K24 motor failure

Hey guys, looking for some extra input. I built this NA K24 motor for a customer. It had H-beam rods with the standard arp 2000 rod bolts. The person who tuned the car set the rev limiter at 8700. The car has been doing amazing and even taking first place in events. Car recently just threw a rod through the block.. bearing clearance was between.0025-.0030 with VR1 20w50 oil. The rod bolts let go.. So I’m leaning towards to much rpm. It’s a full time track car so it doesn’t see street driving. I’ve attached pictures the customer sent of the bearings and failure. Let me know what you guys think

8700 RPM seems to me a bit too high limit for ARP 2000 bolts with 99 mm stroke. I'm reving my 100 mm stroke engine 8600 RPM maximum but I upgraded bolts to 625+ and changed pistons to shorter ones for reducing the weight of reciprocating parts.

For some reason all the pictures didn’t load. I’ve updated the post with more pictures of the bearings!

It does look like one bolt failed, or loosened, and this allowed the cap to be forced open against the remaining bolt, which ultimately failed in bending.

If you check on-line, there are various guides to the failure process in fasteners, like bolts, and by examining the remains of the other broken(?) bolt it should be clear if it did actually break.

TBH, I'd be surprised if that bolt "just" failed, I'd suspect a "money shift", AKA a "high score" over-rev' from a down-shift instead of up-shift.

Also, any chance the owner "adjusted" the limiter?

I doubt it's the case, but these ngines are known to have oil pump gear failures at sustained high rpm, so it'd pay to check that while your stripping the engine.

Are you using an ECU or logger that records a max engine speed tell tale value?

Determining whether a failure occurred within planned operational limits or not really helps choose next steps.

If the driver made an error and spun it to 10,000+ RPM, then you may simply be seeing the result of a one time error.

If you know the engine never went past 9000, then it's more likely changes are needed beyond making repairs.

I would also question the oil weight. 20w50 at high RPM may not be allowing sufficient flow due to tons of oil bypass at the pressure relief.

It’s on a circle track car so it lives in 3rd gear. I chose a 50w oil because of the .0025-.003 bearing clearance. There was another k24 that also threw a rod at the track. Except that was a 4p engine. Both tuned by the same person. Apparently that one was spinning past 9. I can go to the car and pull a log and see what I see

Gotcha.

Are you logging oil pressure to see if you have any drops or signs of aeration?

What are your oil temps?

Are you monitoring for detonation?

The oil choice may not have been a factor, but have you verified oil pressure is inadequate with 10w40 for example, while maintaining reasonable oil temps with a cooler? The K series I've tuned with that range of clearance just beyond stock spec, are usually on the oil pressure relief by 3000 RPM with 10w40 oil.

Oil pressure is not currently being logged. I recommended to setup an oil pressure sensor so we can monitor it through kpro. Oil temps are unknown, but it does have an oil cooler. I’ll have to recommend to get an oil temp sensor also.

Spark plugs looked good with no signs of detonation last time I checked.

I haven’t tried different oil weight yet. What would be the most beneficial way you would recommend to see what weight oil would work the best? This is my first circle track motor build that lives at high rpm its whole life. I’ve been very successful with turbo drag/street cars. Over the last few years. Of course there’s always room for improvement!

You may find this article of interest - http://www.substech.com/dokuwiki/doku.php?id=effect_of_oil_viscosity_on_hydrodynamic_friction_of_engine_bearings

With high(er) rpm engines something that's sometimes overlooked is that there also needs to be suffient oil flow through the bearing in order to carry away the heat caused by the oil's viscous friction. That is represented by the "power loss" graphs, and can be significant. Sometimes this can be seen as discolouration of the bearing, and the deposits around it, while the bearing surfaces still show no sign of damage, or contact - yet.

With the higher rpm, and higher viscosity oils, you will need to run higher oil pressure to counter the centrifugal resistance of the mains', and to ensure enough oil flow, and the higher the oil temperature, the less lee-way available - this is one of the advantages of synthetics, they break-down at much higher temp's than mineral oils.

The trick is knowing where the balance of viscosity, clearance, and rpm lies - and this is one of the reasons professional race teams, and their lubricant specialists and suppliers, spend a lot of time on dyno's, testing.

Gotcha, at minimum logging oil pressure, and being able to watch oil temp on a gauge, would be a start. Oil temp has a big impact on oil pressure, so you really have to know both.

For example if your oil pressure drops 20 psi when comparing data between two heats, it may be purely from oil temp being hotter, or oil temp may be lower/the same in which case you have a mechanical fault to immediately diagnose.

Oil selection CAN get very in depth, but you can also perform a very basic test once you have oil pressure monitoring. Run the car on the 20w50 you've been using for a short stint. You'll likely find oil pressure is at relief pressure of 80 psi the whole time on track. If so, I'd try a 10w40 and see if you still have plenty of oil pressure.

If you're making 80 psi from 5000 RPM up on the 20w50 and the 10w40 at similar oil temperature, you actually have more oil flow doing more cooling work with the 10w40 oil because less oil flow is being dumped out the pressure relief to create that same 80ish psi.

While people often think of water/coolant as cooling the engine, the oil is not just lubricating items. The oil is transferring heat away from and cooling your pistons, bearings, cams and valvetrain.

Oh, one very important thing I forgot to mention - the V-TEC used on these engines is very sensitive to the oil viscosity.

I don't know it that contributed to the issue hour client had - unlikely, though - but it should be born in mind.

Brandon if you attach the onboard log from the race it had the failure, perhaps we'll spot something helpful.

I’ll try and see if I can snag a log this weekend! Unfortunately oil pressure or oil temp isn’t logged. We should at least see all the basics! The motor is also getting dropped off this weekend so I can do a deeper inspection

Sounds good!

Here's the log when the motor let go! I got the motor here now also. Oil pan is pulled and everything still looks brand new. No discoloration anywhere. Ill send more pictures from what I see at my point. Ill Tear into it tomorrow Night and update some more!

Here’s the Rod bolts, & Cap! Very interesting how one of the rod bolts broke. I haven’t seen that before.

It looks as I thought - one bolt snapped and the other was broken by the cap 'levering' on it and bending it - you can see the cap is slightly splayed and the latter bolt bent. Might be wrong but confident that's what happened. The question is why?

Can you get a shot of the remaining, threaded sections in the 'rod - if it's clear it will give a lot more information on the failure mode, as the bolt end is rather smashed up.

I’ll be able to get some better pictures when I start my disassembly tonight! I did manage to get some decent pictures through the window real quick though

I once red statistics saying that overreving is the second overall reason for engine failure after detonation...

I guess some more info to the story, the car just recently switched to a shorter 3rd gear ratio. Before they said the max rpm was 7700. It would fall out of vtec around corners though. Car lives in 3rd gear. It’s been taking first place all season and is championship leader currently. Once they switched the 3rd gear they were seeing anywhere from 8500-8700. I’ve attached the log when the car blew up above. I believe it lasted 1 race, then the next race it blew during the heat race.

8771 is the peak RPM in the log, in the section you sent average RPM is 7179, so it sounds like average RPM was probably 1000 higher than before as well.

I also noticed your battery voltage is 10.5-11.5 volts. With very low voltage the injectors and coils do not work well and the engine is likely running poorly in general because of it. Is that new or did the tuner warn you about it during tuning? I'd check logs from tuning to see how long that issue has been present.

It looks like the engine was tuned to run 28 degrees of timing any time you're over 1/4 throttle regardless of RPM or load. I understand some old circle track V8 stuff used to run fixed timing, but with EFI you generally want to optimize ignition timing as engine speed varies.'

Knock isn't being monitored. I would verify whether knock is being accounted for. What fuel are you running?

Lambda isn't what I've found makes power on those engines, but isn't a reliability concern.

I'd get vehicle speed working, and it looks like air temp is a bit warm in case there's an opportunity to source cooler intake air for the engine, but I don't believe this is related to the engine issue.

Shota mentioned high RPM and knock are the two big causes of this and from the log it's possible you had both.

The battery voltage doesn’t pick up the alternator in kpro for some reason. The charging system is working properly though. There’s a couple cars out on the track that has this same issue. They all have aftermarket wiring harnesses. Whenever I tune a k series car I leave the knock sensor enabled.

Car didn’t have a baffled oil pan, and it only makes left turns at the track. This could be another hint of potential cause of the failure. I would imagine #4 rod would have let loose instead of #2 if it was an oiling issue. I don’t have a ATI crank pulley adapter to get the pulley off currently so I’m kinda stuck on getting it taken apart. So far I’m leaning towards to much rpm for the arp 2000 bolt. Car runs on ms109 fuel.

k24a2, 12.5:1 forged pistons, sanez rods, Dc 3.2 cams. We’re upgrading to 625 rod bolts, and getting a 3 new sensors to monitor fuel pressure, oil pressure, & oil temp

I would still consider lowering down RPM limit below 8700... At that rate of revolutions the mean piston speed with 99 mm stroke is 28.7 meters per second which is too much even for performance parts whilst F1 and most of NASCAR cars (except the most powerful ones) don't exceed 25-26 meters per second. Going much above that number requires very strong valve train parts and very good quality engine oil.

I completely agree. After this setup is back together I’m gonna be the one tuning it this time. Sounds like he’s gonna switch back to the lower 3rd gear ratio. I understand the higher rpms put it in the critical wear state. Any parts you guys recommend changing out so we could potentially gain a bit more power? Current setup is a k24, 87.5 bore, 12.5:1 std forged pistons. 4 piston oil pump, stock valves, aftermarket springs & retainers, dc 3.2 cams. 74mm intake & TB. It made 258 on a hub dyno.

What is valve seat throat diameter- do you know?