Dissected Turbo 3100 Pistons 5 & 6, piston noise

[Guest]

2000 3100 short block purchased with very low miles as a near new take out. Installed the Fiero 2.8 top end and accessories, turbocharged and put it in the car about 5 years ago. Immediately on startup there was a tapping noise that sounded like a loose rocker that I figured out was piston noise right away which would explain why it was removed though hardly used.

I drove the car hard running an average of 7 psi and sometimes well into the high rpm range. Sensing the time was near for replacement given the noise was getting louder and was always annoying I finally removed it from the car in preparation for the 3900 transplant.

I broke the engine down eager to look at the internals to see what effect the turbo had on the engine. The pictures tell the story, GM knew all along pistons # 5 & 6 were a trouble spot, I can't believe they put two different types of pistons in the same engine, pistons 5 & 6 were the only ones that had the antifriction patch and no it wasn't rubbed away on the others.

Nearly all of the pistons had excessive clearance especially # 6 which was probably going to let go any day soon. Despite occassional detonation and high revs the rod bearings looked great, not a hint of copper or the excessive wear that I was use to seeing on 2.8L bearings from a high mileage engine, they looked so unremarkable I didn't bother to pull the main caps.

I'm going to try and link pictures from my FTP site, if I can't get it to work I'll post using standard procedure.



It didn't work, oh well.

The worst of the pistons was # 6 and they are head to head in opposing pairs 1-2, 3-4 and 5-6. Look at the ring gap on number 6 in the bore. It's amazing number 6 was still in one piece, and nearly all of the pistons had excessive bore clearance but 6 was the worst by far.

[Joseph Upson]

Except for the noise the engine ran strong and didn't put out a puff of smoke.
[/img]http://josephupson.60degreev6.com/DSCF0672.jpg[/img]

[CNCguy]

Not trying to take your thread off-topic but it makes sense to me as I've read that most wear and detonation is at the far end of the engine due to the engine being hotter at that end.

Hopefully the oil squirters will resolve some of that problem. Initially I thought the U-flow system would help but I'm not so sure now. I made a little sketch of the system flow and can't see how it would really help. Just like before, the coolant is already hot by the time it reaches the rear of the engine. And with flow being low at lower rpm and pump speed, it seems as though cooling may be worse at the rear of the engine because the coolant will take the path of least resistance. Perhaps an electric pump would be beneficial on the new engines. I guess time will tell.

All of the coolant holes in the new head gaskets are small except one at the rear of the engine. This is to force coolant flow around the cylinder walls to the rear of the engine and return to the front through the head. But if buildup or other restriction occurs around that hole, flow will be restricted to that end of the block and head. With the old system, restrictions only limited the flow around the cylinder walls but the coolant still flowed across the head as it exited the opposite end of the engine. IMO cooling system maintenance is going to be even more important on the newer engines.

Although it wouldn't be easy to do, the better way to route the coolant would be to pump it into the side of the block in the center or at each cylinder and have it exit out both end of the heads. That would give the most even flow of fresh coolant across the cylinder walls.

[Joseph Upson]

Not trying to take your thread off-topic but it makes sense to me as I've read that most wear and detonation is at the far end of the engine due to the engine being hotter at that end.

Hopefully the oil squirters will resolve some of that problem. Initially I thought the U-flow system would help but I'm not so sure now. I made a little sketch of the system flow and can't see how it would really help. Just like before, the coolant is already hot by the time it reaches the rear of the engine. And with flow being low at lower rpm and pump speed, it seems as though cooling may be worse at the rear of the engine because the coolant will take the path of least resistance. Perhaps an electric pump would be beneficial on the new engines. I guess time will tell.

All of the coolant holes in the new head gaskets are small except one at the rear of the engine. This is to force coolant flow around the cylinder walls to the rear of the engine and return to the front through the head. But if buildup or other restriction occurs around that hole, flow will be restricted to that end of the block and head. With the old system, restrictions only limited the flow around the cylinder walls but the coolant still flowed across the head as it exited the opposite end of the engine. IMO cooling system maintenance is going to be even more important on the newer engines.

Although it wouldn't be easy to do, the better way to route the coolant would be to pump it into the side of the block in the center or at each cylinder and have it exit out both end of the heads. That would give the most even flow of fresh coolant across the cylinder walls.

U-flow appears to be new for fwd engines, the Fiero coolant flow is already setup that way with inlet and outlet in approximately the same loctions in the front on the side of the engine. The one unique characteristic here is the likely increased efficiency of coolant flow that results from the following two changes: water flows un impeded through the head at a faster rate that makes filling easier because there is no thermostat up ahead to stop it at the end of the block and hold it over cylinders 5 & 6 having been moved to the pump coolant entrance, and it exits the end of the head through a nice size port instead of channeling through extra turns in an intake.

I'm gonna have to give GM the benefit of the doubt here because they have so much riding on the new platform, all of their fwd cars that have a V6 option. I'll go back and read the lit on their site about the U-flow benefit which would have been even better if it were reverse flow.

[Joseph Upson]

Didn't find anything fancy about U-flow benefits sited. I did notice that the 3900 fuel shut off has been bumped up from 6000 to 6400 rpm retroactively for the 06 and up confirming my theorizing some time ago that the engine could handle higher rpm.

The horsepower should still be the same although the numbers are coming up lower because they are rating the newer engines at a lower rpm than the 6K rpm the first engines were rated at. I had already planned to set the rev limit at 6500 unless my cam shows signs of needing to go to 7k to get short of breath.

[bszopi]

As far as your problems with the FTP, it was a problem with the filename. It is case-sensitive. You had ".jpg" in the thread, but on the server it was ".JPG". You wouldn't think it would make a difference, but it does.

[Joseph Upson]

As far as your problems with the FTP, it was a problem with the filename. It is case-sensitive. You had ".jpg" in the thread, but on the server it was ".JPG". You wouldn't think it would make a difference, but it does.

This is a test to post pics from FTP:

http://josephupson.60degreev6.com/DSCF0672.JPG

Works, had to remove the [img] thingies.

[Chris88CL]

I had similar issues with my 3400. Excessive piston skirt to bore clearances on 5 & 6. Lots of scuffing on the piston skirts aswell.