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Engine Compression Ratio question...

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Brian 89KBlazer, Sep 13, 2001.

  1. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    I was led to believe that if I exceeded the low 9:1 compression ratio range with my TBI 350; I'd be in for detonation problems.

    The machine shop I visited the other day to inquire about a rebuild said that he's done a TBI 350 in a full-size 4x4 chev that is running 10:1 with no detonation problems with the stock computer.

    Does this sound right to anyone? I'd love to be able to bump it up to 10:1 for better torque & HP but I'm curious about the detonation issue. Can anyone help?

    Thanks


    Brian
    89KBlazer

    Build it Right or Don't build it at all!!
     
  2. outlaw612

    outlaw612 1/2 ton status

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    With 10:1 youll need to run like a 92-93 octane, but shouldnt have any detonation probs unless you have your timing way advanced.

    <a target="_blank" href=http://www.geocities.com/outlaw613/K5.html>http://www.geocities.com/outlaw613/K5.html</a>
     
  3. J.R.

    J.R. Guest

    you would be fine with 10.1, but you would need to run a better grade of fuel. and when you do get around that 10.1 mark you can tell when you get a bad tank of gas and then you would want to put in a additive to correct it.

    BIG BLOCK MAN
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  4. dyeager535

    dyeager535 1 ton status Premium Member

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    Id check to see if anyone else has actually done that, with those heads. 9.5:1 is easy on 87 octane, but head design, pistons, cooling, etc., is ALL going to affect detonation.

    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</a>
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  5. Executioner

    Executioner 1/2 ton status

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    Compression ratio by its self is a meaningless #, just like a HP of 300.
    Just like a high torque # of say 400 lbs./ft.
    You need to understand the whole picture !
    Depending on a few things, Timming/Cam Timming and whats happening inside the clyinder, and the way
    CR is measured. Ask more questions ?
     
  6. Twiz

    Twiz 1/2 ton status

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    10:1 you bet. Some of the new engines are running 10:1 +..
    How?
    -Persision fuel delivery
    -Electronicly retarding timeing as needed.
    -Highly voltile/turbulint combustion chambers (fast-burn chambers)
    -Spark plugs located closer to the hot side of the of the chambers (exhaust side)
    -More (and idenical from chamber to chamber) atomized air/fuel mixture
    -High and accurate (S/P?)spark
    -Cooler air intake
    *A study i read from memory*
    A dude built a engine that was designed to detetinte. He ran 3 diffeerent intakes. One standerd exhaust heated intake, one with the heat risers blocked off, and one artificialy cooled. The one that was cooled did not detinate*

    We will stand strong!
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  7. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Thanks all for the help so far.

    Some supplementary info is that I plan on running a Crane Compucam 114122 with 204I/214E @ 0.050" lift &amp; 0.423I/0.446E lift. The builder also says he would use a flat-topped pistons instead of dished.

    Dyeager; he says my stock heads should be 76cc heads &amp; should have no problems with the 10:1.

    I guess my question is; does 10:1 with the mostly stock engine parts seem totally out to lunch or well within the realm of possible?

    Thanks again

    Brian
    89KBlazer

    Build it Right or Don't build it at all!!
     
  8. beater74

    beater74 1/2 ton status

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    i run a 10:1 sbc but it's not tbi and HAVE to run 93 or i get detonation big time even with my timing a 4 deg above tdc.
     
  9. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Brian, if I recall correctly the TBI 350's were 9.3 to 1 stock. I would guess that with a knock sensor and a computer it would keep 'adjusting' itself until there was no pinging...then again i can't see a huge benifit in going from 9.3 to 10 power wise.

    Rene

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  10. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Thanks Rene! That's what I think they are too if I recall the dealership brouchure correctly. That's also the way the machine shop guy looked at it. He said that there wasn't much difference between the stock &amp; 10:1 but he meant that the diff would be insignificant to the computer; not to power gains.

    My big conditon of this engine is that I want to be able to run 87 octane! So according to what's been said here; 10:1 sounds a little high for that! [​IMG]

    Many thanks again &amp; please keep it coming!

    Brian
    89KBlazer

    Build it Right or Don't build it at all!!
     
  11. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I think the only way to run 10 to 1 and 87 octane would be if you switched to Aluminum heads. They tolerate higher static compression without detonation than the same heads in cast iron. Just something to think about...

    Hmmmm...Brodix [​IMG][​IMG]

    Rene

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  12. dyeager535

    dyeager535 1 ton status Premium Member

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    I wonder if maybe he is talking about "pump gas" vs "race gas"? You always see magazine articles where it runs on "pump gas" Yeah. 91 Octane pump gas, not the cheap stuff.

    If you can keep the cylinder cool, you can get away with higher compression. Raising compression alone increases cylinder temps, hence the problem with just bumping a motor up to say 10:1 without head mods or changes, etc.


    Are they really 76CC? I'm not doubting, just seems awfully large. Probably why the original pistons are dished. The Vortecs are 64CC for comparison.

    Personally, to be on teh safe side, I wouldn't run more than 9.5:1 for a street engine. Compression change alone doesn't make enough power for the negative side effects, which can result, such as faster wear, (more pressure on rings, head gasket, etc) problems passing emissions, and the all important, "will it run on 87" : )

    I guess depending on engine design, you could even have problems with 9.5:1 and 87 octane, but in a Chev V8, no matter the vintage, I have never heard of anyone requiring more than 87 octane, if they stayed under 9.5:1 I guess thats more a rule of thumb than an actual rule.



    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</a>
    No anti-theft measures on your truck? No pity when its stolen
     
  13. Triaged

    Triaged 1/2 ton status

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    The way the computer works is if the knock sensor sences pinging it retards timing. It doesn't put the timing back till the fuel gage goes down and than back up (the knock sensor is only ment to help not destroy the motor from a tank of "bad gas"). After it sences the knock power will go down. Just make it easy and stay under 9.5.

    '71 Blazer CST w/ a 400sbc, 4" lift, 36" Supper Swampers, and alot of rust
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  14. CaptCrunch

    CaptCrunch 1/2 ton status

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    I agree w/ Dorian. Keep w/ a nice 9.5 compression ration so you can run the cheap stuff. The power gain in a mild build up for a 1/2 pt of compression isn't worth having to run a more expensive gas IMO.

    -Mikey
    1987 Chevy K5 Blazer- 350 TBI
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  15. mpascino

    mpascino 1/2 ton status

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    I am running 10.5 to 1 in a 350 .30 over. I have cast iron heads and TBI. I run 93 octane with no problems, The truck performs beautifully and doesn't give me problems unless I got some bad gas. I have a stage 1 chip in the computer and a 270 advertised degree cam. So far I am happy with the complete setup.
    mike

    You call that dirty!?
     

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