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Compression Ratio

This is a discussion on Compression Ratio within the Internal Engine forums, part of the LSx Technical Help Section category; nice! however I still want to keep it low... so how about flycutting? what did you do Luos? did you ...

  1. #21
    Member Bouvers's Avatar
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    nice! however I still want to keep it low... so how about flycutting? what did you do Luos? did you dish or flycut to fit your cam?

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    Has anyone got info or reference for the performance gains from milling heads? Sacrificing CUI and gaining CR.

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    Quote Originally Posted by BowTied View Post
    Has anyone got info or reference for the performance gains from milling heads? Sacrificing CUI and gaining CR.
    CUI?

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    CUI- Cubic Inches

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    the difference milling does to displacement is tiny compared to the difference it makes to the compression so it will pay off as long as your quench is tight enough to not cause detination and thus retarding the timing, you should always see gains.

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    OK, I did some calculations. From 10.1 to 11.1 CR you give up 1.31% CUI and gain 9.9% CR.

    A lot more work but I think I'd lean to stroking if I ever get in the block. I'd rather gain both...but then I'd go new heads and cam as well. A lot of cash for crank, cam, and heads. Something I will seriously consider once my kids are gone (4 years.)

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    Quote Originally Posted by Mr. Luos View Post
    Yup.


    I run 11:1 with my 402. 91 octane and 31 degrees of timing.
    ZERO knock.


    That's an unusual amount of timing for such a setup to like! Well, maybe not unusual for an unloaded dynojet but unusual for the real world of driving and racing. I had a *VERY* similar combo to yours the week before Christmas and the gains going from 24 to 26 deg were only about 2 HP and 4 ft/lb and no gain moving higher. Getting it out on the street and putting 4 gears together showed that all was fine down low and we got some (predicted) KR in 4th gear. I believe we left off about 23deg around peak tq moving up to 25 by 6000ish. Every setup likes something different, but I have never had 40X" setup end up liking more than 28.

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    Quote Originally Posted by Frost View Post
    That's an unusual amount of timing for such a setup to like! Well, maybe not unusual for an unloaded dynojet but unusual for the real world of driving and racing. I had a *VERY* similar combo to yours the week before Christmas and the gains going from 24 to 26 deg were only about 2 HP and 4 ft/lb and no gain moving higher. Getting it out on the street and putting 4 gears together showed that all was fine down low and we got some (predicted) KR in 4th gear. I believe we left off about 23deg around peak tq moving up to 25 by 6000ish. Every setup likes something different, but I have never had 40X" setup end up liking more than 28.
    Done some loaded pulls and still no knock.

    Altitude may play a large role though.

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    Yeah Mr. Luos is hit with a 14 to 15% air density reduction if his "Rocky Mountain High" is a Mile High.

    Taking 14.5%, .855 x 11:1 yields a relative to sea level compressed air density of 9.41:1, he runs very good so I'm tempted to think he is driving around closer to 2600-3200ft???

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    Bandimere's official altitude is 5800 feet.

    Last summer the best Density Altitude I got was 8100.

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    Wow...that is s whole lot to comp for.

    What did you get off the dyno?

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    Quote Originally Posted by BowTied View Post
    Wow...that is s whole lot to comp for.

    What did you get off the dyno?
    531.1 RWHP SAE corrected.
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    Lookin' good!

    You are enduring and compensating well I'd say for 7900DA. You go in down 22.91% in air density. (You get 1 thing only that I know out of this...flow!) But air density kills you once in the cylinder. That would be like 8.48:1 CR I don't know how much help it would be but your VE is helping some!

    Without VE corrections you could run 13.1:1 to be equivalent to 10.1:1 at sea level!

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    I was a bit curious at running the 91 Octane that seemed a bit of a stretch. But I guess that runs strong up there. Do you know if you are in the 12.5 to 12.9 fuel ratio range?

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    Quote Originally Posted by Mr. Luos View Post
    .....

    Altitude may play a large role though.


    !doh, that's it.

    Bring that thing down here near sea-level and run 10s N/A

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    Quote Originally Posted by BowTied View Post
    I was a bit curious at running the 91 Octane that seemed a bit of a stretch. But I guess that runs strong up there. Do you know if you are in the 12.5 to 12.9 fuel ratio range?
    12.8-12.9 at WOT.

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    maybe you guys can help me. I have mahle 10.7.1 pistons in my car. If i get the Prc 2.5 5.3 heads, will my compression be streetable. Thanks in advance.

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    Quote Originally Posted by badblazeSS View Post
    maybe you guys can help me. I have mahle 10.7.1 pistons in my car. If i get the Prc 2.5 5.3 heads, will my compression be streetable. Thanks in advance.
    http://www.c5frc.com/calculators/Com...Calculator.htm

  19. #39
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    Quote Originally Posted by badblazeSS View Post
    maybe you guys can help me. I have mahle 10.7.1 pistons in my car. If i get the Prc 2.5 5.3 heads, will my compression be streetable. Thanks in advance.
    need this info:

    bore
    stroke
    relief volume in the piston
    deck height
    compressed gasket thickness
    combustion chamber size

    to figure up static compression ratio

  20. #40
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    Info on those heads:

    Intake Flow Information:

    .300 208
    .400 257
    .500 290
    .600 312

    The PRC Cylinder Heads Feature:

    * Brand New GM 5.3l Based Cores
    * Smaller Combustion Chambers For 10.75:1 Compression
    * Stainless Steel 2.02"/1.575" Valves
    * PRC Double Spring Kit Complete With
    Springs, Titanium Retainers, Seats, & Seals

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