Great article on compression ratio!
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  1. #1
    Fellow Frogger! Karoshi's Avatar
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    Default Great article on compression ratio!

    http://www.popularhotrodding.com/tec...ech/index.html

    All Chevy based theory explanation is fantastic.

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  2. #2
    COL
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    Quote Originally Posted by Karoshi View Post
    http://www.popularhotrodding.com/tec...ech/index.html

    All Chevy based theory explanation is fantastic.
    Very Interesting.
    Regards Col

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    And I found my unclaimed prize at the end...
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    Yes, thats a great article.

    First thing that struck me was the author, David Vizard. Probably best known for his tuning books on British 1960's cars - Minis, Cortinas, etc.

    I read an article about 25 years ago I reckon, where he went to USA for a look at the racing scene. He said he was at some NASCAR gathering/party or something, and in discussion with some quiet spoken old NASCAR engine builder, the old boy made a comment like "all decent racing engines have a compression ratio of 8.5:1" which set Vizard back on his heels. Then the old boy explained what in the current article Vizard is calling "dynamic CR". Vizard said that up to that point he had never even thought about it, but realised it was a great principle.

    It looks like he stayed there and found his place among the big-iron V8's. Maybe he was actually a yank when he wrote his first books on pommy engines - I have always thought he was a pom. His old books make good reading too.

    By the way, there is another way to measure the Dynamic CR and that is by actual measurement. My brother was a Mini Cooper S guru of sorts (the 1960's ones) and raced them at length. We once thought we would check the dynamic CR while building the engine in the stand - with the head off. Simply measured the distance from top of block to piston top with a vernier caliper depth gauge, and finding the point at which the inlet valve closed with a dial gauge on the cam follower. Then calculated CR using the "effective" stroke measurement. And yes, it was something like 8.5:1, where the static CR was about 12:1. Very interesting. Depends on the cam overlap, and we learnt that the CR must be matched to the cam. A good exercise.

    Last edited by Fordman; 23rd May 2012 at 12:14 AM.
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    Fellow Frogger! stew's Avatar
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    That ratio can then be increased by the ram tuning of the inlet manifold length and harmonics. Another 10% or more! Most serious engine builders also do calculations at 0.050" lift off the seat as well. The static CR numbers would surprise you! 13:1 static that runs on 92 octane.

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    Fellow Frogger! Karoshi's Avatar
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    One thing I've found from poking around looking for info is that there's lots about v8s....

  7. #7
    1000+ Posts PeterT's Avatar
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    Absorb this and then install the DCR calculator.

    http://cochise.uia.net/pkelley2/DynamicCR.html

    There's no point in doing a calculation 0.050" off seat, as the compression stroke hasn't started at that point. The compression stoke starts when the intake valve closes. Duration measurements at 0.050" are used for more accurately comparing camshaft profiles. Below 0.050" nothing really happens in terms of flow.

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    Fellow Frogger! stew's Avatar
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    Fair enough Peter.
    I find it useful to get a feel for the dynamics of engine cranking and ignition curve changes, inlet and exhaust design and cold and hot crank amps with a solid cam. Overlap, idle smoothness or lack of, etc......................
    Static and dynamic figures can help to get a feel, for me any way!

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    1000+ Posts Wildebeest's Avatar
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    So... This means my large burette and pot of piston sealing vaseline are now redundant?

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    1000+ Posts PeterT's Avatar
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    Did you read it? You still need to know the static CR before you can calculate the dynamic CR. So yes, vaso and burette is still needed.

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  11. #11
    1000+ Posts Wildebeest's Avatar
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    Default Compression....

    Quote Originally Posted by PeterT View Post
    Did you read it? You still need to know the static CR before you can calculate the dynamic CR. So yes, vaso and burette is still needed.

    PeterT,
    Thank the lord for that. One of the few items I have in my lockup to impress my friends!

    Peter, I doubt whether the theories put forward would be understood by current motor mech apprenti or their instructors.

    Ivan Washington [PCCV] had a great way of theorising to be understood.
    Last edited by Wildebeest; 26th May 2012 at 10:51 PM. Reason: oops!

  12. #12
    Fellow Frogger! stew's Avatar
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    Depends. What you do with the burette is your business, and don't let any one influence you other wise!

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