Part durabilty and rpm limits Q's (Combined threads)

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' (http://www.amazon.com/exec/obidos/tg/detail/-/0679734163/qid=1099690228/sr=8-2/ref=pd_csp_2/002-3147725-3352029?v=glance&s=books&n=507846)

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.

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.

Great thread and very interesting stuff!!

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

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.

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 !

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.

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.

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 (http://www.mustangsandmore.com/ubb/SteveWmaxpistonspeed.html)

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?

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.

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

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

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!

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!!!!!!!!!!!!!!!!!!!!!!!!!!!

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 (http://pg.photos.yahoo.com/ph/pcwright77/my_photos) for examples.

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)

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.

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 450Hp@6500RPM And 450TQ And add a 75hp shot of spray.

Lets hear it.

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

well my 355ci bottom end consists of a new cast crank and some stage 2 stock type rods,( thats peened, beams ground , polished and wave loc bolts.) and trw forged domed pistons. its has about 5k on it . saw 7000 rpm frequently with a 415 hp combo. now i have it at a 550 hp combo with new top end bits but with the same lower end. still goes to 6800 rpm . no probs. yet.
doubt ill ever see one.

had a all stock lower end 327 that went to 7500 all the time. never lost the bottom. 413 hp.

I'd probably opt for the cast crank, better rods, and the best rod bolts. From what I've read, most bottom-end failures begin with inferior rod bolts. Can anyone confirm this?

I've lost a motor before to stock rod bolts, i dont comprimise anymore with bolts, but i still will use cast rods. I have total faith in cast cranks for any motor the normal joe builds, I'd put the extra money into the rod bolts and pistons and the all important balancing, which is what most people neglect, weight matching and balancing is worth more than any part you can put in any rotating assembly.....Doug

For reference on what rod bolt failure does... look at my avatar.

That poor 283 just couldn't take the whooping anymore.

Kev

You can save some ****$ by not buying those expensive SRP pistons and getting a set of Probe pistons.

Shawn, i'm thinking there's a few more factors involved... how strong is the WEAKEST link ??? and how high ya gonna spin it ???

a killer crank and rods in a weak block is still a grenade waiting to explode...


a 2 bolt block with bolts is weaker than the same block with main studs... a 4 bolt block (in most cases) is stronger than the 2 bolt... a 4 bolt with splayed caps is stronger than a standard 4 bolt... a 4 bolt with splayed caps and studs is stronger than splayed with bolts...

see where i'm goin' ??? there's minor things and major things... they all add up in the chain of strength...


all the money in the world towards crank rod and pistons is of no use in a weak block... it all works hand in hand...

What are you going to be doing with the engine? I think that needs to be addressed first. Is it a daily driver, or a weekend thumper?

A good cast crank will take a lot, as will stock rods with good bolts. Remember that what you hang on the end of the rod effects overall strength as well. Put a good quality light piston, there will be less stress on the rods. Put a heavy piston, you add stress.

If you have to invest in something I'd say use the lightest, strongest pistons. The less load the rods and crank have to carry the better. Rod bolts are the weakest link any rod. A stock rod is pretty good piece if it's reworked right. New aftermarket rods are more economical in the long run. Cast cranks are marginal at 450hp and over 6,500 rpm. Some survive and some don't.

When you are on a budget and you start to compromise durability to pay for power, you are gambling. The risk is you could lose the entire investment.

Pick a durability level you can afford and then build the power level to not exceed that.

A 450HP engine with 400hp durabilty isn't a wise compromise. A 400 hp engine with 450 hp durabilty is a better choice.

I would have to say get import crank & rods .Stay away from cast ,for the extra 100.00 for crank and extra 50.00 for rods, DEFINATLY GET FORGED!!!
For the price of both crank and rods imported your lookin at approx 600.00
CAN you buy an american crank for 600.00 (barely).
Also 525 hp wont beat up the import stuff (if its balanced).
IMO pro tru piston kits are a bargain!!

I agree with Paul. Also, most rod failures that I've seen basically came from spinning a bearing first. I've only heard of a couple of bolts break, which can still be from a failed bearing, and I've also seen the nuts fall off and be in the pan when it was taken apart. A good bolt and proper clearance go a looooong way. I run my engines a bit loose compared to tight. Get a reputable crank grinder and have it put on low limit and make sure you check and recheck everything. Dave

Get a reputable crank grinder and have it put on low limit and make sure you check and recheck everything. Dave



For what reason would you have a new crank cut down? The crank Im looking at is forged 4340 nitrated.

I have a 350 block here ready to be sent off to get splayed caps.

This will be a weekend driver with some track duty.


I was looking for probe pistond last night and couldnt find them... Any help

This may end up being a 363 with a 3.562 stroke. with same goal.

Paul did catch on to what im doing with said goal being higher than actual goal.

I was looking for probe pistond last night and couldnt find them... Any help

http://www.probeindustries.com/

Loose clearances are worse for durability. I have a chart that shows the load capability of bearings drops off as the clearance opens up. Too tight reduces protective oil film. Most people think spun bearings are cause by siezure of the bearing to journal, so they open up clearance so this doesn't happen. The problem with that is then the bearing can't maintain the oil film wedge and it's less efficient.
Spun bearings also happen when bolts lose preload. That's also why nuts fall off. A bolt is just a very strong spring. When a bolt stretches and relaxes it eventually loses it's tension and preload goes away.

Oh I see now My mind was thinking wrong. I thought "Low Limit" being cutting the journal down to its smallest safe point. I see you are talking Low Limit of the clearance spectrum...

Actually I interpret "low limit" as the smallest journal size within the tolerance range.....which in turn gives a larger bearing clearance.
When cranks are ground they have a tolerance range even if it's "standard". Same is true of bearing shells. This is why you can get varying bearing clearance with brand new parts. You can specify the journal diameter to have better precision but that costs more. Measuring and juggling shells is how clearance is set precisely but you have to be able to measure to the 4th digit and you have to buy enough sets to get the ones you want. You also need a micrometer with a ball anvil.

Read Lingenfelters book. He has a section on bearing clearance and some charts showing the effects of clearance changes.

I have that book and will read that section closer... Thanks.

So what are the correct uses for "I" and "H" beam rods?
What are the trade offs for one over the other? :chev:


*****EDIT******
Yes I do know the physical differances.

"I" beams are less expensive. "H" beams are stronger. For most street/strip applications "I" beams are sufficient. Stock rods are "I" beam style.

Found this while seaching some info on rods..


• Reciprocating weight accelerates to a stop then changes direction. Rotating weight moves in a single direction. Loosely speaking, removing one ounce of reciprocating weight is more beneficial to longevity and efficiency than removing a pound of rotating weight. In other words, don’t waste time grinding the crank counterweights. Concentrate first on the rods, pistons, and pins.

• Using lightweight rods with stock pistons and pins can lead to disaster. The hefty stock parts can impart excessive tensile loading and fail the rods. Instead, get the lightest pistons and pins that still offer a surplus of durability, then spring for the light rods. Running light pistons on stock rods is safe.

• A too-heavy rod-and-piston assembly running on a lightened and scalloped crankshaft is an invitation to disaster. The crank can flex and fail at high rpm.

• If you’re stuck building an engine with marginal connecting rods, don’t install parts that increase engine speed (hot cam, ported heads, big carb, and so on). Instead, go with nitrous oxide, a blower, or turbo. Adding power through compressive loading is actually easier on the rods than the high-tensile loading that comes with screaming rpm.

• The root cause of as many as 80 percent of catastrophic rod failures actually begins when the big end loses its ability to maintain safe bearing clearance. The oil film gets displaced, the bearing grabs the crank, and the resulting kink-load snaps the beam.

• Contrary to popular myth, titanium rods will not evaporate if they come into contact with aerosol carburetor cleaner. What is true is that prolonged exposure to the stuff will cause oxidation that can cause surface inclusions and eventual fatigue points. To be safe, clean titanium rods with mineral spirits or water-based cleaners.

Here is an answer I found in a long web seach

H-Beam Vs. I-Beam




For years the battle has raged on as to which is better, an I-beam or an H-beam. In pure tension and compression, they are both equally capable, assuming equal cross-sectional area. But when you add the fact that some components of the combustion event attempt to screw the piston down the cylinder, the greater distance from the centerline of the pin to the edge of the beam gives the H-beam an advantage in resisting such twisting forces. We’ve seen both designs used successfully in a wide variety of extreme applications, so the jury is still out. Perhaps the biggest advantage of the H-beam design is that it gives the manufacturer more flexibility when sculpting the rod into the most effective form from a strength-to-mass standpoint.

Found this while seaching some info on rods..


• Reciprocating weight accelerates to a stop then changes direction. Rotating weight moves in a single direction. Loosely speaking, removing one ounce of reciprocating weight is more beneficial to longevity and efficiency than removing a pound of rotating weight. In other words, don’t waste time grinding the crank counterweights. Concentrate first on the rods, pistons, and pins.

This is why lightweight valvetrain parts are important. Not just the valves but the lifters and pushrods.

This is why lightweight valvetrain parts are important. Not just the valves but the lifters and pushrods.

That sounds logical and common sense but consider this:
The camshaft rotates at 50% of crank speed so even at a whopping 10,000 rpm the cam lifters and pushrods are moving at only 5,000 rpm. A pushrod weighs much less than a connecting rod and reciprocates slower.
Lightening up the pushrods is counter productive if it reduces stiffness. Large diameter steel pushrods are better than lightweight pushrods. Ever wonder why you see aluminum rods but you'll never see aluminum pushrods?

Lifters are another curious thing. Which will rev higher? a lightweight solid lifter or a roller lifter. Again it's not the weight that allows a roller lifter to track a lobe at high rpm. It's it's ability to follow the contour. Remember that the cam is going half the crank speed.

The lifters and pushrods are essentially motion transfer devices. They are trapped between the cam and the rocker shaft.

I worked on a valvetrain instrumentation program for about a year. I'm the resident strain gauge expert and I was asked if it was possible to measure valve train forces at very high rpm. They said in all the SAE papers on the subject, no one had had ever succeed in collecting data higher than 4,500 rpm. I thought it might be possble so I engineered some innovative measurement techniques. I instrumented every thing in the valve train. We ran Spintron tests and collected data. It was very interesting and some of it defies common sense until you explore the physics.
I can't reveal much of what I know because of confidentiality and I certainly can't post pictures, but the bottom line is don't skimp on pushrod stiffness to save a few grams.


(I merged all these questions together into one thread since they are all related.)

I agree with everything you said Paul and that was the point I was trying to make in the other thread about what differences there are between pushrods and overhead cams:
http://stevesnovasite.com/forums/showthread.php?t=27702
I never said making the lightest pushrods were most important, I just said the weight was one of the cons to a pushrod valvetrain system. Overhead cams have less parts and reciprocating weight. That's one of their pluses. Here's my quote from that thread:
The main problem with pushrods is weight and strength. The equipment used to open the valves with a cam in the center of the block requires lifters, pushrods and rocker arms, all that have weight. Plus, they have to move in an up and down motion, constantly changing direction. Controling that motion requires stiff springs. To work against stiff springs, the pushrods have to be strong (where the weight comes from) so they don't bend. If they bend, the true lift and duration of the cam is not always reproduced at the valve.

On the plus side, it's not a complicated system. Overhead cams require very complicated control systems (long chains or belts) but have fewer parts controling the valve motion.

I still stand buy my statement that weight saving in the valvetrain will yield benefits as long as there is no loss of strength. Once you make the parts so light that they can hold their shape you are losing power.

My point is the weight of the pushrod is not as important as you think.

For example:
A car handles better if the unsprung weight is very low and the chassis is stiff. Removing stiffness from the frame and rollcage saves weight but sacrifices handling.
By the same token a pushrod engine revs better if the valves are light and the pushrod is stiff. A higher revving engine makes more power than one with a lightweight but unstable valvetrain.

Here's a quote (http://www.insideracingtechnology.com/cosnascar.htm) from
Instead of the jewel-like intake and exhaust valves in the CART heads, NASCAR valves are twice the diameter as the CART valves and are more than twice the weight. This weight difference is critical because valve train mass is the single most important factor in getting the engine to live at elevated rpm. NASCAR does allow titanium valves, retainers, and keepers but instead of four small valves per cylinder controlled by a direct-acting camshaft in the CART engine, the NASCAR "motor" has to use the same configuration as in the Ford vehicles sold for street use-two valves per cylinder operated by rocker arms and long pushrods. This engine is very different from the purpose-designed race engines in the surrounding work stalls. This is the engine Cosworth has begun to develop for the NASCAR Busch Grand National series.
Notice that in this statement all the titanium is on the valve side?The long pushrods are made from steel because of flex, certainly not because it's light weight. Shortening the pushrod reduces flex but you have to move the cam up which effects geometry. Adding diameter to a long pushrod adds weight but increases stiffness. The stiffness benefit outweighs (figuratively) the weight gain. Basically removing weight from the valve side and adding it to the pushrod is a good thing.

Also:
SOHC engines have more parts than a typical pushrod engine even more still if it's DOHC with 4 valves.

Despite what magazines and the internet experts perpetuate, having pushrods doesn't make an engine low tech.
Didn't Penske run a Mercedes pushrod engine that dominated Indy so well that it was later legislated out of the running?

I've discussed these points with my friends in the magazine world and they admit that some GM insiders claim that Ford did a PR number on them to try and discredit the pushrod design and tout the Mod motor. They had a lot riding on it and they wanted it to succeed. GM stuck with pushrods because the data showed it to be superior in cost and performance. The magazine writer's lapped up Ford's "pushrods are ancient" propaganda. GM still gets grief over it even though a 3.8 Series III pushrod V-6 gets 30 mpg and makes over 200 hp.

Ford 2 valve OHC V-8's trail GM 2 valve push rod V-8's in both fuel economy and normally aspirated engine power. So once again advertising has influenced popular opinion despite the facts to the contrary.

To be fair, I might add that Ford has made gains with the 3 valve heads on the mod motors. This narrowed the gap with GM but they had to license the technology from Feuling. The additional parts added to the excessive cost and complexity of the engine. Costs more and more expensive to fix. To me this is not value added.

Just because an opinion is popular doesn't necessarily make it the truth, but that's only my opinion!

Very interesting Paul. What is your take on I beam vs. H beam rods? My engine builder prefers I beam rods (though I don't know why). He builds primarily race engines.

My point is the weight of the pushrod is not as important as you think.

For example:
A car handles better if the unsprung weight is very low and the chassis is stiff. Removing stiffness from the frame and rollcage saves weight but sacrifices handling.
By the same token a pushrod engine revs better if the valves are light and the pushrod is stiff. A higher revving engine makes more power than one with a lightweight but unstable valvetrain.

Paul, you are only reading half of what I wrote:
I still stand buy my statement that weight saving in the valvetrain will yield benefits as long as there is no loss of strength. Once you make the parts so light that they can hold their shape you are losing power.