62Ranchero200
Famous Member
I spoke with Ricky Hults at ATI Racing yesterday about harmonic balancers. He told me that the risk of running a machined-down harmonic balancer would depend on the HP output and RPM range of the engine.
The primary issue that harmonic balancers are designed to address is torsional vibration. Overly simplified, torsional vibration is the flexing of the crankshaft in the direction of rotation, due to the power stroke. The risk is that with too much flexing, a crankshaft can experience metal fatigue, crack, and eventually break.
From online articles (for example, atiracing.com), it appears that in rough terms (not engine specific), less than one degree of torsional vibration is desirable, between one and two degrees is acceptable, and over two degrees is considered dangerous. These guidelines vary according to the length of the crankshaft, the construction of the crankshaft, the specific output of the engine (HP per cubic inch), the RPM range of the engine, and perhaps other factors such as whether a manual or automatic transmission is used (I think torque converters probably act as torsional vibration dampers, since the viscosity of the trans fluid probably damps torsional vibrations to some degree; whereas a solidly engaged clutch may not, depending on the construction of the clutch).
We have zero information from Ford as to how much torsional vibration is acceptable for the components of our Ford Sixes, nor any way to quantify the damping factor of stock harmonic balancers (although Ford engineering guidelines on these topics presumably existed when these engines were being actively designed). Even less information is available on how much more torsional vibration a modified Ford Six might generate and how much damping it might require.
There is also no reasonable way for an individual to directly measure torsional vibration on their engine. I thought that perhaps Autometer or a similar company would make vibration gauges - surely they would be useful in some forms of racing with high output, high RPM engines, like NASCAR engines, as predictors of component failure - but it appears that no one in the automotive aftermarket does.
The complete lack of information on this topic bothers me greatly - torsional vibration and harmonic balancers are truly the proverbial "black boxes" that we know nothing about.
Of course, we could assume that Ford's wise, all-knowing engineers designed harmonic balancers with exactly the needed amount of damping for the worst operating conditions that could possibly occur on these engines :roll: , and that reducing this damping by even the smallest amount would provide insufficient damping, allowing torsional vibration over the threshhold of metal fatigue and eventual breakage. But we need to remember that this is an assumption. Some engines could have been "overdamped" in an effort to "err on side of caution". This is the reason why "out of the box" carburetors are often excessively rich - the manufacturers are "erring on the side of caution", since running rich may cause poor performance, but rarely damages engines, while running lean has been known to damage engines. We're not terrified to modify carburetors, because air/fuel gauges are available, and even without those it's possible to read the plugs. Also, many engine part decisions are made based on economies of scale: if the same or a similar part can be used across engine families (for example, SB6 vs BB6, or SB6 vs SBF), the part becomes cheaper to produce.
Engines are often stroked to provide greater displacements - 200s to 221, 302 SBF's to 347, 350 SBC's to 383, 460 BBF's to 521 - and the stock harmonic balancer is often used on the stroker engine for a low-budget build, if the external balance is correct (internally balanced /28 oz /50oz for SBFs).
I wonder if anyone on this forum has any information about torsional vibration on our Ford Six cranks - specifically, US 250's - and about the degree of damping provided by the stock harmonic balancers. Would machining the second (front) pulley from a stock 250 harmonic balancer (in an effort to gain clearance between engine and fan) mean certain destruction?
EDITS:
The rotating assembly has been statically and dynamically balanced
The engine features ARP main studs
The engine uses "early" 300 rods, which I expect are heavier than 250 rods
The engine uses very short custom pistons, which I expect are lighter than stock 250 pistons
This combo uses an automatic if that makes any difference (if torque converters damp torsional vibrations)
Thanks,
Bob
The primary issue that harmonic balancers are designed to address is torsional vibration. Overly simplified, torsional vibration is the flexing of the crankshaft in the direction of rotation, due to the power stroke. The risk is that with too much flexing, a crankshaft can experience metal fatigue, crack, and eventually break.
From online articles (for example, atiracing.com), it appears that in rough terms (not engine specific), less than one degree of torsional vibration is desirable, between one and two degrees is acceptable, and over two degrees is considered dangerous. These guidelines vary according to the length of the crankshaft, the construction of the crankshaft, the specific output of the engine (HP per cubic inch), the RPM range of the engine, and perhaps other factors such as whether a manual or automatic transmission is used (I think torque converters probably act as torsional vibration dampers, since the viscosity of the trans fluid probably damps torsional vibrations to some degree; whereas a solidly engaged clutch may not, depending on the construction of the clutch).
We have zero information from Ford as to how much torsional vibration is acceptable for the components of our Ford Sixes, nor any way to quantify the damping factor of stock harmonic balancers (although Ford engineering guidelines on these topics presumably existed when these engines were being actively designed). Even less information is available on how much more torsional vibration a modified Ford Six might generate and how much damping it might require.
There is also no reasonable way for an individual to directly measure torsional vibration on their engine. I thought that perhaps Autometer or a similar company would make vibration gauges - surely they would be useful in some forms of racing with high output, high RPM engines, like NASCAR engines, as predictors of component failure - but it appears that no one in the automotive aftermarket does.
The complete lack of information on this topic bothers me greatly - torsional vibration and harmonic balancers are truly the proverbial "black boxes" that we know nothing about.
Of course, we could assume that Ford's wise, all-knowing engineers designed harmonic balancers with exactly the needed amount of damping for the worst operating conditions that could possibly occur on these engines :roll: , and that reducing this damping by even the smallest amount would provide insufficient damping, allowing torsional vibration over the threshhold of metal fatigue and eventual breakage. But we need to remember that this is an assumption. Some engines could have been "overdamped" in an effort to "err on side of caution". This is the reason why "out of the box" carburetors are often excessively rich - the manufacturers are "erring on the side of caution", since running rich may cause poor performance, but rarely damages engines, while running lean has been known to damage engines. We're not terrified to modify carburetors, because air/fuel gauges are available, and even without those it's possible to read the plugs. Also, many engine part decisions are made based on economies of scale: if the same or a similar part can be used across engine families (for example, SB6 vs BB6, or SB6 vs SBF), the part becomes cheaper to produce.
Engines are often stroked to provide greater displacements - 200s to 221, 302 SBF's to 347, 350 SBC's to 383, 460 BBF's to 521 - and the stock harmonic balancer is often used on the stroker engine for a low-budget build, if the external balance is correct (internally balanced /28 oz /50oz for SBFs).
I wonder if anyone on this forum has any information about torsional vibration on our Ford Six cranks - specifically, US 250's - and about the degree of damping provided by the stock harmonic balancers. Would machining the second (front) pulley from a stock 250 harmonic balancer (in an effort to gain clearance between engine and fan) mean certain destruction?
EDITS:
The rotating assembly has been statically and dynamically balanced
The engine features ARP main studs
The engine uses "early" 300 rods, which I expect are heavier than 250 rods
The engine uses very short custom pistons, which I expect are lighter than stock 250 pistons
This combo uses an automatic if that makes any difference (if torque converters damp torsional vibrations)
Thanks,
Bob