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sbc piston help

Discussion in 'The Hokey Ass Message Board' started by Rich Rogers, May 7, 2009.

  1. Rich Rogers
    Joined: Apr 8, 2006
    Posts: 2,019

    Rich Rogers
    Member

    Well guys, I'm starting my engine build and ran out of money due to injury and out of work for now. What I have is a 1973 4 bolt main 350. out of a 3/4 ton van. Engine has flat top pistons with 4 reliefs. I know it's low compression and have the parts to do the build and raise the comp. with 64cc 327 heads. Also planning on using a .028 head gasket to raise it to 10.2 (according to Summit). Will the stock pistons handle 10.2 comp. or should I run a stock gasket to drop it.Thanks
     
  2. JohnEvans
    Joined: Apr 13, 2008
    Posts: 4,883

    JohnEvans
    Member
    from Phoenix AZ

    If it don't ping and knock on what you are running for gas don't sweat it. Pre-iginion will kill any piston.
     
  3. fridaynitedrags
    Joined: Apr 17, 2009
    Posts: 402

    fridaynitedrags
    Member

    If these are stock '73 pistons, I doubt they are true flat-tops. More than likely, the crown is depressed, leaving only a thin ring of flat crown material around the circumference of the piston (see the thumbnail for an example of what I'm saying). In my opinion, there will be insufficient material to mate with the underside of the head to generate good squish to prevent detonation on pump gas at anything over about 9.0:1 with iron heads.

    The other thing is that the piston deck height is probably somewhere in the awful range, with the piston down in the bore somewhere around 0.030" to 0.040" with the piston at top dead center. These motors, beginning in 1972, were de-tuned compared to their 1971 and earlier brethren. I would trash the stock pistons and start over with a real flat-top with at least 1.560" compression height, then dial in the piston deck height for a zero deck by cutting the decks to the piston crown, then using a 0.039" or 0.040" gasket to achieve a 0.039" or 0.040" squish.

    I understand you're out of work and out of money, but trying to crip this motor together with the stock junk pistons will not work as well as doing it properly. You may get the motor together only to find that it pings on pump gas.

    The other thing is, static compression ratio should be adjusted with the piston crown configuration, combustion chamber volume and the piston deck height. Head gaskets are only for mating the head to the block, setting the squish and absorbing minor irregularities, not for setting the static compression ratio.
     

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    Last edited: May 7, 2009
  4. Rich Rogers
    Joined: Apr 8, 2006
    Posts: 2,019

    Rich Rogers
    Member

    Nope, these are true flat tops with 4 valve reliefs with 0 deck height. The stock head gasket that came off of it was a shim gasket. Course that was when it was mated to the 76cc heads. I was more concerned with possibly ruining the pistons than with detonation. I can control some of that with timimg and or additives
     

  5. dbradley
    Joined: Jan 6, 2007
    Posts: 1,036

    dbradley
    Member

    If this is a "street" motor, I'd build it at 9 to 9.5 to 1 and be able to run about 'any' gas. You can get some pretty good hp with that compression. If you had aluminum heads you could go 10 to 10.5 with no problems. Anything higher with your heads and you need to be a chemist or run slow timing causing it to overheat to drive it. Not worth the trouble.
     
  6. Rich Rogers
    Joined: Apr 8, 2006
    Posts: 2,019

    Rich Rogers
    Member

    You could be right about that and that has been a consideration. I can run a stock .039 -.041 gasket that would put me in the right area of compression and at the same time lesson the chance of detonation. Sometimes I forget about the crap they call gas we have to use
     
  7. Hotrod Lincoln
    Joined: Apr 8, 2009
    Posts: 55

    Hotrod Lincoln
    Member

    Your theoretical compression ratio might be above 10:1, but depending on the cam you use, the actual compression pressure will probably be way below that. The only reason to increase compression above about 9.5:1 is to compensate for the loss of compression stroke caused by late intake valve closing with a radical cam. With a 10.2:1 bore/stroke/head ratio, use at least a 290 degree duration cam. 300 or 305 degree would be better suited to that ratio on normal premium gas. If you use "octane booster", you might get away with a slightly shorter duration, but that stuff is sucker bait. A 5% mixture of Diesel fuel to gas, (one gallon of Diesel to 19 gallons of gas) will raise the effective octane rating by 5 points, with no ill effects, and some advantages in the way of valve guide lubrication. Diesel is a LOT less expensive than octane booster!
    Jerry
     
  8. skunx1964
    Joined: Aug 21, 2008
    Posts: 1,455

    skunx1964
    Member

    never heard about using diesel like that. really work? id think it would foul plugs
     
  9. Hotrod Lincoln
    Joined: Apr 8, 2009
    Posts: 55

    Hotrod Lincoln
    Member

    It works on the 600+ HP round track SBC's I build. On the dyno, it allows us to run a little more timing advance without detonation.
    Jerry
     
  10. luckyuhaul
    Joined: Jul 11, 2005
    Posts: 182

    luckyuhaul
    Member

    Never heard of that one, I've always heard about acetone, toulene, MTBE making home brew boosters.
    I run flat tops with 416 , 305 heads, had to slow down the advance rate with the two "medium" advance springs and run 93 octane a little rich.
    Set timing at 4 BTDC with vacuum plugged, then run full vacuum. Lots of bottom end torque. I'm pleased with the way it runs. - luckyuhaul
     
  11. Rich Rogers
    Joined: Apr 8, 2006
    Posts: 2,019

    Rich Rogers
    Member

    Ok, so from what you're saying the 292 dur. cam that I already have should be ok, right ? Damn I'm gettin tired:confused:. Would that cam raise the compression to more that 10.2?
     
  12. ThePuck
    Joined: Apr 9, 2009
    Posts: 116

    ThePuck
    Member
    from Ottawa


    That is absolute nonsense.
     
  13. Del Clark
    Joined: Feb 21, 2007
    Posts: 632

    Del Clark
    Member
    from DeLand,FL

    If you are running 64 cc heads and flat tops you will be able to run pump gas fine without using the thick head gaskets!
     
  14. landseaandair
    Joined: Feb 23, 2009
    Posts: 4,406

    landseaandair
    Member
    from phoenix

    If your talking about a 292 comp cam it will bleed off lots of compression and you'll easily get away w/pump gas especially premium even at 10.5 possibly 11:1 but you'll at least need good springs to use it and you might be better off with a milder cam like a comp 280 which would work w/stock springs. Also w/the 280 I'd use a standard Fel Pro gasket around .039 and run premium just to be safe.
     
  15. fridaynitedrags
    Joined: Apr 17, 2009
    Posts: 402

    fridaynitedrags
    Member

    There are three separate compression ratios to deal with.

    1. Static Compression Ratio. The mathematical computation of volumes in the system. Computing SCR requires 5 values. Cylinder volume, combustion chamber volume, piston crown volume, piston deck height volume and head gasket volume. All five values are added together. Then the four values less the cylinder volume are added together. Dividing the larger number by the smaller number will give you the SCR. In today's motors, these numbers can range from 7.0:1 to 11.0:1 for pump gas and up to about 16.0:1 with race gas or alcohol.

    2. Dynamic Compression Ratio. A computation based on the SCR, crankshaft stroke, rod length and intake closing point of the cam. These numbers will range from 7.5:1 to 8.5:1 for pump gas motors and higher for race gas or alcohol.

    3. Actual Dynamic Compression Ratio. This is a figure based on the motor being operated at the sweet spot of the cam, usually around the torque peak or a little higher. This is the spot of maximum cylinder BMEP and can easily be seen on a DynoSim of the combination.

    It is the Dynamic Compression Ratio that Hot Rod Lincoln is talking about.

    When you're building a motor, you need to know the SCR before you ever think about choosing a cam. Then you choose a cam with the correct intake closing point to coordinate with the SCR and make power in the range you have chosen to operate in depending on converter, gears, etc. Using the SCR, intake closing point, rod length and stroke, you figure the DCR so that it falls into the 7.5 to 8.5 range to make the proper cylinder pressure and you're pretty much good to go on pump gas. Different DCR calculators seem to produce different values that are pump gas friendly. I have relied on the Keith Black calculator for years now and have seen good results with street motors on pump gas using 8.0 to 8.5, so long as you pay attention to the squish and keep it in the preferred 0.035" to 0.045" range.

    With a SCR of 10.2:1, you would want to use these values or somewhere in between them:
    For a DCR of 8.005:1, close the intake valve at 52 degrees after bottom dead center (closing point figured at 0.050" tappet lift).
    For a DCR of 8.494:1, close the intake valve at 44 degrees after bottom dead center (closing point figured at 0.050" tappet lift).

    So, close the intake at 44 or 52 or anywhere in between there for the correct DCR to use with 10.2:1 SCR on pump gas.

    Check the specs on your cam and see what the intake closing point is.
     
  16. ThePuck
    Joined: Apr 9, 2009
    Posts: 116

    ThePuck
    Member
    from Ottawa

    A 10:1 350 with a 274 or 280 duration cam is just about the most popular combination ever built. No problem with pump fuel.
     
  17. landseaandair
    Joined: Feb 23, 2009
    Posts: 4,406

    landseaandair
    Member
    from phoenix

    If it's a comp 292 it should be 74. I'm not trying to be a know it all. I just looked it up earlier.:)
     
  18. fridaynitedrags
    Joined: Apr 17, 2009
    Posts: 402

    fridaynitedrags
    Member

    At what tappet lift?
    And I'm not trying to be a know it all either, I'm just trying to pass on 50 years of doin' this crap (started racing in 1958) so that others can learn by my mistakes.
     
  19. Rich Rogers
    Joined: Apr 8, 2006
    Posts: 2,019

    Rich Rogers
    Member

    Lunati 292 with 480 lift
     
  20. fridaynitedrags
    Joined: Apr 17, 2009
    Posts: 402

    fridaynitedrags
    Member

    Rich, I don't mean this to be offensive, but if that's all you know about the cam and all you want to know about the cam, then screw the motor together and hope for the best. Good Luck.
     
  21. Rich Rogers
    Joined: Apr 8, 2006
    Posts: 2,019

    Rich Rogers
    Member

    fridaynitedrags, none taken, right now I don't have the paper work in front of me to be able to spell it out. The main reason for the post was to make sure I wouldn't hurt the stock low compression pistons by raising it to 10.2 :1. That was all. At that point that was all I wanted to know. It is a street engine that was tired out but the bores were good and it's ready for re-assembly. I already have all the parts to do it but was unsure if I should wait to be able to buy more.When I can have the cam specs in front of me I'll pm you if you don't mind. Man it's late and I'm whooped, have a great night and thanks for all the help
     
  22. lippy
    Joined: Sep 27, 2006
    Posts: 4,834

    lippy
    Member
    from Ks

    There were a ton of older engines with cast pistons at 10 to 1. The cam will be the deciding factor and where you put it, follow your cam card and degree the cam as per specs. I understand the old 10 to 1 engines had better gas back then, but in my opinion, you can run pump gas and 10 to 1, just adjust the timing accordingly and keep an eye on the plugs. If you are worried about the comp being high, go to a thicker head gasket. If your question was about a stock piston living at 10 to 1, don't worry about it they will be fine, just like any engine, don't let it detonate. Throw a bottle of 104 plus octane booster to every tank and jack the timing up where you want it and go on down the road. :D JMO. Lippy.
     
  23. I think you will be on the ragged edge of detonation with flat tops and those heads, but keeping the quench tight would do more to prevent it than a slightly lower C.R.
    Also, spend plenty of time getting the spark curve optimized (follow the instructions, and don't cheap out on the vacuum advance, get an adjustable one), and an old trick; vapor injector! Worked for me!!!
     
  24. 1961CadillacHearse
    Joined: Apr 23, 2009
    Posts: 16

    1961CadillacHearse
    Member
    from Vegas

    def listen to friday night drags this dude knows his shiz!!! jus sayin :)
     
  25. fridaynitedrags
    Joined: Apr 17, 2009
    Posts: 402

    fridaynitedrags
    Member

    BBBBZZZZTTTT, wrong answer. John Erb, chief engineer for Keith Black pistons has stated that you will make the motor detonate easier by widening the squish with a thick head gasket to lower static compression ratio. He says the motor will detonate less with the higher static compression ratio and thinner squish. 0.035" to 0.045" squish is the key to an efficient motor that will run on pump gas without detonation, as long as you pay attention to the dynamic compression ratio and keep it in the 8.0 to 8.5 range with iron heads and 8.5 to 9.0 range with aluminum heads. (on the Keith Black calculator)
    http://www.kb-silvolite.com/calc.php?action=comp2
     
  26. falcongeorge
    Joined: Aug 26, 2010
    Posts: 18,341

    falcongeorge
    Member
    from BC

    If they really are 0 deck, I would think twice about running a .028 head gasket, unless the other clearances have been checked VERY closely,wall clearance is on the tight side, and the deck hieght is DEAD TO NUTS in EVERY hole, and you are running forged pistons. If a hyper taps the head, its "goodbye charlie".
     
    Last edited: Apr 21, 2013
  27. fridaynitedrags
    Joined: Apr 17, 2009
    Posts: 402

    fridaynitedrags
    Member

    Excellent post. To those who do not know, hypereutectic piston are cast pistons. They differ from a run-of-the-mill cast piston by the fact that an excess of silicon is used in their manufacture, but they are still cast pistons.

    David Vizard found the pistons kissing the heads on a SBC at, I believe, 0.027" squish. You must understand that the crank flexes a little, the rod stretches a little and the piston compression height grows a little from heat. All of this action together will reduce the squish to zero in a motor that's put together with a very tight squish on the engine stand. Most everyone agrees that 0.035" to 0.045" works on a SBC, with the norm being 0.040". If I had a zero deck motor, I'd use a readily available composition gasket at 0.039"/0.040". If, when I was planning the build, I wanted to use an 0.028" gasket, I'd cut the block decks for a 0.012" piston deck height, so that I'd have a 0.040" squish with a 0.028" gasket.
     
  28. henryj1951
    Joined: Sep 23, 2012
    Posts: 2,304

    henryj1951
    Member
    from USA

    lots of info here



    :cool:
     
  29. raengines
    Joined: Nov 6, 2010
    Posts: 227

    raengines
    Member
    from pa.



    pay attention to what they said ^^^^^^^^^^^^^^^
     
  30. gary terhaar
    Joined: Jul 23, 2007
    Posts: 655

    gary terhaar
    Member
    from oakdale ny

    I think Kb uses .020 valve lift to calculate dcr. So his effective dcr is lower than a .050 number. Your spot on,just a note for the .020 number. Good info,sometimes it's hard to get it across.just looked and its more like .035.
     

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