We've lost 2 engines due to #2 rod bearing failures this past year.

We're doing just about all the known preventative methods.

Wondering if using a ferrous detection system similar to what
Flat6engineering is using for the IMS Guardian used for Boxsters/996s

We do endurance races, and if we could detect the bearing failing and shut the car down before loosing the whole engine -- that would be phenomenal!

thx,

mike

 

I cant say exactly how long to failure but I know that in 200 hours these are likely to need rebuild. Some say 100 hours of use like yours. And could be even less of course.

One thing is the installation and getting the correct clearance(s) could have an impact. Sustained runs like in endurance could exacerbate any problem quickly. Might be sure there are no other starvation factors occurring too although those would generally be further up like on the crank etc

Additionally you might look in to using coated bearings as we use in other high stress applications. When my 951 was re-done, I had the rod bearings polydyn coated and although it is a street car and a sometime track car it is ginning like a top.

 

the defect is the bearings. too soft. way too soft.

 

Rod bearings last 30 seconds at most... Then boom.

Seriously... once you lose the oil flim and can no longer support the rod over the crank you have crank to rod contact. At speed the bearing can only last seconds like that. What happens is force of the rod against the crank causes the bearing grap and very shortly spin. Once it spins it develops copper from the bearing which can burn up. More heat cause the journal to expand and then sieze on rod. Then boom.... bottom end blows out. I had one go on my and from the time I felt the engine drag to point where rod contacted crank the motor lasted 5 seconds more. I was exiting a slow corner in upper 2nd gear (45 mph). Felt it drag a second and then freed up so I kept going. The drag was the contact and free up was the bearing spinning. I made to 85 mph before it siezed and blew a part in cloud of smoke.

I had another one go on me, but this one I caught it. I felt the drag and then rather then doubting my feeling put the clutch in and shut it down. Bearing spun on that one and I bent the crank, but the block was intact.


Now the reason people replace rod bearings is that bearings do wear over time. Both main and rod wear over time from start up and this leads ever increasing gaps bettween the journal and the bearing. Larger gaps need more oil flow to maintain the hydrodynamic effect. So old rod and mains need more oil. Fresh bearings then need less oil. Also remember that oil is feed from the main bearing journals on the crank and the excess is feed through crank to the rod jounrnals. So bigger main bearing clearances mean less oil goes to the rod journals to start with.


The best way to ensure you don't have a failure is to zero time the bearings by replacing them. Never trust orginal bearings in a race motor. It is a time bomb.

Then you need to ensure you are using a x-50 or at minimum a x-40 wt oil. 30wt oil get thin at high temps are a due to their thin nature you need even more oil flow to support the rod loading.

Always run the car at the full mark on the dipstick, but not overfull. These motor can burn oil and lots at 5000+ rpm. If you don't check you can be a qt low at speed at the end of a day. Low oil level can expose the pick-up tube to air in high G cornering. Full is better. However overfull can lead to excessive oil frothing from oil getting whipped around be the crank.

If you pull the pan a trap door baffle is great idea to reduce the risk of oil moving away from the pick up tube. In this system every little bit of prevention helps.

Cross drill the crank. There many ways to cross drill the crank, but every one seems to improve the situation by creating more flow at rod journals. Main bearing journals never fail because they get more than enough flow. The rod aways fail first. Mostly the number 2 and 3 journals and some, me included, have run with cross drilling in just 2 and 3 to counter balance this.

Run an external oil cooler. NA cars use a water bath oil cooler. Works fine, but in racing conditiion heat develops and the system could benefit from either 944 turbo oil cooler or any other additional cooling system. Hot oil lowers viscosity at the journal and that cause a need for more oil flow to support the rods.


Few other things.
HP and RPM. More HP will put more load on rod and more RPM puts alot more heat into the joint due friction of oil shearing. So the more power and more RPM you run the geater you oil flow needs will be.

On the maintence side. I have seen oil pick-up tube crack. The time my motor blew up it was cause by broken oil pickup tube. Only after I studied my incar video did I realize that for the 2 laps before my oil pressure was dropping to zero in hard braking zones. Then would come back up under acceleration. I never noticed this change as I generally did not watch the gauges in braking zones, but did watch on the straights. I believe that my the tube finally failed and oil starved the motor. The number 2 went due to it haveing the lowest margin on oil supply. I believe the pick-up tube failed due to not having my balance shafts timed properly.

So check the pickup tube when you do rod bearings. My 2nd failire I believe was from too much sealant from when I installed the crank girddle. I believe some got picked up and disrupted the oil flow. Not 100% sure but that is what think it was.


Point is any thing that might reduce or disrput the oil flow can incease the risk of a bearing failure. I have found that there is no cure, but doing all that I listed helps alot.

I do the following
Cross drilled crank, replace rod bearings every 2 years. Main bearings fresh on rebuild, trap door baffled oil pan, turbo oil cooler, check oil before each session, run 15w50 AMSOIL Dominator racing oil changed every 3 events ( have seen mobil 1 lose alot of pressure when hot), Stock rev limit.

 

Update motor

 

Always stirring up trouble, huh X......

 

I knew some real knowledge would get on this - thanks Joe.

Have you or do you use the coated bearings BTW?

 

No.. my goals is to maintian the oil film and never had contact at all. Contact should only occur on initial start up. Once you have oil pressure the journals for the mains and rods should all be floating. I just pulled rod bearings on motor was rebuilding. I guess there was 2-3 years of racing on these rod beaings. They looked new. I changed them since the motor was out of the car and pulling the pan and changing the bearings just makes sense with the engine in a stand.

Anyway what is the coating going to to? Make the life 60 seconds instead of 30? If you feel a drag on the motor and hear deep taping shut it down ASAP and pray you did not damage too much. I toasted a 47k mile block because I doubted my feelings. Since the motor "freeded up" I thought it was fine. Then bang.

So again... focus on keeping a supply of fresh clean cool oil to the number 2 and number 3 rod journals. Do that any you are fine. If the flow is restructed for any reason the one day you may not longer be able to float the bearing and BANG....

Most of the time bearings fail on car with old main and rod bearings, thin oil, low oil level, or broken pick-up tubes. Any one of those could cause a failure so best to deal with all of them. 80% of the time a bearing fails I will start asking about these issues and often find it was something that could have be prevented.

There was a guy who built a car and spun the bearing 40 mintues. Car was great in that it was fully preppred.... Then started asking questions... What oil were you running ... Answer 5-30 ( Strike 1). Where did you get the motor from? Answer friend's spare. Did you change the rod bearings like he suggested? Answer No (Strike 2) ... Never made it to strike 3... did not need to.

 

Here's some more input, some repetitive...

My 'race prep' for the 944 cranks includes checking for straight (.001 runout max.), cross-drilling #2 and #3, properly chamfering the holes, balancing and polishing. Then it gets measured so the oil clearance (critical!) can be set. The rod big ends are also checked for round, the ID sized for the oil clearance (varies with application), and each rod checked for length and twist. After corrections the rods are weighed and balanced.

The above processes are critical to maximize rod bearing life. Keep in mind the rod bearing is a street design that is being asked to survive a severe performance application. Ignoring proper preparation is an expensive mistake.

Here's a mod that's worked out great for the next step: Cut the rod journals to Chevrolet size (2") and use NASCAR type performance rod bearings. These bearings are designed (dimensions and construction) for racing and are light years ahead of the Glyco street bearing. A custom rod is required.

Of course dry sump eliminates the oil supply problems and even adds power due to windage and crankcase vacuum control. It's the best solution (along with the prep above), but also has to be done properly.

Hope this helps,
MM

 

How about lightening and balancing? The stock crankshaft being out of balance only helps squeeze out oil on the bearing surfaces at high rpm's. Or so I've read on here :P

 

Michael, do you think oil temp plays a factor? Also, what are your thought on an Accusump?

(I ask, because I'm in the process of both upgrading my oil cooler and installing an Accusump...)

 

Thanks for the great info!

Well guys -- these are ChumpCars -- so you have to keep that in mind :-)

Oil temp is a factor -- with a small 6x9x6" external cooler and a accusump
on 90+ degree weather the oil temp was pegged at 280+.

The first motor had new rod bearings and new main bearings, and a lyndsey baffle.
Probably had 40 hours on the engine when the rod bearing seized.
We got lucky that time and only lost the #2 rod and crank shaft.

We did a complete rebuild. all new bearings, rings, etc. Added the accusump and oil cooler.
Did a DE and an 18 hour race. Had 4.5 bar through out the race. Change oil after every race.
Second race we made it 12 of 14 hours. (double 7) before the #2 went through the block.

We run 20w/50 or straight 50. I think at the time the last one went we were running VR1.

I wasn't driving either time, so I can't say how it felt or sounded.

This time around we're adding crank scrapers and one of the two cars will have a cross-drilled crank.
I'm considering adding a big *** oil cooler too.

We're probably going to preventively replace the #2 rod bearings after every other race, maybe after every race???

The #2 on the last was burnt to a crisp -- while the other bearings looked pristine.

thanks for your suggestions -- hopefully we don't have any repeat events....
starts to get painful from a time and wallet point of view.

I like the idea of the Nascar -- maybe we'll try that with the back up motor we're building out.
But then again -- I'm sorta with the opinion best to never have bearing contact.

cheers,

Mike

 

Mike,
Had the 'pristine' rod bearings from your last engine lost their 'spring'? Did they fall out of the rod caps or rods when you disassembled the engine?
MM

 

Van,

Yes, I think OT plays a major factor. You can run a small cooler, but it has to be efficient. A proper air inlet and a proper outlet is required. Also make sure oil enters the bottom of the cooler and exits the top.

You might also consider an oil/water heat exchanger. I ran one for years with great results. Other benefits: no ducting required, warms the oil quicker.

I'm not a fan of the Accusump. I feel its only benefit is being able to pressurize the engine with oil before your start it. That's a good thing. The negatives: It won't respond quick enough to save a rod bearing if the pick-up gulps some air, and you have to carry the additional weight of the system.

 

Good info here. Bearing failure is a major drawback in developing a 944 track car. If I may add to the discussion; a couple of minor details that may be helpful.
I think the preventive replacement is key. If a bearing fails the damage is done, as I suspect the rod suffers. Catching it in time is not happening as the bearing is not simply sacrificed, but the rod is hurt.
Also, as M758 points out, good mains help hold it all together. Crank thrust may be a fair measure of bottom-end integrity and can be referenced while everything is all together with a dial gauge on the crank pulley. Go easy on your thrust bearing by keeping your foot off the clutch pedal...especially at start up.