Going from 2bbl to 4bbl. Hp increase? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-27-2008, 07:29 PM
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Going from 2bbl to 4bbl. Hp increase?

I have a MOPAR 383 with magnum heads, but a 2bbl carb. Obviously the carb is the "bottleneck".

I'm considering replacing the 350cfm 2bbl with a 650-750cfm 4bbl with vacuum secondaries.

So my question is: what kind of HP increase could I expect, and what kind of gas mileage drop. (I'm still using the car as a semi-daily driver.) Let me know if you need more information about the engine setup.

Any recommendations are welcome too. Thanks.

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Old 04-27-2008, 07:39 PM
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A guess would be about 30 h.p, as for mileage it should stay the same if ya keep your foot out of it.Which is hard to do with new found power.
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Old 04-27-2008, 07:47 PM
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Thanks for the fast reply barnym17. Does 650-750cfm sound about right, or would it be over/under carburating a 383 with 350 HP? Also, would my torque change?
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Old 04-27-2008, 08:21 PM
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You may notice a drop in torque at WOT, but not much. As far as HP I say 15 tops, and that would only be at or near redline with your foot to the floor, and honestly - how much time do you spend there?

A 383 making 350 hp probably tops out at around 5000 rpm. Do the math, carry the one, multiply by the date of the vernal equinox... It should ingest about 471 cfms at 85% VE.
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Old 04-27-2008, 09:34 PM
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Quote:
Originally Posted by curtis73
A 383 making 350 hp probably tops out at around 5000 rpm.
MOPAR 383, factory says 6500 RPM.

MOPAR Muscle magazine said they got good results with 850cfm on a stock 383 magnum with headers, though they said it was a bit over-carburated.

Last edited by Sir Joab; 04-27-2008 at 09:43 PM.
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Old 04-27-2008, 09:52 PM
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You may be able to take it to 6500 rpm but you're not making any power there. And since it was a 2 bbl' engine where did you come up with 350 hp? The old 4 bbl' 383's that I remember were rated at 335 hp but were out of steam at 5500 rpm.

Good luck with whatever you do, just don't expect too much......and your gas mileage will really go south because those old engines sound so good when the secondaries open up!!!
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Old 04-27-2008, 10:03 PM
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Quote:
Originally Posted by 327NUT
And since it was a 2 bbl' engine where did you come up with 350 hp? The old 4 bbl' 383's that I remember were rated at 335 hp but were out of steam at 5500 rpm.
350 HP was after adding barnym17's 30 HP gain estimate to my current HP (optimistic...) estimate. The 2bbl 1962 383 was rated at 305 HP, then I added Magnum heads, bored .030 over, electronic ignition, better flowing exhaust, 10.3:1 compression, etc. The Magnum heads made a noticeable increase in the high-end torque. Even with the 2bbl it's effective to hold it in 1st gear 'till 5500. (I do realize 2.91 rear end gears don't help acceleration, but it's for mileage right now.)

Last edited by Sir Joab; 04-27-2008 at 10:09 PM.
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Old 04-28-2008, 02:17 AM
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The magnum heads are a nice upgrade, but 6500 rpms? no way. If you're only making 350 hp, there is no way you are spinning that fast. I have a 383 chevy making 425 hp that never sees more than 6000. Granted, the bore/stroke ratio is different, but my guess is 5300 rpms tops.
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Old 04-28-2008, 06:17 AM
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A good 750 vacuum secondary such as a 3310 holley will be perfect.
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