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Old 10-29-2005, 11:31 AM
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Carb choosing???

Need some help on choosing a carb. I am getting ready for my winter project. I'm going to spice up my 383 stroker a little with a hydraulic roller cam it will have a .571 lift on the intake and a .594 on the exhaust with the 1.65 roller rockers.The Cam has a 236 duration on the intake at 0.05 and a 108 lobe seperation. I'm also installing a new set of aluminum jegs heads that have 197cc High flows with 2.05 intake and 1.60 exh. I have a 700r4 with a 2800 stall converter and its all in a 1979 Z-28 that ways around 3400lbs. I will have a guestimated compression between 10.5.1 to 11.1 also a single plane intake. Will be a weekend crusier every once in a while but not a daily driver.(unless gas prices go down.....ALOT. I really am drawn towards a 750 Mighty demon VAC sec. But I would like to hear from anyone that has a set up similer and what they are running. Any suggestions will be greatly appreciated.

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Old 10-30-2005, 08:37 AM
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a 750 sound like the right choice....
now double pumper...or vacuum secondays......
i say vacuum secondarys....my self......
sounds like your right where you need to be....
great job....your pal mark
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Old 10-30-2005, 09:41 AM
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I have heard that it might Stumble with the vac sec due to the cam size and the low vac at low rpms. but with the high Stall ,I don't think that would be a issue. Wish I had one of each to try out. Also keep in mind my elevation is around 6500 feet above sea level and where I would be taking it to the strip is around 5200 I believe (Salt lake city).From what Ive been reading a vac. sec. will give the motor what it wants not to much.. but do they give it to little? So this might be the harder dicission of the 2. VAC or MECH any other imput from anyone.. thanks

flaming79

Last edited by flaming79; 10-30-2005 at 09:48 AM.
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Old 10-30-2005, 10:47 AM
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I just had the opportunity to try a 670 vac secondary and a 650 double pumper on a slightly less performance oriented motor than yours. 351 windsor with 233* duration @ .050" cam with a 110* LSA. 10: 1 comp ratio. 3400# car with 2500 stall speed converter. The double pumper really woke the motor up. With the Vac carb the throttle response was crisp and it pulled really good with a fair amount of torque. there was not much snap to it though. It pulled like a truck. You could feel the secondarys open up and it pulled a little better but not much. Bury the pedal from a dead stop it and would not hardly even chirp the tires. Put the 650 DP on it and the difference was outrageous. From a dead stop it will let all the smoke out of the tires until you let go of the gas pedal. The midrange was equally improved . I don't think that I would want the vac secondary if I wanted to have any kind of fun with your car. Your gas mileage will suffer for sure with the DP instead of the Vac sec. I think the 750 size is ok, although
possibly a little small. I would want to talk to someone at Barry Grant regarding the high altitude thing. I recently had the pleasure of talking to Eric @ BG and he was very helpful, informative, and did not seem biased at all. I have used Holleys for along time now but the tech guys there are all burned out or something. I think the next carb I buy will be a Demon. I hope this helps.
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Old 10-30-2005, 10:57 AM
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I think thats what I needed to hear. I have been told by a lot of people that the Mech.sec are the way to go for a strip car. I had a buddy of mine do the calculations from my motor on cfm I would need for it and 750 turned out to be really close. Only thing Im wondering about is if the Elevation will make a differance in carb choosing. Thanks for the info.

Flaming79
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Old 10-30-2005, 09:15 PM
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Living more or less in the flatlands of the Sonoran Desert of Arizona, I never gave high altitude carb selection much thought until now.

Going on seat of the pants experience tells me that the air/fuel ratio can be adjusted with jetting, but choosing cfm at altitude could be a whole different matter. If we consider that a motor loses 3% power for each 1,000 ft. of altitude over sea level, then wouldn't it be because the air weighs 3% less for each 1,000 ft.? Continuing with this line of thinking, wouldn't the air weigh 85% of sea level at 5,000 ft.? (3% X 5K ft. = 15%, subtracted from 100% = 85%). If this makes any sense, then wouldn't the motor ingest 85% of what it would at sea level? And if this is true, wouldn't you multiply .85 X 750 to find the actual cfm that would be needed at altitude? (750 X .85 = 637 cfm). Would using a 750 result in a weak signal at the venturis? (assuming that the 750 would have larger diameter primary venturis than a 650 carb for instance). I'd like to read some opinions.
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Old 10-30-2005, 09:29 PM
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Exactly

This is what I was thinking. The air is to thin up here. I'm wondering if the 750 will kill it up here, or if the altitude will change the cfm of the carb. Can it still suck in 750 cfm at this high level? or should I go with what the motor will want at sea level, and assume the carb will be affected the same as the motor. Are the 650..750 cfm ratings taken from sea level. if it can suck in 650 cubic feet per minute at sea level, I doubt it can at 6500 feet. And as far as the jets my thinking on it is all they do is meter the flow of gas ,, then it mixes with the 650-750 cfm of air. So if your car need 650 cfm you can't make a 750 work on it by jetting it will always have to much air. My thinking could definitely be wrong though. I think I'm making this harder then it is..

flaming79
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Old 10-30-2005, 11:04 PM
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"Are the 650..750 cfm ratings taken from sea level?"

To the best of my knowledge, yes.
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Old 10-31-2005, 10:22 AM
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750 cubic feet of air is 750 cubic feet regardless of altitude. The difference is there is less oxygen in the air at higher altitude -- the air is "thinner" in oxygen content. So you theoretically need to ADD percentage for higher altitude. What requires 750 cfm at sea level may require a 1000 cfm at higher altitude. Run the math, then add 3% per 1000 feet. Jetting can make a big difference. Since there is less oxygen at higher altitude, I'd think smaller jets would be required? There's just as much air being sucked in, but less oxygen in that air, so less fuel needed to get a good ratio. Sounds good, but I may be wrong!
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Old 10-31-2005, 05:49 PM
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Sounds right to me..
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Old 10-31-2005, 06:04 PM
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"There's just as much air being sucked in"

That's where our opinions differ. The motor does not suck in air. When the intake valve opens and the piston descends, it is the weight of the atmosphere that pushes air into the cylinder because nature abhors a vacuum. I contend that if the air weighs less, less air will be pushed into the cylinder at 5,000 ft. than at sea level for the same valve-open period of time.

And maybe I'm confusing volume of air with density of air. Maybe the motor will ingest the same 750 cfm like you said, but the charge will be less dense than at sea level, requiring jetting the carb leaner. I'm no engineer, so just trying to figure it out the best I can.

I know one thing for sure, I need to talk to Barry Grant
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Old 10-31-2005, 08:55 PM
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wow.. this is getting complicated!! if im not mistaken though, vacuum would be the same as suction so it would be sucking air in...right? I give, I'm just going to get a 750 mech. sec. Mighty demon..THE END..

flaming79
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Old 11-09-2005, 01:13 PM
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Quote:
Originally Posted by techinspector1
"There's just as much air being sucked in"

And maybe I'm confusing volume of air with density of air. Maybe the motor will ingest the same 750 cfm like you said, but the charge will be less dense than at sea level, requiring jetting the carb leaner. I'm no engineer, so just trying to figure it out the best I can.

I know one thing for sure, I need to talk to Barry Grant
You're hitting on it here. The engine will have the same amount of Suction, or Draw regardless of where you're at. Now here's the main difference: The change in Air Density will affect how much oxygen is in the air. This is why it is much easier to breath at sea level then up in the mountains. Secondly on a performance engine the ram affect is decreased due to less air pressure pushing the air into your engine.

Flaming79,

You never sent back the other information we needed to make a recommendation.
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Old 11-09-2005, 09:18 PM
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Quote:
Originally Posted by Tech @ BG

Flaming79,

You never sent back the other information we needed to make a recommendation.

What information? "I take it you got my e-mail". Let me know what is needed... jas1konobi@msn.com

Thanks, Flaming79
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Old 11-10-2005, 06:18 AM
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Flaming79,

I replied to your PM here on the board.
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