Need educated guesses

Discussion in 'High Performance Modifications' started by biker, Oct 5, 2017.

  1. G72Zed

    G72Zed Veteran Member

    376
    21
    Sep 8, 2015
    Canada
    Yep, Tom nailed it, I thought sure it would have been higher.

    I come in at 193 psi, give or take 2 psi, tried 3 gauges just to make sure. Carb open, warm, and 5 pumps, I have 10.57 SCR and 260 dur @.050.
     
  2. biker

    biker Veteran Member

    Ok, that is higher than my readings for sure. My vacuum reading at 850 rpm is 7 inches. at 1000 it is 11. Going to do a leakdown test as well. I'm suspecting that my static compression may not be where I think. Pistons were slightly above deck when installed, so machinist recommended a fairly thick MLS head gasket. If leakdown test checks out ok, I may re-think the head gasket thickness to up the SCR.
     
  3. G72Zed

    G72Zed Veteran Member

    376
    21
    Sep 8, 2015
    Canada
    Yes, it's high for the SCR and effective Dur @ .050, it's actually (262 @.050) but I am using a custom ring pack from Total Seal, and it included the 2 piece Gapless Top's.
    Hence my reference to the +12 to 15 psi cranking pressures.

    How much was it out of the hole, and how thick of an MLS was used?
     
  4. biker

    biker Veteran Member

    If I remember correctly, pistons were .017 out of the hole and we used a .061 gasket. I will look up old emails to be sure. I believe the valve reliefs in the pistons amounted to 12cc. Were I to do it again myself, I would use the gapless rings for sure. I was skeptical due to past reviews of them, but I see after last night's research that they have made improvements. Any thoughts or recommendations on my combo are welcome! I'm thinking I can bump the static compression considering the cam duration.
     
  5. biker

    biker Veteran Member

    Crankcase pressure does not seem to be an issue, I'm using a fixed orifice PCV. Proform aluminum slant valve covers with a very basic PCV baffle. Still seems to pull some oil through PCV, so I'm adding a moroso grommet with integral baffle to see if that helps. Performance Distributors HEI, 32 degrees total timing at 3000rpm, vacuum advance kicks in at about 4.5 inches of vacuum. I was running ported vacuum advance, but just switched to full vacuum which seems fine as long as I can get the pilot circuit rich enough. If not, engine tries to die occasionally which kills vacuum advance which then kills idle.
     
  6. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

    8,806
    130
    Nov 4, 2004
    Ringgold, GA
    You might do better if you set up for ported vacuum advance. My cam is similar to yours and I've never been able to get a stable idle with full vacuum.
     
  7. G72Zed

    G72Zed Veteran Member

    376
    21
    Sep 8, 2015
    Canada
    Well, that gets the piston to head clearance around .044, take it down 6 thou to .038 and your in the sweet spot, but you only go from 10.09 to 10.2 with the 1 cc less. Quickest way would be to get the right spec thickness (.055) HG, and shave the head to get the SCR up to where you want it.

    More involved would be to shave the piston tops 17 thou, (if safe by the manufacturer), use a 1003 or 1010 HG, and go with the Total Seal Gapless rings. The Gapless Top rings will in fact add 12-15 psi at the min, that would get you at 189-192 psi. I got 20 psi in my testing.
     
  8. biker

    biker Veteran Member

    I've had the same experience, I was running ported for the past couple months and idle was fairly stable. Any issues I had were mostly due to lean primary circuit. I do like the cleaner idle and improved throttle response when full vacuum is pulling on the vacuum advance, so I'm trying it again. I know it's maybe not the most user friendly cam, but I should be able to get a consistent turn-key start and idle with no drama. It's a 4 speed car, so it's nice to have a consistent idle at stop lights etc.
     
  9. biker

    biker Veteran Member

    Wow. Had no idea the rings were that good. And yes, I agree with your assessment on the quench zone. I will see how the leakdown goes before I make a final decision. Thanks again for your help. Any thoughts on make of head gasket?
     
  10. COPO

    COPO Veteran Member Lifetime Gold Member

    15,836
    163
    Sep 15, 1999
    Ontario, Canada
    One head of mine avg is 213
    Other head avg is 200

    Roller Cam, 10.2 cr
    • Advertised Duration (Int/Exh): 278/288
    • Duration @ .050 (Int/Exh): 232/242
    • Gross Valve Lift (Int/Exh): .507/.507
    • LSA/ICL: 112/108
    • Valve Lash (Int/Exh): Hyd/Hyd
    • RPM Range: 2000-6400
     

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