440 compression test numbers question

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    Offline tommyg29

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    440 compression test numbers question
    « on: March 11, 2010 - 08:37:20 PM »
    Ok, I took my car to my mechanic to take a look at a couple things, and wanted to have the compression checked, especially since my plugs look a little black and wet.

    Anyway, all 8 came out from 176 to 180, which is a nice tight range, but I thought they were on the high side. He told me its probably because it's been overbored, and maybe even stroked.
    I know it's bored the typical .030 over, plus .030 down in the hole, but I would think that would lower the pressure it holds, or at least have nothing to do with it.
    How does that work? and how do those numbers stack up against a typical healthy motor.
    btw, it has about 4,000 miles on it since rebuilt 8 years ago.
    Thanks.
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    Offline dodj

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    Re: 440 compression test numbers question
    « Reply #1 on: March 11, 2010 - 09:08:07 PM »
    The bored 030 over doesn't alter the compression ratio. It's the piston selected for the application, cylinder hear volume, stroke and gasket thickness.
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    Scott
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    Offline Strawdawg

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    Re: 440 compression test numbers question
    « Reply #2 on: March 11, 2010 - 09:19:50 PM »
    bore size does affect compression ratio :)

    The volume of air being compressed increases while the combustion chamber stays the same size assuming the same gasket being used.

    Compression ratio is computed by calculating the total volume of the cylinder and combustion chamber of the assembled engine and dividing that number by the volume of the combustion chamber above the piston at tdc.

    An overbore can substantially affect CR, therefore :)
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    Offline Changin Gears

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    Re: 440 compression test numbers question
    « Reply #3 on: March 11, 2010 - 09:28:02 PM »
    Those numbers sound fine.  A radical camshaft will lower them.
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    Offline tommyg29

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    Re: 440 compression test numbers question
    « Reply #4 on: March 11, 2010 - 09:28:42 PM »
    I understand how it would effect compression ratio, but how would it make for a higher compression test pressure?
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    Offline Strawdawg

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    Re: 440 compression test numbers question
    « Reply #5 on: March 11, 2010 - 11:14:21 PM »
    the more air that is compressed into a given volume, the higher the resultant pressure. 
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    Offline tommyg29

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    Re: 440 compression test numbers question
    « Reply #6 on: March 12, 2010 - 07:58:25 AM »
    strawdog:
    that makes sense. Thanks for the easy laymans explanation.
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    72 Roadrunner 400-4 Auto-3.23 Gear-Black Cruiser
    71 Cuda 440-6 Tribute-Limelight-A833 Close Ratio-4 Sp-Pistol Grip-Dana 3.54 Powr Lok-Rally Dash-Shaker (Sold)
    92 Dodge Stealth RT-Twin 15g Turbos-SAFC2 Tuned-Mystic Blue-5 Sp-AWD-Rear Wheel Steering-AutoX'r (Sold)
    12 Dodge Charger SXT Plus Blacktop Package-3.6L-8 Sp-Leather-Nav (the wife's)

    Multiple SRT's, Rams, Dakotas, Caravans and Neons

    ...the lines on the road just look like dots!....

    Offline moper

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    Re: 440 compression test numbers question
    « Reply #7 on: March 12, 2010 - 08:05:25 AM »
    Tom,
    It's a 446 (440+.030). Not stroked. The pistons are flat tops, the heads are milled to get the compression up. IIRC the static ratio with the steel gaskets is 10.8:1 The cam is the Crane H302-2 (232°@.050 intake; 242°@.050 exhaust). It's got 83° of overlap and will have a rough idle and the plugs may get fouled if it's idled a lot or driven lightly all the time. Last I checked it was around there too (170-175 IIRC). The reason the cylinder pressure is what it is is because the intake valve closing event officially seals the cylinder as the piston is moving up the bore. The larger the cam, the later that intake valve closes, so the lower the effective cylinder pressure will be with the same given mechanical static ratio.
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    Offline tommyg29

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    Re: 440 compression test numbers question
    « Reply #8 on: March 12, 2010 - 08:17:57 AM »
    Hi Moper:
    thanks for the numbers. I like knowing all I can. You have a great memory,
    and that makes perfect logical sense.
    I'll try not to drive it too easily :bigsmile:
    Thanks again
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    72 Roadrunner 400-4 Auto-3.23 Gear-Black Cruiser
    71 Cuda 440-6 Tribute-Limelight-A833 Close Ratio-4 Sp-Pistol Grip-Dana 3.54 Powr Lok-Rally Dash-Shaker (Sold)
    92 Dodge Stealth RT-Twin 15g Turbos-SAFC2 Tuned-Mystic Blue-5 Sp-AWD-Rear Wheel Steering-AutoX'r (Sold)
    12 Dodge Charger SXT Plus Blacktop Package-3.6L-8 Sp-Leather-Nav (the wife's)

    Multiple SRT's, Rams, Dakotas, Caravans and Neons

    ...the lines on the road just look like dots!....

    Offline moper

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    Re: 440 compression test numbers question
    « Reply #9 on: March 12, 2010 - 08:27:21 AM »
    I have a couple hints: Run the highest octane you can get (unleaded 93/94) and don't keep snapping the throttle at idle or pumping too much while it's warming up. It's never going to idle perfectly or smell clean at idle but it's not bad and you could always set it a little faster idle speed to help that. You can also try going with a higher heat range plug too.
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    Offline Moparal

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    Re: 440 compression test numbers question
    « Reply #10 on: March 12, 2010 - 08:34:23 PM »
    Thats a good crank compression test. I got a little bigger cam and a 499 and mine was 215 lbs on the engine stand fresh n new.  Besides warming my engine up at around 2k  my engine really never runs under 2500 rpms and I mostly cruise at 3k.  I dont let it idle much, so the plugs stay pretty clean. Sounds like you have a good built engine. Keep good gas in it, hot plugs, and the right timing and jets and it should serve you well.
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