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    1. #1
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      12.5:1 compression on pump gas???

      I remember reading through a magazine, not sure which one it was, but it said it was possible to run 12.5:1 Compression on pump gas. It gave information about Cylinder head combustion chamber shape, it explained about what quench is, the total cc's above the piston at TDC etc. I lost the Article but if anyone has information on this that would be really helpfull

      Thanks.


      1993 Camaro Z28
      2001 Camaro Z28
      1969 Camaro



    2. #2
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      Never mind is 12:1 compression, and i found the article here.

      http://www.popularhotrodding.com/tec...ression_ratio/

      Any insight on this theory? Is it actually possible?

      I also see that David Vizard is the Author of this theory so it must be true then?

      1993 Camaro Z28
      2001 Camaro Z28
      1969 Camaro


    3. #3
      Join Date
      Nov 2005
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      It's absolutely possible. It all depends on how much cylinder pressure is bleed off by the intake valve. Big cams = lots of pressure bleed.

      Having all the details correct, like Vizard points out, can make or break the scenario as well.

      Or, use direct injection. IIRC, Audi was running 12:1 on their 3.XL V6. And that's a production engine.
      Matt Jones
      Mechanical Engineer
      Art Morrison Enterprises

    4. #4
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      I had a buddy with 14:1 in a 302 sbc '68 Camaro. HUGE cam really bled off pressure. Loped like an sob as well.
      Nick R.
      69 Camaro - 383, 700R4, 12 bolt 3.55, Hotchkis, Bilstein, Global West, Morris Classic
      08 HHR SS - Still Stock for now
      Do you still believe in all the things that you stood by before? Are you out there on the front lines, or at home keeping score?
      Do you care to be the layer of the bricks that seal your fate? Would you rather be the architect of what we might create?

    5. #5
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      Well, I'm up there on the cam end of things (.700 lift, 262/268 duration @ .050), and at 11:1, I think it's close to maxed with 93 octane. The heads were massaged too on top of the cnc work- no sharp edges, etc. At times I have to add some booster to keep it from diesel-ing on shutoff, etc.

      For some of these pump gas shootout motors with high compression - they are ok at WOT with the propper tune, but I wonder how they are under real-world conditions? (i.e. part throttle loads, shutting off without diesel-ing, etc.) just my 2cents
      1968 Camaro RS/SS, LS7 with Katech mods, T56 Magnum, C6Z06 Brakes
      1968 Camaro RS Convertible project LS3/480hp/4L70E
      1962 Corvette 327-340hp stock
      1972 Corvette LT1 Stock
      2006 Corvette Z06
      2011 Corvette GS convertible


    6. #6
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      So im guessing that inorder to run high compression with 93 pump gas, you'll need a huge cam to bleed off cylinder pressure. If thats the case would there be a way to tell how much pressure there will be with a given combonation of parts? Im trying to rebuild my 350 to the best that I possibly can, and if i can some how fully understand this i bet it would be one hell of a motor.

      So what heads, and cam would you use on a 357 sbc with 12:1 compression to run on 93 octane?

      1993 Camaro Z28
      2001 Camaro Z28
      1969 Camaro


    7. #7
      Join Date
      Aug 2007
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      Normal, IL
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      234
      I think it's very inefficient to run a bunch of compression and then bleed it off with a cam or retard the timing. I'd want to use an appropiate amount of compression with an appropriate cam for the octane you're planning on using. For a street vehicle, it means you'll have to run a big stall or bring the revs up for it to start going. Just seems like a waste to me.

      68sixspeed: with that cam and what I assume aluminum heads, even with the appropiate timing (34-6), as long as you have the amount of added timing by vacuum limited, I think you'd be fine with 93 especially in a lite camaro.

    8. #8
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      So there would be no benefit from running a high compression cam on pump gas with a huge cam?

      1993 Camaro Z28
      2001 Camaro Z28
      1969 Camaro


    9. #9
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      badbu68 - thanks- yes, aluminum heads, even aluminum block (big old heat sink!); and about 3500 miles on it so far, no vacuum advance, I've had mixed results with that on aggressive motors, so go without. If you think about it, the diesel-ing is not timing related- that is on shut-down, it is pre-igniting on poor fuel and it keeps the motor running.
      1968 Camaro RS/SS, LS7 with Katech mods, T56 Magnum, C6Z06 Brakes
      1968 Camaro RS Convertible project LS3/480hp/4L70E
      1962 Corvette 327-340hp stock
      1972 Corvette LT1 Stock
      2006 Corvette Z06
      2011 Corvette GS convertible


    10. #10
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      Quote Originally Posted by Project69
      So there would be no benefit from running a high compression cam on pump gas with a huge cam?
      For a controlled dyno only engine. Yes.

      For a real world, multiple conditions in car engine. Not worth the gamble IMO.

    11. #11
      Join Date
      Aug 2004
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      Boston MA
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      686
      Folks are refering you to the concept of Dynamic Compresion Ratio. If your valvles closed at BDC, your cylinder presure would be wholely determined by your compression ratio. In reality the intake valve closes some time after BDC. The formula for DCR attempts to acount for this. If you subscribe to this theory, you're looking to find a combination of static compression and cam duration that yeilds around 8.2:1 DCR for pump 93. A google search should give you all the information you'd ever want.

      My $0.02, don't ask the internet for head and cam advise. Especially with out pertinant details like: weight, rear end gear, transmission, torque converter, intended use, AL or steel.... Build a relationship with a professional engine builder and ask them.
      1967 #s RS

    12. #12
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      11-1 max on a steel block, alum header motor, 11.5-1 with all alum are starting point numbers

    13. #13
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      I used to have a dieseling issue on an old engine that had high compression. I found a throttle solenoid that I think was used from the factory to up the idle speed when the AC kicked on. I had it set to idle speed when energized, but when the ign. was shut off it closed the carb. Worked great.
      1969 Camaro..getting closer to being done..I think
      1994 Camaro... Future N.A.S.A racer... maybe
      Victory Circle South West Tour race car (SCCA)
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      Greg is my other name...

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    14. #14
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      If you're going that high you're going to want to have the intake and exhaust valves thermal coated as well. I'd all also consider an oil cooler.
      Red Forman: "The Mustang's front end is problematic; get yourself a Firebird."

    15. #15
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      Quote Originally Posted by hotrdblder
      11-1 max on a steel block, alum header motor, 11.5-1 with all alum are starting point numbers
      Would i need a huge cam, to bleed of some compression or would i be safe without one? And by huge cam what are they talking about? Long duration, Max lift or both?

      1993 Camaro Z28
      2001 Camaro Z28
      1969 Camaro


    16. #16
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      Quote Originally Posted by ITLBTU
      I used to have a dieseling issue on an old engine that had high compression. I found a throttle solenoid that I think was used from the factory to up the idle speed when the AC kicked on. I had it set to idle speed when energized, but when the ign. was shut off it closed the carb. Worked great.
      That's a very cool idea-- I think I have the holley kit around too - I was going to use it as a high idle for warming the car up.

      Rob- on the cam front; for ref- the 358 I have in the for sale area, steel block, flat top pistons w/2 valve reliefs (coated), aluminum heads, 10.4:1 compression, also ran well on pump 93 - occasionally dieseled on shutdown on poor fuel too, probably acted about the same as 11:1 on the aluminum motor I now have (same basic cam spec, I think the LSA is 112 vs 108) 260/268 duration, .634 lift on the 358.
      1968 Camaro RS/SS, LS7 with Katech mods, T56 Magnum, C6Z06 Brakes
      1968 Camaro RS Convertible project LS3/480hp/4L70E
      1962 Corvette 327-340hp stock
      1972 Corvette LT1 Stock
      2006 Corvette Z06
      2011 Corvette GS convertible


    17. #17
      Join Date
      Apr 2006
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      1,027
      get a dcr calxculator and figure head cc,head gasket cc, compression ratio, and cam specs, like wendeall said 8.1-1 dynamic compression is maqx on pump gas, which basically equates to 11-1 with iron block,alum head(with 10.8-1 being better max, or 11.5-1 all alum,

    18. #18
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      Sep 2006
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      Ok guys forget the static compression and go for the dynamic cranking compression.
      If you get an engine over 200 psi its gonna need some serious octane.
      Now the best idea is to use a decent compression for the engine design.
      Figure old style sbc w/iron heads (ie stockers modified) you will push it with high 9s, aluminums you can push as far as 11 to 1 if you choose the cam right.
      LSx engines are more tollerant.
      For best power production, I prefer to massage a set of medium sized heads, keep my valves moderate, and be frugal with cam selection as too much will hamper the port velocity.
      I took a set of 991 truck heads and rebuilt them as 194/150, I throated the bowls under the valves, I raised the guide height 5/32 and throated righ to that.My throats were the same size as the inside diameter of the valve face.
      This engine had a split lift/split duration cam that was computer friendly, as it was designed by John Lingenfelter for the Accell SuperRam FI kit.
      I built this engien as a 355 with flat tops down in the hole 018, steel shim head gaskets 015 and the chambers were something like 78 or 79 cc and this thing was MEAN and it would only run on 94 Sunoco Ultra or 93 Super.
      I ran a 600 AFB, old Edelbrock Scorpion intake, HEI, HOOKER Monza Headers, 2 1/2 pipes to rear axle and a set of Flowtech Afterburners.
      This BIOTCH ran and hard, I used a TCI saturday Night special converter in a TCI th350, and 3.42 rear gears.
      I prefer to keep my street engines in the 180 to 190 range or you have knock issues, generally.
      Now that engine had some AWESOME prot velocity,,, a friend of mine had me do another set of those heads, and he was getting some serious port velocity from them and told me that it was within the 10 percent or so of raching the Hemholts resonance point, but he never had my intake and I bet it would have hit it.
      That is where the speed of the air coming in has reached the speed at which the mass of the air molecules are going so fast as to liken them to a car going over a long tall steep hill, thier weight actually has them accelrating to a degree.
      I wish I had those heads back now.
      Lee

    19. #19
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      Yea ive heard something about if you keep the cranking pressure around 180-185 your safe with your fuel.


      Say i want to run 11:1 or a little more compression with my iron head small block, could i get away with using 93 octane if i have a big enough cam to bleed of compression?

      1993 Camaro Z28
      2001 Camaro Z28
      1969 Camaro


    20. #20
      Join Date
      Aug 2004
      Location
      Boston MA
      Posts
      686
      Read the posts above and start researching DRC. There's more to it than static compression and cam duration. Take inititive and figure it out.
      1967 #s RS

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