threw another #3 rod, how to prevent this from happening again. Options

[crash914]

So blew up my #3 rod. wondering what caused this. lack of oil, due to rpm? poor return? oil pump sucking air?

last time it was #2 rod. I really don't want to live with this again.

Motor specs,
Dual oil coolers with full flow plumbing
102 bore by 80mm stroke
long rods,
ham heads and valve train.
Running to 6800 rpm
g loads to 1.5
windage tray modified at pushrod tubes for oil flow
tuna can with 1/2 extra oil

Pics of horror below. First are this failure, then old failure.

[Chi-town]

What kind of rod is that? Looks aftermarket?

Odd that it's snapped in half and the cap bolts are snapped

[HAM Inc]

Man that sucks!
I think the rod bearing seized on the crank. I see micro-welding on the journal. That happened before the rod broke.
Any warning? Weird vibration, change in engine tone? Anything?

Is the oil pressure stable in corners?

State of tune? Detonation can cause bearing failures. Bearing failures can break con rods.

[crash914]

I did get some warning (that I ignored) running hotter than normal, low oil pressure at idle, knock, etc.

I heard something happen part way through a run. but nothing visible outside. I knew better than to run it, but only 2 runs to finish the season....No detonation, running 100+ octane at 12:1 afr. this was a really fast motor. revved quick and put the power down. I figure close to 200hp and also 180ish torque.

[Superhawk996]

Not trying to be a jackass . . . but sometimes it is what I do.

And we're discussing how/why this happened?

It does suck but when playing at those power levels and racing, this sort of failure shouldn't be a surprise. Especially when combined with ignoring the warning signs.

You have my utmost sympathy though. This is an expensive failure. But . . . racing is an expensive hobby. Ask me how I know.



(IMG:style_emoticons/default/lol-2.gif)

[Superhawk996]

On a more constructive note.

Play with this centripedal force calculator.

https://www.engineersedge.com/physics/centrifugal_force.htm

In this specific case, RPM's have increased from 5500 rpm to 6800 rpm. RPM is directly related to the velocity term.

The force on the rod bearings is going up as the SQUARE of velocity. Said differently, the force increase isn't linearly proportional to the crank journal velocity, it is increasing exponentially.

Double the velocity term in the calculator and watch what happens to Force, it goes up by way more than double.

Bottom line, although the top end was likely beefed up to control the valve float, the stock lower end rod bearing journals are getting overloaded. This squeezed out the very thin oil film that separates the rod bearing from the journal and prevents metal on metal contact.

First part of this occurs as dramatically increased bearing material wear. Eventually it results in steel on steel contact and the micro-welding Lew mentioned.

If you are going to run this type of engine at these elevated power levels I'd recommend that you do professional oil analysis after every race. If you establish a baseline immediately after engine break in, you WILL be able to tell when the bearing material is being shredded off into the oil more rapidly than usual.

At that point, you need to stop running the engine and replace the bearings at predetermined intervals. Ultimately you will likely get a crank failure in the long term after some unknown number of bearing replacements.

[wndsrfr]

With the stroker crank, do a really thorough inspection/magnaflux of the crank, especially if it's a DPR or other built up then machined OE one. If it's a new forging it might have survived, but I'd be wary of a reworked OE. Had two DPR's fail on me due to the resulting closeness of the oil channel to the web after offset grinding. Went with FAT performance new forging crank after that.

[Chris914n6]

I can't think of a motor with nearly a 4" bore that turns more than 6k.

[falcor75]

Pushing an engine with low oil pressure and knocking sounds. Sorry but thats just dumb. As for what caused the failure I think Superhawk is onto a good lead. Sure a T4 can push 200 hp with some reliability but it will always need alot more attention and preventive maintenance than a less high strung engine. Changing the oil after every race with an oil analysis is a good start. Measuring is knowing. To me this isnt as much an engine failure as an maintenance/owner failure, the signs were there and you saw them and chose to ignore them.

[crash914]

I accept the responsibility for the failure. No need to run it in.
What I would like to generate is what to do to have a high rev engine live more than 20 hours? Bearing coating? Super secret windage tray mods? Better balance? Lots of folks build big bore motors, hopefully we can help them with longevity.

[mepstein]

Regarding bearings, I’m curious why 6800 is high revs on a 4 and normal for a six. Are the bearings loads or stresses different! I always thought valve train was more the limiting factor to revving a four.

[crash914]

Exactly what I thought also. That the valve train was the weak point. I did address that. Let the high revers chime in.

[johnhora]

Herb..

20 hours is pretty good for MTTF
were both engine failures at the 20 hr mark?
I see a sheared rod bolt...that is definitely a failure point on a high rev engine.

[jd74914]

I accept the responsibility for the failure. No need to run it in.
What I would like to generate is what to do to have a high rev engine live more than 20 hours? Bearing coating? Super secret windage tray mods? Better balance? Lots of folks build big bore motors, hopefully we can help them with longevity.

Dry sump with a few suction points...

It would be interesting to measure and log gallery oil pressures at high speed and see what happens in relation to lat./long. acceleration. As noted, there is likely a story on the bearing overload side which is related to oil pressure.

[crash914]

I will know more after the post. Engine is ready to drop. I should have it apart Saturday. Small thin bearing material in the sump. Correct on 1 sheared rod bolt. The other is ok, I haven't found the rod cap yet

[ClayPerrine]

I accept the responsibility for the failure. No need to run it in.
What I would like to generate is what to do to have a high rev engine live more than 20 hours? Bearing coating? Super secret windage tray mods? Better balance? Lots of folks build big bore motors, hopefully we can help them with longevity.

Use a 1.7L crank and bigger bore to increase the displacement. Shorter stroke/bigger bore allows more usable RPM with the same displacement.

For example... a 76 930 engine with a 70 911S counter-weighted crank and 906 cams gives you a motor with about 2.5L, but will rev to 10K. AS explained above, it lowers the stress on the rod bearings. It is a screamer of an engine, but not very streetable.

So get a 1.7L crank with counterweights, and bore it as big as possible. you can probably get 7K out of the motor.

I would suggest asking Jake Raby about this. He knows more about it than I do.

Clay

[sixnotfour]

76 930 engine with a 70 911S counter-weighted crank and 906 cams gives you a motor with about 2.5L, but will rev to 10K.

Clay that's a 66x95 2.8....76 911 66x90 2.5+...but get your point... (IMG:style_emoticons/default/beerchug.gif)

[ClayPerrine]

76 930 engine with a 70 911S counter-weighted crank and 906 cams gives you a motor with about 2.5L, but will rev to 10K.

Clay that's a 66x95 2.8....76 911 66x90 2.5+...but get your point... (IMG:style_emoticons/default/beerchug.gif)

HEY!! Give me a break. I am doing this from memory.

[sixnotfour]

All I Know is; Type Four Sale (IMG:style_emoticons/default/idea.gif) (IMG:style_emoticons/default/lol-2.gif) (IMG:style_emoticons/default/driving.gif)

[HAM Inc]

I did get some warning (that I ignored) running hotter than normal, low oil pressure at idle, knock, etc.

I heard something happen part way through a run. but nothing visible outside. I knew better than to run it, but only 2 runs to finish the season....No detonation, running 100+ octane at 12:1 afr. this was a really fast motor. revved quick and put the power down. I figure close to 200hp and also 180ish torque.

Herb, you can't assume that because you have a fat AFR that you aren't experiencing detonation. Indeed, because a fat tune takes extra time to react completely in the combustion chamber it's common to have elevated temps around the exhaust valve and higher egt's since the fuel burned at the end of the cycle does so around the valve. That concentrated heat can definitely contribute to detonation.

You said it was running hotter than normal. CHT's, EGT"S, oil. . . what was running hotter?

You may need an accusump. Your rod failures might be a combo of state of tune and inadequate oiling. Could just be oiling alone.