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Old 12-20-2009, 11:14 AM
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383 and a 625 road demon

I have put together a 383 cast iron vortec heads roller cam with dual pattern 216 duration and 0.5 lift. I live in Colorado above 6000 ft and many have told me that 600-650 cfm is plenty of carb at higher altitudes. I have a road Demon 625 would that be a good combination?

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Old 12-20-2009, 03:09 PM
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What's your maximum RPM? A 750 vacuum secondary carb works well on a 383 from my limited experience. Assuming you won't rev over 6000 rpm, V.E. of 85%, you theoretically need 565 cfm, so a 625 should be fine. You will probably need richer jetting etc. When we tried a 600 holley on my brothers 383, it wouldn't rev over 4500 rpm. I've been told the cause was lean jetting, that if we gave it more fuel it would have worked well.(the carb came off a 70's smogger.)Good luck, hopefully somenone with more experience can help you.
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Old 12-21-2009, 02:46 AM
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Quote:
Originally Posted by gundi_cad
I have put together a 383 cast iron vortec heads roller cam with dual pattern 216 duration and 0.5 lift. I live in Colorado above 6000 ft and many have told me that 600-650 cfm is plenty of carb at higher altitudes. I have a road Demon 625 would that be a good combination?
Given the cam and your location, I think that carb would be fine- IF it flows 625 CFM. But Demon carbs are sized differently in some cases than Holley carbs, so calling them or looking for info on their site might be your best bet.

Less air at high altitude means less fuel is required for a given A/F ratio- try it as delivered, let the plugs and performance determine the need for a jetting change.

FWIW, Holley has recommendations for tuning for altitude and ambient temperature: "Carburetors are calibrated at 70 at sea level. Decrease the jet size one number (approx. .002) for approximately every 2000 ft. increase in altitude. Increase jet size one number for every 35 drop in temperature."

Demon may also have recommendations for high altitude. If so, they may or may not coincide w/Holley's recommendations.

Good luck.
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