The Road to Drag Week 2016 - July 24, 2016

[jayb]

On Wednesday this week I picked up the last parts from the short block, including the rod and rod bushing that Blair had sent me Monday as a replacement for the cracked rod in #5.  Wednesday night, Thursday night, and Friday night I spent the time getting the short block put together; here's a picture:



I was anxious to test fit the old left side head on the short block, and see if I could find any evidence of contact with the piston; at least that would tell me if that was the problem.  Saturday morning I installed the head studs and head gasket, and clayed up the pistons to see if I could find the issue.  I installed the head and torqued the nuts to 90 foot pounds, then rotated the engine through two complete revolutions.  I removed the heads and carefully inspected the clay; here's some photos of the clay on the heads:







Long story short, I had at least 0.070" clearance everywhere on the dome that I checked, including the area where there was evidence of piston contact to the chamber.  However, on #5 it did appear that the flat portion of the piston was closer to the deck than anywhere else on the whole bank; see the last picture.  I measured slightly less than the 0.042" head gasket thickness on the clay between the flat portion of the piston and the head, at the front of the #5 cylinder.  It's tough to get a real accurate measurement with the clay, but it was definitely thinnest on #5, on the flat portion towards the front of the cylinder.  There was also a kind of matching ding in the cylinder head in the flat area, although in the area of the ding the clay was thicker.  See the pictures below of the chamber, showing the ding from one piston crown and also the ding from the flat part of the piston, visible just to the right of the exhaust valve in the second picture:





At the spot where the piston crown hit the chamber, there was at least 0.075" clearance all around, and more in the ding itself.  Based on that I think that it is reasonable to assume that my original conclusion on that was correct, that this ding occurred when the rod let go in 2014 and the piston flew up and hit the chamber.  I'm not so sure about the ding in the flat area, though.  It may be possible that this was an interference point, and is causing the whole problem, because the clay was definitely thinnest there.  Of course it is also possible that the damage occurred at the same time as the other ding; no way to tell.

Just as a final check I set up a dial indicator on the heads and measured the depth of the chambers at their deepest point, between the two valves.  I was thinking that maybe the head had been machined off axis, and the chambers were shallower towards the front of the head as a result.  But that didn't pan out; the depths of all the chambers were within 0.003" of each other, near as I could measure.  Then I did the same thing with the pistons installed in the block, measuring the top of the dome when each piston was at TDC.  Again, everything was nearly the same. 

So, I don't know why the clay looked a little thin on #5, but even so it wasn't a big difference with the other cylinders.  And I'm not sure why the problem is being repeated on the #5 cylinder.  Basically, I have no smoking gun here  .  But I think to be safe I'm going to go up about 0.010" in head gasket thickness; maybe the 0.042" thick gaskets are not enough.  It will cost me about a quarter point of compression to do this, but its probably a worthwhile compromise, on the chance that with the heavy pistons, long stroke, and 7700 RPM the rod and piston stretch is exceeding 0.042" and causing the problem.

After getting these checks done I spent the rest of this weekend working on the new sheet metal intake.  I had previously completed a Solidworks model of this intake; here are a couple of screen shots:





Notice that the ports on these heads are no longer round, but more squarish instead.  The heads flow about 30 cfm more than the last set, so I'm hoping with these heads I'll be able to get to 1000 HP with this engine.

In order to build the sheet metal intake I am machining plates for the plenum where the tubes weld in, and also plates that the tubes weld to that bolt to the heads.  I pretty much spent all weekend programming these pieces on the CNC, and am working on getting them machined now.  I also cut the tubes for the runners, but they need more work to put in a taper and give the squarish port shape at the head.  Hopefully by next weekend I will have all this stuff done and have most of the intake welded together.  I will take lots of pictures as I go to help anyone who might be thinking about doing this themselves.

There were a bunch of other smaller projects on the car that I wanted to get done with this weekend, but just ran out of time; seems like the big projects end up expanding to fill all the time slots I have allotted for this project.  Next weekend I should finally have the heads, so those have to go together and get installed next weekend also.  Right now it looks like late August before I will have everything done and the engine on the dyno.  Its going to be another last minute thrash, all right...

[cjshaker]

Interesting design on the intake. Are the runners lengthened on this one? Also curious if you're hand making the tubes, machining them or if they are something you bought? If bought, where did you get them?

I'd still be nervous on the rod thing. It's just hard to imagine the rods stretching at a +.042 per cycle and having any lifespan at all. Has anyone come up with experiences as to what distance they start to see piston to head contact from rod stretch? Seems that I've always heard about .030 as a safe minimum, but that's with much lighter pistons. That last piston picture, with the thin clay, seems a bit close, especially if there were piston rock due to the offset chamber. But I'd surely think there would be witness marks if it were hitting the head.

Good luck getting everything together. I'd really like to see the car in person at Drag Week.

[jayb]

The runners are shortened compared to the last intake.  Last year HP peak was in the 6600-6700 RPM range.  I wanted to move that up a few hundred RPM, so I took a half inch off the length of the runners.  I will show some details on how I make the runners next week, assuming I get them done by then.  For this intake I am starting with 2-3/4" OD tubing that I buy from Summit.  It gets cut to the required length, then a pie shaped section is cut out of it lengthwise to give it some taper.  Then I will use some forms I'm machining for the tubing to wrap around; the tubing is tightened up to the shape of the forms with a bunch of hose clamps.  Then I tack the seam together, pull off the hose clamps and remove the forms, and finish welding the seam on the tube.  Then it is ready for welding to the base of the intake.

"Nervous" doesn't begin to describe how I am feeling about the connecting rod situation.  Without that smoking gun, I really don't have a good feel for what is going wrong there, or if the steps I'm taking will fix the problem.  I've never gone thicker than 0.042" on head gaskets, but maybe that is what I will need to do on this one.  I hope so; between thicker head gaskets and the other steps I'm taking to try to address any harmonics issues, maybe the problem will be resolved.  Hopefully I'll find out on the next teardown, and not at the track 

[machoneman]

+1 on a thicker head gasket. IIRC, once together, more dyno time is in order.

Then, although it's a major pain, maybe a partial teardown to pull #5 to check the chamber, piston top and most importantly, the rod's pin end. 

[jayb]

Wish I had time to dyno, then tear the engine down and check again.  If it runs good on the dyno, I'm putting it directly in the car.  No time to make the event if I tried to do anything else.

The next two copper head gasket thicknesses that are available are 0.050" and 0.062".  If I went with 0.062", I'd drop compression from 13.4:1 to 12.75:1.  Too much, I think, so I ordered a set of 0.050" head gaskets today.  Hopefully they will be ready by the end of this week, so I can still get the heads on over the weekend...

[My427stang]

This is a bit of an odd idea, but what if crank centerline was off compared to the bore, or the #5 rod journal itself was off enough to try to bend the rod?

I'd think that you'd see it in a scuffing piston, but if it was off in the direction of the pin bore, maybe you wouldn't?

Spit balling for discussion, not anything I have seen or would expect but trying to understand what was trying to reshape the small end of the rod that you haven't already checked

[jayb]

I would think that if the rod journal was off like that, it would show up in rod bearing wear.  And it would also show up on the #1 cylinder.  But the bearings looked great, and the #1 rod doesn't have any problems.  And for the crank centerline idea, I'd think it would affect #1 and #5, not just #5.

But who knows?  I'm guessing on this too.  FYI the crank was checked for straightness after the rod broke in 2014, and it was perfect according to the shop.

[ScotiaFE]

The little devil, mechanical resonance?

[Cyclone03]

another spit ball from left field....
is it possible the doom itself is distorting into the head?

Tuned to the limit 2 strokes will "just" sink the dome ,toss em after the race , can the weight/rpm cause the dome to get flexxy and tap the head?   


How about an ignition miss lowering the cylinder pressure allowing the piston/rod to fly unloaded?

[machoneman]

Hard to do with an auto tranny but....lifting off the gas pedal easy at the stripe while hitting the brakes may also help. I don't know of Jay's technique but avoiding an instant "snapped 100% shut" throttle at the finish line is always a good idea, although it is easier with a stick tranny, and keeps the pistons/rods from extreme rpm deceleration.   

[mike7570]

I think it was Heo who used small ball bearings to keep the studs from bottoming and possibly side loading some of the threads.
How are your head studs around #5 anything look a little off?

[Cyclone03]

Jay looking at the second picture ,with the head gasket,right on the edge the piston is very clean,quench ? I think the thicker head gasket will help. The piston has the look on that edge of "just" touching.

[jayb]

I think it was Heo who used small ball bearings to keep the studs from bottoming and possibly side loading some of the threads.
How are your head studs around #5 anything look a little off?

The Shelby studs are actually tapered on the end that goes into the block, and go into a conical seat below the threads to center the stud.  Shelby says torque them to 10 foot pounds.  They look OK to me...

[jayb]

Jay looking at the second picture ,with the head gasket,right on the edge the piston is very clean,quench ? I think the thicker head gasket will help. The piston has the look on that edge of "just" touching.

I noticed that too, and it is actually present on several of the other pistons.  Plus #5 measured closer to the deck than any of the others.  I'm hoping the thicker head gasket resolves the issue.

[Joe-JDC]

Picture #6 seems to have some interference on the chamber edge from something.  There is a definite indent in the picture.  It matches the piston top in picture #2.   Maybe from something else, but looks suspicious.  Joe-JDC

[jayb]

You mean at the edge of the chamber just below the exhaust valve centerline?  I saw that too, but the clay didn't show any closeness with the piston there.  I think that may have happened during the rod throwing incident in 2014.  Who knows how that piston was flying around in there...

[Joe-JDC]

The problem is this:  An iron/steel rod typically needs .050" growth room in an engine, and the aluminum rods need more.  Checking this in a cold setting will give you a clearance that is not there in a running hot engine.  That piston seems to be touching the head where it is shiny.  I have seen 427 pistons with the combustion chamber imprinted on the top of the piston when hot, but check OK cold, with .038" being minimum head to piston clearance with steel rods.  That aluminum block may be moving more than you imagine when under load and stress of combustion heat.  This may be a detonation slap causing the piston to rock just enough to hit the combustion chamber on that side.  I would check piston movement at TDC with the clay in that area and then measure it again.  Joe-JDC

[Barry_R]

I can verify JDC's dimension comment to a point.  On one of my wedge deals I ran the piston out of the hole a bit.  If I recall correctly I was at +/-.036 piston to head.  When we tore that one down it had very clear images of the head surface on the piston - right down to the machining marks from the old Van Norman we were using back then to deck the heads.  No measurable deformation or stepped raised area - just like a photograph or etching.

We were obviously - and accidentally - as close as we could get.  Some kind of transient or occasional side load or contact still feels like a more likely cause for the rod failures than does sheer luck or harmonics.

[machoneman]

Jay, some outfit makes a polymer (?) that one can literally pour into a combustion chamber to make a copy of the dead space. I did see this some time ago in the JE Pistons site but can't remember the name or type of compound. This would make a great backup to the clay method as this flexible, semi-stiff material, when set, could even be calipered.

Found it!

https://www.jepistons.com/TechCorner/Mold-Making.aspx