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Old 10-28-2019, 09:11 AM
hberwald hberwald is offline
 
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Default Type IV rod bearing failure

Anyone have thoughts on why the #3 rod bearing would fail on a type 4 race motor?

This is a high RPM motor, with plenty of cooling and proper clearances. I am wondering if I was sucking air or not returning oil to the sump.

Running a tuna can, extra oil, 2 coolers, modified windage tray with large holes at the end of the push rod tubes.

This is the 2nd time I threw #3 rod and its getting expensive. I might have to go with a 6.
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Old 10-28-2019, 10:00 AM
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I'm sure you posted on 914world, but maybe this older thread has some ideas:

http://www.914world.com/bbs2/index.php?showtopic=332273


How big of a motor is it? compression? hp? I've known folks over the years running type IVs up to 3.0Liters and the larger they are, the more frequent the rebuilds (both pro-actively and um, re-actively). An ax application can subject to high g instances which could lead to starvation.. Also, high revs on a cold engine might be an issue?
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Old 10-28-2019, 10:40 AM
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High rev makes for high G forces that can influence oil pressure on one bearing passage more than others. This is dependent on how the crank is drilled.

Did you reuse the crank? Could be a runout or slight machining issue that caused the same failure. High rev and increased displacement causes all kinds of strange things with split case motors. Has the case been improved to handle the increased stress?
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Old 10-28-2019, 03:43 PM
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I was wondering about the G load. I pull around 1.5 easily.

Crank was reused. It was reground and balanced. Thinking I need a "6"...
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Old 10-28-2019, 05:57 PM
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And, you'll never go back. My 914 had a 2.7 -4 (built here in San Diego). When we got to DC in the mid-90s, I put a 2.7 -6 (at that time, GT-4 spec) in it. ALL THE DIFFERENCE IN THE WORLD. You'll see.
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Old 10-28-2019, 06:37 PM
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Was thinking of the g load on the oil IN the crank. . G load in the corners is a factor too.

Watched a track bud go through more than one big bore 914 motor before running out of $ and tears.
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Old 10-29-2019, 09:29 AM
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I wonder if at AX, there is enough sustained G-loading to cause starvation, considering the volume of oil being run.

I will note, that most large bore 4 "race" motors I've seen (back in the late 1990s primarily), whether for insurance or practical purposes, tend to also run an Accusump

https://www.cantonracingproducts.com/accusump

I've only run smaller type IV (stock 1.7, stock 2.0 and a 2056) and never had a starvation issue. All motors used a tuna can. All ran 1/2qt over and the 2056 also had a front mounted cooler. The stock 2.0, the previous owner installed an accusump. The other two motors ran no accusump. All subjected to AX and DE thrashings.

Did this happen at an AX or a race?
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Old 10-29-2019, 04:14 PM
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no accusump, typically 1.5g load.

This grenaded at the last Chesapeake AX. I never noticed any problem with oil pressure when running at speed.
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Old 10-29-2019, 06:06 PM
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You look at the oil pressure gauge while on a AX run?! The pressure at the sensor is one thing the Flow and oil film at the metal to metal interface is another. 914s have VW bus motors for the most part. Adding in rpm and case stresses beyond the design makes for a range of issues. Air can cause the oil to foam and foam does not act like a true oil film in critical locations and the rod to crank interface is often the most critical. Once the rod bearing and crank meet without the proper oil film the metal to metal contact causes damage. During the low pressure and ramp of a start is one thing. During a 10 second 6000 rpm WOT event is another. The damage adds up and can help cause the thin oil film to shear off and leave the metal unprotected in even lower stress conditions.
Accusumps are good when installed properly but even they can be too slow to cover a air infiltration or full on pressure loss.
The further beyond stock build and use The shorter the lifespan. That said I would look at any other known 3rd rod issues before building another Bus based Big Bore
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Old 10-30-2019, 08:23 AM
hberwald hberwald is offline
 
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I am thinking the same thing. Something caused loss of oil film. The result is seized bearing at full speed. when I have looked at the oil guage, it was on a straight road at full throttle, not the AX course. Just trying to identify the cause of lack of oil and how to address it so I don't have this happen again....
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