Understanding Compression Ratio(s) And Forced Induction

Discussion in 'General Supercharger Discussion' started by Lowlyslows, Aug 29, 2016.

  1. Lowlyslows

    Hello,

    I am building a 408 stroker with EFI and timing retard. I plan to run 93 octane fuel and ~8psi of boost.

    I am trying to figure out what compression ratio I should run. I see others with a similar setup running 9-10:1 static compression and running 8-15psi of boost. However, after doing tons of reading, are you not supposed to go over 12:1 expected compression ratio?

    Please see this chart and the note below:
    upload_2016-8-29_14-43-21.png #ad


    What am I missing?

    Thanks
     
    Lowlyslows, Aug 29, 2016
    #1
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  2. vinces427bb Staff Member

    welcome to the site
    what are you building this engine to install in???:woot:
    basically the chart is a rough guideline for roots type blowers : which have instant boost at low rpm
    centrifugal build boost at higher rpm and have many intercooling options to help raise the the tolerable total/effective compression
    will you have aluminum heads,
    which supercharger is in your combination planso_O
     
    vinces427bb, Aug 29, 2016
    #2

  3. Lowlyslows

    Hello,

    This is going into a 1967 Mustang. I wasn't originally planning on installing an intercooler as I do not have the tools to create the piping (I have been looking for a reason to buy a tig welder). Yes, I will be using TrickFlow 11r 190s or 11r 205s (not sure which one to use). I will be using a paxton centrifugal superchager (NOVI-1500).

    Thanks
     
    Lowlyslows, Aug 30, 2016
    #3

  4. vinces427bb Staff Member

    https://www.summitracing.com/parts/pax-1001847
    i think if you keep the compression below 9.3:1 with aluminum heads [either one's] you will be golden:D
    the blower is rated[roughly] at 7.5#boost on a 289-302 engine = but on a bigger 408 engine with good heads/exhaust
    you may not develop that much boost o_O
    you should not need an intercooler or meth/h2O system at this boost level
    only pull some total timing at high rpm's:cool:
    http://www.paxtonauto.com/product.php?id=157
     
    Last edited: Aug 30, 2016
    vinces427bb, Aug 30, 2016
    #4

  5. TwistedCrankCammer



    How many cubic inches, which engine platform?

    Roger
     
    TwistedCrankCammer, Aug 30, 2016
    #5

  6. Lowlyslows

    Building a roller 408.

    Slows
     
    Lowlyslows, Aug 30, 2016
    #6

  7. vinces427bb Staff Member

    SBF Windsor?? not Clevelando_O
     
    vinces427bb, Aug 30, 2016
    #7

  8. Lowlyslows

    Windsor
     
    Lowlyslows, Aug 30, 2016
    #8

  9. vinces427bb Staff Member

    what was the 351 block bored to and what is the new stroke on the 408 build?:blackalien:
     
    vinces427bb, Aug 30, 2016
    #9

  10. Lowlyslows

    bore: 4.03
    Stroke: 4
    Rod Length: 6.2
     
    Lowlyslows, Aug 30, 2016
    #10
    vinces427bb likes this.

  11. vinces427bb Staff Member

    that's pretty square engine;
    I've seen a few out to the track that run nice
    even NA...:blackalien:
     
    vinces427bb, Aug 30, 2016
    #11

  12. Lowlyslows


    Yep, I just want this thing to move when I hit the throttle, sound good and take the wife and kids for a fun ride in. I have always wanted to force induct something so I decided I am doing it on this.
     
    Lowlyslows, Aug 30, 2016
    #12
    TwistedCrankCammer likes this.

  13. vinces427bb Staff Member

    I think you will have a solid ride when done:wideyed:
     
    vinces427bb, Aug 30, 2016
    #13
    TwistedCrankCammer likes this.

  14. TwistedCrankCammer



    Why not a Cleavor SBF?

    Roger
     
    TwistedCrankCammer, Aug 31, 2016
    #14

  15. SCF/HAIC Staff Member Admin

    Yeah, we've been through that same chart before, and the fundamental problems are a) the chart doesn't take into account differences in engines or b) metals, c) fuel ratios, and d) the chart goes by boost rather than actual air volume. That's why you'll see boost go down on a gauge when a blower is ported, but power is going up.

    It should go by volumetric efficiency, but that's not something the average person can measure (unlike using a boost gauge). The end result of our discussions if I recall correctly is the charts are good for worst case only.
     
    SCF/HAIC, Sep 2, 2016
    #15

  16. TwistedCrankCammer

    For clarification purposes, a Cleavor is a Windsor block with Cleaveland heads and intake on it.

    It offers more flow potential in the heads.

    Roger
     
    TwistedCrankCammer, Sep 3, 2016
    #16

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