can think of no other advantage.
To internally balance most of these engines, it requires adding malory (SP?) metal, which is quite a bit more expensive. |
The intial engine disigns pretty much had internal balance but as cubes went up, with longer stroke cranks, the counter weights got cut down so the pistons would clear the crank weights so some compamies put extra weight in the flywheel and front damper--- using a constant amount and location of the weight added to the flywheel allows for exchanging flywheels without having to rebalance the complete assembly----
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Cheers, Glen |
Hi, Glen.
I'm confused on what you say. ALL rotating assemblies should be balanced, in balance so to speak, in a crank balancer shop using bob-- weights etc. A 302 crank can be one of three (if you disconcern any oddities) either (OE Ford or aftermarket) externally balanced on either 28 or 50 OZ (that's 2) OR internally balanced 0 counterweight/ neutral (aftermarket/ Ford racing) (3. possibility). Wait a second- my memory seems to be dim... I am not sure; if you have an externally balanced crank: Can you switch between 50 and 28 oz imballance damper/ flywheel (in a pair, ofcourse) without issues? I've read on this topic earlier, but conclusion I do not remember. If you have a factory Ford crank, it will need the counterweights in balancer and flywheel. Eventually the rotating assembly, even if stock parts are used, SHOULD BE be taken to a shop for balancing. (Or don't do so and hope for the best- lots have done, and mostly with luck, as I have seen.) |
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And I agree with your comment about taking the rotating assembly to a shop for balancing. The cost would be small compared to the overall investment. Cheers, Glen |
5 Attachment(s)
This is a photo of a sbf flywheel and the portion of a front dampner for an external balanced 302 ---you can see how there is a differance in the amount of weight because of the distance from tyhe crank centerlineAttachment 20582
Attachment 20583 these next 2 are pics of early sbc flex plate for internal balance and next for a one piece seal crank with internal balance--the use of one piece seals doesn't allow for any offset flange weight in the rear Attachment 20584 Attachment 20585 these two pics are of a flywheel for an sbc LT1 and you can see what the aftermarket flywheel uses compared to a weight off a factory flexplate Attachment 20586 |
1 Attachment(s)
Bob weights are set up to equal the weight of the rods, bearings, wrist pins, rings, and pistons according to whether rotating or recipacle weight mass---the 2 bearings and 2 rod big end weight(rotating) plus the small end of one rod, one piston & pin &rings
(recipercating weight)---they are put on the 4 crank throws for the balancing operation--- |
On balancing machine
2 Attachment(s)
Crank with bob weights on balancing machine----for internal balance crank assy is balanced without dampner or flywheel
As you can see--there is a pretty good sized rear flange that helps with the balance at the rear--this is a BBC stroker crank that also took one piece of heavy mallory metal in the front counter weight to acheive internal---- .In the left photo you can see the one piece of 1 inch heavy metal in the front counterweight. |
2 Attachment(s)
Callies 302 Ford stroker
Holes drilled and reamed for 1 inch pieces of Mallory heavy metal to internal balance |
Thanks Jerry. That does seem more sophisticated than an external balance, but no doubt horses-for-courses.
Would a corrective imbalance flywheel and front balancer tend to induce some flex into the crank, as the masses are applied some distance from the imbalance they are trying to correct? Cheers, Glen |
SFI balancer?
I was doing a bit of trolling around, and saw reference to an SFI balancer for an internally balanced engine - why do you need one for an internally balanced engine?
"Pioneer part number 872034 ... for... Ford SB 302-351W with 6.37" Dia Internally Balanced" I'm some way off talking to an engine builder, so I'm 'chewing-the-fat' a little here... Cheers, Glen |
You need a harmonic balancer to take out the harmonics of a rotating assembly...which even internally balanced assemblies have harmonics.
The SFI rating just shows that it's been approved by SFI guidelines, which is a standard for most racing sanctions. |
Understood....so if an engine is internally balanced or not, it still needs a harmonic balancer. The harmonic balancer for an internally (zero) balanced engine would obviously (yes?) be different to that of say a 28oz/in engine?
Cheers, Glen |
Well, not that obvious. It would look the same, except it would have a weight bolted/welded/molded into it.
Some of the aftermarket balancers have bolt on weights (some flywheels do too), so you can interchange between 0-balance, 28-oz, etc, just by removing the bolts and removing the weight. |
Just a "DOH!" moment. Sorry.
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Thanks. The light is slowly coming on...
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