Gridlife Round 5 Engine Failure Analysis
Often times when pushing your car beyond its factory-designed capabilities you find the weak points, those who race in time attack are no stranger to this. The weak links usually come in the form of oem components, however sometimes even the best products fail from time to time and in our case this came in the form of a broken connecting rod in our race engine at Gingerman Raceway for the 5th and final event of the Gridlife Series. In this article, we will be taking you through a step-by-step analysis of the failure path and ultimately the destruction of our primary engine. We hope you can use this information to thoroughly think about your failures to not only see what failed but also to understand the process in which it happened to make you a better racecar or engine builder in the future.
The engine analyzed is based off the Subaru EJ257 platform. This engine in specific was redone with Iron Sleeves Bored to 100.5mm, I-beam High HP Conrods, Race Bearings and a Forged crankshaft. The bottom end was built to handle upwards of 1000bhp. Chassis dyno testing put the engine at 714whp, which correlates to approximately 820bhp, not leaving a ton of safety factor for the components. The engine ran through the second half of the season in the Gridlife series and met its end on the Second session of day 1 in Round 5. During the time of failure, the engine was in a sustained sweeping corner hovering at 6000rpm, 31psi of boost, and 67psi of oil pressure, then BANG!! Oil everywhere!!!
When going over the failure path of this particular race engine you start at square 1. As seen, there is a hole in the upper, and lower block casing- this right off the bat tells us we have a connecting rod failure of some type. A connecting rod can destroy a block in this manner from 3 different ways:
-spun bearing from poor oil clearance or lack of oil pressure (eventually leading to conrod fracture)
-broken rod bolts from poor conrod bolt tension
-bent connecting rod (eventually leading to conrod fracture)
After the initial teardown, it became obvious the failure mode was not from a spun bearing. A spun rod bearing would show distinct signs of heat and oil coking, our scenario did not have that. In addition to the lack of heat marks, the rod bearings themselves looked very good outside of the obvious damage after flying around the crankcase.
Knowing we didnt have a mechanical failure sourcing from oiling, we can now look at the next common failure mode which is a broken rod bolt. Improper tension on a rod bolt can be a devastating engine failure. The purpose of a rod bolt is to keep the cap side of the rod rigidly mounted as if the connecting rod was one piece, only to be removed when installing engine bearings. Joint separation (an instance when the cap of the rod loses contact with the main connecting rod) can occur when the tensile force of the conrod bolts is lower than the dynamic loading of the conrod from combustion/rotating forces. This dynamic loading scenario will eventually weaken the connecting rod bolts to the point of failure leaving the connecting rod free floating in the crank case waiting to get hint by the crankshaft on the next revolution. Though this scenario seems likely given the circumstances, a few things hint otherwise. The first piece of evidence is one rod bolt is broken in 2 pieces and the other is broken in 3. This is nearly impossible with low tension rod bolts. When fatiguing a bolt, it is similar to bending a paper clip back and forth until finally you have 2 pieces, in the process you only weakened one part of the paper clip till it fails. The rod bolt will show similar behaviour and eventually break by the threads where is lowest cross sectional area is. Having the idea that the conrod was the first to fail, we need to step through the failure path.
To confirm the rod bolt broke after the connecting rod, we need to find the evidence. Witness marks on the top section of the conrod confirm the contact of the bolt head and upper conrod. The marks left were nearly 5mm deep into an already forged forged rod. To leave a mark like this would mean the bolt struct the conrod with a large amount of force. This would mean it would be necessary for the rod bolt to be firmly bolted or supported then struck into the other part leaving a new forging- in the shape of the bolt head.
Knowing that the conrod bolt could not break into 3 pieces without the introduction of an additional force, we can draw up the scenario that cause the bolt head and the bolt shanks to breakaway from the conrod.
Seeing the way in which the cap broke away from the connecting rod, we need to look at why the rod broke in two pieces before hand. The first sign of this is found on the side or "thrust" face of the connecting rod. Upon assembly, connecting rod side clearance was measured, and was in spec at 11 thousandths of an inch. It is not suspected that this wear occurred because of either lack of clearance or lack of lubrication, the engine never suffered from low oil pressure. The wear marks also occur on opposing sides of the rod, as if it had a side force exerted on it at the top of the rod.
Thrust wear of this nature can be a tell tail sign of a bent rod. Further indication of the scenario comes from witness marks on the piston and wrist pin. The markings show a distinct shift of the conrod on the wrist pin. It is no longer centered and moves toward on side of the piston until it makes contact and begins rubbing.
When we have the rod bend and kink like this we will also see abnormal wear on the rod bearing and conrod bushing surfaces. The wrist pin bushing shows wear corresponding to the direction of bend, as well as the bearing show evidence of side loading in their wear pattern.
If you take all of the wear indicators and put them together into a picture you will get the following image.
Racing can be a brutal sport, and as you can see even the best components can fail. In this case we saw it from running 720whp in a road race application eventually weakening the rod enough to bend and break it in the process. These failures are not uncommon, they actually tend to be part of the running costs of the sport. The important thing is to learn from the failures you experience and look at them in intricate detail to know the failure process and avoid doing it again and again.