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Old 11-13-2009, 08:05 PM
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Compression ratio for 383

I have a sbc 383 and the man who owned it before us took the compression ratio down to 9 to 1 so he could run a blower. we took the 8/71 off the motor and went back with an edelbrok airgap intake and a 770 cfm holley to feed the pretty much camed out small block but it runs pretty rich and has fouled a plug or two. I plan to take the compression upstairs anyway but if i wanted to take the compression ratio up to say 12 to 1 and run a hotter plug would it run better than if I were to just get a smaller carb?

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Old 11-13-2009, 09:05 PM
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9:1 is too much compression for an 8:71 motor, when he lowered the c.r. why he would have stopped there is beyond me. Looks as if you have issues with this motor and your 3,250 rwhp 454....
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Old 11-13-2009, 09:17 PM
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This is getting good let er ride, I love watching people make ***** of themselves,, a kid and a computer,, what a combo for dreams
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Old 11-14-2009, 07:30 AM
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This is getting good let er ride, I love watching people make ***** of themselves,, a kid and a computer,, what a combo for dreams
What are you talking about?
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Old 11-14-2009, 07:38 AM
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I bet it is running rich due to a bad tune.

First, Set the timing to 20 or 25 initial, 38-40 total. If the cam is really big then don't use the vacuum advance. And disconnect the vacuum advance when setting the timing.

Then set the idle mixture screws for best idle. if the idle mixture screw don't do anything then pull off the carb and drill a 0.090 to 0.100" hole in each front throttle blade.

See, the carb and timing aren't set up correctly. so the carb runs rich because it is idling out of the idle circuit and into the main circuit because the throttle blades are open too much at idle. drilling holes lets more air in and allows you to shut the throttle blades. Plus more timing is required to raise up the idle and allow a leaner idle.

do a web search "how to tune a holley the scientific way". read it, learn it, live it.
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