All I gotta say is...

Rod bolts do break!!!

http://www.socalmuscle.com/~mleaf/en...d/DSCN4840.JPG

This was a 383 that was spinning at about 6100 when it let go...

 

A 9500+ Cup engine has about 5000 gs on the piston at TDC reversal, while BMW quoted their 19,200 rpm 2003 F1 engine at 10,000 gs. Since the F1 rpm is about twice the Cup engine, and the strokes are about a 2:1 ratio, piston speeds, both average and max are similar, but the gs are double for the F1. Witha bore about the same as an LS1, the pistons have to be VERY light to keep the rod tension loads within reason. There was a "lively" discussion of this here a while ago.

I cheat and use an engine simulator to calculate piston gs. .

The worst condition for rod bolts is lifting completely with the engine at max rpm, like at the end of a straightaway in circle track. Under power the combustion reduces the tension load for 1/2 of the revs, but with closed throttle and high manifold vacuum and practically no combustion, every rev is a max tension load on the rod/bolts. If you visit short tracks enough you'll see an engine let go at the end of a straight. It's not fun running over your crank while trying to turn in.

 

Hmm, spend a few hours with Roark calculating stress/strain to come up with a rod bolt that just does the job or go to something that might be considered overkill?

I'll take 8 of those marginally adequate rod bolts please!

That's craziness....

Then you are doing something wrong. Please show work.

7000rpm, ~1.7 r/s ratio, max piston velocity should be in the neighborhood of 6600 fpm... tensile loads ~8200 lbs, compressive ~4500 with a 1200 gm bobweight.

I have a spreadsheet you can have if you're interested Jon. From there you can track through the formulas.

-Mindgame

 

Eight? Are you building some sort of killer four-banger, MG?

 

Alright heres what I got:

first off.
7/16" rod bolt=.6013in^2 cross sectional area
using a 1150g rod/piston/pin/ring combo = ~2.53lbs

assuming a max piston acceleration at 7000rpms of 119385ft/sec^2
that becomes 3699g's for the force equation

F=MA> 2.56x3699 = 9472lbs of force

9472lbs/.6013sq/in = 15,786psi ?????

But like I said earlier, it doesnt sound like this accounts for the pressure involved in the rod first stopping from the say 2500fpm to 0, then back to 2500fpm in a matter of 3* crankshaft revolution.

I also am not saying I would not be one to overkill, I love overkill, my 381 was all forged with L19 bolts, splayed billet caps, everything, all for a sub 7k redline N/A LT1 casting engine, totally pointless, but I knew it wasnt going to break.

But if say you could get this down close to a science, I mean you have to be able to figure it out, how did GM decide what kind of rod bolts they needed in the LSx engines and so on? I doubt they just bought the best most expensive bolts they could so they wouldnt break, they probably designed them to work right up to and a little after the levels the engine would run at....then if you were say an engine builder, you could offer better engine combinations to customers for reduced prices, and this doesnt go just for rod bolts either, anything metal in the engine could be calculated to how much it will handle I would assume.

"Every engineering material is rubber" , "all solid materials behave like springs"-sir henry royce.

And it seems that the properties of all metals are known, and if the stresses applied to those pieces is roughly known, then you could more accurately pick a part. Obviously if you have unlimited budget then the strongest metal is best since you can make it so much lighter in return for the strength it gives. Like the crankshaft that baffled my mind in the BMR project SB2 engine, an LA Billet Kryptonite crank....28lbs!!!!! I dont even know if thats the best, but thats insane...assuming a good lightweight forged unit is ~43lbsish.

Anyway, rambling now....am I way off base here, have I gotten too excited about something for nothing or am i on to anything here? I also tried to apply this to a rod, but im sure the design/shape of the piece affects things in this situation.

jon

 

Jon: I am sure that there are very sophisticated ways to model this. In the end, they still do testing though! Sims only go so far.

Rich

 

Just to keep you thinking, jon...

Jon

 

From ARP's web site.

Examining the “Over-Kill” Fallacy
If there’s one thing I’ve heard over and over from visitors to trade shows and races it’s,“Your fasteners are great. I’m not having any problems but I’m being told, by your competitors, that ARP® is over-kill and therefore I’m wasting some money when I buy ARP® pan bolts, manifold bolts or just about everything except for certain critical engine, drive train or suspension fasteners.” My first instinct is to say they are full of _ _ _ _.

But the subject is worth talking about. Cost is an important consideration when you choose a particular vendor’s offering. Still, if you use lesser quality fasteners and they were not subject to many assembly and disassembly cycles, by people with varying skills from professional to rank amateur. Maybe, just maybe, you could make a case for minimum grade fasteners that are over designed, size-wise, to allow a reasonably safe application for conservative usage.

Now, lets get back in our world. The real world. We can expect the engines and vehicles to be leaned on, from a little to beyond any sensible extreme. We can expect 10 or more assembly/disassembly cycles. We can expect over-torquing, which will leave the fastener looking 100%—but actually in a condition RED, semi-failed mode. We can expect some fasteners that are minimal in quality to end up in a critical, high stress area. We can’t expect everyone to be able to look at a fastener and determine its quality—by looks, or even by markings.

So we leave ourselves wide open for expensive and possibly dangerous results. For the amount of money saved by “type rating” every fastener’s capability, and consideration of a long range view of the best mix of customers—I recommend all fasteners be of a quality that does exceed the minimum standards.

 

Jon..Why would I have divided it by 4? The CSA of a cylinder (IE rod bolt) is nothing more than the area of the cirlce created by chopping the bolt in half correct? If so then A=pi.4375^2 = .6013, I looked but cant understand why that wouldnt be correct?

And I didnt even realize I had to do the conversion between mass and weight. To do that I would simply multiply it by the acceleration of gravity (~32fps^2)correct?

And is there a more detailed chart of values of "E" for selected materials than the one given in the Auto Math Handbook online somewhere? Like for specific grade materials? Also it would probably include, but would want the typical strengths for different grade materials as well.

Thanks for putting up with my crazy head guys, I think Im doing all this more to keep me busy than anything, talking about brake systems in class this week.... Now I just wish I could get my TI83 that I painted back in highschool with permanent marker working again so I dont have to use these sin/cos/tan tables to do trig on paper

jon

 

This I know is true Rich, but I think more or less it applies more to the typical at home garage wrencher that doesnt have either the resources or (dont like to say this) but knowledge to pick the CORRECT part, instead of what will DEFINITELY work. I am really starting to backfire on myself and realize that this discussion should apply more towards things like rods and wrist pins, since things like that you can actually reduce the strength therefore reducing weight and actually gaining something if you can do it safely. A rod bolt will always be the same size for the most part, no matter the strength, and weights do not change all that much, so choosing the LEAST strength needed bolt will not really gain you anything in the end, except more $.

Again Rich, I dont want you to take this like Im bashing you, or anyone else for that matter, since like I said I am a victim of overkill as well, just thinking out loud, and my thinking needs help from you guys which is why i posted.
jon

 

i don't know if i'm reading your post right, but you seem to be blurring racing applications with street apps.
the quote rich posted was talking about the bolts being used by average joes of different skill. if you are talking a custom application just for you specific engine, then i'm sure all sort of oddities come into play like depth and spacing of the threads. the correct part is what works, now if you wanna push it to its limits, you can do as they said earlier in the post and rebuild it ever few thousand miles. i kinda of like the way rich put it about his car with one type of rods, and his race car with another type of rods. kinda like fork in the road, you can only go one way.

 

Where the hell do you guys learn all this stuff

 

magazines, articles, talking to folks, etc.
its funny cause the more things i learn, the more things i find i don't know.

 

If you don't get the basics correct, everything you derive from a bad start is bad. I'm not bashing you, but don't get ahead of yourself. Go back to the basics.

 

When some typical 8740 rod bolt is stretched to spec,
what is the preload {aka clamping force} ?