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Hemi compression supercharger?

Discussion in 'The Hokey Ass Message Board' started by marq, Apr 25, 2006.

  1. marq
    Joined: Aug 22, 2003
    Posts: 1,423

    marq
    Member

    Ok all just wondered as Hemi's seem to be able to run a higher compression than other motors and get away with it ,what could i get away with compression-wise for low supercharger boost say 5 lbs................Marq
     
  2. Gotgas
    Joined: Jul 22, 2004
    Posts: 6,984

    Gotgas
    Member
    from DFW USA

    Lots of variables. What gas do you plan on running? What cam specs? How hard do you intend to run it, and for how long?

    Even though you can theoretically get away with more compression, don't. Just run a mild dome, shoot for 8-8.5:1, and if you need more power then spin the blower faster.
     
  3. SinisterCustom
    Joined: Feb 18, 2004
    Posts: 8,269

    SinisterCustom
    Member

    Yeah....8.0 to 1 should work well.
     
  4. marq
    Joined: Aug 22, 2003
    Posts: 1,423

    marq
    Member

    Over here premium is 97 octane and regular is 95.we also have an airport up the road and we can easily get octane booster.........Marq
     

  5. Gotgas
    Joined: Jul 22, 2004
    Posts: 6,984

    Gotgas
    Member
    from DFW USA

    That's high enough to support some boost. :)

    But like I said, keep the static compression relatively low and up the boost to make power. I wish I was building a blown Hemi. :)
     
  6. ol fueler
    Joined: Oct 6, 2005
    Posts: 935

    ol fueler
    Member


    With the fuel you can get there & 5 lbs boost 9:1 With conserative timing would not be a problem, 10:1 would be possible . At even 10:1 if you get detonation, water alcohol injection would probably cure it.
     
  7. repoman
    Joined: Jan 2, 2005
    Posts: 1,276

    repoman
    Member

    Higher than what? A flathead? An unmachined chevy chamber?

    The hemi head is LESS tolerant of compression vs a good wedge chamber.

    I'd keep it under 8:1. It's easier to change pulleys than pistons.
     
  8. Gotgas
    Joined: Jul 22, 2004
    Posts: 6,984

    Gotgas
    Member
    from DFW USA

    Top fuelers run em for a reason.

    A smooth chamber with no sharp edges (read: hot spots) will be less likely to pre-ignite. That's why Hemis work so well, you can go with high compression and lots of spark advance and they are very tolerant of it.
     
  9. imperial
    Joined: Feb 14, 2006
    Posts: 63

    imperial
    Member

    Have the same q here.
    Liqued propane powerd Injected under the 6-71 to cool the air.
    14 PSI boost. ( 12% overdrive on the 6-71)
    392 Hemi
    8 to 1 pistons or 8.5 to 1
    Still have to order the pistons can't decide.
    Remco
     
  10. Flat Ernie
    Joined: Jun 5, 2002
    Posts: 8,406

    Flat Ernie
    Tech Editor

    95 in the UK is roughly equivalent to 90 in the US - the US uses the R+M/2 method & the UK only uses RON (which is always higher) so don't let his 95 octane fool ya...it's equivalent to most mid-grades in the states - I know, I run enough of it in my car here & our on-base pumps show both numbers!... :(
     
  11. Candy-Man
    Joined: Mar 21, 2006
    Posts: 1,713

    Candy-Man
    Member

    Not that I am an expert, because I do not much when it comes to hemi's, however; I a building a nostalgic drag car, 392 powered and huffed... I will be purchasing the rotating assembly from ""Hot Heads"" who appear to be an excellent company to deal with... Has any one else had dealings with Hot Heads?
     
  12. Bass
    Joined: Jul 9, 2001
    Posts: 3,350

    Bass
    Member
    from Dallas, TX

    What information are you basing that claim on?

    From what I have read (and experienced), the total OPPOSITE seems to be the case. I.E., a hemispherical (iron) combustion chamber will tolerate MORE compression than an equivalent (iron) wedge chamber. At least a half a point more in most cases.

    I agree that compression should be kept between 8:1 and 9:1 for a blower motor.

    But for comparison's sake, current Top Fuel Hemi's are now running right at a 10:1 compression ratio. This comes from a reliable source, Bob Brooks at AFT.
     
  13. Bass
    Joined: Jul 9, 2001
    Posts: 3,350

    Bass
    Member
    from Dallas, TX

    Yes, I just spent a good chunk of change with them on my 331. I have nothing but good things to say about Bob Walker and Hot Heads.
     
  14. Gotgas
    Joined: Jul 22, 2004
    Posts: 6,984

    Gotgas
    Member
    from DFW USA

    I've heard that they run it that high to ensure that the engine will diesel after the spark plugs have vaporized. :eek:

    And I've been around '50s Mopar people a long time and have never heard of ONE single unsatisfactory dealing with Hot Heads. They have a great reputation and are proud of it. That is also reflected in their pricing. ;) :p
     
  15. I'm bringing this back to the top because I've got a similar question. I'm gathering goodies for my digger. I'm selling my old Stromberg setup and will be using a 6/71 and Hilborn four port on alcohol instead.

    I've been told that I want 10 to 12 to 1 compression for the alcohol with the blower. Does this sound right? I've got a used set of forged 392 flattop Venolia pistons that would make a lot less compression than that and another set of forged domed Aries 392 pistons that have been machined down some to lower the compression. I know that the 392 pistons will make less compression in my bored out 354 just due to the lower swept volume.

    All that to say running a blower and alcohol should I wimp out, use the flattops and give her more boost or go with the domed ones?
     
  16. Jmountainjr
    Joined: Dec 29, 2006
    Posts: 1,366

    Jmountainjr
    ALLIANCE MEMBER

    We are starting to get apples and oranges here - gas vs. alcohol. Yes, alky will want and need more compression than gas. You said it was for a digger, talk to some A/D guys.
     
  17. Kerry,
    If it were me I'd stay around 8:1 or so static compression and use my blower to fill the volumn. Look at it this way you can only get so much crap in a 5 lb bag, but if you have a 6 lb bag you can get more crap in there.

    Maybe I'm wrong in theory, but that's the way I've set 'em up before without any problems.
     
  18. Yo Baby
    Joined: Jul 11, 2004
    Posts: 2,812

    Yo Baby
    Member

    Hey Kerry the SOTI today with alchy is an assload of static,then add boost.
    The less boost you have to run the less hp loss to spin the wheezer.
     
  19. blown41
    Joined: Apr 6, 2008
    Posts: 139

    blown41
    Member

    Not an expert either but from everything I've read ;
    8:1, forged pistons, high quality rods and good head gaskets. Even with low boost you can have cylinder pressures equivalent to 13:1 compression.
     
  20. 65 impala
    Joined: Jan 12, 2007
    Posts: 1,086

    65 impala
    Member

    on blown motor you dont want to go over 8 to 1 whole point is to be able to put boost to her
     
  21. I think some of you are thinking of gas. Remember I'm going to be running this thing on alcohol which puts up with and likes a lot more compression, or so I am told. Any racers out there want to tell us how you are setup?
     
  22. recycler
    Joined: Mar 27, 2001
    Posts: 661

    recycler
    Member

    Kerry, with alky use the domed pistons. But check the dome thickness if they have been milled. OR you could use the flat tops and run nitro :)
    I run Hot Heads solid cast aluminum heads on my fueler -albeit heavily modified. I also use his blower manifold and valve covers - all modified but that is just the way it is with a race car. Bob has been fine to work with.
    Brad
     
  23. Yeah, Bob's a great guy but I'm a tightwad and new pistons are $$$. He'll be getting enough of my money on all the other little goodies I need. Looks like I'll be using my domed pistons and a pretty hefty overbore. I'm mentally picking through my parts stash for the right block and heads this afternoon. :)

    The thought of running honest to gosh nitro would be awesome except for two things. There's none to be had and it's price is alarming. How long before the NHRA repeats the sixties fuel ban to slow the field down?
     
  24. Yo Baby
    Joined: Jul 11, 2004
    Posts: 2,812

    Yo Baby
    Member

    Kerry talk to Ross for your slugs. You'll find them helpful and more than competitively priced. Like I said earlier,the SOTI these days is big static 13/14:1. O-rings and copper gaskets are a must.
    This is considering alchy. The old way was keep static low and spool up the wheezer but it takes a lot of HP to run the wheezer when spooled up so hp was bein' pissed away and blower maintainece ain't very cheap either.
    There is one of those sand drag guys that is running 75 count 'em 75Lbs of boost on a high static engine. When asked how that could be,the reply was simple,I get by with a lot of things drag racers don't because my engines only have to survive 2,5 secs. at a time under power. This an EXTREME example.
    Obviously 75 is a lot of boost and takes a lot of blower to do it but still,13/14:1 and not having to spool the blower to bump the cylinder pressure alot equals a lot of easily (so to speak) HP if for no other reason than the static makes pretty good cylinder pressure and hp on its own so the blower then becomes a 'help make more' as opposed to a crutch to make some.
    Short answer run high comp. ratio.
     
  25. E.C.
    Joined: Apr 7, 2007
    Posts: 559

    E.C.
    Member
    from Tx

    I'm bringing this back to the top because I've got a similar question. I'm gathering goodies for my digger. I'm selling my old Stromberg setup and will be using a 6/71 and Hilborn four port on alcohol instead.

    I've been told that I want 10 to 12 to 1 compression for the alcohol with the blower. Does this sound right?

    Yes that sounds right because of alcohol. If you never tuned a blower with Hilborn injection before I would not worry to much about what pistons you put in because it will probably lift a ring land anyhow. Trial and error. You cant wimp out now this is a Digger. Good Luck

    -E.C.
     

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