Update on my over-rev'ed head...

[VWRacer]

Jake, I definitely vote with the "throw the head on it and go" crowd. In fact, I have done it myself, though not with a diesel. A while back my son had my Talon TSi up at Thunderhill for a driver's school when he dropped an exhaust valve head at about 7000 RPM. The Talon engine is an interference fit, so the valve head beat the heck out of the head and severely and repeatedly hammered the top of the piston.

Rather than pull the engine and do a proper rebuild, after consulting with my favorite mechanic I decided to install a replacement head and see how it ran (the engine turned over fine and the tops of the pistons all reached the same height within the limits of our measurements). On my mechanic's suggestion, I did take an air grinder to the top of the piston to smooth down the high spots and ensure there were no exposed sharp edges (to reduce the chance of hots spots developing into early detonation). I used a "fine" porting and polishing stone  for the work -- which took me all of 5 minutes.

So far the car is running great (I've had it up to 6500 RPMs), passed California smog with no problems, and there is no sign yet of knock.

I say go for it, Bro!

[fspGTD]

Here's a test of my new digital camera - a close-up pic of the carnage in the over-revved head...



[edit: replaced picture with more reasonably sized, lower-res version]

[fspGTD]

More to update along these lines...  Last night I measured my piston projections.  I got, from #1 - #4, protrusions at the exact center of the pistons being approx .025", .022", .025", .021".  Previous measurements that I have on file (measured before the over-rev incident) were .022", .018", .0225", .0175".  So with the later measurement the piston project more by +.003", +.004", +.0025", +.0035".

Now two observations about these sets of measurements... it doesn't make much sense for the pistons to project farther now than they used to, the later measurements were probably because I measured a different spot on the piston or varied the measuring technique somehow.  Ther are on average about 3 thousands more projected in the later measurements, which is pretty darned close and probably within my margin for error of the two measuring techniques.  Comparing the differences of the latest measurements to the previous, it it encouraging that they are tightly clustered, within one and a half thousands of each other.

This information is a good sign I'd say, and it suggests that my bottom end is fine and didn't sustain internal damage from the over-revv incident (IE: the rods didn't get bent, or bearings didn't get mashed out of place when the pistons hit the valves.)

By the way, the worst valve damage on the piston top is #4, although all the other cylinders looked like they kissed the valves at one point or previous incident (when timing belt broke.)  I am working on cleaning the cylinders and piston tops, and am using a razor blade to shave off the high aluminum spots and the bulk of the carbon build-up on top of the piston... I've got two more pistons to clean, then I'll post more pics!

[web]

Nice work Jake... I am new here and must say, from reading the archives, your work has been an inspiration. I just wonder, don't you have any plans of raising boost again?

I've found uncoated piston tops or any other carboned up part are easily cleaned with a combination of scotch brite and a solvent (like gasoline or perhaps diesel). One does not work worth a damn without the other, but together they work wonders. Of course you scrape off the bulk first.

Instead of scotch-brite (which I found in body shops, used to ever-so-slightly rough up old paint before spraying over it) you can use those slightly abrasive cleaning pads you may find in the kitchen. Clean up the shavings well though, you don't want your bores "scuffed clean" as well.

Marcel

(Who has also found a bit of extra useable powerband now. Not nearly as much, but a good start indeed.)

[Turbo DS]

More pics!  

Are you getting another head in order?

[fspGTD]

Marcel - thanks for the compliments.    I won't be raising boost pressure, because my race class (Street Prepared class, SCCA Solo 2) requires an unmodified wastegate... but, I've got plenty of other things left to experiment with to try and get more power, and it's surprising how far this little IDI diesel has come already on stock boost (not to mention stock cam, and stock head)!

That scotch-brite method does well (I've used it before)... I use scotchbrite sponge cut into squares, as I found the sponge holds some solvent and delays the release of the solvent so you can scrub longer before it completely evaporates.

I've got the piston tops fairly clean, IE: surfaces are flat and all the big chunks of carbon scraped by razor blade off... I'm not sure if I'm going to sweat scouring off every little last bit of carbon - maybe!  There was a touch of it on the top of one of the cylinder walls I wanted to attack before putting the head on, might need to break out some scotchbrite for that.  Solvent and paper towel was not working very well.

TurboDS - I've got a new, freshly rebuilt head here in the garage, ports matched to manifolds, still wrapped up in plastic, ready to go.  I even measured valve face recess on it (not sure where I put the measurements... but I do remember calculating that the CR varied as a result of varying valve recesses from one cylinder to another by no more than .1 )

[fspGTD]

OK - on request - here's an image of the piston tops following the over-rev incident that I stitched together (no my engine isn't some sort of weird off-balanced thing where all pistons go to TDC simultaneously!  :wink:



The piston tops as pictured have been scraped clean with a razor and the high edges of aluminum has also been shaved off with the razor.  The worst damage was the exhaust valve groove pounded into piston #4 (at far right.)  These pistons have had piston-valve contact twice.  The first time was a broken timing belt (which really didn't mar into the pistons too deeply, IE although the valve contact is visible, it's hard to even feel it with your fingernail), and this latest time was the over-rev incident (some exhaust valves hit pretty good, especially #4 at right, sort of "dug in" sideways a bit as I suspect the valve bent sideways slightly as it happened...)

[web]

Marcel - thanks for the compliments.    I won't be raising boost pressure, because my race class (Street Prepared class, SCCA Solo 2) requires an unmodified wastegate...

Ah, right. Figured it had to be something like that. Still even for a streeter a good idea to get as much as you can without upping the boost, and THEN add more boost to the equation. Much better results than just upping the boost. (well, that's how it works for gassers, and I suppose it's no different for diesels!)

Marcel

[web]

BTW those pistons have certainly received quite a beating... you sure you're not going to pull the pan and inspect the rod bearings (at least #4)even though piston protrusion seems ok?

Something I'd recommend for a gasser in a similar situation anyway (where my experience lies).

Sure, it's a messy job but for peace of mind... bearings are cheap, too, although it's probably better not to mess with them if they're allright.

[fspGTD]

Last time I had piston-valve contact on this motor (resulting in bent valves and the camshaft breaking into several pieces!), I didn't change any bearings in the bottom end, and it seemed to run just fine!  Oil pressure was good, etc.  Especially since I don't have any evidence that the the pistons are projecting less now than they were before, and the bottom end rotates nicely and freely, and there has never been any oil starvation, I'm inclined to just bolt the head on to the block as-is.  (Eventually, the plan is to get a blueprinted, full-rebuilt bottom end to replace this beaten one with... but nurse this one for at least a season longer!)

Oh yeah I thought it would be also a good experiment for the good of the GTD board readers to see how well the bottom end of a 1.6lD runs after having reached probably 8-9+K RPM!  (IE: we'll see what kind of RPMs they are built to really take.   )

I also wanted to mention that the way I've shaved the pistons clean with a razor while not scrubbing them 100% clean emphasizes even the tiniest marks (for example, notice how clearly you can see the radial marks in the piston tops, which were from when the piston were tuned at manufacture, or how apparent the number stamped into the surface, also done at manufacture, are.)  Many of these marks show up very clearly here in this picture, but you would not be able to feel them (or would only barely be able to feel them) with your fingernail - including some of the valve contact marks.  Really the only significant depressions are where some of the exhaust valves hit, everything else is very minor.

[web]

... emphasizes even the tiniest marks (for example, notice how clearly you can see the radial marks in the piston tops, which were from when the piston were tuned at manufacture...

Good point. It probably does look a lot worse on the picture than it is.

I still fear that the huge forces involved in leaving even a slight "impression" would also break through the bearing oil film, but time will tell. If it runs OK it's probably fine. Also it's not like you can't change rod bearings with the head on, so it's not really important at this stage.

Marcel

[fspGTD]

Synthetic oil has got to only help in situations like this!  Also the way I think the VW bottom end was designed (beefy, big bearing surfaces, etc) can only help too!

Question for you - if I just slap the head on there and it turns out that the rod bearings were bad... what would happen?  Would it be rod knock noise that develops?  Thanks...

[web]

Yes, you could get an audible rod knock if the bearings are damaged. Also, if you have oil pressure and/or oil temp gauges, watch for substantial changes in their behavior since the "incident".

Absolute pressure/temperature readings usually don't mean much, but changes in their behavior can allow you to spot a bearing problem early (greatly reducing the risk of crankshaft damage).

I once had an engine that ate a set of brand new rod bearings in just 20 miles. It didn't even knock although the bearings were down to the backing metal in just 20 miles. I tore it down the 2nd time after the bearing job because it felt sluggish and exhibited high oil temps / lowish pressures after the bearing job. Indicating high friction (and likely excessive clearance). I have NO idea why it never knocked, I consider it one of those exceptions every rule seems to have... Turns out the root cause was an oil supply problem. Live and learn But that's beside the point, just an extreme example of gauges being a Good Thing.

Don't let me scare you though ; as you say, likely the VW diesel bottom end is very strong.

Marcel

[fspGTD]

I scoured clean the piston tops today - here's #4 with the worst of the valve damage:



The valve damage on most of the other pistons I could indeed only barely feel with my finger or with my fingernail, so I'm not worried about any of those.  This #4 piston however (pictured) is the one that I am most worried about.

you can see I prepped it for dremel grinding action... then I ground out some metal where it was pushed from the valve into the right "cloverleaf" swirl pattern...



And to wrap off the pics, here's one of the perfect blueprinted factory head, patiently awaiting installation...  

[fspGTD]

IT'S ALIVE!!!


I got the motor running today!  There is a coolant leak at the front water nipple I'm going to have to tend to before I can drive it, also need to change the cam break-in oil out soon, and flush the coolant... but it's getting close, and it sounded pretty good when it ran too!