Down in a hole

Discussion in 'Engine Topic' started by frankenZ, Aug 1, 2016.

  1. frankenZ

    frankenZ Member

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    My 355 has some two relief flat tops, 1094 felpro head gaskets and 62 cc camelhumps. Problem is my flat tops are below deck about .062 and I am wondering what my compression ratio might be with that lack of quench ?!?:(
     
  2. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Wow, that's way down in there, almost sounds like a piston/rod mismatch. I don't think jobber pistons are that far down. You are measuring on the block deck, not the head gasket?
     
  3. frankenZ

    frankenZ Member

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    Block deck without gasket. Dial indicator. 1/16 of an inch
     
  4. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    I'm guessing it could be down that far with a rebuilder piston. Normal quench is somewhere in the .040 - .045 with gasket, but there are a lot of caveats to that number. Did you check all pistons, all down the same? Is this a new rebuild you are doing?
     
  5. fatkid_racing

    fatkid_racing Veteran Member Lifetime Gold Member

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    .062 in the hole?!?!? Cripes that's deep. Anyway assuming the reliefs in the pistons are typical rebuilder depth my guess is around 8cc, with your .015 gasket you should be in the ball park of 9.4:1 static. .077 quench concerns me you might get away with it if you've got a functional egr system. I would make sure you had it at true TDC, and measure with a good straight edge and feeler gauges to back up what your dial indicator said. If you've got the information post up the valve timing events off your cam card and I'll crunch the numbers for your dynamic compression.
     
    Last edited: Aug 1, 2016
  6. CasperCasper

    CasperCasper Veteran Member

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    I just ran through the exercise of measuring quench on my LS and would not have been able to find TDC properly without a dial gauge. Did you measure more than one hole ? Did you use a dial gauge to find TDC ?
     
  7. frankenZ

    frankenZ Member

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    I picked up the motor for cheap and it was a fresh rebuild and the heads were done but were a set of 336. I took the heads off to slap on my set of camelhumps which are milled down. The cam going in is a comp cams xe268. Cam coming out is an unspecified rv grind. I was surprised to see number one that far in so I checked 1-8-4 to the same results.
     
  8. thrasher

    thrasher Veteran Member

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    I've definitely seen crappy rebuilder pistons that far down in the whole.
    Good quench (top of piston to flat cylinder head surface including head gasket) is supposed to be what .035-.045 and you're already past that by a longshot.

    I haven't added anything up but I'm guessing compression somewhere around 9:1 compression. That's IF your combustion chambers are actually 62cc's. Many times combustion chambers are larger than stated.

    If you're hell bent on using that engine with those pistons I'd back your cam choice down a step for sure. I know guys that will say it'll run good on that compression but I don't follow those rules and I don't pay attention to an advertiser/manufacturers bull_ _ _ _. I like torque all over the place.
    When you run compression that's at the low end of the scale for a particular cam profile it gives up throttle response, bottom end torque, and midrange torque. Add those up and you get an engine that will be accelerating through it's usable range slower. In my mind it doesn't make for a good performance engine.

    I'd want a TRUE compression of 10:1 with the XE268, but that's me.
     
    Last edited: Aug 10, 2016
  9. fatkid_racing

    fatkid_racing Veteran Member Lifetime Gold Member

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    As thrasher said, it needs less cam if you're going to run those slugs. He guessed 9:1 and my math says 9.4:1 so I feel we're in the ball park. With the comp grind you mentioned you'll only achieve 6.8:1 dynamic compression. It wouldn't be fun at all. You either need different pistons or a lot less camshaft.
     
  10. frankenZ

    frankenZ Member

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    I came in today and cleaned up all the mating surfaces and bolts. Checked twice and got .048 on all pistons. Which is better...
     

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