The Road to Drag Week 2016 - July 17, 2016

[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...