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308 compression ratio

Discussion in 'Technical Q&A' started by gonzo350, Dec 5, 2004.

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  1. gonzo350

    gonzo350 Karting

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    #1 gonzo350, Dec 5, 2004
    Does the change in comp ration across the 308 series come as a result of piston design change....or head a head volume change. What I'm wondering here is will a set of "older" 308 pistons fit an 84 QV without hitting the valves or loosing compression. Any ideas?
     
  2. mk e

    mk e F1 World Champ

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    #2 mk e, Dec 5, 2004
    You can't put 2v pistons in a QV. There are a couple companies who can make you whateven compression you want. JE and wisco come to mind. Norwood (site sponsor) may have them in stock, or certainly could order them for you. 10:1 is about the limit for pump gas unless you are changing cams too, then you might be able to go to 11:1
     
  3. gonzo350

    gonzo350 Karting

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    #3 gonzo350, Dec 5, 2004
    what is the difference between the 2v piston and the 4v piston?
     
  4. mk e

    mk e F1 World Champ

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    #4 mk e, Dec 5, 2004
    The combustion chambers are quite different shapes. The valve in a 2v are much larger diameter, so to make them fit, the angle between the valve must be larger. In the 4v engine ferrari takes advantage of the smaller valves by standing them up to get straight flow paths and a more compact combustion chamber.
     
  5. gonzo350

    gonzo350 Karting

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    #5 gonzo350, Dec 5, 2004
    so is that why the QV piston is dished? To allow the valve more room to travel downward considering is "straight up" design? (Sorry just trying to figure out the actual differences in the pistons themselves.)
     
  6. Newman

    Newman F1 World Champ
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    #6 Newman, Dec 5, 2004
    The qv piston is not dished, it is dome shaped with 4 valve reliefs for valve clearance. The 2 valve has 2 reliefs. The 2 valve engine has a hemisphericle combustion chamber like a good 'ol 426 hemi but with over head cams. The QV is most likely a pent-roof design chamber going by memory.
     
  7. gonzo350

    gonzo350 Karting

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    #7 gonzo350, Dec 5, 2004
    hmmmm.....the mystery deepens.....my QV pistons are dished slightly....take a look at the pic. I am replacing these because the car sat for quite some time and the rings "froze" themselves in the ringlands. I've taken them to several machine shops as well as a ring manufacturer and nobody can get them out. Now I'm searching for replacements...and have found some pistons from a 76....thus all these questions. Do you guys think these pistons I currently have (note the pic) are NOT the original pistons?



    Thank you to everyone for your input!
     
  8. gonzo350

    gonzo350 Karting

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    #8 gonzo350, Dec 5, 2004
    Pic attached
     
  9. mk e

    mk e F1 World Champ

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    #9 mk e, Dec 5, 2004
    They look right to me...but my was apart almost 4 years ago so I could be mistaken. As far as I know the euro cars are a basically flat top piston and 9.2 :1, the US got 8.6:1 which required dishing the piston. So you could just go with euro pistons, but it will probably be cheaper to go afermarket, and then you might as well bump it up to 10.

    The good news iabout the stock pistons and compression ratio is that it's about perfect if you want to add boost, either a turbo or supercharger like I did....mine hits 22 psi at redline on pump gas :)
     
  10. Newman

    Newman F1 World Champ
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    #10 Newman, Dec 5, 2004
    Well, guess I was wrong about the QV pistons. I know mine (2valve) are a dome and only 8.8:1 now they are big domes with 10.5:1 compression. I guess the smaller chamber in the QV head dictates the flat or dish. The euro QV was higher, I believe 9.2:1.
     
  11. gonzo350

    gonzo350 Karting

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    #11 gonzo350, Dec 5, 2004
    LOL....now I'll add the twist. This is a Euro car. I'm sure I'll end up going aftermarket....does anyone know if Weisco (or anyone else) has done a high comp QV piston for a reasonable amount of money? Reasonable being under $1,000. And are we now correct in saying that the '76 piston will NOT work in my application.
     
  12. Newman

    Newman F1 World Champ
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    #12 Newman, Dec 5, 2004
    Weisco made my pistons for $1400 cdn including rings and pins, shipped.

    You need QV pistons and they have also made them for another member on this board in the past so they have all of the info. Tell them your bore size and what ratio you want, thats it.
     

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