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compression ratios and pump gas

This is a discussion on compression ratios and pump gas within the Internal Engine forums, part of the LSx Technical Help Section category; Ok, so if my math is right on this= 4.030 bore 4.000 stroke 58 cc chamber 6 cc valve relief ...

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    cutting and welding mark21742's Avatar
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    compression ratios and pump gas

    Ok, so if my math is right on this=


    4.030 bore

    4.000 stroke

    58 cc chamber

    6 cc valve relief

    Would give me 12.49:1 static compression


    But with my cams intake closing at 85 degrees abdc my dynamic compression would sit at 8.22:1......
    Camshaft is a comp 305 255/263 .624/.624 115lsa

    This would be the perfect compression ratio for 93 octane and the most power if I am correct.


    Any thoughts?

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    12.5:1 is too high for pump gas...better off dropping it to 11.5:1. Most tuners and builders will agree that 11.5:1 is the limit for pump gas unless you want to pull A LOT of timing to make it work, and then you're just loosing power.

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    cutting and welding mark21742's Avatar
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    That's what I originally thought too, till I took my can into consideration.... my dcr was pretty low compared to scr.

    To get 8.22:1 dcr I would be looking at 12.49:1 according to a few different calculators I was looking at.

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    Quote Originally Posted by mark21742 View Post
    Ok, so if my math is right on this=


    4.030 bore

    4.000 stroke

    58 cc chamber

    6 cc valve relief

    Would give me 12.49:1 static compression


    But with my cams intake closing at 85 degrees abdc my dynamic compression would sit at 8.22:1......
    Camshaft is a comp 305 255/263 .624/.624 115lsa

    This would be the perfect compression ratio for 93 octane and the most power if I am correct.


    Any thoughts?
    Just my 2cents,

    Well, for shits, assuming the block is zero decked, and I gave .035" for head gasket I got about 12.75, that can change a decent amount though depending on diameter of head gasket and piston deck height. So I would say were pretty close.

    12.5 Seems a little high to me on pump gas, That cam is going to call for some serious timing too. So if your planning on spinning that girl past 6500 which that cam will easily do... I would back off my compression, I would also say 11.5 is the limit for pump gas(with aluminum heads, and a great cooling system, and a spot on tune) Something that is going to be putting 600-700hp to the ground, like you I assume, is at a higher risk of detenation... so I would feel safe at 11.1 that way you don't have worry about one little thing blowing it. When an engine gets real close to the compression limits in my experiance... any thing will trash it I.E. low fuel pressure, to much timing, clogged injector, bad tune, high temp, free rev, ect...

    Also it is very heard to calcuate dynamic compression, to do it accurately you would need to know the exact velocity of air through your exact intake, heads, headers, efficiency within the cylinder ect. there is alot of variables to dynamic compression, so I just stick to static calcuations.

    Hope that helps
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    cutting and welding mark21742's Avatar
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    Looking at spinning to or past.7,500rpms.... Il determine peak shift points and fuel cut rev limit when I see where it falls off power on a dyno

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    Member Jay37's Avatar
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    Smile

    Thats going to be a mean, SOB

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    cutting and welding mark21742's Avatar
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    Quote Originally Posted by Jay37 View Post
    Thats going to be a mean, SOB
    With all the time, money, blood (literally), and effort I have put into this thing it had better be lol

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    midnight blue
    1998 Pontiac trans am

    What intake are you going to use to spin the motor that kind of rpm's?

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    Quote Originally Posted by Kevin Wilson View Post
    What intake are you going to use to spin the motor that kind of rpm's?
    Edelbrock Proflo xt


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    midnight blue
    1998 Pontiac trans am

    Looks good, I haven't seen anyone post results with that intake yet. Curious to see how it performs. Did you have to cut much out of the cowl to get it to fit?

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    Quote Originally Posted by Kevin Wilson View Post
    Looks good, I haven't seen anyone post results with that intake yet. Curious to see how it performs. Did you have to cut much out of the cowl to get it to fit?
    On my goat I didn't have to cut the cowl, but it was close...I did have to make a scoop on the hood to clear it.

    The intake is supposed to really shine up in the higher rpms


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