cranking pressue too high

Discussion in 'High Performance Modifications' started by my71bmf, May 6, 2011.

  1. my71bmf

    my71bmf Veteran Member

    314
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    May 26, 2008
    Tulsa, Ok
    I posted an issue to engine vibration last year, and I just now had a chance to figure it out. My cranking psi is 220 lbs on the low side and 230 lbs on the high side. I'm willing to bet on detonation. So I called Comp cams and they recommended retarding the cam 4*, since they grind them at +4*. I put that -4* figure in my Desktop Dyno and it actually showed an increase in power and torque. How much psi should I expect to loose? There guy said around 25 to 30 psi, which would put me back in pump gas range. Any suggestions??????
     
  2. AJ_72

    AJ_72 Veteran Member

    5,122
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    Jul 3, 2003
    Chebanse, IL USA
    IIRC, 180-190 cold cranking cylinder pressure is OK for pump gas (91 octane).

    Should get you close. Only one way to find out.

    [​IMG]
     
  3. Rich Schmidt

    Rich Schmidt Veteran Member

    3,966
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    Mar 27, 2010
    Denville NJ
    It really depends on the combination. My first car was a67 Olds Cutlass with the factory 10:1 330 in it. With the very mild luxury cam it had 220 psi of cranking compression even with rings so bad that it smoked a little. It ran just fine on the cheap 87 octane gas which was all my 17 year old broke azz could afford. If it does detonate then the recommendation to move the cam back 4 degrees is a good one. Keeping it cool and keeping the timing at or below about 30 degrees total and using cold spark plugs will help too.
     
  4. rustbucket79

    rustbucket79 Veteran Member

    You could try retarding the cam but other than reducing the compression test numbers it won't fix the issue of excessive compression ratio. The best fix would be to put larger chamber heads on the engine.
     
  5. 73 Z

    73 Z Veteran Member Gold Member

    713
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    Jun 9, 2007
    Los Angeles, Ca
    whats the lobe center of that cam
     
  6. hhott71

    hhott71 Veteran Member Lifetime Gold Member

    14,373
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    Mar 30, 2001
    Joplin Mo. 64801
    I don't understand the loose... Loose psi? Is that when an air hose fitting breaks?

    Try the Comp recommendation. They should know their products best.
     
  7. 1980RS

    1980RS Veteran Member

    1,513
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    Jun 17, 2006
    Do what Joe Shrerman does add more duration to lower the cranking pressure and make more power. I read one time that he did this on a customers engines wirh 265psi. changed cams the engines psi fell to 215psi. and the thing picked up around 75hp. That guy knows what to do when he see stuff like this. Joe said the engine was fighting itself.

    When I have had an engine like this I always use a 114° LCA cam and have never run into problems. I don't think retarding you're cam is going to help other than giving you more drivability troubles and less low end tq.

    On a side note, I retarded a 292 CC 9° one time, The engine was a pig N/A but on the spray OMG the 350 was a monster. Even with some tuning issues it was still 11 flat that day.
     
  8. my71bmf

    my71bmf Veteran Member

    314
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    May 26, 2008
    Tulsa, Ok
    The lobe separation is 110. I will probably get to it this weekend. On a side note, I would have posted sooner but I didn't get the notifications. The intake is 106.
     
  9. Mike-78 Z-28

    Mike-78 Z-28 Veteran Member

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    Mar 4, 2003
    ElDorado Arkansas
    I don't think your going to see a vibration fix by lowering the compression
     
  10. my71bmf

    my71bmf Veteran Member

    314
    0
    May 26, 2008
    Tulsa, Ok
    I know that an engine that is detonating wil not run as smooth as one that isn't. Also, the guys at comp don't think my combo should have cranking compression that high. I'm still learning, so I just try to talk to people to limit trial and error.
     

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