Tahoe Motor Is Blown Up

[turbonator]

Well I took it all apart,I'm leaning towards machining a huge dish in the piston. I drilled a hole in an old piston,they are .650 thick in the center,which is the thinnest part.

When I was studying Mechanical engineering, we always discussed destructive and non-destructive testing....you my friend are the master of destructive test methods...LOL. Great info on the crown thickness by the way.....Jeff and I have been discussing a method of having a type of flapper with an opening to be able to put a heat gun in to be able to start the engine.....we still have our new 6.2L pistons here (not machined yet) and are tossing around the idea of a modified pre-cup only to seal the fire-ring in the head and maybe trying a DI.....(picked up a Bosch inline 8 cyl IP today for $25.00 and the labour in the bin to remove it from the motor....LOL)

 

[6.2 turbo]

I calculated the compression,it was at 17.68-1 . I think I'll take it down to 12.8-1 .

 

[buddy]

Dishing the piston is the best way, to keep the ring height on the outside to protect the ring. I really cannot believe 40psi of boost and 18:1 CR would bend rods. Like you said, you had run it before and not had bent rods. Maybe it had something to do with the IP setup and an affect on advance the modifications may have had. My suggestion would be aiming for 16:1 CR if lowering it to run about 40psi of boost. That should put you at similar cylinder pressures as those of us with 20:1 CR and pushing 25psi of boost. Except you would have significant lower cylinder temps with the lower CR as long as IATs are kept reasonable.

To do that, depending on how decked your block has been, or if you are starting with stock compression height pistons, you could leave a 1/3" ring on the outside of the piston and go down 0.12" to get down to 16:1 CR. Or leave 1/2" ring on edge and go down about 0.17"

But lets say block has been decked .01" and you use a copper gasket that is only .035" thick (stock gasket is .045") and your pistons are already shaved .03"

Then you would be right back to 20.2:1 CR, so the piston dishing is the same required.

 

[3500GMC]

Would one be too concerned about the recardo bowl area not being there? Or the quench area that the pre-cup needs?

 

[buddy]

I think the swirl bowls will have more of an affect on an NA engine, but once we start boosting past stock levels they dont do as much guiding of air and combustion. The dish will probably help keep more of the force down having more piston surface to catch before getting on the edges down to the ring.

 

[turbonator]

Not sure how well it would run at 12-13:1, but I like the idea of a new style bowl. Also believe that it would be of benefit to reducing somewhat the potential of blowby, with the explosion being better contained centrally than with the shallower Ricardo bowl......think that the pre-cup is needing some drastic opening up too. In our discussions here, we are vacillating between 16:1 and 17.5:1 and have talked in depth about totally opening up the cup and using a DI system for fueling. Our rods this time (so far anyways), will be smoothed, weighed and ground accordingly for balancing purposes and have cryogenic treating to try to aid in their strengthening. As of yet, no plans to go over 40 psi boost.

 

[6.2 turbo]

I think the small precup hole might be a problem,and cause a pressure spike,on the compression stroke. I did run it around the block one time with too much timing,maybe that bent the rods. A motor with high boost and not much fuel might last longer. I should get my pump cc's checked.

 

[buddy]

What precups do you have? If you have small ones that is a problem.

I would not want to completely hog them out, because I would still want them to guide the flame front towads the center of the piston and not any more right below it into the piston ring. I had a set machined out slightly larger than diamonds. Not sure if I am going to use them now, because I would rather start with a set of 6.2NA cups that are thicker and hog the mouth bigger on them, so I lose less CR.

 

[6.2 turbo]

I think they are O stamped,not as big as the T stamped or diamond.

 

[buddy]

O yeah, you need to get some diamonds or a little larger in there. The smaller openings slow your combustion expansion, so I cannot see how you would be getting complete burn.

I would expect really high EGTs and high cylinder temps because of it. Diamond precups will lower the CR some and probably give you faster spooling for a larger turbo, more power and lower EGTs.

What were your EGTs like under high acceleration/load.

 

[sshewins]

Jim, The only other place I've heard cryogenic and diesel in the same sentence, is over at great lakes diesel (.com) in New York. They are doing this kinda stuff with 15/16 liter caterpilar engines. According to their site, the do a lot of engines for the trucks that run in 'el rodeo' up by you in Quebec. Whats involved in that process and how does it help us?

 

[littleboy]

Lower the compression the lower the power and the less street friendly an engine becomes. I wouldn't go under 15-1

 

[6.2 turbo]

The egt's were fine,as long as I didn't lug it below the turbo. I know what happens with very low compression,but the power can be gotten back plus more. A bigger pre cup hole might help,but probably not to the point of saving the rods. I ran the smallest precups in my 6.2,it didn't throw rods,but had very low compression. I estimated 16-1,but was probably a good bit lower.

 

[buddy]

The egt's were fine,as long as I didn't lug it below the turbo. I know what happens with very low compression,but the power can be gotten back plus more. A bigger pre cup hole might help,but probably not to the point of saving the rods. I ran the smallest precups in my 6.2,it didn't throw rods,but had very low compression. I estimated 16-1,but was probably a good bit lower.

Do you have a range of what fine EGTs are for you? Everyone has a different idea there, like would you think 1500F preturbo were fine because of all the boost and fuel you were running, or they were normal fine like 1100F :hihi:

Was there any difference in the bearings used on the crank mains or journals? Any signs they were getting hotter than normal?

Thinking about your oil cooling/filter setup. Do you have a sketch of the flow of oil? Can you be sure the oil cooler is not restricted with so many rebuilds or has it be changed? With the concrete in the lower block I am just trying to think of a combination of things to heat up the rods making them more prone to bending.

 

[turbonator]

I think the small precup hole might be a problem,and cause a pressure spike,on the compression stroke. I did run it around the block one time with too much timing,maybe that bent the rods. A motor with high boost and not much fuel might last longer. I should get my pump cc's checked.

:agreed: This is exactly the route that we are proceeding in......we would like to do a tweek, test run it to verify usability then flow bench it to see what the #'s actually are. This combined with a dyno and all our gauge readings will give me plenty of info to crunch for a least a couple of days without sleep.....LOL.

So many theoretical #'s get tossed around, but without actual flow testing, we will never really know. :agreed:

 

[6.2 turbo]

The rods and cyl walls look brand new,nothing is blue or black,the oil even smelled good. The bearings are in fair condition,not smashed at all. One of the rods had a bi-metal bearing,it was good also. The highest egt's were 1400 full throttle and boost in overdrive,about 2500 rpm's. Full throttle in any other gear was lower egts . If my oil cooler was restricted it would show up on the oil psi gauge,there is no bypass anywhere except in the oil pump itself. For sure the top of the oilpan gets a good bit hotter than the bottom.

 

[6.2 turbo]

The cyl wear marks start exactly were the concrete stops at the top.

 

[NVW]

Is the block toast?

 

[6.2 turbo]

The piston that's gouged matches the factory clearance notch at the bottom of the cyl. Both times it blew up was after 30-45 min of constant driving,water temp was a little under 180 degrees. I have an oil temp gauge but not installed. Not so worried about the wear marks,except that it starts with the concrete. It could also just be because I didn't get the cyls bored ? When I assembled the motor the first time I checked piston to wall clearance,was just a hair under .005 .

 

[6.2 turbo]

I can still use the block,but I'm not sure if I should.