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Compression vs. pump gas

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by ftn96, Nov 13, 2001.

  1. ftn96

    ftn96 1/2 ton status

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    Can someone give me the break down of the 3 types of pump gas in relation to how much compression I can run with each?



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  2. blackwidowk5

    blackwidowk5 1/2 ton status

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    I read an good article on this in Hot Rod last spring i'll see if i can't find it and give you the exact numbers. I think the numbesr went something like this. 87 up to about 8.9:1
    89 up to 9.4:1 and 92 for everything higher
     
  3. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    The TBI 350's were designed to run on 87 and they were 9.3 to 1. A lot has to do with things other than compression ratio. Aluminum heads will allow more compression due to the amount of heat they transfer for example. My 350 is 9 to 1 and I always run 87...even when I tow.

    Rene

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  4. TX Mudder

    TX Mudder 1/2 ton status

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    Remember that aluminum heads can have higher CR with the same octane because they cool more efficiently.
    Also, different head designs are more efficient and can take a higher CR.
    I believe that 882 heads would be on the verge of detonation with 87 and 9:1. Vortec heads would be fine because of the more efficent combustion chamber design.
    Always trying to muddle the answer... ;)
    -- Mike

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  5. CaptCrunch

    CaptCrunch 1/2 ton status

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    Yeah a lot has to do with other things as Rene said. Like I have had 11 to 1 compression cars run all day on pump gas, but that was with alum heads and zero deck height. Usually stick w/ 9 or 9.5 to 1 and you will be fine on 87 or 89 octane. Sometimes you will have to play with the timing in hot weather to keep with 87 or else just run 89 depending what components you have. (I am kinda assuming that since it is a daily driver you will probably want to run 87 and not have to worry about alum heads)

    -Mikey
    1987 Chevy K5 Blazer- 350 TBI
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  6. ftn96

    ftn96 1/2 ton status

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    We are talking about the S/R torquer heads with 76cc, 2.02/1.60 IRON heads.
    Mainly was wondering if I could go to 9.5-1 and still be fine with say the mid-grade gas.
    Hell, I'll probably just leave it at 9.3 and be done with it.

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  7. CaptCrunch

    CaptCrunch 1/2 ton status

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    If you are willing to buy 89 or 90 you should be able to manage 9.5 IMO. You can always play with the timing a touch, but I don't think you will have any problems unless you run a huge advance in your ignition curve (in your case most likely running a Jet or other Hi-po chip that makes it like that)

    -Mikey
    1987 Chevy K5 Blazer- 350 TBI
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  8. muddin4fun

    muddin4fun 3/4 ton status

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    my heads are 72cc (cast) with 9.75:1 and I have to run 93 or it'll ping when under a load.

    Peace and stuff [​IMG],

    Muddin

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  9. TX Mudder

    TX Mudder 1/2 ton status

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    Muddin, doesn't that make it too expensive to drive the truck?
    Although a motor warranty won't approve, you can drop the CR about 1.0 point by simply installing TWO head gaskets in place of the one. WOuld sure make it more affordable to drive.
    Right now 87 is going for 99 cents a gallon here in Houston.
    -- Mike

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  10. ftn96

    ftn96 1/2 ton status

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    Yeah, Lara, cause you got what? A 100 foot stroke in that damn thing! [​IMG]

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  11. muddin4fun

    muddin4fun 3/4 ton status

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    <blockquote><font class="small">In reply to:</font><hr>

    Muddin, doesn't that make it too expensive to drive the truck?

    <hr></blockquote>

    Nope [​IMG]

    93 is like 1.29 right now. I was paying 1.46 for 89 a while back so I'm still making out good [​IMG]


    I dunno about 100 ft, but it goes vroom vroom real well [​IMG] [​IMG]

    Peace and stuff [​IMG],

    Muddin

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  12. hammer

    hammer 1/2 ton status

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    I have 10:1 in my 350 LT1 crate for my camaro and run 93 no knocking cast heads 1.45 gallon. Truck has 8.5:1 or so stock just a daily driver for the snow and offroad during hunting season 87 1.27 gallon
     
  13. 84_Chevy_K10

    84_Chevy_K10 Banned

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    Vortec 350s are 10:1 I believe.

    Many 305s were 9.2:1 from the factory.

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     
  14. ftn96

    ftn96 1/2 ton status

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    So does this mean I can get away with a 9.6-1 compression running the vortec heads and run on middle grade 89 gas?
    I was just looking at thicker head gaskets adn WOW!!!! 44.95 each head. Hell I can almost buy a new set of pistons for that!!! And you'll want to know whay I keep asking questions?[​IMG]

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  15. dyeager535

    dyeager535 1 ton status Premium Member

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    With what my speed shop buddy calculated my CR at with the Vortec heads and 350, (9.3-9.5:1) I am certain that I will have no problems running on 87 octane.

    Keep the engine cool, (right about 195 or so) timing in check, and you should have NO problems. They were running 9:1+ compression on some of the 80's 305's that had marginal at best heads...that should give you some confidence, especially with the much better Vortec head design.

    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>www.dorianyeager.com/index2.html</a>
    Why insist on counting when the ring gear has the tooth counts stamped in?
     
  16. ftn96

    ftn96 1/2 ton status

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    Will aluminum pistons help with detonation? But they dont come in contact with and water so I wasn't sure if that would really help.

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  17. schwinnd

    schwinnd 1/2 ton status

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    TXMUDDER - .99 cents a gallon!!!! Man, here in SE Washington we are still at 1.50 a gallon for 87 octane. I gotta move, we've been getting screwed all year, even Seattle has cheaper gas.[tounge]

    Just collecting rusty mid-70's chevy's
    1976 Blazer, 1975 Vega SW
     
  18. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    Just to add some more confusion to this thread, I think you can't just do a static compression ratio calculation to know the whole story. I was reading on an engine builders website that you need to take the cam profile into consideration too....something about the "absolute cylinder pressure" and the fact that those valves are opening and closing all the time, and the amount of pressure you are building in the cylinder is related to the cam profile too....

    I don't know enough about engines to elaborate, but I remember at the time it made sense to me. He sets up engines with seemingly HIGH compression for the street &amp; pump gas, and because of the cam choices, they work just fine.

    Something else to consider....I wish I could explain it better, but I'm sure this will make sense to someone else here who can clarify.... [​IMG]

    -Greg72
     
  19. dyeager535

    dyeager535 1 ton status Premium Member

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    AFAIK&lt; all modern pistons are aluminum. Don't think you are going to find anything other than aluminum alloy as a piston material. But coating the top of pistons is something that is being done now, I believe also in an attempt to decrease detonation. I'd think it would reflect heat, but perhaps thats the intent, keep piston as cool as possible, so as not to add heat to the next intake charge.

    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>www.dorianyeager.com/index2.html</a>
    Why insist on counting when the ring gear has the tooth counts stamped in?
     
  20. 84_Chevy_K10

    84_Chevy_K10 Banned

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    It depends on a lot of factors.

    Thicker head gaskets will make your detonation condition worse, not better.

    Quench distance, or the distance from the top of the piston to the bottom of the cylinder head, is the most important thing to worry about when you want a pump gas engine that will run a good compression ratio and on regular fuel.

    Camshaft choice also has a lot to do with it as more duration/wider LSAs will bleed off some compression at lower RPM due to more valve overlap, minimizing the chance of detonation.

    I've been told that optimum quench distance is from .039-.045" on an SBC. It has been known since the '30s that quench makes a huge difference as far as detonation is concerned. It's a constant talk over at Chevytalk.com in the performance section.

    Vortec heads are very efficient. I think with proper quench, the right camshaft, and 89 octane, you can run 9.6:! compression...however, with .060" or more quench, a very small camshaft, or if you have the improper rear gears or something, you're looking at potential problems.

    i wouldn't be worried as long as your cam isn't real small and your quench distance is correct.

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     

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