how can you find out how much compression

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    Author Topic: how can you find out how much compression  (Read 855 times)

    Offline 73 challenger 440

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    how can you find out how much compression
    « on: July 15, 2006 - 06:58:24 PM »
    is there any way to tell what a motor has for compression? never got to ask previous owner....
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    Offline JRoss22

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    Re: how can you find out how much compression
    « Reply #1 on: July 15, 2006 - 07:11:44 PM »
    You can do a compression test by simply removing the spark plugs and using a compression tester and crank over the engine. :2thumbs:
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    Offline 67Vette427

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    Re: how can you find out how much compression
    « Reply #2 on: July 16, 2006 - 12:54:46 AM »
    A compression test is going to give him a PSI result. I think he's looking for a compression ratio. You can look up the heads by the casting number and if the pistons haven't been changed or the heads shaved, you can get an idea. Or do a cc volume with a gauge.
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    1 of 1 Mr Norm's T/A/ 2015 Hellcat, sublime/ 13 Power Wagon

    Offline Chryco Psycho

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    Re: how can you find out how much compression
    « Reply #3 on: July 16, 2006 - 01:13:52 AM »
    the compession PSIwill give an idea on the compressiong ratio , for example 140 lbs devided by 14.5 will give a ballpark comp ratio
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    Offline 67Vette427

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    Re: how can you find out how much compression
    « Reply #4 on: July 17, 2006 - 12:23:54 AM »
    lol...there you go...get out the calulator  :22yikes:
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    1 of 1 Mr Norm's T/A/ 2015 Hellcat, sublime/ 13 Power Wagon

    Offline moper

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    Re: how can you find out how much compression
    « Reply #5 on: July 17, 2006 - 06:24:54 AM »
    Static compression can not be inferred or "figured" from the results of a compression test. It's because the camshaft is what really determines the results on a compression test. A "Compression Test" is what it's always been called, but it's a little deceptive. It's a dynamic test of that particular engine in that particular location. An example: a 10.5:1 static ratio engine (typical 440 running Ross flat tops, small chamber Edelbrock heads, and Fel Pro gasket). For a cam, lets say we use an MP .509. Because of the long duration and 76° of overlap (that cam is designed for mid range torque and upper rpm power, not low speed torque) a measured PSI of 140 (roughly) is the result. The same engine, but with the P4286677 cam with it's shorter duration and less overlap of 48° will produce a test pressure of roughly 190psi. Same engine, the difference is the exh valve is being closed sooner in the cycle. So the piston has more "time" (in degrees) to compress the mixture with the smaller overlap. Static ratio is a volume based measurement of the total distance from bottom dead center to top dead center with both valves closed and the head and gasket torqued. Cylinder presure is also a volume based measurement, but takes into account the  fact that in most cases and all performance cam uses, the exh valve is still open when the pistons begins to move up on the compression stroke.
    The only way to truely know what you have no, is to pull a head, cc a chamber, use the 1" down method to cc the piston in its bore, and measure the old head gasket. With those, you can figure it out correctly. Anything else will be just guessing, and they will never be higher, they will always end up lower than you think.  :smilielol:
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    Offline Chryco Psycho

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    Re: how can you find out how much compression
    « Reply #6 on: July 17, 2006 - 10:19:35 AM »
    I agree that it is not that accurtae & cam profile will scew the results but it can give a ball park idea , most guys here are not running 300* duration cams in 8 :1 engines
     
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    Challenger - You`ll wish You Hadn`t

    Offline moper

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    Re: how can you find out how much compression
    « Reply #7 on: July 17, 2006 - 01:22:09 PM »
    Most engines that are "8:1" are a little over 7.5 when measured...
    I guess it's just a difference of opinion of the size of a ball park :cheers:
     One could almost say, "if you have to ask, it's too low" :bigsmile:
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    Offline 73 challenger 440

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    Re: how can you find out how much compression
    « Reply #8 on: July 17, 2006 - 02:35:49 PM »
    I am changing the cam, it has a very rough idle, the guy had to swap in a 2500rpm stall, he had 3500 but it was a pain he said in stop and go traffic, he has good bottom end but it really screams in the mid to upper rpm, I have no problem smoken my 295s.. but My dad thought if he took out the cam there may be a part number on the cam, this way he could possibly find out the lift and duration, there is a purple mark on the cam, he noticed that when we changed carbs and intake, I want to install a comp extreme cam 274H, but before I do that i'll get some input from the forum, I am going to take it to the track next month, I might have it dyno tuned, to see what I am getting from this cam, so when I swap out the cam and compare it to the new one, I can see where the difference is in the power band, I am looking for more bottom- mid...not rreally concerned about the high rpm power
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