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Max CFM for a 283 SBC?

Discussion in 'The Hokey Ass Message Board' started by poboyross, Jan 19, 2010.

  1. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    A question for the experts:
    Does a 500 CFM carburetor flow the same on his 283 as on a 427 (no, I'm not talking about the secondary opening point, or the air valve)?
     
  2. mart3406
    Joined: May 31, 2009
    Posts: 3,055

    mart3406
    Member
    from Canada

    -------------------------------------
    No. Air flow for 4-bbl carbs is usually rated by the manufacturers at a standard 1.5 hg. pressure drop. (Most 2-bbls are rated at a 3 hg pressure drop by the way).The cfm number is what is what the carb is capable of flowing at WOT at that pressure drop.. If engine is too small, or the carb too big to produce that amount of pressure drop at wide open throttle , the airflow through that carb will be less than the rated CFM number. Likewise, if a bigger or stronger engine produces a greater pressure drop - ie - it sucks harder - the carb will flow more air than the rated cfm number. CFM is is always what the carb is *capable* of flowing at WOT at a set pressure drop - not what it necessarily does flow on a given engine. Obviously, a big block 427 running at WOT at say 6000 rpm is going to create a greater pressure drop and thus flow more air than a 283 running at the same speed.

    Mart3406
    ===============================
     
  3. You may be unfamiliar with large use of Powerglides in drag racing. Also, maybe you have no actual experience SBC equiped cars running a Powerglide.

    No urban legend here, just a guy that knew how to race his car even with a stock ring and pinion. He would run the car to around 70 mph before shifting to 2nd. Somethings just work. Just like gravity.

    He did not sit around doing all the math that has been thrown around. He may have changed the jets and metering rods in the Qjet, but did his tuning was done with a dwell/tach meter, timing light, and a vacuum gauge. Also, a good bit of seat of the pants testing was involved.

    I would bet a lot of the "experts" here have never rebuilt a carb, but sure know how to parrot back everything they Google.


    Tony
     
  4. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    Given that a 64 Chevelle is pretty small (test weight from 1964: 3250), but how fast could a stock 220 hp SBC + QJ be, with a nearly-stock axle (if he reached 70 in 1st, 26" tire, 5,500 RPM, the axle ratio was about 3.36)?

    Perhaps, on a good day, at sea level, with ice on the manifold: 15.0.

    For those not familiar with those numbers, that's pleasant, fun, and able to keep up with most street traffic.
    Bat out of hell? If the bat were carrying a big sack of window sash weights.
     
  5. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    mart3406: thanks for posting that, I was begining to think I was here by myself.

    To summarize:
    A given 4 bbl. carb flows its rated CFM if, and only if, it draws the same vacuum on your engine as it did in the Holley test: 1.5" Hg.
    If the engine is smaller, and cannot pull that much vacuum, the carb flows less. Example: a 265" SBC only pulls 2.0" Hg. on a 500 CFM carb. The carb actually delivers 433 CFM.

    If the engine is larger (or more VE), and pulls higher vacuum, the carb flows more. Example: a 454" BBC pulls 1.0" Hg. on a 500 CFM carb. The carb actually delivers 612 CFM.

    We will always find people who explain that "Somethings just work", and that math is pointless.
    1. they "just work" because someone else already did all that math for you.
    2. biggest reason not to use math: you can't.
     
  6. I'm always amazed at the knowledge of the HAMBERS and once again I think you've gotten some really excellent answers. Maybe a couple not so much, but for the most part, lots of good ideas. My only suggestion would be, don't spend much money, experiment, and you'll come upon the best answer for your particular application. Have fun.

    And remember, Will Rogers said:

    "Some people have a thousand different experiences......others have the same experience a thousand times".
     
  7. el Scotto
    Joined: Mar 3, 2004
    Posts: 4,183

    el Scotto
    Member
    from Tracy, CA

    Hahaha!!! Awesome!!!! I love you guys!!! :)
     
  8. Runnerhard
    Joined: Jul 23, 2008
    Posts: 8

    Runnerhard
    Member
    from So Cal

    poboyross just wait for your 500cfm it will give you all you need .. I never heard if your 500 is a two or four barrel...We use the 500 cfm two barrel on race cars with over 400 Hp ..
    I have started up engines with a gas filled rag with no problem (make sure your timing is right)
     
  9. The 56 chev stock dual quad setup used two 380 cfm carter wcfb,s 760cfm total. Just depends which way you want to go?:cool: OldWolf
     
  10. burnout2614
    Joined: Sep 21, 2009
    Posts: 612

    burnout2614
    Member

    "Take a can of starting fluid and.... you writing this down son?" this is how my questions were answered when I first started fooling with carbs. My only input for this is: I have had poor luck using the "race" nylon gasket for power valves. Use the old standard type cellulose (paper) gasket to get that valve to seal. AND whatever you do DO NOT bang on the carb with a wrench or hammer you start flooding! peace
     
  11. burnout2614
    Joined: Sep 21, 2009
    Posts: 612

    burnout2614
    Member

    that should be IF you start flooding.
     
  12. 53sled
    Joined: Jul 5, 2005
    Posts: 5,820

    53sled
    Member
    from KCMO

    I don't think a worn out 283 with a leaky cylinder needs a 1050 dominator with a powerglide and 3.08 gears. No matter how much drag racing you say you can do.
    500 seems closer to reality.
     
  13. The Hank
    Joined: Mar 18, 2008
    Posts: 779

    The Hank
    Member
    from CO

    Got a stock 2 brl on mine.
     
  14. 333 Half Evil
    Joined: Oct 16, 2006
    Posts: 1,440

    333 Half Evil
    Member

    Hey Panic...wtf made you think of a big sack of window sash weights?!!!? That shit is funny as hell!!!
     
  15. poboyross
    Joined: Apr 29, 2009
    Posts: 2,134

    poboyross
    Member

    Man...this whole conversation has just gone south :p I don't even *want* to run the 650 on my 283....I just wanted to know if I could just use it to TEST FIRE the engine to see if it's a brick or not.

    Most all ya'll done turned this into a pissin' contest......I'm OUT! (to clarify....I'm not pissed....just done :) )

    P.S. To those who weren't arguing with each other and gave some great constructive criticism and replies....I sincerely appreciate it
     
    Last edited: Jan 20, 2010
  16. 333 Half Evil
    Joined: Oct 16, 2006
    Posts: 1,440

    333 Half Evil
    Member

    Panic and Mart3406,

    You guys must have at least a GED in math....am I right? JK. Math is not pointless, but sometimes can be misleading, especially when people find an article or post someplace explaining the formula. I'll use cfm rating for cubic inches as an example, and 350ci. If you plan max the rpm at 6500 then you would end up with a required 658 cfm. This may be right for the basic or maybe one could say stock engine. But this is also what they say is for 100% VE. Then as they are reading the article usually it goes on to say that most engines operate in the 70-90%VE range. Sometimes it is even written that the best highly modified engines may only reach 95% VE.

    Given that input, this makes the same 350ci now only need from a 460cfm(70%VE), 526cfm(80%VE) to a 592cfm(90%VE) carb. Mathematically this might be right, and under the law of physics it should also be correct, but there are just too many factors that can effect the efficiency of an engine therfore this math versus actual conditions does not always make the mathematical answer correct. I am not ever going to claim to know much about physics, formula, or VE, but I do know from years of experience that something in all of this math just does not add up.

    I do not know how the person or people who came up with the actual VE of any given engine did that, but to me this seems to be the furthest from accurate of all the engine math I've came across. If one bases the the amount of cfm for a carb off of VE being what they claim then how can it be that some engines need a certain amount of cfm to produce maximum tq/hp output that would equal out to them having a VE of more than 100%? I have done a lot of engine building over the years, and if I use the formula for cfm for some of these engines and compare it to what actual cfm carb was used to reach it's max potential then some of these would be 130-140% VE.

    I am not sure if I read someplace or was told years ago, but what I remember was somewhers along the lines of VE being like 75% for stock/low performance engines, 80-85% for most high performance engines, and 90-95 for all out, highly modified racing engines. If this is so, then how come actual results seem to be way off from this? Do you guys know how they figure VE? Can either one of you explain this? Like I said earlier, I am not claiming to know my math, nor do I understand how they come up with the formula for a lot of different applications, but from my experience I never seem to be able to support the theories, formula, etc by my actual results.

    Now I know, like this threads originally started out with a guy wanting to know about a 500cfm and a 650cfm carb on his stock 283 that this would be much closer to a correct answer based off of the math, but when someone has an engine that has had an intake change, or cam and intake change, or even more mods than that how does the math add up or what does one have to do mathematically to figure out what will work best for thier application? I'm not sure that the math alone can do this, I would think this is where experience of actually building and testing would be more accurate.

    So my biggest reason for not doing the math is/was not because I can't, but more because it doesn't add up to me in the "REAL WORLD"!!! I had to fit real world in here somewhere!! Seriously, to me once someone has done much if anything to an engine, then the math does not seem to be as accurate. I find that too many people take the math/formual as gospel and that is is always right....but I do not think it is.
     
  17. Brad54
    Joined: Apr 15, 2004
    Posts: 6,008

    Brad54
    Member
    from Atl Ga

    More than likely, the 500cfm carb will be just fine for what you're going to do. I just did some dyno testing on a 316ci 283 with a 500cfm Edelbrock, the C4B intake you have, and waaaay more cam than a reasonable street engine needs. I doubt a 650cfm carb would have make too much more difference.

    That being said, if the 650cfm carb has vacuum secondaries, the engine will open them up as far as it needs.

    Then again, I'm normally running a tunnel ram and a pair of 660cfm center squirters on that 283/316 to go with that cam. 403hp @ 6,000. (it made 338hp with the 500 carb and the C4B).

    -Brad
     
  18. el Scotto
    Joined: Mar 3, 2004
    Posts: 4,183

    el Scotto
    Member
    from Tracy, CA

    While we have the attention of the carb smart guys and this thread seems to have run its course, I have another question...

    My combo is a stock rebuild 1962 283 with an Edelbrock Performer RPM and headers, topped by a supercharger that puts out 6 psi. Not sure about the heads, as I don't have the casting #s.

    I have a Holley 650 vaccum secondaries on it that has been partially converted to work as a supercharger carb, but isn't totally "right". I picked up a 750 mechanical secondary Holley to build as dedicated blow-through supercharger carb.

    I'm pretty the 750 would work good on this combo, but what are your thoughts?

    I'd just like to be pretty sure before I drop a few hundred on carb parts...

    After I get the new carb dialed in so that the drivability of the car is great, I plan to upgrade the cam, valvetrain and heads, then retune the carb as necessary.
     
  19. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    Over 100 VE?
    Very short list:
    1. blower: all the time
    2. long overlap cam + highly tuned intake and exhaust systems (wave resonance): fairly narrow period when the wave returns all "line up", and generally causes some lost power elsewhere.
    3. very modern electronically controlled multiple-path manifolds and automatic LSA cam adjustment (as in my Toyota 3MZ).

    Engine has a mild cam, dual-plane intake and shorties?
    95% even with ported heads.

    Math always gives the right answer, the problems arise when:
    1. you have no reliable data to input (like VE)
    2. you're oblivious to important variables, and looking for a simple answer
    3. you use the wrong math (like using .050" IVC for DCR calculation)
     
  20. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    partially converted to work as a supercharger carb

    Power valve referenced to boost pressure?
    Read my article: http://victorylibrary.com/mopar/blower-carb6.htm

    An example of math helping:
    283, let's assume (good heads) 95 VE, 6 psi, 6,000 RPM peak power, and ATM = 14.7 psi.
    The usual CFM math: 283 × .95 × 6,000 ÷ 3,456 = 467 CFM.
    The boost makes a pressure ratio of (14.7 + 6) ÷ 14.7 = 1.41.
    You need 1.41 × 467 = 657 CFM.
     
  21. Hi!
    Joined: Oct 4, 2006
    Posts: 730

    Hi!
    Member
    from SoCal

    Weres the bunny with the pancake:D its time.
     
  22. holeshot
    Joined: Sep 18, 2009
    Posts: 1,519

    holeshot
    BANNED
    from Waxahachie

    ROSS...it comes down to how big your cam is, size of heads, and exaust system...POP.
     
  23. 42hotrod
    Joined: Nov 3, 2005
    Posts: 811

    42hotrod
    Member
    from S.E. Idaho

    LOL....The dyno don't lie.

    a bunch of the "experts" swore this car was WAY overcarbed and would never run....

    This was my car before I bought it, it runs even better now and is painted, hood, etc...

    Its a 355 with a Holley 850 CFM carb.

    Impossible right?

    You be the judge...

    Quick mention, the car is NOT turbo charged as stated on the listing. Read the comments and sevral times my friend got on and stated it wasn't turbo'd. Just a typo from the dyno shop. It is however being sprayed on this pull, but runs very very well motor only with a 850!!

    Talk about a sleeper though, hahahaha....Car got the name the joe dirt camaro and people respected it and still call it that.

    <object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/sbdgQr1e2QQ&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/sbdgQr1e2QQ&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>

    Over 500 HP AT THE REAR WHEELS....

    Every appplication and engine combo takes different carbs...Don't get sold on all the mathematical B.S....there are entirely toooooo many variables in setting up carbs. This engine topped out with the 850. several dyno sessions and carbs and what you see is what ran the best..
     
  24. 42hotrod
    Joined: Nov 3, 2005
    Posts: 811

    42hotrod
    Member
    from S.E. Idaho


    MUUUHAHAHAHAHAAAAA...


    [​IMG]
     
  25. JoJo O.
    Joined: May 12, 2009
    Posts: 170

    JoJo O.
    Member

    That is the funniest shit i've seen in a long time!!
     
  26. SYCO620
    Joined: Jan 26, 2009
    Posts: 96

    SYCO620
    Member
    from Merced, Ca

    Its almost 2am and I couldnt sleep so i jump on here to read a relaxing bedtime story...... Now my head is gonna explode w/ all this info! Im sure mama has already taken up the whole bed so I might as well see if I can find a re-run of Sanford and Son.......
     
  27. 53sled
    Joined: Jul 5, 2005
    Posts: 5,820

    53sled
    Member
    from KCMO

    This is the most important part of your post. a 355 tells me its a bored 350.
    Variables: lets start short- cam? compression? spray? heads?

    I'll bet you can't put that same carb on this worn out 283 with bad rings and tiny valves and get that number. Size matters, math doesn't lie, women do.
     
  28. Hi!
    Joined: Oct 4, 2006
    Posts: 730

    Hi!
    Member
    from SoCal

  29. Powerglides are used in high horsepower/torque low weight combinations because there is not as much need for gear multiplication due to the high power to weight ratio. A stock 283 in a street car is not a high power to weight ratio machine.

    Somethings just work, like gravity, so did he race downhill all the time?



     
  30. el Scotto
    Joined: Mar 3, 2004
    Posts: 4,183

    el Scotto
    Member
    from Tracy, CA

    Thanks Panic, I really enjoy your technical articles! Didn't know you had one for blow-through carbs, too!
     

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