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Old 09-22-2003, 09:10 PM
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Need help!! WHAT CARBS??

I need help on carbeurator recommendations. I called Holley tech line and he recommended me two 850 double pumpers. I think that is too much carb. The only information he asked for was engine size(496) and max rpm(8000). I told him that it was a race engine. But what I don't understand is that with a single carb they once recommended a 1250 dominator but two 850's is 1700 cfm's. The intake is a Weiland Pro High Rise Tunnel Ram. My cam rpm range is 3000-8500 and my heads have 320 runners with 2.25in and 1.88ex and 119CC. And my compression is 10.5.1
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Old 09-22-2003, 09:38 PM
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It may seem like a lot but IF your engine is designed to take full advantage of the tunnelram then your Volumetric Efficiency will be in the 110%-115% range. Thats like feeding a 545 CI engine at 8000 rpm at 100%VE.
A single plane intake will not give you 100% VE- more like 90-95% so the carb size will be smaller to match this reduced airflow requirment. Also peak torque will be at a lower RPM with the single plane intake, than with the tunnelram.
I hope this makes sense.
I personally wouldn't use less than a pair of 750CFM carbs to start out testing and work up from there.
good luck
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Old 09-22-2003, 09:43 PM
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Is there a formula for a tunnel ram set-up! I just don't want to over carb my engine!
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Old 09-22-2003, 10:16 PM
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I would use a set of 600 - 650 cfm carbs for starters, you have lots of cubic inches, but not a lot of compression, I think this would likely put you in the ball park.
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Old 09-22-2003, 10:21 PM
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right carbs

Quote:
Originally posted by ss crazy
Is there a formula for a tunnel ram set-up! I just don't want to over carb my engine!
Is your mind set on holley if not i would give barry grant tech a call.They will help you every step of the way.

tech-sales@barrygrant.com


Phone: (706) 864-8544
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Old 09-23-2003, 01:15 PM
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Barry Grant just recommended me 750 tunnel ram carbs through an email response. But they also quoted that these will give me good throttle response with drivability and overall good performance. I don't know if they misunderstood about this being a drag car not a street ride to be worried about driveability. I want maximum race performance without over carbing. Any more advice out there?
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Old 09-23-2003, 03:21 PM
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Well, lets see how my math goes here. ( no guarantees, correct me if I'm wrong here.)

496 cubic inches times 1/2 of max rpm. ( pistons only pull air in every other revolution.)

So, 496 X 4000rpm =1,984,000 cubic inches per revolution.
There are 1728 cubic inches in a cubic foot...so,,,,
1,984,000 divided by 1728= 1,148 cubic feet per minute. So in theory, a pair of 600 cfm carbs should do it.
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Old 09-23-2003, 06:09 PM
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Calculate Required Induction Airflow CFM
Use to determine how large a carb or throttle body to run
Engine Displacement (cubic inches only):496 cubic inches
Manifold type:Tunnel Ram, Volumetric Efficiency @ red line:
Red line RPM: 8000 RPM
Required CFM: 1515 to 1791 cfm (calculated)
calculator site I used.
http://www.newcovenant.com/speedcraf.../intakecfm.htm

With a small plenum area, a larger carb can be used.
I have two reworked 650 Holly DP carbs on my 427 small block and runs great, gets 10 mpg on the xway @ 70mph.
Go with the 850's its all in the tuning, go for it.

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