#62391 - 12/28/10 08:49 PM
Cast 250 Crankshaft durability.
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TheSilverBuick
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Registered: 12/22/10
Posts: 25
Loc: Ely, NV
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#62454 - 12/31/10 12:16 PM
Re: Cast 250 Crankshaft durability.
[Re: CNC-Dude #5585]
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TheSilverBuick
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Registered: 12/22/10
Posts: 25
Loc: Ely, NV
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Would you expect the same cracks/breaking on the 250 crank considering the same compression, rpm and abuse?
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Rebuilding an OHC Pontiac 250 with EFI and a Turbo
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#62456 - 12/31/10 04:03 PM
Re: Cast 250 Crankshaft durability.
[Re: TheSilverBuick]
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CNC-Dude #5585
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Registered: 09/27/08
Posts: 1732
Loc: N. Georgia
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Would you expect the same cracks/breaking on the 250 crank considering the same compression, rpm and abuse?
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There's no such thing as too much cam....only not enough engine!
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#62457 - 12/31/10 06:24 PM
Re: Cast 250 Crankshaft durability.
[Re: CNC-Dude #5585]
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TheSilverBuick
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Registered: 12/22/10
Posts: 25
Loc: Ely, NV
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If I can get my oil system worked over enough, I'd like to see 2-2.5hp/cid and 9000rpm range. That'd be my ideal goal, I intend on using a turbo to help out. I figure I'll be rpm limited though.
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Rebuilding an OHC Pontiac 250 with EFI and a Turbo
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#62460 - 01/01/11 06:56 AM
Re: Cast 250 Crankshaft durability.
[Re: TheSilverBuick]
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panic
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Registered: 01/11/05
Posts: 1340
Loc: Hempstead, Long Island, NY
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The 230 & 250 cranks differs from the 292 by both stiffness (more journal overlap: .50 square inches vs .32 square inches vs .07 square inches), and the RPM at which destructive harmonic vibration occurs.
The 292 bad spots will be at lower RPM due to the lower resonant frequency. The really bad ones will happen in all these engines, but at different speeds (the next order in RPM is far above anything you would use).
The actual RPM is a direct function of the crank's resonant frequency, which is difficult to calculate from engineering data. Roughly? Between 225 and 275 Hz.
Using the low number (292), the bad spots are at
1,500 (faint)
2,250 (weak)
4,500 (worst)
5,400 (very bad)
6,750 (bad)
Using the high number:
1,833 (faint)
2,750 (weak)
5,500 (worst)
6,600 (very bad)
8,250 (bad)
For comparison, a V8 crank at 350 Hz:
2,333 (faint)
3,500 (weak)
7,000 (worst)
8,400 (very bad)
10,500 (bad)
As you can see, all the numbers are different: no resonance occurs at the exactly the same RPM in any 2 cranks, and where there are similarities they're at different harmonic orders and amplitudes.
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#62463 - 01/01/11 08:45 AM
Re: Cast 250 Crankshaft durability.
[Re: strokersix]
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TheSilverBuick
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Registered: 12/22/10
Posts: 25
Loc: Ely, NV
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I've been looking at fluid dampeners or the TCI Rattlers, as they are designed to absorb the harmonic energy, not dampen, so finding one for a a V8 my thoughts are it'll work good on this engine. Looking at ones with the Serpentine belt grooves built into it, then see what I can find for water pump and alternator pulleys.
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Rebuilding an OHC Pontiac 250 with EFI and a Turbo
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#62465 - 01/01/11 09:24 AM
Re: Cast 250 Crankshaft durability.
[Re: TheSilverBuick]
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panic
Active BB Member
Registered: 01/11/05
Posts: 1340
Loc: Hempstead, Long Island, NY
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The key is to determine what the crank's resonant frequency is, and that's also somewhat variable based on the flywheel weight, type (and method of attachment), balance weight removed from the CW, and even the fixed part of the damper (the hub).
The math is very messy, the closest guess will be from an engine with about the same:
1. length, or at least similar config like another L6 with similar bore pitch. A 235/261 vs. 270/302 GMC will not be too far away, the 235 will be slightly higher Hz (shorter length and stroke, same journals = stiffer)
2. stroke length
3. journal sizes
4. number of main bearings (not directly, but it affect where the CW are)
I got an engineering guy to run an analysis of a 235 crank for me based on factory drawings, material specs, weight, fasteners etc. and it's pretty low (= low RPM for all relevant orders), lower than the 250 etc.
I suspect that a clean crank can be tested for frequency the same way you would test for cracks: suspend it, and smack it with something soft but metallic like a brass hammer.
A mike attached to an oscilloscope will pick up a strong trace at at least 1 point.
Alternative: a tone generator that produces a strong signal between 200 and 350 has its horn aimed at the crank, and spun up through the range. A vibration probe on the CW will jump when the frequency is correct.
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#62466 - 01/01/11 09:34 AM
Re: Cast 250 Crankshaft durability.
[Re: panic]
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panic
Active BB Member
Registered: 01/11/05
Posts: 1340
Loc: Hempstead, Long Island, NY
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Lighter flywheel or crank frequency change can be roughly estimated if the frequency is known. In general terms, the extra mass reduces the frequency in inverse proportion to the square roots of the weights:
ω ~ 1 ÷ (M2^.5 ÷ M1^.5)
If the crankshaft and its attached components (damper, flywheels, bolts, &c.) weighs (example only) 100 lbs. (M1), and a 10 lb. lighter flywheel were substituted (new weight: 90 lbs., M2) the frequency would be 1 ÷ (100^.5 ÷ 90^.5), or 94.874%. If the original frequency were 250 Hz., it decays to 237 Hz, &c. and the RPM drop at each order is proportionate.
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#62467 - 01/01/11 09:54 AM
Re: Cast 250 Crankshaft durability.
[Re: strokersix]
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Greybeard
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Registered: 01/27/06
Posts: 75
Loc: Sequim Washington
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I don't have drag racing experience with inlines but I did do a few years with Limited Class Hydros with inlines as well as bent motors. Although I have 250s in two of my cars, I've never built a Chevy 6 to race, but I have broken crankshafts in the race boats.
In the PNW we've always tried to have big fat race courses where we were able to keep our engines wound up. As a single heat would be 5 miles and the engine required to run 3+minutes @WOT in a good handling boat, we couldn't build our engines like one would for the strip. The first engine I used was the 144" Falcon. Injected on methanol, the 4 main bearing motor would break frequesntly in the bottom end. We had to safety wire everything on the motor or attached to it, because the harmonics unbolted everything. I made 175-180hp and got about 30-40 minutes of 7-7200rpm before suffering severe pan failure. I switched to Nissan 2.4, same fuel and induction, 235-240hp. I redlined it @7500 the first year and got a full season, but the next year turning 78-8000 I unwound two bottom end$. I've always felt that harmonics at high rpm was the real killer in the L6s. CNC Dude's experience of 20 runs, approximately 3 minutes of full throttle, shows what happens when you really turn'm up.
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#62476 - 01/01/11 10:10 PM
Re: Cast 250 Crankshaft durability.
[Re: panic]
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panic
Active BB Member
Registered: 01/11/05
Posts: 1340
Loc: Hempstead, Long Island, NY
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A boat is a really severe test. Sometimes your hull, required speed, and prop pitch make it impossible to choose your RPM to avoid a know-to-be-bad spot.
High RPM is no worse than in anything else - if it passes quickly through any lower resonances, or they're well damped.
What's fatal is running hard at a continuous RPM, which is at or very close to an "order" (multiple of frequency) without enough damping.
7,000 may be just fine for 1/2 hour, and 6,600 break the crank after 1 minute at WOT.
Sometimes this happens by using an SFI quality damper - but designed for the wrong engine. It will react (convert torsional twist into heat), but at a speed harmless to your crank, and remain passive when yours needs protection.
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#62477 - 01/01/11 10:15 PM
Re: Cast 250 Crankshaft durability.
[Re: panic]
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panic
Active BB Member
Registered: 01/11/05
Posts: 1340
Loc: Hempstead, Long Island, NY
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The small Ford L6 has had problems in the past IIRC because there are both 4 main and 7 main engines (all 144 and 170 are 4, 200 made both ways, 250 7 only?) - but the dampers will be different based just on this point. Run the wrong one, expect a failure.
There are perhaps 9 orders of consequence in an L6, but only a few are strong enough to do anything: the 2nd, 2-1/2th (yes, what I said), and 3rd. The others either happen so high up (over 10,000 RPM) or are so weak that they have no effect.
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#62496 - 01/04/11 07:45 AM
Re: Cast 250 Crankshaft durability.
[Re: strokersix]
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Bruce
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Registered: 05/03/00
Posts: 670
Loc: Boise, ID
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I lost two 292 cranks; both broke at the front main. Very likely it was due to using a small block Fluid Dampner. Went to a lightweight aluminum balancer and have gone 6 years without an issue. It is in a drag car and use 6600 for the shift point.
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#62502 - 01/04/11 09:33 AM
Re: Cast 250 Crankshaft durability.
[Re: Beater of the Pack]
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panic
Active BB Member
Registered: 01/11/05
Posts: 1340
Loc: Hempstead, Long Island, NY
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JM2¢?
Any quality damper is going to have periods of harmonic suppression wide enough to do something useful, even if it's not where you want.
The OEM damper isn't as accurate if the RPM range goes up significantly (you may reach another "order" that the OEM wasn't designed to handle), or the crank has been modified (lightened, light flywheel, stroked), and not blow-proof at high RPM (cast-iron) but will do a good job if new or excellent.
Remember: SFI cert. is only a safety check (it won't explode) and doesn't test for actual damping - let alone damping on a specific engine.
The closer the similarity between your engine and the model from which the damper was designed, the better the match. I'd rather have a Ford 300 damper on a 292 than none.
If you can't find one really close, the next best thing is to try to anticipate where the crank will suffer, and avoid using it either very hard or continuously at those points. For a street car, the engine may feel "buzzy" as certain speeds, and better slightly above and below regardless of what gear it's in, and regardless of power. Those are probably suspect points - drive through them, or around them. Don't do 200 miles at that speed.
With a drag car, they's pretty easy to avoid them since you pass through them. An LSR or boat is much worse, circle track very bad.
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#62504 - 01/04/11 09:53 AM
Re: Cast 250 Crankshaft durability.
[Re: panic]
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panic
Active BB Member
Registered: 01/11/05
Posts: 1340
Loc: Hempstead, Long Island, NY
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For a 292, the frequency may be as low as 250 Hz. If yes, the only destructive orders you will reach at that speed are the 3rd, 6th and 9th (the 2-1/2 is bad, but it's at 6,000):
3rd 5,000 RPM
6th 2,500
9th 1,667
This means that 4,500 is safe, and you can cruise at any speed between (slightly above) 1,667 and (slightly below) 2,500, or between (slightly above) 2,500 and 4,500.
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