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Thread: L84 with LT4 blower at 10psi blew out rear seal around 3k rpm

  1. #1
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    L84 with LT4 blower at 10psi blew out rear seal around 3k rpm

    What?s everyone?s recommendation for a rear seal replacement type for a SBE 19 L84 with an LT4 blower @10psi, looking to possibly go down a pulley size for another +2 psi in future.
    I?ve found the oem one 12665076 but any way mechanically retain it from pushing out again?
    I understand this root cause is likely very sigh crankcase pressure (wasn?t timing related as it?s been pretty conservative in this range for months with no spark table changes and have many pulls under the same conditions) during that mid-rpm pull so that leads to my next, more important question of: what catch can is everyone running on their LT engine after going from NA to FI? I do not have the LT4 style valley under the blower.
    Thanks in advance,
    Robert

  2. #2
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    Quote Originally Posted by rbass View Post
    What?s everyone?s recommendation for a rear seal replacement type for a SBE 19 L84 with an LT4 blower @10psi, looking to possibly go down a pulley size for another +2 psi in future.
    I?ve found the oem one 12665076 but any way mechanically retain it from pushing out again?
    I understand this root cause is likely very sigh crankcase pressure (wasn?t timing related as it?s been pretty conservative in this range for months with no spark table changes and have many pulls under the same conditions) during that mid-rpm pull so that leads to my next, more important question of: what catch can is everyone running on their LT engine after going from NA to FI? I do not have the LT4 style valley under the blower.
    Thanks in advance,
    Robert
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    These engines seem to have ring issues from what im seeing with higher mileage. They don't seem to be as good as the l83s or l86s in that regard. They're good to 7 or 8 psi then can head downhill after that if they have any miles much. I just tuned a rear mount with super safe timing that didn't have issues until 6 psi and hes running both covers via large hoses to catch cans and atmosphere with the turbo intake pulling vacuum on the system to boot. He tossed in another seal and its holding till he can build the motor for the moment.

  4. #4
    Senior Tuner edcmat-l1's Avatar
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    It's not the rear main seal's fault. It's the hurt piston. Time for a leak down test. A catch can isn't going to fix it. Big valve cover vents aren't going to fix it. If it has enough crank case pressure to push the seal out the engine is hurt.
    Last edited by edcmat-l1; 3 Weeks Ago at 07:31 AM.

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  5. #5
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    I know the one I was tuning didn't damage the pistons. It runs way too smooth plus I always run safe timing when tuning in the fuel. His was running 9 degrees of timing plus the OE's timing pullers for tip in and temperature when the rear blew out on 6psi. It's currently handling 8.1 psi with slightly more timing with a new rear main. No misfires, no roughness. The owner claimed to have found a bunch of people talking about the same thing where the rings aren't holding their tension after so many miles. I know if you don't clean them and flush them regularly they have issues. Also heard a lot of people having rod bearing issues with them in stock form. Guess the oe components aren't what they were, so I'm more cautious in general especially during the preliminary stages of tuning with these newer ones... Compression check would say for sure for the OP, but I doubt the pistons are damaged unless something was really wrong in the cal...
    Last edited by GHuggins; 3 Weeks Ago at 09:22 PM.
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  6. #6
    Senior Tuner edcmat-l1's Avatar
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    That would be pretty crazy if it's a just a matter of the rings not sealing. Can definitely say I have not seen that and it would scare the shit out of me if it happened on my watch.

    Reminds me of the old stories about the earliest small blocks rings not sealing. "Yeah GM's fix was throw some Borax down the carb". Oy Vye. Can't imagine LOL

    It would be interesting the see the results of a compression and leakdown test. Wonder if this is, can be the result of a broken piston ringland between the top and second ring. Seen that many times. The top of the piston will look fine, the rings intact, but when you pull the piston a piece comes out from between the rings.
    Last edited by edcmat-l1; 3 Weeks Ago at 09:10 AM.

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    Thanks everyone for the input. I?ll perform compression and leak down tests, fully document and report back. Pulled the trigger on the Mighty Mouse ? 2014+ Gen V LT Truck Wild Catch Can with -12 filler connector directly from MM. Will take me some time but should have results toward end of this week. Not going to crack the engine open just yet unless necessary based on findings above. Really appreciate everyone and their insight!

  8. #8
    Senior Tuner edcmat-l1's Avatar
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    If you don't have one a decent bore scope is a great tool to have. I've had a few over the past 20 years or so. The current one I have is an Amazon special costs like 89 bucks or something. It uses your phone as the screen and it is articulating. You can stick it in a plug whole and look at the entire cylinder is you turn it right.

    Something like this right here.

    https://www.amazon.com/Articulating-...zcF9tdGY&psc=1

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  9. #9
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    I blew out my RMS also. and after speaking to friend that runs engine testing on a dyno for gm. he gave me a different seal and also said to read up on oil amount. gm reduced the oil amount on 2 engines. so i run 8 but really it is less with my huge filter and massive oil line to the remote filter.

    Anyways that was at 15lb but spiked to 19 on expressway pull up to 135mph.
    What also works it my elite dual(boost) catch can with oil cap vent. then after the issue I did like most higher boost gen v do and added a dual breather tank with 5/8" line to each valve cover. cheap option(~$100) and works better then anything aside from a dedicated vac. system.
    I also deleted the brake pump.

  10. #10
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    The front seal on LT engines is weak. I've read it's a low friction blah blah blah..

    Most shops know to use the LS3 crank seal on them.

    I've never heard anything about the rear being easy to blow out.
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  11. #11
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    probably one piston is cracked,
    i have seen l83 runing smooth with 3-4 cracked pistons,
    i dont think it will push rear main seal with that low boost.