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Discussion and questions related to the course Engine Building Fundamentals
Not sure if this is the correct place or tuning section but, I have had two rb26 motors on my dyno which were built somewhere else and have been driven to the dyno that both spun main bearings. My conundrum is if its my fault or just my luck. One happened last year and one this passed week. Not sure what could be causing it. Is there a reason why main bearing go? The last motor ran fine with no knocks but the low pressure at idle after 3 pulls made me worried so we took it off the dyno. Oem calls for 22psi at idle and I saw 12 psi so I got concerned, took the filter apart and its covered in copper shavings, yet motor didn't knock and cold start was at 74 psi. Taking the motor apart now. But I want to avoid it again. I just would like to know what can cause a main bearing failure. Thanks
Probably too much bearing clearance, but it could have been an improperly cleaned block (or crank) that clogged an oil gallery feeding the bearing, or a bad line bore, bent crank.
Do you get the oil up to temp (say 200 degF) before loading the engine at WOT? Not sure on those engines specifically, but with the dry-sumped race engines I work on, 5-10 psi at idle with 220 deg oil is typical. Needs to be 30psi at 4000 RPM though.
If you can't maintain pressure as the oil temp climbs, using a higher viscosity oil is the first thing to try.
I don't have a oil temp gauge in the car, but the car had a tune in it, I had to erase it as it was locked. I did two pulls on old tune and shut her down. had an appointment took her off the dyno. Put her back on, erased the tune, started from scratch, did all the steady state and then started to do wot wastegate pulls, so I would say the oil was up to temp.
Nissan calls for 22psi at idle and when I saw 10-12psi I knew something was off. The engine was rebuilt a year ago but some other shop. The customer came to us to do flex fuel and retune a hesitation.
The previous shops tune wasn't bad at WOT but its OFF idle throttle and cold drivability sucked. Plus they tuned the High Z injectors with OEM resistor box still attached.
In my opinion a bearing issue is seldom tuning related. A spun bearing is either the result of insufficient crush on the bearing shells (unlikely), or a severely damaged bearing that has resulted in so much of the bearing being worn away and so much heat being generated that the shells spin in the housing. Neither of these are easy to generate via the tune. It is possible to damage a big end bearing through too much timing which creates knock, however in my experience this will almost always show up on the piston crown before you see it in the bearing shell.
in short I'd be very surprised if this was your fault, particularly given the RB engine's propensity for lubrication problems. Did the engine have a restrictor in the oil feed to the head or an external drain back from the head? Without these, sustained high rpm operation can literally pump all the oil into the head.
In the most recent one, have you actually confirmed it is a failed main as your comment "taking the motor apart now" makes it sound like like it is not confirmed yet?
The first thing I would be checking is the crank straightness as RB's are pretty bad for bent cranks, that will take out mains pretty quick.