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Highest Compression for Pump Gas?

Discussion in 'The Garage' started by Ryan B., Sep 11, 2002.

  1. Ryan B.

    Ryan B. 3/4 ton status GMOTM Winner

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    Alright....
    Before i build my next motor (most likely a 383) i could use some first hand knowledge...

    I've heard that you can run as high as like 10.5:1 or even higher with the right camshaft....

    But realistically... who has built up an engine with like 9.5:1 or higher and can attest to how it'll do on 91 with what timing settings, cam profile, etc...

    This will be for my daily driver blazer so i don't want any pinging on 91! I don't mind mixing in race gas for my 11:1 camaro but not everyday with the blazer! /forums/images/icons/grin.gif
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I'd say 9:1 with cast iron heads and 10:1 with Aluminum heads...any cam that'll allow you to run higher is just a crutch fix for a motor with too much CR for the available fuel IMHO. It's bleeding compression off to prevent pinging and detonation.

    I'd stick with 9:1 and not much higher...

    Rene
     
  3. BorregoK5

    BorregoK5 1/2 ton status

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    Rene hit it on the nose. I'm running 9.5:1 on 87 octane with cast iron heads and the most conservative towing Comp Cams Extreme 4x4 cam on my 415. I had my hypereutectic pistons ceramic coated to help in detonation control. I custom tailored the advance curve to accomodate the low octane as well. I run a static timing of 4 degrees (350ci would be more), mechanical timing of about 22 degrees and vacuum advance up to about 32 degrees total advance. Before I recurved the HEI I was getting about 50 degrees total advance and had part throtle ping on 92 octane.
     
  4. Emmettology 101

    Emmettology 101 3/4 ton status

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    I agree with Rene. Keep the comp low to avoid problems. I am in that perdicament now with the truck I bought. The comp is high and I have to mix. I lowered the timing monday to try and avoid the race fuel, but inorder to eliminate the pinging I have to run the timing really retarded and the power goes down. My friend had a shady guy build the engine and thinks the comp is around 10.5:1 - 12.5:1 with Aluminum Trick Flow Twisted Wedge heads.

    Although my Probe(94 GT) has aluminum heads with 10.2:1 comp and I run 93 in it without problems.(I have run 91 at times without problems also)...

    I would go as low as you can allow.
     
  5. Ryan B.

    Ryan B. 3/4 ton status GMOTM Winner

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    Right on. Thanks for the responses. Thats the info i'm looking for...
    So if i go with 9.5:1 i should ceramic coat the pistons to help with detonation and then play with the spark advance curve.
    A friend was telling me that i shouldn't go over like 9.2 or 9.3:1.

    The engine that came in my truck when i bought it was between 10:1 and 10.5:1, and i had problems with that motor pinging at WOT.

    And for my car, that 108+ additive stuff is crap, helps a little but i buy 5 gallon cans of 114 and mix it in.
     
  6. florida4x4

    florida4x4 1/2 ton status

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    What you want when you do the buildup is the longest rod you can afford. Hot rod did a build up of a 400 SBC stroker where they used 300 6-cyl rods (get over it /forums/images/icons/grin.gif ) to get a very streetable 11:1 comp with +400ft lbs. While the frd 6-cyl are 6.2" long you could do something similar using a 350 block and the more commonly available 6.0 steel rod and custom piston.

    Here's a link to the hot rod article:

    http://purplesagetradingpost.com/sumner/techinfo/350%20chevy%20engine.html


    Now the qwestion is... Speed is money. how fast you want to go? /forums/images/icons/grin.gif
     
  7. BorregoK5

    BorregoK5 1/2 ton status

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    If your going to tow or pull your weight around, you would be better off staying below 9.5:1. It really depends on what your building too, a 327 and a 400 are going to react completely different pushing a 5000lb truck at 9.2:1.
     
  8. Mr.Chevy4x4

    Mr.Chevy4x4 1/2 ton status

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    Combustion chamber design, the shape of the top of the piston, flame travel, altitude, and the quality of the fuel delivery(atomization) all play into this question. With the improved combustion chamber design of the aluminum Fast Burn cylinder heads from GM, flat tops or slightly dished pistons, and a good fuel delivery system you can get away with 10:1 or maybe a little more at a low altitude with 91 octane fuel. If you are in Denver on the other hand, forget about it! Run 8:1 compression and buy a supercharger! /forums/images/icons/grin.gif


    Mike
     
  9. TopOff

    TopOff 1/2 ton status

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    Hello my name is TopOff
    ****waiting for response***

    My 383 has spark knock... /forums/images/icons/frown.gif

    I have KB Hyperuetetic 11:1 pistons (when in a 64cc chamber).

    I am running around 9.3-9.4 compression ratio with a Wolverine WG1137 cam.

    Edelbrock performer intake, and a Howell TBI fuel-injection.

    Stock iron heads (76cc chamber) for reliability. I was afraid to go aluminum.

    I should be running a thicker head gasket.

    I am running 91 octane ALL the time.... this hurts /forums/images/icons/frown.gif

    But I also should be
    getting around 440 lb/ft ~1500rpms
    but only ~300 hp at 3200 rpms.
    Makes for easy boat towing!

    I want to advance my curve about another 4 degrees. But I can't in the summer... I get WAY too much detonation.

    In the winter I can advance it, and it drives SO MUCH better!

    HEI ignition:
    8-10 degrees initial. Slow curve 30 degrees at 2500 rpms. and only 12 degrees of vacuum advance on adjustable vacuum can.

    I REALLY need to speed up my curve. I also need to backoff my vacuum probably down to 8 degrees total vacuum.

    But then again, it looks like i will be dropping in a Crane 260 cam.... primarily because the NOS lifters (got these free) have 4 or 5 bad ones = tick... well someday. /forums/images/icons/frown.gif

    I would keep it around 9:1 Besides you don't REALLY gain a whole lot in a truck. If you were going for more HP with a bigger cam (which is hard to do ANYWAY becuase of the stroked crank hitting a high lift cam), then you are going to start spending some CRAZY amounts of money. NOW you might as well get a Big Block!
     
  10. Ryan B.

    Ryan B. 3/4 ton status GMOTM Winner

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    Cool. Thanks for the info TopOff. /forums/images/icons/grin.gif /forums/images/icons/tongue.gif /forums/images/icons/cool.gif
     
  11. TopOff

    TopOff 1/2 ton status

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    TOO much info???

    /forums/images/icons/crazy.gif
     
  12. lizard

    lizard 1/2 ton status

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    Nobody believes in the long rod!!!
     
  13. BorregoK5

    BorregoK5 1/2 ton status

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    You reminded me of the "quench" factor but it really only plays in to flat top pistons and closed chamber heads or domed pistons ond open chamber heads. Getting the quench right will allow you to push the compression ratio a bit as well, not just from the turbulence it creates in the chamber at top dead center but also from the cooling effect it has on the piston with its close proximity to the cooler head, especially with a longer rod.
     

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