compression ratio
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Junior User
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compression ratio
I am confused about figuring compression ratios for my 410. I always thought more was better. Is this a true statement a bigger cam decreases compresion but increases power, and still alows the use of pump gas. So if you built your motor with to much compression from stroke and pistons to run pump gas, you could then use a bigger cam to decrease your compression. let me know guys if I am a little slow or is way of thinking correct.
Posting Guru
Joined: Aug 2005
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From: NE Ga
the way i understand it (correct me if i'm wrong) is the larger cam allows the piston to push a small amount of air/fuel mixture back into the intake on the compression stroke thus lowering your final compression ratio. it opens the valve further and holds it open longer. thats what i'm looking into on my 390. my final compression ratio was calculated at 9.9:1 which is too high for 87. if i go with the crane 941, it'll hold my intake valve open longer and lower my final compression but still (how i'm not sure) give me more power. i guess b/c it's allowing my engine to breathe and move more air. sound half way correct?
Posting Guru
Joined: Nov 2005
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From: the side of a mountain
that is how i understand it too. the static CR doesnt take into account the cam which is where you got your 9.9 from, im guessing 1131s? but with the 941 the dynamic CR is down to about 8.4 because it pushes some of that air fuel mixture back into the intake therefore decreasing the amout it is compressing. i am going to do a build with 105s and a 901 which gives me 9.2 static and 8.3-4 dyanmic and be able to run on 87 just fine. if i were to use the 941 in my build it would hold the intake valve open longer ABDC and i would end up with just under 8.1
also about the more is better it is but i dont think you want to be running a daily driver on alcohol. (well you might want to but dont have the money to and it isnt pratical)
also about the more is better it is but i dont think you want to be running a daily driver on alcohol. (well you might want to but dont have the money to and it isnt pratical)
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Junior User
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What i meant to say was that prior to becoming a member of this site I thought more was better and did understand any of this stuff. This site has really taught me alot. Instead of just being able to bolt things back together I am starting to actually understand how they work. Thanks guys.
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