Engine Failure, What do you think?

T

Team Rocket

New member
I had a massive engine failure in my 84 GTI at WGI last weekend.
frown.gif
The engine is prepared to EMRA ST3 rules (similar to ITB, not particularly overstressed). The engine has 6 races since its last rebuild: new bearings and rings, a general engine overhaul. Here’s my theory, feel free to weigh in with other opinions!

Hard facts:
Engine failed at 6700 RPM in third gear in the left hander coming out of the “boot”.
#1 and #2 rods separated from the crank. Not broken but the big end journal caps came off. I have one of the rod bolts, and it was a tensile failure from what I can tell. What was left of the bolt wasn’t bent.
#1 and #2 pistons separated from the rods at the small ends.
Intermediate shaft just disappeared.
Bearing material welded to crankshaft at the #2 rod journal. None on the #1, crank journal looks OK.
Running largish piston to wall clearances: in the 3 to 5 mil range. (I know, too big, a usually pretty good Drag racing shop does my machine work).
#1 rod exited the block. Exciting! To say the least.


“Soft” observations (no hard supporting data):
I had missed a shift earlier in the day: went to 3rd instead of 5th. Ouch. Real big over-rev. In retrospect, probably hit 9K+. Maybe more…
I measured how far the piston protruded above the block and remember about 16 mils or so. But not 100% sure. Head gasket is 72 mils in compression?
#2 piston had a crack where it hit the head. Not hard to convince myself there was blow-by on the middle ring at the opposite edges of the crack (Wish I had a digital camera to include picture).
Engine sounded a little ratty and people started to pass me about 1 to 2 laps before it blew.


Here’s my theory: stretched the rod during the early over-rev and the piston hit the head (big piston wall clearances didn’t help this). But more importantly, when the rod stretched, it ovalized the big end, which wipes out the oil film clearance. From what I understand, this is the common failure mode on high revving engines. About 2 laps before it failed, the bearing began to spin and seize, generating lots of heat, which weakened the rod bolts. Eventually the bearing seized against the crank, breaking the rod cap off. All the rest of the damage was secondary.

Does this sound reasonable to our engine experts?

BTW, Granracing was behind me when this happened. He said it was pretty spectacular, with a big fireball coming out from under the car.
eek.gif



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Jim
Black Rabbit Racing, #154
 
Jim,

Did you use new rod bolts when the motor was rebuilt? The VW rod bolts are 'torque to yield' bolts, like the head bolts, and are not resuable. If they were the old rod bolts, that's probably the root cause of your problem.

I had the exact same mechanical over-rev w/ my old car, and the motor ran another dozen races w/o any problems.

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MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608
 
I don't think the piston/cyl clearances had anything to do with it. I'm not sure what the optimum clearance is with our VW motors but I run .005" clearance in my Ford 2.0 "Pinto" German motor. I actually built one with .006" and it was the fastest motor I ever ran although it wore out after 20 races which still isn't too bad.

One shop only left .002" and I ended up rebuilding that motor after one weekend due to piston to cylinder galling.

#37 ITB
 
I just checked the Bentley, and list .001 as the spec. clearence and .0025 as the wear limit.

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MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608
 
It blowed up real good; you need a new motor.

Suggestion: Throw out the whole thing and get a good JY motor to rebuild.

(Now that was real analytical)
 
Jim, when the engine was running "ratty" for those last laps, did get a chance to look @ any gauges? What was the Oil Pressure? What was the water temperature?

It does sound like when you zinged it, that one of the rod bolts failed, and then while your were running it "ratty" (piston to valve contact) you lost oil pressure to everything else, and the rest was inevitable.

I had a similar problem this April @ Laguna Seca. Was about that same RPM but in 4th gear, heading into turn 5. Noticed my oil pressure idiot light was on, and Oil pressure was @ ZERO. Shut it down and pulled over to the side and parked it for the rest of practice. Changed oil and filter after that, and everything was OK. I think that I got an air bubble somewhere in the cooler/filter.



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Tim Linerud
San Francisco Region SCCA
#95 GP Wabbit (Bent)
http://linerud.myvnc.com/racing/index.html
 
Bill
Yes, I used new rod bolts. Not ARP though, stock VW. Torqued to Bentley spec, though I didn't measure bolt stretch.

Racer_Tim
I do regularly check my guages (I think!), and I have a big red light right in the middle of the dashboard and a pressure sender (15 psi) seperate from the VW stock sender. But, your comments had me thinking. So, I went and checked the rod end surfaces of the 2 broken rods. The big end of the #1 rod is pretty mangled, but you can see the mating surface of the rod and bearing cap are fairly clean. Not so on the #2 rod end. Those surfaces are deformed as if they were banging against each other. Mmm.... I like your stretched/broke one of the rod bolts idea. There's evidence to support it, too. Either the over-rev got it, or I didn't torque properly. Thanks!

Moral of the story: I'm getting a tach with a tattle tale. Serious over-revs mean the day is over and the bearing caps come off!

Thanks for the replies.

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Jim
Black Rabbit Racing, #154
 
Jim, since this motor had 6 races on it, I would have to assume that if it wasn't properlly torqued that you would have seen failure much sooner.

Just my $0.02 worth

Tim
 
Jim,

I agree w/ Tim about the torque question. Improperly torqued bolts would have failed sooner, IMHO.

Since you're running EMRA, I assume you're somewhere in the Northeast. I live in NJ, and have an extra JH motor that I could hook you up with if you're interested. It's stock, but it's got fresh set of rings and bearings in it.

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MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608
 
Tim/Bill

You're probably right about the torque. But those chafe marks on the mating surfaces of the rod/bearing cap make me think that the rod bolts were a contributor, if not the root cause. Thanks for the input!

Bill, let's talk. I'll shoot you and email. Thanks a million.


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Jim
Black Rabbit Racing, #154
 
For those who might still be following this thread: I brought some of the broken parts (2 rods and a piston, or what’s left of it) to my machine shop. This shop is primarily a drag racing shop that builds 700+ BHP drag engines. Hey, it’s local, they usually do good work, and they understand race engines. So the Owner, an engine builder and a part time employee come out to view the carnage. (The part time employee is a friend of mine who is a retired Engineer/Product manager at a major gas turbine manufacturer - where I happen to work). This was the conversation:

Engine builder, “Have you been using crappy gas Jim?”

Jim thinks, “Uh oh,” but says, “No! The usual.”

Owner says, “Look at all the cracks on this piston. These aren’t from a rod failure.”

Jim thinks, “Uh oh,” but says, “Really?”

Retired Manager looks out from his loupe and above his glasses with a look he has perfected that implies, “Didn’t you learn anything I told you?” and says, “Jim, these are fatigue fractures at the wrist pin bosses. See where it fatigued and failed and started hitting the head? This piston has been hammered to pieces.”

Jim thinks, “Uh oh,” and says, “Uh oh.”

Well, the consensus at this shop is that detonation was a major contributor to this failure, if not the root cause. Either I was too aggressive in advancing the timing, or not using enough octane. Their feeling is that once the piston crown separated from the rod, all sorts of good things can happen. Maybe the rod bolts were weakened by the detonation, and the over-rev didn’t help any. In any event, it failed.

New moral to the story: beware of detonation. Ask me how I know.
smile.gif



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Jim
Black Rabbit Racing, #154
 
Tim

I was running 34° total advance. I use a 50/50 mixture of Cam II and Sunoco 94. The shop says go to an 80/20 Cam/94. They say keep some Unleaded 94 because of the oxygenators. (?)


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Jim
Black Rabbit Racing, #154
 
I must be lucky my motor is still running! I run my GTI motor on 91 pump gas and 35 deg total advance. I mistakenly ran my motor with over 40 deg advance for 2 whole events with me trying to figure out why my car was so slow. Motor is about 8.8:1 compression which must be part of the reason I got lucky.

#37 ITB
 
Detonation certainly does cause bearing and big end rod/crank failures. Read Smokey's book!

Why? The uncontrolled explosion front meets the piston on its way up the bore transmitting a terrible hammering effect through the head of the piston down the rod to the big end.

As I said before, you blowed it up real good; start with a fresh JY piece.

Listen to your engine builder. Those drag racers have seen just about anything in driveline failures.
 
Just so we're clear here:
smile.gif


EMRA "ST" class rules allow 10.5:1 CR. And I use all of it!

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Jim
Black Rabbit Racing, #154
 
So, with only 34 degrees of timimg, I would have to say that it's the fuel that caused the pre-detonation.

I'd go with something a little bit higher on the Octane rating. Leaded or un-leaded shouldn't make any difference.

Just my $0.02 worth




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Tim Linerud
San Francisco Region SCCA
#95 GP Wabbit (Bent)
http://linerud.myvnc.com/racing/index.html
 
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