Anyone have failure analysis data on oem miata rods?
 

Just curious, because Im wondering what the theoretical limits of operating cylinder pressures are.

Assuming 100% knock resistance.

You say "analysis data" and then you say "theoretical limits".
So, which is it?

Well, it has to be both, since rod failure data is tested by itself. Then we can calculate what cylinder pressures are necessary to match that failure, so its still a theoretical limit.

Or maybe not, you always make me second guess myself, but I love you for it!

Arent we already satisfied with using torque as an analog for cylinder pressure?

What data, exactly, do you want? Except for a very few of us, all we have is personal anecdotes, however well documented.

On that note, from personal experience (the car Laz) - 230 ft/lbs in a heavily tracked environment bends stock rods. I have in my posession said bent rod(s). They will eventually become tap handles.

Now, one might argue that in any engine environment, especially a turbo engine environment, ft/lbs produced of torque is not 100% reliable indicator of cylinder pressures and of force applied to said rod, and one would be correct.

So, 230 ft/lbs when run through stock MSM manifold through stock IHI turbo with upgraded compressor wheel equals ... bent rods.

Greddygalant or Curly can chime in on exhaust sizing, but, really, the manifold and the turbo itself were the major contributing factors to back pressure IMO.

Agreed with Mobius, the real answer is "depends".
I've tuned many many miata's at this point, and seen everything from 230wtq bending rods to 270wtq stock rods holding up for over a year with non-stop abuse.

There are eleventy billion variables. In no particular order:
How early does the car make the torque? How does it come on? How is the car used? How much backpressure/restriction is it working with? etc etc etc there's way too much to list.

The latest car I tuned that bent stock rods is sshamrock. No det, and I don't recall if they bent or snapped, but all 4 were banana'd. 250whp and 230wtq on a mustang dyno if I recall (you can look it up, you even posted in that thread). This was a few months ago.

So we have our (based on my 10 year history here) mt.net axioms: (all based on man usage, track environments)

5 speed dies before usable power is achieved, except for sonofthehill
6 speed is good for 230-250 wtq road race environment. At 300wtq w/hoosiers, gear destruction in almost any gear is virtually guaranteed (see bbundy posts, use google fu)
junkyard vvt 10:1 compression engine is good for lots of turbo race time, at 200whp, if boost is controlled (see rover gets a turbo)

and in general, keep wtq < 200, stock rods survive. And the transmission will survive.

Marcello, what's the goal here? I don't think anybody on this forum has recorded actual cylinder pressures. I don't know how that's possible outside of an OEM environment.

If you want a record of "this rod bent at this wheel torque, given these parameters and usage" then that's a reasonable goal.

I do a shitload of conversions and I tune them to 210-220 ftlb on a dyno dynamics on shootout, not strapped, chocks only. I have done 100+ and have only a couple of failures in total. I use an 0.86 rear and 3" exhaust on everything and the dyno sheet always has a graph you can lay a straightedge against.

Dann

No, torque doesn't bend rods, cylinder pressure bends rods. You could probably bend rods on a stock motor if the tune was bad enough and detonated enough to raise cylinder pressures too high and buckle the factory rods.

I am just curious of there is someone out there that did fatigue/failure testing on oem rods. They do it for aftermarket rods, all you'd have to do is put a stock rod in the machine.

Im doing a rods only rebuild anyways, so its pure curiousity.

I agree, but outside of detonation, if we are all running basically the same engine (a stock BPw4/vvt) then torque will represent cylinder pressure. No? You're one of the smartest bastards on here so rip me a new one and learn me.

Yes, but I think detonation is much more prevalent than we know of in our stock motor turbo builds, so our torque numbers arent cylinder pressures.

Thats one of the only explanations I can think of as to why an abused engine lasts at 250wtq, and a babied engine can snap rods at 200wtq.

Nope. I can go run 20 PSI boost on my car now with a soft tune and make say, 350tq. Then drop the boost to 17 PSI, and adjust the tune to be more aggressive and hit the same 350tq. Same torque, same average cylinder pressure (more or less, ignoring crank angle and whatnot). But peak peak cylinder pressure will be lower on the soft tune running 20 PSI than the more aggressive tune running 17 PSI, even at the same torque level. More boost is safer at the same tq if the turbo hardware is up to the task (backpressure is in check, compressor is happy operating at that pressure, etc). Also the softer tune is less likely to detonate, and detonation spikes cylinder pressure, raising peak cylinder pressures which breaks parts.

If goal is to maximize reliability of a given part under load, minimizing peak loading will improve life.

Because boost increases the duration of cylinder pressure as well as increase it?

More boost instead of timing = lower peak pressure but the same average?

Dann

If anyone has exhaust manifold pressure data and their spark and fuel maps, that bent stock rods we can calculate cylinder pressure. Failing the emap data an accurate turbine map from the maker of the turbo would suffice.

In general if you're in it for big power numbers on stock rods you want to run a big lazy turbo with a ratio of map to emap well over 1, run conservative timing, use e85 and run leaner.

If you really wanted to figure this out, do a simple buckling calculation on the rod. Nobody tests rods for buckling failure, even OEMs.

In simple terms, power is directly proportional to average cylinder pressure. More boost means more power, and higher average cylinder pressure. Double the power, you have doubled the average cylinder pressure more or less.

Bending stock rods is directly proportional to peak cylinder pressure. Peak and average are not the same. Leafy described it pretty well on how to keep a stock motor alive. I should drop a stock motor in my car and see what I can get out of it. Just need a VVT motor that's stock and ready for a beating!

Find me 2 customers in Houston. I have a stock VVT motor with sub 100k on it sitting in a shell in my garage. I got to have more then just an engine to deliver to make the trip.

The crazy part is that Pat will have that engine in and making power in less than a weekend. Then purposely exploded by Sunday, just in time for dinner.