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Compression ratio : 76cc vs. 67cc

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by RaisedK5, Dec 26, 2001.

  1. RaisedK5

    RaisedK5 1/2 ton status

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    I'm looking at buying a set of heads on the for sale board, and had one question about the heads, I have a stock rebuilt 350, so figure the crappy 8.5 : 1 compression with the stock 76cc heads. If i were to swap on this pair of 67cc heads, what would my compression come out to be? i know there are other factors, gasket thickness and all, but a ball park figure would be good, because i want to be able to run cheap gas and not have a 10:1 ratio or something like that, the 92 octane bills for my Buick are high enough. Thanks Everybody!

    Leland aka RaisedK5

    "Of all the things I've lost, I think I miss my mind the most!"
    "Are the voices in my head bothering you?
     
  2. Andreas

    Andreas 1/2 ton status

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    If you have 8.5:1 compression with 76cc heads, going to 67cc heads will boost it up to 9.3:1.

    Andreas

    406cui of pure sh*t[​IMG]
     
  3. RaisedK5

    RaisedK5 1/2 ton status

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    So now, with a 9.3:1 ratio, you think i could still run cheap ass gas? just keep the timing back a little? or what, what are you guys running? Anybody? Thanks in advance!!

    Leland aka RaisedK5

    "Of all the things I've lost, I think I miss my mind the most!"
    "Are the voices in my head bothering you?
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    If your vehicle is in good tune, you should be able to run 9.3:1 on 87 octane. (lowest grade in this state) Timing and head design will factor in, as will other things, but you should be able to do it as long as you can keep the vehicle cool. My 8:1 motor pings on 87, but only when it reaches about 220*. (long story, thankfully its not the truck motor) 9.3:1 is pushing the "commonly accepted" line of 87 octane, but it is possible. The 305's used in some GM cars in the 80's were 9:1(as were some truck motors I have heard) so even with your extra .3CR, you should be fine.

    Again, other factors besides compression ratio are going to have an impact, so if it "pings" on 87, you can't blame it on the compression. Kind of a disclaimer, but I don't plan on running anything but 87 in my motor when its done, and it should be right at 9.4:1.



    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?
     
  5. RaisedK5

    RaisedK5 1/2 ton status

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    Excellent, Thank you for your help. Mine usually runs nice and cool, and when these go on it'll be tuned as perfectly as i can get it, so i guess i'll be ok, Thanks again.

    Leland aka RaisedK5

    "Of all the things I've lost, I think I miss my mind the most!"
    "Are the voices in my head bothering you?
     
  6. Andreas

    Andreas 1/2 ton status

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    There are a few things you could do if you have the $$. One is to take advantage of the increased compression by installing a slightly larger cam with a little more overlap (it´ll bleed some pressure out). Not talking about 8000 rpm roller stuff but one of the Xtreme Energy or Xtreme 4x4 from Comp Cams selection. Another thing is to cool the intake charge by using one of Edelbrock Air-Gap intakes. Don´t retard the timing. You´ll loose a lot below 3500 rpm. I have 10.14:1 in my 406 but I have a pretty big cam (for being a 4x4 vehicle). Also, we have 98 octane at the pump in Sweden [​IMG].

    Andreas

    406cui of pure sh*t[​IMG]
     
  7. Slayer

    Slayer 1/2 ton status

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    My crate motor heads are 62cc, and that's 9.1:1 compression ratio.
    The motor was designed to run 87 octane gas (the cheapest stuff), but I've got the timing advanced so far that I have to run premium (91 in Colorado) plus octane booster to keep the valves from slamming shut (and that's not a good sound).


    <font color=blue>I don't know everything, I just think I do.............</font color=blue>[​IMG]
     
  8. CaptCrunch

    CaptCrunch 1/2 ton status

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    Since we are on the subject... what about the Vortechs? I know you are planning to run them Dorian. They say you get a a little extra compression without detonation because of the CC design, but no one really has seemed to figure out how much? You think 9.4 is do-able?

    -Mikey
    1987 Chevy K5 Blazer- 350 TBI
    <a target="_blank" href=http://coloradok5.com/gallery/captcrunch>http://coloradok5.com/gallery/captcrunch</a>
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    www.dorianyeager.com/index2.html
    Why insist on counting when the ring gear has the tooth counts stamped in?
     
  10. CaptCrunch

    CaptCrunch 1/2 ton status

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    Thanks for the link Dorian. I have been looking at buildups for next spring to add some punch to my rig. Been leaning towards a tpi based vortec motor, but we'll see. I don't have the time to mess around with mild build up short blocks so I usually buy just a short block unless it is gonna be a racer w/ beter rods, pistons, and/or is getting majopr machining work on the bottom end. Seems like it is just as cheap too after machining costs on a regular block, but back to the topic. I'm might be kinda old school... think 9.0 to 1 on an iron headed motor is about right... I have heard from some people they are running vortecs with as much as 9.6 to 1. Neither are direct experience so I'm skeptical that that wouldn't be 90 and possible 92 octane in the summer heat. I do know both VC and ROL have thicker gaskets that will reduce compression, but obviously that gives you more quench. Normally wouldn't care but this motor is my main ride so it would be nice to keep it to 87 as it already guzzles enough gas. Any vortec runners know what their compression is? Anyone know what compression the Vortech truck run?

    -Mikey
    1987 Chevy K5 Blazer- 350 TBI
    <a target="_blank" href=http://coloradok5.com/gallery/captcrunch>http://coloradok5.com/gallery/captcrunch</a>
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    Checked on the Monte Carlo list, heres what I got back (and a snippet of my post):

    &gt; I'm curious if any out there have some hands on &gt;experience with known
    &gt; compression ratio, and running 87 octane gas.

    ...I have 9.6:1 in the SS and it will run on 87.

    Again, lots of other variables play into this, (cam, timing, cooling etc) but thats pretty authoritative.

    If mine won't run on 87, I'll be ticked lol. Personally, If they can be run with 87 at 9.6:1, taking it down 3/10ths should only help make SURE you don't have problems towing or wheeling.. I was contemplating smoothing the combustion chambers just in case, but everyone said to just leave them as is, and I think I will.


    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?
     

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