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Thread: compression ratio questions

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    VR-3747 :D Not Verified Feedback Score 1 (100%) futurevr4man's Avatar
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    compression ratio questions

    the infamous compression ratio debate...

    i understand (and agree) that a lower compression ratio, in a boosted application, will yield bigger horsepower numbers.

    my questions are:
    1) hoe big of a difference does .5 extra compression really make? for example, 8:1 compared to 8.5:1... what about 8.5:1 compared to 9:1?
    2) is there a limit to when it really doesnt do any good to lower compression? or better yet, is there a limit when you start sacrificing more than you gain (torque for top end power)?
    3) if you had the option what ratio would you select to get the BEST combination of low end torque and high end boosting ability? right now (from my research) it seems 9:1 is a good balance, but would 8.5:1 really be better? remember im not talking in general, im talking about for our v6 engines.

    debate.

    -nate-
    rise and rise again until lambs become lions

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    Now with more poop-smear Not Verified Feedback Score 8 (100%) IPD's Avatar
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    subscribed. i need to know if running 90 psi on 6:1 is for me.

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    Forum User Feedback Score 0 AdamVR4's Avatar
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    Quote Originally Posted by futurevr4man View Post
    the infamous compression ratio debate...

    i understand (and agree) that a lower compression ratio, in a boosted application, will yield bigger horsepower numbers.

    my questions are:
    1) hoe big of a difference does .5 extra compression really make? for example, 8:1 compared to 8.5:1... what about 8.5:1 compared to 9:1?
    2) is there a limit to when it really doesnt do any good to lower compression? or better yet, is there a limit when you start sacrificing more than you gain (torque for top end power)?
    3) if you had the option what ratio would you select to get the BEST combination of low end torque and high end boosting ability? right now (from my research) it seems 9:1 is a good balance, but would 8.5:1 really be better? remember im not talking in general, im talking about for our v6 engines.

    debate.


    -nate-

    You don't need a debate full of anecdotal evidence and speculation, you need a thermodynamic model... Get on it.

    In the mean time google provides some information...

    Engine Compression Ratio - Tech - Popular Hot Rodding Magazine
    '93 VR4 | 10.57 @ 135 on C16 | 11.29 @ 125 on 93 | ~3275 lbs

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    heres my input. 8.0:1 is best. the higher comp engines always run into knock at lower power output vs octane.

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    VR-3747 :D Not Verified Feedback Score 1 (100%) futurevr4man's Avatar
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    LOL at IPD.

    adam, i am so sick of thermo.... uuuggghhhh lol

    ray, i hope you are getting ready for me and my engine/head/tranny build. and you still need to check out my email!

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    The Infamous Mr. Chocolate Feedback Score 0 AudibleSilence's Avatar
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    Quote Originally Posted by IPD View Post
    subscribed. i need to know if running 90 psi on 6:1 is for me.
    LuLz!!!
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    The Infamous Mr. Chocolate Feedback Score 0 AudibleSilence's Avatar
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    Quote Originally Posted by Ray Pampena View Post
    heres my input. 8.0:1 is best. the higher comp engines always run into knock at lower power output vs octane.
    I thought the power 10:1 creates at 9 psi was relative to the power 8:1 makes at 12psi (obviously these psi ranges are approximations) because of the extra heat/compression generated?

    Oh and someone had a 13:1 engine once on 3si iirc.

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    VR-3747 :D Not Verified Feedback Score 1 (100%) futurevr4man's Avatar
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    audible you may be right... but extend that thought to 10, 11, or 12psi on 10:1... you will hit knock pretty quick.

    on 8:1 you can boost to 18-20psi before ever seeing knock... there is more power to be made at the higher boost like that.

    i was just wondering if 8.5:1 would be worth it, if there was a happy median on raising CR without killing your boosting capabilities

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    Advanced Tech? verified Feedback Score 0 i3igpete's Avatar
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    Quote Originally Posted by AudibleSilence View Post
    I thought the power 10:1 creates at 9 psi was relative to the power 8:1 makes at 12psi (obviously these psi ranges are approximations) because of the extra heat/compression generated?

    Oh and someone had a 13:1 engine once on 3si iirc.
    you're right. here's a better diagram we can use to discuss...
    THERMODYNAMICS 101 IN 3 MINUTES



    in general, the energy for an engine cycle is the area in this box.

    advancing ignition timing or increasing compression increases the area in the box by moving point 4 higher and higher, while keeping point 5 the same. but keep in mind that higher pressures cause bent rods and lifted heads.

    Instead, let's lower point 4 by decreasing the compression ratio and retarding ignition timing. We can instead raise point 5 by increasing the air mass in the cylinder - higher boost and/or larger turbos. so yes, it's theoretically less efficient because 4 isn't as high as it could be. but the amount of energy is higher, and you do it more safely because peak cylinder pressures are lower.

    Of course, it takes more $ to buy the turbos that can raise point 5 higher. It's easier to play with your emanage and increase your ignition timing and raise point 4. ***I should mention at this point that knock is something different: it's an uncontrolled burn where point 4 spikes sky high, but point 3 moves to the right. So the net effect is that you get the same or less area in the box, with a ton of damaging cylinder pressure.



    on a side note, this diagram explains why engineers find the debate of torque vs HP kind of dumb.
    -Torque is simply "how big can you draw this box?". this alone doesn't mean anything, if you draw it slowly.
    -RPM you can think of as "how fast can you draw boxes?". this alone doesn't mean anything either, if you draw a bunch of tiny boxes.
    -Multiply the two, and you have power: "how much AREA can you draw in a second?".
    -This is where the Torque*RPM/5252 equation comes from.
    Last edited by i3igpete; 07-06-2011 at 02:53 PM.
    Maddog Performance Engineering

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    The Infamous Mr. Chocolate Feedback Score 0 AudibleSilence's Avatar
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    I never got going turbo and still having the engine do the compression for you. There is a kid on the goat board that is going to push 13:1 psi and try to run at least a bar vs. keeping away from the knock and running more boost (less compression). Why people make their lives difficult, I have no idea.

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