The Road to Drag Week 2016 - July 17, 2016

[jayb]

I don't know what to say about this week except, holy crap I was not expecting this! 

Friday I went back to my machine shop to pick up the short block so I could get it assembled over the weekend.  The good news was the honing on the cylinder bores came out pretty well.  The #4 hole, which had the dings in it from the air cleaner stud incident last year, came out looking pretty good; there are only two small areas that didn't hone out.  See the small dark areas in the picture below:



Those dark areas used to be about an inch wide and half inch high, so going 0.008" oversize cleaned the bore right up.  All the other bores also honed nicely with almost no material removed.

Then I got some completely unexpected bad news.  I had brought the rods in to get checked out of an abundance of caution.  These were new rods in the spring of 2015, and heavier than the standard Crower rods, just to avoid any potential issues like the one I had in 2014, when the #5 rod broke at the Drag Week test and tune.  Well, my machinist informed me that I had a cracked rod!  Not only that, but it was the same rod, #5, as the one that broke the last time!  I just couldn't believe it.  The rod was cracked so badly that I'll bet it would have let go in one or two more passes down the track.  I guess I was lucky this time, but it sure doesn't feel that way.  Here are a couple of close up pictures of the rod:





Both sides of the rod were cracked, and the cracks extended through the edges all the way to the center section of the rod.  That center section was the only thing holding the rod together.

The news wasn't much better after that.  My machinist also pointed out that on most of the pistons there was evidence of contact with the exhaust valve.  I had completely missed this when I took the engine apart, probably because I was so focused on what I found with the oil ring support rail.  But he was right; here's a picture of one of the pistons:



Well, at least the piston to valve contact was explainable.  On the dyno I didn't have any indication that there was a valvetrain problem, but I only ran the engine to 7200 RPM on the dyno.  At the track, I was going through the lights at 7700, and I must have just run out of spring.  This was unexpected but probably not too serious, and a set of upgraded valve springs should solve the problem.

The connecting rod situation, on the other hand, is completely baffling to me.  Why this one rod is taking so much abuse is a mystery.  I ran this engine from 2008 through 2012 and didn't have this issue, so something must have changed after that.  I spoke with Blair Patrick about this on Saturday, and it was his opinion that there is some kind of resonance or harmonic in this one cylinder that is causing the problem.  He thought I should focus on upgrading the valve springs to change any resonant frequency that may be causing a problem.  Also, last year at Drag Week I had not been real careful about maintaining the timing chain tension, and it had gotten rather loose by the end of the event.  Blair thought I should try to monitor that more closely this year.  Finally, he suggested a call to ATI to see if a larger or heavier harmonic balancer might be in order.  My balancer is several years old anyway, so maybe rebuilding it might help.  Also ATI suggests bolting any accessories to the harmonic balancer, rather than using a long bolt to the crank.  It just so happens that I am using a long crank bolt to hold the arbor and drive pulley for the dry sump in place, so getting one of ATI's drive arbors that bolts to the balancer might be a good improvement to make.

But in any case, when I get this thing back together I'm going to minimize the dyno time, take it as easy as possible at Drag Week, and then when I tear the engine back down to look at the pistons, I'm going to have to look at the rods again too.  Fortunately for me Blair has a replacement rod that I can get my hands on next week, so this won't hold me up too much.  But, back to the assembly, the piston rings didn't make it on Friday, so I wasn't able to assemble the rest of the short block.  Looks like it will be next week before I can get that done.  I did get the block cleaned up, main bearing clearances checked, and the crank installed this weekend; pic below:



Right now I'm wondering what else could possibly go wrong with this project in the next eight weeks, before I hit the road for Drag Week.  I'm still hoping I can make it with this car and engine...

[427Fastback]

Is the crack on the same side of the rod ?

[jayb]

There are cracks on both sides of the rod, you just can't see them very well in the photos.  To be a little clearer, all four thick areas on the rod, front and back, and left side / right side, show the cracks that you can see in the photo.  The only thing holding that rod together is the center portion of the I-beam.

[cjshaker]

Wow, talk about dodging a MAJOR bullet! That crack is visible to the eye so I'm amazed it was still holding. It's a darn good thing you didn't try one last run for the fun of it!

It seems odd to me that harmonics would affect #5 while not doing the same on #1. Is there a working hypothesis as to why harmonics in the valvetrain would affect the rod, or was that just in relation to the piston contact? The contact doesn't seem very severe, so it's hard to imagine the relation between the two.

I have to say, you're shaking my confidence in the Crower rods a bit; the rod of choice in my next engine. The larger balancer and drive arbor certainly seem like good ideas, as does a vigilant maintenance of the timing chain tension. I had mentioned before that chain and belt harmonics can get brutal, and I imagine that to be even worse on the unsupported lengths of the SOHC chain. Are there any signs of abnormal wear on the nylon guides?

[jayb]

I think I'd be looking at a different set of rods also, if there were time.  But having the failure on the same rod twice in a row makes me think that its not the rods, its something else. 

Blair thought that since the front of the engine is different from side to side, relative to the timing chain configuration, this may explain why only one rod is affected.  His suggestion was to start changing things up, and since resonances can appear in the valvetrain, plus I have a P-V contact issue, that was the first place to start.

Just got off the phone with ATI and the guy I spoke to said that going to a larger harmonic balancer (they make an 8" diameter, 14 pound balancer) was not necessary, that a 585" engine at 7700 RPM would be just fine with the 7" balancer.  He said I should rebuild it though, since the O-rings are old (balancer was new in 2008).  Not sure that Blair agrees with his assessment regarding the larger balancer LOL!  But in any case, getting a custom balancer is not in the cards for this engine because of the lead time required to get one (6+ weeks). 

Chain guide wear looks normal...

[Hemi Joel]

Sorry to hear of all these problems Professor Brown. 2014 was the year that you busted the rod on Drag Week, and if I remember right, that was the first year that you ran the dry sump. So maybe the harmonics are coming from the oil pump?

[jayb]

I was thinking the same thing, Captain Stabbin'.  It could at least be a contributing factor.  Getting the drive pulley mounted to the harmonic balancer, rather than through the long bolt to the crank, will be a good step I think.

I was also thinking about the crank target I am using, also new in 2014, which is 3/8" plate steel, 8" in diameter, with 36 teeth.  But one tooth is missing, to allow the EFI computer to sync up once each revolution.  So, that piece of steel is not balanced.  I think I'm going to drill some holes to try to lighten that wheel, and also balance it...

[Barry_R]

I'm having a hard time blaming the mid-beam rod failure on the balance/harmonics stuff.  Just seems like a really long reach.  Have you discussed this with Crower yet?  That almost looks like a fatigue/flex location for a break.  How do the bearings look?  Pin fit in the pistons?

[machoneman]

Rod stretch is my guess, maybe induced by slamming the throttle closed at the line and using heavy engine braking to slow the car, stretching the rods. Still, why only #5? Checked on the rod centers for the other seven rods? 

[jayb]

I'm having a hard time blaming the mid-beam rod failure on the balance/harmonics stuff.  Just seems like a really long reach.  Have you discussed this with Crower yet?  That almost looks like a fatigue/flex location for a break.  How do the bearings look?  Pin fit in the pistons?

Haven't talked to Crower yet, but I plan to.  Rod bearings were beautiful on #5; no significant issues, the coating was still intact.  I haven't checked pin fit or rod length, but I will...

[jayb]

Still, why only #5?

This, to me, is the key question.  Why the #5 rod, twice in a row?  If it was engine braking and rod stretch, what are the odds it would affect #5 and no others?  None of the other rods show any evidence of problems.  The harmonic/resonance theory seems unlikely to me too, but what else is there?

[MRadke]

With your block being offset bored, is there simply more angle load on 5 than on the other cylinders?

[jayb]

Nope, #1, #4, #5, and #8 all have the same offset bore (0.105").  #2, #3, #6, and #7 are all offset 0.035".

[cjshaker]

It's certainly possible that it was a coincidence that it was #5 both times, but that just leads to a Quality Control or materials issue. I'd certainly be interested in what Crower has to say, but I'd guess they would place the blame somewhere else, and with all the successful racers out there running bigger power it'd be hard to argue against.

With such a long stroke, and running nearly 8000 rpm, I wonder if the side loading of the rod is just too much? It would also explain the piston coating scuffing, and possibly even the ring issues. Maybe a slight gear change to keep the RPMs down in the traps would be a good call?

[jayb]

After Drag Week last year I knew I needed less gear, so I've already gone down to 4.11s (from 4.29s).  Should keep the engine in the powerband I want.  Last year peak power was at 6600 RPM, and fell off rapidly after 7000, so I definitely had too much gear.  I'm also considering dropping down to 3.89 gears, depending on how things look for this engine on the dyno.  I've got to believe that RPM exaggerates this problem, whatever it is that is causing it.  I never intended to run the kind of RPM that I did at Drag Week last year, but who wants to let off in the last 100 feet LOL!

[JamesonRacing]

I'm running 4.00:1 gears in my race car and my street car...the gears can be found if you want something between 3.89 and 4.11.  I went the cheap way and found used NASCAR REM polished lightened gear sets on eBay for a good price.

[jayb]

Thanks David, are they pro gears, with the 35 spline pinion, or do they have the normal 28 spline pinion?

[Joe-JDC]

Just looking at the cracks, it seems to be a lateral rocking issue.  How is the head/combustion chamber on #5 in relation to the piston valve notches?  Is it offset any different than the others?  The combustion seems to be hitting the piston off center to cause those cracks.  If the combustion chamber is offset any, it would be different from the bores being offset.  I find many combustion chambers are not evenly spaced or sized even when a CNC program has been run on them.  Definitely worth checking.  Flip the head gaskets over end to end to see if everything measures exactly the same in the chambers.  A few thousandths may be the culprit.  Joe-JDC.

[JamesonRacing]

Thanks David, are they pro gears, with the 35 spline pinion, or do they have the normal 28 spline pinion?

Standard 28-spline input, don't think they're pro gears.

[jayb]

That's a good point, Joe, the cracks do indicate a lateral load.  However, the heads have the normal FE bore spacing, so all 8 of the combustion chambers are offset from the bore.  The pistons compensate for this with offset domes.  So again, I would assume that if this was a problem, it would show up on all four corner cylinders, not just #5. 

You bring up an interesting point though about the heads.  I changed to these heads on this engine in 2013, prior to running them at the Drag Week test and tune in 2014.  So, the list of changes starting at the failure in 2014, from the previous version, is the EFI system and crank trigger, dry sump system, and the cylinder heads.  I wonder if the heads could have something to do with it?  I do know that there was no gross piston to valve clearance issue on that cylinder, because I clayed all of them to check for clearance between the valves and the valve reliefs in the piston the last time the engine went together.

[Barry_R]

How heavy are those pistons?

[57 lima bean]

It would be interesting to check timing on each cylinder.

[jayb]

How heavy are those pistons?

592 grams.

Also checked all the rod lengths tonight, and they are all right on the 6.700" spec.  Even the cracked one measures right on.

[Barry_R]

ooooph. 

Cammer pistons are always heavy because of the design.  Still a lot of weight at 8000.  Make sure to mention that to the folks at Crower when you talk to them.  So far I am only aware of a couple of failed Crower rods - and you own both of them.  Has to be a "root cause" somewhere in that combination.  Probably the same thing that is causing that ring/piston wear issue.  The fact that the rod is still perfect to length kinda points to Joe's comment on side to side rock from combustion or ?  Wonder if the piston is "balanced" in that plane???

[machoneman]

Based on what Barry said, how thick are the piston pins? Even though they may be straight, I wonder.......

From IIRC Yellow Bullet forum replies, some racers said they had some deformation of pins in nitrous engines that more or less snuck up on them. I didn't know a pin could bend under load yet not break. Maybe this was pure speculation but......

[jayb]

They are very thick pins, because they are SBC size (0.927").  If this was a problem, why is it only happening on one cylinder?  It doesn't make sense to me that it could be related to the combination, whether its the pin, piston, rod, whatever.  This is basically the same in all 8 cylinders, and the problem has shown up twice on #5.  It has to be something specific to that cylinder...

[Gregwill16]

I'm way out of my league here but I'm with Joe, I would be looking at that chamber on the head.

[jayb]

The #5 chamber got dinged in 2014 when the rod let go; there is a nice imprint of the piston top on one side of the chamber.  That original ding is still there, but I looked at the chamber last night and didn't see anything fresh.  But for sure when I check for piston to valve clearance on this build I will put some clay on the piston tops, and look for piston to chamber clearance also.  I suppose it is possible that on that one cylinder, the piston can touch the chamber at higher rpm and induce the cracking...

[machoneman]

One more idea: block is deforming under load.

How is the block mounted (I forget) to the chassis. Motor plate only, OEM engine bosses and motor mounts, using a torque strap near #5 to limit engine movement?

[jayb]

Now there's an interesting idea.  Seems a little far fetched, but who knows.  The engine is mounted with factory mounts, and the torque strap is mounted right up front on the driver's side, to one of the #5 exhaust bolt holes.  Hmmm, maybe I should be putting a matching torque strap at the back of the block...

[jayb]

And another thought occurred to me after talking with a friend of mine today.  Back in 2014 when the rod let go, I disassembled the engine and found the ding in the combustion chamber mentioned a couple posts back.  I assumed that this was due to the piston flying up and hitting the chamber after the rod let go.

But what if the ding in the chamber was due to the piston repeatedly striking the head at high RPM?  If I had piston dome to chamber interference for some reason on that cylinder, that would explain why the first rod failed in 2014, when I had been running it successfully in the engine since 2008.  In 2014 I put on a new set of heads, caused the piston to chamber interference because of some issue with either the piston or the chamber, and broke the rod.  Then, put it all back together again for last year, and the same thing started to happen.

What's more, the ding is on the front side of the chamber, so a fore-aft crack in the rod makes sense under this scenario.  This is actually the first really rational explanation for the problem, I think.  I can test it by installing last year's head on the short block when I get the short block together, and clay the piston top to check for clearance.  Should be enlightening...

[JERICOGTX]

Why not mount it with a motor plate?

[jayb]

Why bother?  A motor plate would add complexity and cost, cramp the already cramped engine compartment, and the factory mounting points would still be sitting there.  The Modified class requires stock front frame rails.

[WConley]

Jay - The head whacking the piston makes a lot of sense.  An off-center hit could be fatiguing that rod.

Years ago a friend rebuilt his 240-Z engine and decked the block to clean it up.  At high revs the engine made a strange ringing sound that we couldn't figure out.  Finally the head came off and all of the pistons had witness marks from smacking the quench areas of the chambers.  Statically he had enough clearance, but stuff was stretching when he zinged it.

Interesting stuff!  Glad I'm not writing the checks.

[machoneman]

And another thought occurred to me after talking with a friend of mine today.  Back in 2014 when the rod let go, I disassembled the engine and found the ding in the combustion chamber mentioned a couple posts back.  I assumed that this was due to the piston flying up and hitting the chamber after the rod let go.

But what if the ding in the chamber was due to the piston repeatedly striking the head at high RPM?  If I had piston dome to chamber interference for some reason on that cylinder, that would explain why the first rod failed in 2014, when I had been running it successfully in the engine since 2008.  In 2014 I put on a new set of heads, caused the piston to chamber interference because of some issue with either the piston or the chamber, and broke the rod.  Then, put it all back together again for last year, and the same thing started to happen.

What's more, the ding is on the front side of the chamber, so a fore-aft crack in the rod makes sense under this scenario.  This is actually the first really rational explanation for the problem, I think.  I can test it by installing last year's head on the short block when I get the short block together, and clay the piston top to check for clearance.  Should be enlightening...

The plot thickens! Sounds most plausible (fore-aft cracks). What is the nominal head-to-piston clearance that you run? Heavy piston+heavy pin+lotsa rpms+ lotsa hp = rod stretch.

[Barry_R]

I am signing in on this theory - best guess so far. 

When I built the SOHC that I intended to run in EMC I had the chambers digitized at Diamond and tried to create a +/- 1/4" wide quench band around the outside diameter by bringing that surface within .010 of the chamber and having a tiny degree of angle tilted toward the center before adding in the gasket.

My initial test assembly with a gasket showed it to be too close - touched the head - and ended up tweaking things just a bit.  With the radius it left no marks at all - just kind of snuggled up into the chamber & pushed against the head.  If you were close but not hitting hard you would never feel a thing turning the engine over.

[cjshaker]

If that were the case, I'd think there would be some other tell-tale sign that was specific to that cylinder. Different markings on the piston skirts, the cylinder, the chamber, the rod bearing or something. If it's shocking the rod enough to cause it to crack, I just can't see it not leaving any other mark. Being nearly all aluminum, the rod is the strongest part. Granted, it's also the most brittle, but aluminum is much more malleable so I'd think there would be some sort of deformation somewhere.

I'll stop being a Debbie Downer now.

[jayb]

Picked up the pistons and replacement rod today.  I had the piston pin bores checked for roundness, and they were perfect; no re-honing required.  Sometimes if you have a serious detonation problem the pin bores will go egg shaped, but these pistons did not have that issue.  If they had been out of round, it would have been another piece of evidence that the engine encountered severe detonation, and that could help explain the movement of the oil ring support rail.  But, they looked fine.  I still don't have an explanation for the oil ring rail issue.

Also, when I picked up the pistons I gave #5 a very, very close look.  There is zero evidence on the dome that it was hitting the chamber.  Maybe it still was, and the cast aluminum chamber was weaker and gave in, but I would have thought I could see at least some contact evidence if I looked closely at the top of the piston.  There isn't any.  Guess I won't know for sure until I clay it. 

At least now that I have all the pistons back, and the replacement rod, I can get the short block finished up over the next few days...

[RoyceP]

Let's say the engine puts 400 Lb - ft of torque on that strap. I think you may be onto something here Jay. Perhaps increase the clearance on #5 piston to head, and add the other strap.

Now there's an interesting idea.  Seems a little far fetched, but who knows.  The engine is mounted with factory mounts, and the torque strap is mounted right up front on the driver's side, to one of the #5 exhaust bolt holes.  Hmmm, maybe I should be putting a matching torque strap at the back of the block...

[Royce]

As unlikely as it seems, I think you have to consider the possibility that the rod cracked due to an internal flaw or metallurgic deficiency.. I would send that rod to someone for failure analysis..
 Somebody said that "once you eliminate the likely possibilities you are left with the improbable or seemingly impossible as the truth".

[jayb]

As unlikely as it seems, I think you have to consider the possibility that the rod cracked due to an internal flaw or metallurgic deficiency.. I would send that rod to someone for failure analysis..
 Somebody said that "once you eliminate the likely possibilities you are left with the improbable or seemingly impossible as the truth".

That was the conclusion the last time.  We thought that in 2014 the rod broke due to an internal flaw.  Now, here it is again, one year later.  Same rod, crack in the same area.  I'm not buying the two defective rods in a row theory; it is just too coincidental.  I'm certain at this point that it is some problem with the engine, and now I am equally certain that the failure in 2014 was not due to a rod defect, despite me (and nearly everyone else) thinking that it was.

[jayb]

Let's say the engine puts 400 Lb - ft of torque on that strap. I think you may be onto something here Jay. Perhaps increase the clearance on #5 piston to head, and add the other strap.

Definitely going to add a second torque strap this year, Royce...

[machoneman]

Good to hear on the 2nd strap.

Jay, does the tranny have a rubber mount under it or a solid metal mount? Just asking....as I used solid engine mounts AND a solid tranny mount long ago in my old doorslammer 10 second BBC '69 Camaro. Big  mistake as I broke a few ST-10 cast iron tranny cases before I realized chassis twist was the real culprit.

[jayb]

Rubber mount on the trans also.  This is a STREET car 

[machoneman]

Hah! 

[Royce]

Jay, Wouldn't a metallurgical analysis pinpoint if it was a stress crack, and determine if it was from stretch or side flex, or if it is an inherent flaw in the metal?   

[Cyclone Joe]

Yes Royce it would. 

Jay, after all the fun of Drag Week is done, if you want me to take a look at one of the rods and do a bit of 'science' on it, please let me know.  I'm always happy to help.

Joe

[jayb]

Thanks Joe, I appreciate the offer, and also the other work you've done on rockers for me.  I will send you that rod sometime after Drag Week - Jay

[Heo]

Just a thought....What if number five is just a litle bit
more stressed than the others. And the other 7 is just
a couple of rounds away from start cracking......

[Cyclone Joe]

After 2016 Drag week is done, there is a few things we can do with accelerometers and installing the SOHC on the dyno (might work while still in the car to check out the rotational frequency theory.

Joe