Inliners International Forums General Discussion Hi Performance Cast 250 Crankshaft durability.

Since I'm new to inline engines, and plan on potentially being quite hard on mine I'm hoping to figure out the weakest link build to below that (size the turbo, HP target, etc accordingly). So most my current ??? are about the crankshaft. A search on crankshafts surprisingly didn't come up with anything disasterous (good thing?) so I thought I'd ask about the limitations of the 250 crankshaft. I am hoping to be school'd in crankshafts here

HP\Torque:
How much HP/Torque are some of you guys reliably putting into a stock or lightly worked 230/250 crank?

RPM:
Barring the limitations of intake/exhaust air flow, does any one know the breaking point for a 250 crankshaft in the rpm department? Assuming a good balance job done and good harmonic balancer.

For another question, how high are some of you revving your 230/250's?

Failure points:
With seven mains it looks like a fairly stout design, any info on anyone that has broke a crankshaft? Cause? Any particular thing that should be watched out for or modified for endurance?

Anything anyone wants to throw out there about the differences in the Pontiac 250 crank from the Chevy 250 crank? I've got spotty info, just looking to centralize some information. Thanks for any info.

The stock crank seems to hold out fine to 1000 HP w/stock maincaps it seems.
Guys in Brazil have done it,anything over 1000 HP, then they mod the maincaps by adding w/steel straps.

The weakest link to the 250 or Standard blocks,(230,250) is the left hand front corner head bolt area,they tend to crack,or in Douglas's case exploded that corner piece off,he was making a 8.0 pass when it blew up before the finish line. & ran an 8.6.
Leos book is good to read & get info about the Chevy L6's.
You can get it through this forum or other members I believe.
http://www.amazon.com/Chevrolet-Inline-S...93599912&sr=8-7

RPM wise, people go to 7000-8000 RPM with a built engine & seem to hold together.
My supercharged 250 liked to turn to 7000+ rpm & never broke.



MBHD

The Pontiac OHC engines I've taken down had Chevy part numbers on the cranks. From what I've seen, the cranks are the same.

It's also my understanding that you will be better off with a 250 12 weight crank instead of a 230 which only has 6 weights. Less internal stress in the crank at high speed with 12 counterweights.

You should consider using a 292 crank in your Poncho. No camshaft in the way! I'm pretty sure it's a drop in except for wristpin clearance. You will need to alter the counterweights according to your choice of connecting rod but that's not a big deal.

The 250 crankshaft is far more rigid than the 292 crank is. It has greater bearing journal overlap and doesn't have the bad harmonic issues the 292 can have from twisting and flexing. As has been mentioned, the fully counterweighted cranks are more desireable, and can handle most any amount of HP you can throw at them either N/A or with forced induction.

Under what circumstances does the 292 crank breakage problem occur? I'm asking because I don't know. I haven't broken one yet in my stroker and would like not to break one.

My guess is it occurs at a certain rpm(s) and worst case is holding that rpm over time. Like a 1 ton truck on the highway for example.

And further, what damper choice has been shown effective to reduce breakage risk? I ran a SBC StreetDampr on my last build.

And lastly, which cranks are the worst? I read somewhere that the early steel 6 weights are the worst, maybe because lower internal damping of the steel material really let's it ring? Plus the bending loads of 6 weight versus 12 weight? The steel forging sure is attractive versus nodular if the harmonics can be controlled.

Some technical data on this subject from General Motors would be great if it's available. Real world experience equally valuable.

Sounding like good news so far for me. I don't see me cracking 600HP, or probably even 500HP, all depending on cam/turbo, etc.

To give you some pictures of what I'm playing with:

That is a 292 crank on the left and my 250 crank on the right.


And the Pontiac 230/250 have a deeper skirt around the mains.


I've thought about going to stroker route, but because I'm going with forced induction to gain more rpm (in theory) I think the comprimises needed for the additional cubes aren't benificial enough and will likely just stick with the 250 crank. There is oil baffle clearance issues as well with the 292 crank.

Keep the crankshaft info coming! Thanks guys.

With forced induction and higher rpm goals I think you are wise to use the 250 crank.

What damper will you use?

I don't know about the dampener yet. The Pontiac one has the pulley grooves on it, and I "think" it is different than the chevy ones. The rubber on mine is definately comprimised, so I need to get mine rebuilt at a minimum. Not much out there in the aftermarket for these. What ever I use I'll get before I send the assembling for balancing.

Here are two non-ideal pictures of mine when I test fitted it on the 292 crank snout. The belt cog is not part of the dampener.

That damper looks similar to some dampers found on 292 Chevys.

So, has anyone actually broken a 292 crank? I'd like to hear the details if you have.

Putting what you think of "Leelites" aside..he did post up a very good post on crankshafts some years back with a ton of photos and casting number data. I have been unable to find it in a search.Maybe some one with better computer skills than I can dig it out. Just a thought !!! Jerry

I had a book (Thawley?) a few years back detailing putting a 292 crank in a 250 block for the roundy guys. He recommended the cast crank over the forged for endurance because the forged crank tended to crack before the season was over. Unfortunately the book disappeared into the depths of my 'brown bag filing system' and is nowhere to be found.

Tim

In our racing program some years back, once we reached the 575-600 HP range, our cranks would only last 20 1/4 mile passes before breaking. Several broke in half, and those that didn't break into 2 pieces were visible cracked so bad, you didn't have to magnaflux them to see it. These engines were at the uppermost extreme of compression, RPM and abuse you could ever put an engine through in N/A form, so I doubt that would be a concern for you to worry about.

Would you expect the same cracks/breaking on the 250 crank considering the same compression, rpm and abuse?

Thanks CNC. That's good info.

I had a bad feeling that I was destined to report breaking my crank. I feel better now since I'll not work mine anywhere near that hard.

We had a lot of racing customers that used the 250 engines, and many were the same as far as compression goes(15-1/2 to 1 range), the RPM's for the 250's were in the 9000 RPM range, and several were over 2 HP per cubic inch. The 250 seemed to be less suceptible to the damaging harmonics as the 292 is, and I can't recall any of our customers ever having the crank breaking issues we had with their 250's. Also, from what i've seen some of the guys making for HP with forced induction on 292's, they seem to have not reported any of the same breakage issues we had with our N/A application, probably because the operating RPM's they are observing are much less than we used, and stayed out of the harmful harmonic ranges that can make them self destruct. I think that either the 292 or 250 can be a viable engine for N/A or forced induction usage, providing a reasonable and sane range of RPM is observed. And should be able to enjoy much trouble free service out of them as well.

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.

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.

Thanks for that panic. Next question: From those numbers, I would target a damper tuned to 250Hz corresponding to 5000 rpm. Probably more to it than that but lacking sophisticated lab test equipment is it that simple? Anyone have a damper reference with this type of info?

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.

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.

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.

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.

Panic,

You wrote in post 62460 that a 292 crank resonant frequency is roughly between 225 and 275 Hz.

Then in post 62465 you wrote that the key is to determine the resonant frequency.

Which is it? Earlier post suggests that the answer is known, later post suggests the answer is unknown.

Clarification please.

Panic, thank you for the formula. I'm taking the tilde to mean "approximately equal to".

Striker, "severe pan failure".

I try to avoid that as well.

The answer is known.
I don't know it.

Tilde means "varies as".

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.

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.

"Varies As" good to know.

Boat application, only slightly more abusive than Bonneville.

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.

So the wrong dampener can give the same results as no dampener? Is there a best guess? Lets say for a turboed 292 12CW crank with a 700R4 in a a truck that will probably top out at around 4,500 rpms? I know there are a lot of variables but is there a safe bet for a street engine?

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.

If I had to track down a commercial product, search the catalog listings (duh).
BUT: check the part number to see if it's also listed for other not-very-similar engines.
An L6 won't match any V8, but a 194/230/250 is probably not identical to a 292 - the extra stroke does quite a lot - and the RPM at the same "danger points" will be somewhat lower.
In L6s, the entire arrangement of weight in the crank is different between 4 main bearings and 7, and I suspect it affects stiffness.
Anything that reduces stiffness (smaller bearings, longer stoke, longer crank) reduces the RPM at these points, but exactly how much is the $$$ question.

BHJ advertises one specifically for the 235/261 (which have the same dimensions in the important stuff).

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.