head gaskets: take two

[AaronGTR]

Time for an update. I got the heads back a couple weeks ago but hadn't had time to take anything off them and double check them. When I picked them up the guy who worked on them told me he cleaned them and remeasured everything and nothing had changed. He said the deck surface measurements were the same, and the run out on all the valve guides was within specs. He went ahead and decked them .002" to make sure they were perfectly flat, touched up the valve job to make sure they were perfect, replaced the one bad spring shim, put all new valve seals on, and reassembled them.

When I found the problem and took them in, I had told him about using the LS1 shims and 3.1 seals with the LS springs and that the shims didn't fit my stock valve guide diameter without drilling them out. He said he used a later model LS spring and it dropped right on. Well duh.... he didn't replace the valve guide and the outside is smaller than it used to be. This made me suspicious so I pulled off several of the other springs on both heads as well to check them out. Every single valve guide is like that. Here's a little better picture.



The shims used to be a fairly close fit, but now they are all loose. He said the bottom of the springs shouldn't move around that much... but I'm not so sure. Maybe it was enough to let that one get worn down? That doesn't explain why none of the others did that, or why that valve guide was leaking oil and none of the others were. There is still about 1/8" between the shim and the bottom of the seal and the spring would keep the shim pressed against the head so it wouldn't touch the seal. And all the seals seem pretty tight on the guides and around the valve stems, so I don't know why they would leak.

Since every guide is like that and they are far too smooth and uniform for it to be some kind of wear, I can only assume it's something the shop did, but why? I don't know if it was required for the tool they used to machine down the spring pocket, if it was accidental. Or maybe they just did it to fit the seals they were going to use? But if that was the case, why wouldn't they have called me and asked what seals I was using instead of just machining the guides and changing seals without notifying me? Anyway, for now I'm still not going to use these heads. The ones on the car don't seem to have any issues and I care more about reliability at this point than outright power. I might use them again in the future if I get a different cam again and have another car to drive. I might try getting a new set of LS1 shims and seeing if they fit tight without drilling them out now... or I might see if the stock one piece shims/seals will fit over them and stay tight and swap out the LS6 springs for Comp ones. That will be in the future though.

[AaronGTR]

On another note, I think I have an idea of why the original set of head gaskets might have blown.

I've been doing some more street tuning since getting the blower back on the car. Even though I'm using a bigger pulley and running less boost than before (2.5" vs 2.2") I'm actually getting detonation showing on the knock sensor much earlier now than before. The total amount of KR is no more than before, about 2-3 degrees tops, but it start almost as soon as I enter boost and before it didn't start until around 5psi+. My only guess is that since the old engine was worn and had more piston to wall clearance that the rings were also worn and allowing more blow by and that peak cylinder pressures were lower. If they are higher now that could contribute to the KR. Anyway...

While trying to log some WOT pulls to adjust the timing, I noticed it wasn't shifting like it should. It seemed like the trans was slipping or waiting to shift into the next gear and the engine was bouncing off the rev limiter 2-3 times. I don't really do any WOT shifting on the street like I would do at the track (it's not very safe) so I hadn't had a chance to figure this problem out, and it blew on the second run at the track before I had a chance.

I was using an excel sheet from the original DHP tuning forum to figure out my shift points. It was originally made for grand prixs, but you could put in whatever tire sizes and gear ratios you wanted and it would figure out RPM vs MPH, which is what you need to plug into the WOT shift tables. Well apparently something in the formulas still isn't right for grand ams. It was telling me I would shift from 2nd to 3rd at 84pmh at 6000rpm. Well on my scans I was seeing 6000rpm at 78mph. Went back and looked at my scans from the track and it did the same thing. Rev limit was set at 6300, but it would bounce off the limiter until the cars acceleration momentum carried it to 84mph and then grab the next gear, and it hit 6500 rpm at one point.

It blew right when it shifted into 3rd so I can only assume that the combination of maxed out injectors and the high rpm along with fuel cut from the rev limiter caused a sudden massive detonation. If it was bad enough and sudden enough it might not have shown on the knock sensor (I didn't see any KR at that point in the scan). I also have been having a problem with the analog input on the scanner that reads the wideband freezing up and not refreshing the data. This happened at the track too, so unfortunately I have no actual data about detonation or the AFR when it happened. That's the best explanation I can come up with though.

For now I changed the shift tables and set my 2-3 WOT shift to 78mph and now it shifts into gear crisply and right away. Now if I can get rid of the KR so it's not pounding my pistons/rods/bearings all the time I'll be all set.