Compression ratio question

Discussion in 'Street/strip 400/430/455' started by jr_, May 18, 2022.

  1. jr_

    jr_ Well-Known Member

    10.6 ok for pump gas?

    Below is my calculations for a flat top piston design 0.040 in the hole, 1971 iron heads, .050 offset on crank, bbc rod.

    903FD0D6-5371-42E9-BB20-4F9BFECBEBD7.png 7D03C16B-AD23-4051-807D-C344638DE601.png
     
    Stage 2 iron likes this.
  2. sean Buick 76

    sean Buick 76 Buick Nut

    I would mill 40 though off the block so you have a 40 thou quench zone.
     
    Schurkey likes this.
  3. hugger

    hugger Well-Known Member

    What cam do you plan on running? You'll need some overlap to run pump with the iron head
     
  4. jr_

    jr_ Well-Known Member

    Either,

    TA_286-08H
    455H.516"-234'/.520"-248',112'

    Or

    TA_290-08H
    455H.525"-238'/.520"-248',112'

    Or

    comp cam 234/249
     
  5. Stevem

    Stevem Well-Known Member

    The iron heads and there very open chamber shape on the spark plug side with no squish are a mess on a running motor in terms of wet flow vortex's and burning all the fuel they take in.
    As stated in post number 2 you must get rid of that .040" deck clearance to generate more squish , in fact even though you would need a deeper dish I would have the Pistons made to be.005"above the deck.

    The piston you have made need to have whatever depth of a dish in it to not give up squish area!

    Another thing you need to do when running a dish piston and many times even if your not is to run a long plug so you can get the flame front propergating from the center of the air and fuel mass.
    You will need to factor this into your CC calculations .

    Plug wise you want to mock things up since any plug threads that stick out passed the camber wall will need to well rounded over as to make for no hot spots.

    You also want to start out with one heat range colder on the plugs and 2 to 3 degrees less total timing since the burn will take place faster, this is another side benefit.

    Also these heads when run with lifts above .500" start to give up a lot of there pressure recovery that they have at lower lifts, and this can't be seen on a flow bench, only in a A/B test on a running motor.

    Unfortunately theres nothing you can do about this since it's a factor of the depth of the combustion chamber.
     
    Last edited: May 19, 2022
  6. rkammer

    rkammer Gold Level Contributor

    For a street engine you need to calculate the Dynamic Compression ratio which should be kept at or below about 8.0 for a pump gas motor using 91 or higher octane. The cam overlap has a large affect on this. Also, as previously mentioned, you need to get the pistons up close to zero deck. Plug your numbers into this calculator to see what dynamic compression is with whatever cam you want to use.

    https://www.gofastmath.com/Compress...atic-And-Dynamic-Compression-Ratio-Calculator
     
  7. wkillgs

    wkillgs Gold Level Contributor

    Yes, Dynamic Compression ratio is what you need to be concerned with.
    But cam overlap doesn't affect that calculation, it's the closing point of the intake valve (and static CR) that determines the DCR.
    If your DCR is too high for street fuel, you can lower it by retarding the cam or by putting in a bigger cam (more duration). If it's too low, advancing the cam will boost the DCR by closing the intake sooner.

    The whole point of the DCR is that the cylinder doesn't start building compression until the intake valve closes. (unless your volumetric efficiency is over 100%, such as a supercharged engine)

    Overlap doesn't directly affect DCR. But if you try two different cams with the same duration, but different lobe separation, the one with closer lobe separation (more overlap) will have higher DCR but only because it closes the intake sooner.
     
    rkammer likes this.
  8. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    DCR with 10.6 SCR for the TA 286-08H is approximately 8.0 with the cam installed at 108* ICL

    DCR with 10.6 SCR for the TA 290-08H is approximately 7.89 with the cam installed at 108* ICL

    Need ADVERTISED DURATION for the Comp Cam.

    Your bore is 4.350" Your head gasket bore will be more than that. The Felpro gaskets I use have a 4.385" bore.

    I get 10.65 SCR, but your calculator adds the volume to the top ring land.

    JrSCR.JPG

    I think that motor with iron heads, and a .08 quench distance might be very iffy on pump gas. If you were using aluminum heads, it would be better.
     
    Last edited: May 19, 2022
  10. Bigpig455

    Bigpig455 Fastest of the slow....

    I have a true 10:1SCR/8:1 DCR 462 with a .045 Quench, 23 cc dish and 36 degrees total timing, it race it regularly on 93 Octane fuel with no race gas mixed in. 87 Octane works just fine for driving around town, but will detonate at WOT.
     
    sean Buick 76 likes this.
  11. 87GN_70GS

    87GN_70GS Well-Known Member

    Thank you!!
     
    wkillgs likes this.
  12. sean Buick 76

    sean Buick 76 Buick Nut

    The overlap effects the cranking compression, which effects octane requirements
     
  13. wkillgs

    wkillgs Gold Level Contributor

    No, it doesn't. Compression doesn't start until the intake valve closes. It could care less if the exhaust valve is open longer while the intake is opening.
     
    chrisg and 87GN_70GS like this.
  14. hugger

    hugger Well-Known Member

    My reference to overlap is my way of saying duration,..because as we know longer duration equals more overlap,..so hand in hand so to speak,...you want to run 10.6 with iron heads you will need to run a healthy cam,...you poke a 230 at 50 cam in it and it's gonna sound like a Ford 7.3 ha
     
    sean Buick 76 likes this.
  15. hugger

    hugger Well-Known Member

    I myself have never had an issue not having zero deck,...for instance the engine I just bolted together that I drove to the nationals,...it's old TRW dished slugs that sit at least .040 I'm sure,..then add a .041 gasket,...34° basically locked ,...113 cam not one hint of knock,..12.6 at 105 3900lb

    My old 464 in the orange car had wiescos those sit .050+ with a .100 gasket to get compression down for pump 93,...no knock,..11.6 at 115 4100lb

    I'm understand the reasoning behind it but I'm not convinced it correlates to detonation
     
    jr_ likes this.
  16. hugger

    hugger Well-Known Member

    Look at the old engine master cam profiles those guys were running 12 even 13to1 on pump 93,....chamber design plays a roll but cam timing and events let it happen
     
  17. hugger

    hugger Well-Known Member

  18. sean Buick 76

    sean Buick 76 Buick Nut

    The reason you didn’t have any detonation issues is because once you are over about 80 thou quench distance it’s fine. It’s the “danger zone” of 50-80 thou that increases detonation.
     
  19. hugger

    hugger Well-Known Member

    Now my brother engine that's essentially the same engine literally as the one mentioned first would ping like no other,...different cam,..no idea on the specs but it would ping at anything over 28°
     
  20. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Everything is combination dependent. Zero deck is best for maximum efficiency, and detonation resistance. Calculating DCR is just one tool in the tool box. It can help you avoid the mistake of not properly matching your cam to the static compression ratio. Cam descriptions usually mentioned a desired minimum static compression.

    https://www.enginelabs.com/engine-t...ormance-squished-between-the-piston-and-head/
     

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