Cam variables that can affect detonation in a high compression engine

Engine, Trans & Driveline

  1. Moparfeind

    Moparfeind FBBO Gold Member FBBO Gold Member

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    Hey guys how ya doin? Now that you gave your honey her valentines gift and dinner its time to get back to reality lol.

    I have two questions:

    1. What variables can I use in cam shaft selection that can help soften the effects of detonation in high compression motor?

    2. If I am running a 12:1 piston and a 64cc head (unknown gasket about 017 out of the hole) how can I estimate compression? Is there a good formula that I can use?

    Thanks for reading.......
     
  2. wyrmrider

    wyrmrider Well-Known Member

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    12:1 piston could be all over the place
    do you have a brand and model number?
    or dome cc's
    what is .017 out of the hole ?
    the flat of the piston or the dome?
    do you have a cam in it now and motor together?
    run a compression check and post back
    where did you get the 64cc number for the head
    what head?
    who measured or is this a stock head number from the service manual?
    cheers
     
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    • andyf

      andyf Well-Known Member

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      You just have to measure the compression ratio to know what it is. Mfg numbers don't mean anything unless you know all the assumptions they used in their calculation. If the engine is together on the stand you can measure the chamber size at TDC and then calculate the compression ratio.

      You can't really compensate for compression with the cam. Dynamic compression only occurs at low engine speeds so it is a mostly useless concept. Once the engine is at higher speeds the wave dynamics will fill the cylinder and if your octane rating isn't high enough you'll cause engine damage.
       
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      • dryheat

        dryheat FBBO Gold Member FBBO Gold Member

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        Hey Ron ... welcome back!!!
         
      • wyrmrider

        wyrmrider Well-Known Member

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        How about a little help here
        like bore and stroke
        what motor?
        Remember Crower tried the late closing cam and high compression years ago
        only worked in some very special cases
         
      • Cranky

        Cranky Banned Staff Member FBBO Gold Member

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        Just don't use an RV cam! :D
         
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        • Don Frelier

          Don Frelier FBBO Gold Member FBBO Gold Member

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          A compression check will give you some indication.
          If what you say is correct, you are going to need some larger chamber heads.
          That's not a street motor.
           
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          • Cranky

            Cranky Banned Staff Member FBBO Gold Member

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            I've done a couple of pump gas 11.5 motors and even ones with more compression will run on pump gas but you can't stick your foot into it unless the cam really bleeds off a lot of PSI in the lower rpm ranges....and even at that, you're taking a chance....
             
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            • Curiousyellow71

              Curiousyellow71 Well-Known Member

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              https://www.summitracing.com/popup/calcsandtools/compression-calculator
              Plug your numbers in and find out your compression.

              Figuring Dynamic compression does have variables that affect the final outcome. Certainly factors such as quench come in to play. But it is a good tool when combined with real world experience. Without experience it's just a number that can be misleading. The cams intake valve closing intensity is a factor. Slow ramp hydraulic vs a roller cam will detonate at a different dcr. DCR is useful for pump gas builds, high compression builds like your mentioning I don't think it's going to be helpful unless you are trying to run a specific octane and have a dcr number in mind that someone has experience with.
               
              Last edited: Feb 14, 2020
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              • Cranky

                Cranky Banned Staff Member FBBO Gold Member

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                • Moparfeind

                  Moparfeind FBBO Gold Member FBBO Gold Member

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                  • Moparfeind

                    Moparfeind FBBO Gold Member FBBO Gold Member

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                    Thanks everyone I am still reading some of your replies.

                    Motor 1973 340
                    Unknown cam
                    Stock heads
                    64cc came from the book I know I know.
                    Piston TRW L2322 12.5:1 I spotted it inside the motor with a bore scope.
                    Out of the hole measured at the edge. This is a guestimate.

                    I need to find someone here in AZ that has a Whistler to measure compression as the motor is in the car. I am going to try and run it on 110 octane but at $8.50/gal thats a bit much. So the plan is to see if a different cam might help. The cam thats in there is too aggressive and maybe for a 4speed. I am using this with a 904 automatic.
                     
                  • 33 IMP

                    33 IMP FBBO Gold Member FBBO Gold Member

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                    Has it got a cam in it now?
                    Ive got a 1989 trw catalog, i can get you some info on the pistons, but not till i get home later tonight. We can make an educated guess as to comp ratio then.
                    If that were my engine, i would contemplate a big roller, a thick head gasket, and an e85 carb.
                     
                  • wyrmrider

                    wyrmrider Well-Known Member

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                    you can reduce the dome on those pistons
                    YR also has done some high compression builds
                    E85 Propane
                    doing it with a cam would be really tough
                    what andyf said
                    what's the casting number on the heads
                    Those pistons are really heavy
                    did you ever find out what cam is in it now?
                    most likely your heads have more cc than 64 and whatever they are you might get another 5cc out of them
                    if no quench now nothing lost with a thick or stacked gaskets
                    did you post a compression check
                     
                  • Moparfeind

                    Moparfeind FBBO Gold Member FBBO Gold Member

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                    Yes its an unknown cam pistons can be cut but that means a complete overhaul. Trying to see if a cam could help.
                     
                  • Moparfeind

                    Moparfeind FBBO Gold Member FBBO Gold Member

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                    Yes the pistons are heavy for the 030 they are something like 750g if I recall correctly. I looked up the heads a while ago so I forgot but recalled they were 64cc stock. I could be wrong and I know thats what they say and could measure differently.

                    Static cylinder pressures were high at 210 psi +/- 5%
                     
                  • Kern Dog

                    Kern Dog FBBO Gold Member FBBO Gold Member

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                    For your sake, please learn from the mistakes and efforts of others.
                    For years, I dealt with spark knock in my 440/493. I posted here, at FABO, on Moparts and DodgeCharger.com. LOTS of advice followed. Lots of smart people suggested a piston swap. I was at almost 11 to 1 and even with the less than optimal total ignition timing of 31 degrees, it knocked at anything over 3/4 throttle.
                    I was stubborn and didn't want to pull the engine to swap pistons so I took other bits of advice. ONE was to swap to a bigger cam based on the belief that the later intake closing event would reduce lower rpm cylinder pressure, reducing the risk of detonation. A trusted guy at FABO suggested a pretty big Lunati cam. It ran great and made MORE power but actually knocked worse. 110 octane was the only gas I tried that totally eliminated detonation.
                    I ended up slipping in thicker head gaskets to lower the compression. Lots of guys warned me that the engine would run worse with the reduced quench but it made all the difference. No knocking now. I went from 10.9 to 1 to 10.07 to 1.
                     
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                    • Moparfeind

                      Moparfeind FBBO Gold Member FBBO Gold Member

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                      Sorry meant 12.5:1 piston TRW2322L cylinder pressures are high 210 psi 5%
                      Stock 1973 cast heads j type I believe I will check casting numbers tomorrow
                       
                    • Curiousyellow71

                      Curiousyellow71 Well-Known Member

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                      I am curious what 2 cams you tried at 10.9:1. I assume you have aluminum heads and what elevation your at.
                       
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                      • Moparfeind

                        Moparfeind FBBO Gold Member FBBO Gold Member

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                        Hi KD Trying to learn from others thats why I am posting on this. I am trying not to pull the motor even though I know its the right way to deal with this issue. I want to estimate the CR and see if a cam could help along with thicker gaskets just like you did actually. If I am at indeed 12.5:1 not sure I can do anything other that pull it and rebuild it with some KB hypereutectics. But if I am lower maybe thick gaskets and a different stick could get me by. I know there would be little tuning ranger and it may not be optional with respect to getting all the advance I need.....
                         
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