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Thread: Static Vs. Dynamic CR

  1. #1
    Hey all, I was wondering is there a formula or a way to get a real close guess as to what your dynamic compression ratio will end up at when you plug in CR, cam and timing?

  2. #2

  3. #3
    I was hoping somebody would have a better answer than I can give. I have seen a chart used for supercharged engines in a book/catalog from Blower Drive Service. In a normally aspirated engine you would not have the blower boost to consider, however, there are a few variables to consider. Some are; large cubic inch motors with more cylinder volume, cam timing,as you stated,and volumetric efficiency. The term Dynamic means changing.What are you trying to figure out?

  4. #4
    I'm trying to get a good ball park as to what static compresion ratio i should shot for with a specific cam, timing etc So my dynamic compresion ends up close to my planed static ratio.. reason i ask is I plan to use a rather large cam and use a good set of heads with large valves.. I'd like to make sure i don't spend a ton of money and end up with a lower then optimum dynamic compresion ratio and now that we are on 91 octane it's a bit more critical to also check that it wont be to high.

  5. #5
    More cam=more static comp needed. Big cams bleed more cylinder pressure.

  6. #6
    There is no formula really, other than calculating where the piston is when the intake closes. You would need to trig out the rod length and stroke to find the exact figure. In general, larger displacement and / or higher compression cylinders can tolerate longer duration and maintain intake tract velocity. In a smaller boat with a fairly big engine, there is not much load below 2500 rpm. As an example, my 454 was set up with 10 to 1 and a 222/233 cam, ran 84 mph. Went down to 8 to 1 and added a 240/244 cam, did not lose enough on the low end to even notice, and ran the same 84 mph. It still idled ok at 650. Once you get up around 250 duration at 0.050 you will get a rackety high idle and lose low end unless it's a 502 or bigger.

  7. #7
    tahitijet, there's a Great article by Ed Isky, at the Isky Cam Co. website, that deals with just that....!!!i don't have the link, but i just read it the other day...
    I think ?

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