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Discussion Starter #1 (Edited)
A common topic that comes up in this section is something along the lines of : "How strong does a certain part need to be for a horsepower level or weight?"

One of the many things I do at Roush is test and analyze part durabilty. I often use strain gauges, tensile testers and examine broken parts.
Part breakage is a very common problem in the racing industry but it also happens in other fields as well. Because of our experience in material science we often get asked to consult on engineering problems.
The LV monorail recently had some unfortunate part failures that lead to grounding the fleet until the root cause was found and addressed. The locals are perplexed why a 60 million dollar piece of equipment isn't more durable. The answer is as complex as the machinery.

On the way back from Las Vegas I picked up a book by Henry Petroski to read on the plane.

'To Engineer is Human'

He teaches mechanical engineering at U of T Austin. He has an interesting demonstration for new students showing the problem of predicting part failure.

He hands out paper clips to all the students and asks them to bend them until they break, while keeping track of the number of bends. He records the numbers on the board and does a graph of the data.
The results show a classic bell curve with most breaking at a certain number but some lasting longer and some breaking sooner. All the paper clips came from the same box.

There are many reasons why one part breaks sooner or lasts longer but it's difficult to predict EXACTLY when a part will fail.

A part that is designed to last a long time may be very heavy and rob horsepower. A lightweight part may spin faster but need regular maintenance and or replacement.
A strong and light part is probably also very expensive.

There are many tradeoffs and decisions but there isn't one finite answer to how long a part will last, just statistics. Keep that in mind when a part is claimed to be good a certain horsepower level. Just like the paper clip experiment, some level of stress applied over certain number of cycles is what breaks parts. Horsepower is one factor to stress but as RPM increases so do the number of stress cycles.

The statistical bell curve may say that an axle is good for 400hp but in the field it may break with only 300 or it may hold up to 450.

Choosing the largest margin of durabilty is safest. Choosing the average has it's pro's and cons, but running a part at the extreme is very risky and ultimately expensive and potentially dangerous.
 

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I have always enjoyed reading and learning from your posts.

Thanks.


I have always tried to make it a point to go way overkill on parts like that, such as axles, cranks, rods, etc etc. The way I figure is its easier to spend more money once than spend less money over and over and over again.

Anyway, thanks again.
 

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Funny,I was thinking the same thing as I had my pull tab on my beer break off!
 

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The key is to engineer and design, not to guess.

A couple of weeks ago at work we heard a lecture on the failure of the Space Shuttle Columbia. I will have to get the full report. 2 main points of the lecture. Nasa had been engineering by success. "It worked last time". Same problem with Challenger. Also a more recent trend. Engineering by powerpoint. Reports that could be studied, verified and reviewed by other engineers had been replaced by powerpoint presentations. Sales pitches not engineering. I read the report on Challenger. By the time the report was done, NASA didn't want to be bothered with it. They had come up with a "fix" even though it still burned o-rings. Hopefully Nasa learns from the latest report.

Also heard a lecture recently from one of the head engineers at RCR racing. About $14 Million per car in the Nextel cup. He said they spent that much because that is all they had..... It jogged my memory as to why I got out of circle track racing.
 

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rpms w/cast 400 crank?

Im racing this weekend w/fresh 400. Cast gm crank,ballanced and gm 5.7(o) rods, all
reconditioned and ballanced, 10.8 to1 hyperutectic pisons I know the 535 "solid cam made power in my last motor,(327) all the way to 7000.
but was told by a few guys to keep this one below 6000. Im running a 4.33 gear with 30"tall tire and a turbo 350/3500 stall. I hope Im not revving to hi thru the traps.. I want this one to last !
 

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I get a little worried above 6K with the stock GM crank. Very rarely does it even see it though. I'm guessing you'll trap at around 5600.
 

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From what i have alway's heard you are safe w/cast up to around 7 grand for short periods at a time as long as it is balanced well.
 

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Discussion Starter #9 (Edited)
The 400 crank could spin to 7,000...if it didn't have rods and pistons attached. With the longer stroke piston speed increases. The G force of a piston reversing direction at TDC goes up dramatically. This increases the loading on the crank.

Piston speed can be calculated with the following formula:

Mean Piston Speed(FPM)= .166 x stroke" x rpm

.166 x 3.75" x 7000 = 4,375 feet per minute

Your 327:

.166 x 3.25" x 7000= 3,776 feet per minute.

To keep you piston speed the same as your 327 you'd have to limit rpm to 6,065 rpm.

Here's a link to a discussion on maximum piston speed and limits:

Maximum piston speed
 

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Paul Wright said:
The 400 crank could spin to 7,000...if it didn't have rods and pistons attached. With the longer stroke piston speed increases. The G force of a piston reversing direction at TDC goes up dramatically. This increases the loading on the crank.
So with the rods and pisons attatched what do you think Paul is 6000 ok?
 

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Discussion Starter #11 (Edited)
I was still typing. I have to save periodically and come back to the post because I have a real job to do.

Your safe limit depends on a lot of factors. There's no way I can stick my neck out on what's safe. The metallurgy of the GM rods can vary depending on how old they are and how many cycles they've already experienced.
G force loading depends on the bob weight and failure often depends on how good your rod bolts are.
Keep in mind that the rod bolts bear the brunt of the piston coming to a screaming halt at TDC every revolution. Kev's avator pix shows what happens when you overrev a stress fatigued bolts.

Crank shaft stress is mostly rotational. It twists back and forth every 90 degrees. This torque is a long term killer. It make take a little awhile but your longevity is definitely reduced. Detonation is a short term killer. It's like hitting your crank with a sledge hammer.

I would limit your rpm to no more than 6,000 to start but I don't guarantee that's safe.
 

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I would say 6000 would be a safe area to shift. You could probably push 6300 and a little more but I would guess you would be past your power range based on your cubic inches and cam size. Definately not 7000 with cast crank and rods. I am also assuming you don't have an internally balanced 400 crank rather an externally balanced engine. Forged and internally balanced is OK to see them kind of RPMs with a 400. Otherwise, play it safe or be sorry. :D
Dave
 

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Yah, my 283's connecting rods didn't appreciate having to move up and down in coolant filled cylinders. The block developed a fairly large crack around cylinder number 5 and the engine was fighting hydrolock. I was going down the highway at 75 mph and didn't really see/feel/notice the problem until it was too late. The crank tossed 4 connecting rods and scored the cylinders beyond recognition. The crankshaft was unuseable. The engine was a total loss.

Kev
 

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Thanks guys. I think ill shoot for about 5800 for a limit, Tach may be not so accurate. I just hope at the end of the track Im still in my power range. I guess Ill know sunday for sure!
 

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Throwin in my 2 cents

I personally will only build 400 sbc's I have never had any bad luck with them at all. My Camaro motor uses a stock cast crank, stock rods with arp rod bolts and manley pistons. I have a huge roller in the car that will make power way past what I take it to, but spinning the motor past 6500 doesn't make that much of a difference in the performance. So I set my shift light point at 6300 and after my reaction and the split second it takes to shift it spins about 6500-6600. Even with the 350 hp direct port fogger I have never had a problem. I know the stock crank in mine even with the big nitrous kit will go 6500 no prob. Oh, always balance it!!!!!!!!!!!!!!!!!!!!!!!!!!!
 

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Discussion Starter #16 (Edited)
novaboy009 said:
Yah, my 283's connecting rods didn't appreciate having to move up and down in coolant filled cylinders. The block developed a fairly large crack around cylinder number 5 and the engine was fighting hydrolock. I was going down the highway at 75 mph and didn't really see/feel/notice the problem until it was too late. The crank tossed 4 connecting rods and scored the cylinders beyond recognition. The crankshaft was unuseable. The engine was a total loss.

Kev
Hydrolock doesn't typically break rod bolts like that. I think the rod bolts were weakened from fatigue, lost tension and eventually failed but I don't think it was caused by the coolant leak. I could be wrong but it sure looks like a classic rod bolt fatigue failure.

Look at my rod failure photo album for examples.
 

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Well, when I pulled off the road there was ZERO coolant in the radiator and it was blowing coolant out of the carb. Perhaps the rod let loose and CAUSED the cracked block. That doesn't suprise me, considering how hard I was on that motor. It had a forged crank and pistons, but I think I found the weak link winging it up to 6600 rpm:).

Kev (Younger and stupider at the time)
 

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I take my 406 up to 6grand regularly.If it had anything left i would take it higher.At the track and on some road's i shift at 5500RPM.I guess we will see how long it last's.
 

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Where to comprimise? Rotating assembly

This is more a what if than a what should I do question.

Ok you have X amount of money ."PERIOD" No waiting to save or what ever just X amount.

Would you go for the strongest set of rods (American) you can get but still be able to get a 4340 crank (imported) and SRP forged pistons.

Or would you get a decent Rod (import 4340) and a more exspensive crank (American 4340) with same SRP pistons?

Would you comprimise on Crank or Rods?
Make Goal of 383CID with [email protected] And 450TQ And add a 75hp shot of spray.

Lets hear it.
 

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Well, it all depends on the rest of your combo. If the car is all set up and you want to build the engine, you can live with a cast crank. Buy a decent set of rods and a set of forged pistons. On my 327 I'm going with the stock rods, bush them, arp bolts and balance, a forged piston, and of course the stock steel crank. But, I wouldn't have a problem running a cast crank up to 6500. As long as you're not putting a ton of nitrous to it, it'll live. Just make sure all the clearances are correct. If you're on a budget, I'd go with the rods and pistons and just have the cast crank ground to low limit. That's my opinion. I'm running a steel crank in the 327 and I have a 350 with a cast crank. I don't have a problem running a cast crank to 6500. It's been done like that for years before all this forged stuff came around. My first engine I built, a 396, had a cast crank in it and it ran 7500 for two seasons before I sold the engine. And I raced twice a weekend all season. Dave
 
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