carb size - Team Camaro Tech
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post #1 of 11 (permalink) Old Jan 13th, 05, 04:17 AM Thread Starter
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I am installing a new 350 cid 290 h.p. in a 67 this is close to the original 295 hp. that would have come in a ss model. I am tring to keep it stock as posible using a q- jet. The motor spect. with 650 cfm holly and headers to get 290 hp. I was told by the carb builder that the 750 q-jet is the right carb because the 650 holly and the 750 q-jet are the same total, and the q-jet has smaller primary than holly so I would get power when I want and gas milage at light throttle using q-jet 750. I dont want to over carb I know it can loose power and milage and dump raw fuel into blowby. I am using stock exhust 2 1/4 inch.
Anyone with toughts?
Thanks team Camaro.
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post #2 of 11 (permalink) Old Jan 13th, 05, 04:42 AM
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You should be fine. Vacuum secondary carb only opens enough to feed the motor what it needs.

68 Camaro~LSx ~all motor
1.54 60'--6.95 @ 98.45 660'--10.96 @ 121.53
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post #3 of 11 (permalink) Old Jan 13th, 05, 04:48 AM Thread Starter
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Thanks doug G
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post #4 of 11 (permalink) Old Jan 13th, 05, 05:17 AM
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1. A q-jet isn't a vacuum secondary carb, it has an adjustable air valve above the secondaries that opens on demand. When you floor the throttle, the secondaries are wide open, it's the air valve that controls the flow.
2. A large carb doesn't 'dump raw fuel'. Fuel is attracted out of the carb by vacuum in the venturi caused by air flow through it. A larger carb requires more air flow to achieve the same vacuum in the venturi as a smaller carb.

"For those that will fight for it...FREEDOM ...has a flavor the protected shall never know."
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post #5 of 11 (permalink) Old Jan 13th, 05, 12:30 PM Thread Starter
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do you think I will have more fuel than I need?
Is the relation from holly and q-jet the same?
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post #6 of 11 (permalink) Old Jan 13th, 05, 05:19 PM
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I use this formula and a fudge factor:

CFM = (RPMpk * CID / 3456 ) * VE

Where RPM pk is the peak RPM, CID is cubic inch displacement, and VE is the volumetric efficiency.

VE is almost always going to be less than 100%. Although, with the right heads and cam you can get well into the 90% range at certain RPMs. That's the fudge factor. Make a value judgement of your motor's VE, peak RPM, and you'll have the size.

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post #7 of 11 (permalink) Old Jan 13th, 05, 05:34 PM
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The Qjet will work fine.The 650 and 750 are air flow numbers not fuel.The fuel is determined by jet sizes.Not being a samrta$$ but you answered your own question if you thnik about it the stock 290hp. 350 came with a Qjet.

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post #8 of 11 (permalink) Old Jan 14th, 05, 04:08 AM Thread Starter
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Thanks everyone for replys
Let me state the question this way.
What was the carb air volume for a original 295 hp in 67 650 or 750 or less using the o.e. q-jet
That would give me a good guide line.
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post #9 of 11 (permalink) Old Jan 14th, 05, 05:22 AM
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The original q-jets were rated at 750 cfm, if you have enough motor to pull them open. Some of the later ones were limited in the secondary opening and were rated somewhat less. The q-jet was standard equipment from 250" 6-cylinder Pontiacs to 500" Caddys, so it is a very versatile carb.

"For those that will fight for it...FREEDOM ...has a flavor the protected shall never know."
Semper Fi! L/Cpl Edwin L. "Tim" Craft, B Co 3rd AT's, Khe Sanh Combat Base, February, 1968
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post #10 of 11 (permalink) Old Jan 14th, 05, 04:31 PM
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BTW my formula above is more critical with a square bore carberator. Spread bore carbs are nice because you can get a decent size secondary while still having a reasonably small primary. For example, I'd recommend a 650 square bore carb for a small block 350, but a 750 or 800 cfm Q-jet would work well also.

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post #11 of 11 (permalink) Old Jan 15th, 05, 04:05 AM Thread Starter
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Thanks Everyone for the info I know how to deal with this now. You have all been a great help!

Thanks Team Camaro!
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