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Thread: compression ratio's and cam combinations

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    compression ratio's and cam combinations

    I was wondering what combination of compression ratio's and cams people were running?
    and tuned for what fuel?
    cranking compression?
    and if the felt they left power on the table by pulling timing? or setting the fuel rich?

    also stock 2.0 engine 10.8c/r with a a few drops of oil I was getting 220psi compression average, which I feel the pressure gauge was calibrated wrong. I was expecting it to be high but that seems unrealistic. has anyone got cranking pressure numbers for stock engine.

    My planed setup is 11.2 with a 276 degree cam mainly to get the cranking compression under 200psi
    but it will lead to some intake reversion at low speed. But It has benefits with not retarding timing or going over rich to stop knock.

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    Re: compression ratio's and cam combinations

    from what i can find Paul, looks like the norm is about 170-180psi cranking pressure for a standard engine.
    sounds like your gauge is a fair bit out. i think you could run more compression than that and still be under 200 if thats what you want to do...
    i used this calculator and looks like 12:1 would still put you under with that sort of cam.
    http://www.wallaceracing.com/dynamic-cr.php

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    Re: compression ratio's and cam combinations

    Surprised this topic didnt get more replies.
    I looked into it some more and tend to agree 175 - 185psi is more in line with what this motor needs.
    My original thought process was smaller pistons, small efficient chamber and all alloy would allow more dynamic compression, compared to what is considered safe for an large piston iron block. Much like how motorcycle engines employ very high static compression ratio's in the 13's currently.
    What has changed my mind is someone pointed out vehicle weight and converter lockup is a large consideration , And most bikes employ ecu controlled secondary throttle limiting efficiency at low revs, which in turn lowers dynamic compression.

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    Re: compression ratio's and cam combinations

    Stock duratec 2007 year from ST fiesta should be in line with 220-230psi or 15-16 bar

    I've recently completed my first duratec builds from two tuned engines I assembled one stock and one tuned. As usual I measured compression on both engines.
    Stock engine was bit off since the head was skimmed as previously was run on E85, now high octane 100, so figures were 18+ bar... All 4 cylinders showed max recordable pressure so actually I don't know how high is it since chart is labeled to max 18bar. I immediately noticed how strong it pulls in lower revs, little bit worried will it knock on high revs but so far so good

    The tuned engine is in deck tolerances and it showed healthy 220-230psi or 15-16 bar. This tuned engine is producing in range 170 to 175hp


    Both engines were tested with same new battery.

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    Re: compression ratio's and cam combinations

    Well 220-230 was what I had recorded on a stock engine as well. I expected it might be wrong but I used my tester on a known engine (old pinto) and it was reading correct numbers to my other gauge.
    These number seem high but I can see how modern electronics can get away with it.
    Because of electronic throttle body will not let you open the butterfly all the way until you reach a point in the rev range where detonation becomes aceptable risk. ie the flame front beats the detonation point to the end gases.
    NASCAR was famous for it with drivers who stepped on the throttle too soon leaving the pits blowing the motor below 5krpm.

    Knock is most dangerous at High load and low rev's. it not good at any point but does the most damage low down. Now this all comes down to what I think is high rpm compared to your definition of high rpms.

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