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BRINGING IT ALL TOGETHER.... A's, G's & E's


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Big W
Scott
6 posters

    Carb. Sizing..?

    Scott
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    Scott


    Number of posts : 1651
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    • Post n°1

    Carb. Sizing..? Empty Carb. Sizing..?

    Post by Scott Mon Mar 05, 2012 2:08 pm

    Now that I've had the van on the road for a few weeks now I am fighting my carb adjustments. I knew before I got it on the road I would probably need to drop to a smaller CFM carb, from 600 to 500. But now that I'm digging around I think I should be running closer to a 350 CFM carb.

    I can't seem to find any solid answers so I'm putting up here for a little help.

    Online Carb. Calculators typically want the engine size, and Max RPMs.

    I'm running a 250 bored .060, which as near as I can tell = 258 CID

    I don't know the Max RPM for a 250, looking around the internet it looks like it varies, depending on how the engine was built.

    Hopefully with the right carb. I can get better then 11 miles per gallon. I looked at a Holley 350 2BBL, but it says not to use it with an Automatic Transmission w/Overdrive. I don't know why it would make difference if the transmission doesn't require vacuum.

    So, what are some of you running for carb size on your straight 6, and what is the basic setup of your engine?
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    Big W
    Big W
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    • Post n°2

    Carb. Sizing..? Empty Re: Carb. Sizing..?

    Post by Big W Mon Mar 05, 2012 4:06 pm

    Is it possible to re-jet the carb you have to a smaller size, or are these carbs not able to do something like that. I am not a carb guy so I thought I would ask.
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    Scott
    Scott
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    Number of posts : 1651
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    • Post n°3

    Carb. Sizing..? Empty Re: Carb. Sizing..?

    Post by Scott Mon Mar 05, 2012 4:20 pm

    Re-jetting won't compensate for that because lowing the CFM also requires lowing the amount of air that passes through the carb.

    At least that's the way I understand it.
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    Big W
    Big W
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    • Post n°4

    Carb. Sizing..? Empty Re: Carb. Sizing..?

    Post by Big W Mon Mar 05, 2012 4:43 pm

    Ah I see, That does make sense. I guess re-jeting would just lean it out I suppose, and that's not always a good thing.
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    Nightmoves
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    • Post n°5

    Carb. Sizing..? Empty Re: Carb. Sizing..?

    Post by Nightmoves Mon Mar 05, 2012 5:42 pm

    Have you ever crunched numbers on a Holley 390?Its a smaller four(which seems to be the way your leaning).I had one on a Amc 232 many years ago,with Offy intake.Ran pretty dog gone good.Just my.02
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    Scott
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    • Post n°6

    Carb. Sizing..? Empty Re: Carb. Sizing..?

    Post by Scott Mon Mar 05, 2012 6:03 pm

    Thank you Nightmoves, thats the kind of feedback I'm looking for..

    My concern is how close the carb. is to the doghouse wall. The 2 barrel 350 is small enough to fit, I'm not sure about the 4 barrel 390. Clifford Performance also listed the 390 as a good size too.

    If I can find someone that stocks the 390 locally maybe I can see if it would fit before I buy it. I have to turn it 90 degrees to the right to keep the linkage out of the wall, so the question is if the bowl will be too close.
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    Digz
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    • Post n°7

    Carb. Sizing..? Empty Re: Carb. Sizing..?

    Post by Digz Mon Mar 05, 2012 6:22 pm

    If my math is right,You may really only need something like a 290 cfm carb. It can only pull as much air as the CI and what ever cam you have in it allow. If I'm thinking right by bumping up the cfm capacity of the carb you may loose the ability for the fuel to mix as well as you may be cutting down on the velocity of the air thru the throat. Just my quickie number crunchin',,LOL
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    jrinaman
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    • Post n°8

    Carb. Sizing..? Empty Re: Carb. Sizing..?

    Post by jrinaman Mon Mar 05, 2012 9:03 pm

    its been a while since i crunched the numbers but 390 is what i needed for a 292. max rpm that you will use is what you want, not what redline is. a smaller carb is more responsive for reasons digz mentioned. thats why some 4 barrels have small primarys. i think i used 5000 in my math and 85% effeciancy. the 350 2 barrel is plenty but will run better with the 390 4 barrel because it would spend most of its life using the front 2 barrels. the other 2 are there when you need them.
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    Scott
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    • Post n°9

    Carb. Sizing..? Empty Re: Carb. Sizing..?

    Post by Scott Tue Mar 06, 2012 7:37 am

    Thanks Digz, the Summit calculator shows 290 also, and I could probably run fine with it too. But since most everyone that has had a similar setup is, or was, happy with the 350 or 390, I went with the 350.

    I ordered a Holley 2 barrel 350cfm 2300, along with an adapter block, from Summit. Should be here on time for this coming weekend.

    Thanks to all of you who gave your opinion..
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    Scott
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    Number of posts : 1651
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    • Post n°10

    Carb. Sizing..? Empty Re: Carb. Sizing..?

    Post by Scott Fri Mar 09, 2012 10:33 am

    About 15 minutes before I received my new carburetor Tom Langdon called me about an email I had sent him last weekend. After pouring over my engine setup he had a number of recommendations. The first, biggest one, is to replace the carburetor.

    Since the one I ordered is a 2 barrel 350cfm, it's going to dump more then the 600cfm 4 barrel that I have. His point is that in normal driving on my existing 4 barrel I am only using two barrels, which means about 300cfm. So switching to this new 2 barrel 350cfm is going to put more down its throat then its already getting. So, the Holley is on it's way back to Summit.

    Since the carburetor is close to the doghouse, and because the linkage would be the same, what I really need is a Carter 400cfm Model 9410. But they haven't made them since the 70s', so they are hard to find, and pricy. The next one he recommended was the Holley 390cfm, but I don't know if I have the room for it.

    So, I'm on hold until I can figure out the space problem, find a Carter, or see what Tom says about the Edelbrock 500cfm.

    Most of you probably already know his site, if not you should poke around.

    http://stoveboltengineco.com/
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    Stoopid john
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    • Post n°11

    Carb. Sizing..? Empty Re: Carb. Sizing..?

    Post by Stoopid john Sat Mar 17, 2012 11:13 am

    I've got a 390 on my 225ci slant, i really like it. My new engine is 10.5:1 with a pretty large cam and it gets better gas mileage than the old 8:1 engine with a 1 barrel. Go figure.

    The difference looking down the throat of a 600cfm and a 390cfm is huge. The 390 is about half the size (witch is to be expected) but that means your cruising around on 195cfm. Your 6 will like that better.

    I drive my van 50 miles a day and would't change anything.
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