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  1. #1
    casual poster BasicQ's Avatar
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    Default Holley CFM selection

    I have posted two of the below images on another thread and I thought I would start a new thread as it could invoke a bit of discussion. Sometimes when selecting carby sizing is bought up it can be like throwing a chip among seagulls and we all want a piece of the discussion. The first image below is from the technical section of the 1998 Holley catalog. I also stumbled across the chart online where it was published in an SA Design book about Holley carburetors. The chart is almost a paradigm shift to the old formula we know that uses displacement, rpm and volumetric efficiency to help select a carby for our engine. I have on this thread added an image of the opposing page that is based on that formula. There is also a third selection method that those familiar with David Vizard work may have seen before. This one involves not just cubic inches but includes a factor for .050Ē cam timing and head flow. The last graph is also from Vizard. It shows testing of three different cfm carbies on a sbc 383. Interesting results as the highest cfm produced best torque and peak hp. It is noted that none of these methodologies take into account weight or gearing of the vehicle the engine is in.

    My take? The old formula will tell you how many cubic feet per minute of air your engine will ingest at the rpm entered into the formula. That tells me you want to have a carby larger than that figure to not be a restriction. Most will and are advised to choose the closest cfm rating above the figure the formula gave. It doesnít however give a maximum, itís just advised to stay close to the number the formula spat out. Read closely at the different selection chart below and it says Ďmaximum cfmí and seems to be more for WOT driving that street ability. Back to the tried and true original formula, it does not make a recommendation or cap what the upper cfm limit should be but any literature accompanying the formula warns that too large would cause drivability problems ie lack of response among others. Example; if the formula spits out say 590cfm for your engine combo and rpm input then donít jump up to an 850 or above cfm carby for that application or problems may be encountered. I am curious if anyone has changed to a smaller cfm carby in the past from what they had, what was the determining factor to do so and what tuning attempts were made before coming to the conclusion the cfm rating was too high? In other words empirical evidence that it was the cfm and cfm alone that was the problem.

    Many of of us probably see no reason to change the cfm size carby we have but when new information comes to hand for decision making it should always have some consideration. Are there cases where we are too anchored to the old formula for our own good and could the alternative methods below actually serve better for intended application?

    Three other questions
    1) Why are fuel injection throttle bodies a bigger rated cfm than we would select our carbies? Why donít they just stay within the cfm rating as though it were a carby?

    2) Quadrajets only ever came in 750 and 800cfm and were put on everything from 253 to well over 400ci? Did the OEM engineers use the classic formula? Certainly different calibrations but the scrutiny of cfm of Holley type carbies is put before its calibration.

    3) on a 350ci engine if the formula advises a 650cfm, but a pair of 465cfm or even 600cfmís can be made to work quite good on a tunnel ram on the same engine? Thatís an enormous amount of rpm and VE input to come up with 930 - 1200cfm.

    (Sorry, 5 questions)

    [img]https://i.ibb.co/6gkrmdm/479-C2-AF6-6-E88-4-F10-8294-E09-DA204-B3-D5.jpg[/img]

    [img]https://i.ibb.co/Vtk9HKN/10-B227-A4-D0-B8-4203-B653-646570-EBB693.jpg[/img]

    [img]https://i.ibb.co/fngQNQy/0-F3-CD3-FB-EE4-A-4695-9-D3-E-7-AE02-C2-F734-F.jpg[/img]


    [img]https://i.ibb.co/RDr6zpC/E900-E8-EA-9-A56-41-D2-9371-A83-AE7-CB82-AA.jpg[/img]
    Fig. 6.4. Assuming the compression ratio and the exhaust are appropriate for the engine, the heads and cam duration are the most influential factors in selecting the right size carb. To obtain the correction factor for a particular application, first choose a curve for the cylinder head spec from the list below. Next, locate your 0.050 cam duration figure along the bottom scale. Then go straight up the graph until you intersect the previously chosen curve. Now go to the left for the correction factor on the vertical scale.
    Red = super race heads such as ProStock and NASAR Cup Car heads
    Orange = top-of-the-line race-ported heads such as used by pro racers
    Green = race-ported conventional heads
    Blue = street-ported heads
    Magenta = pocket-ported pre-1990 heads or stock Vortec or aftermarket heads
    Black = stock OE heads of pre-1990 design

    Figure 6.4 gives a correction factor (CF), which takes into account cam duration and cylinder head flow capability. Using this correction factor, here is the formula for predicting the required carb CFM:CFM =ci x RPM x CF / 2 x 1,728 As an example, letís use one of my 482-ci Chevy big-block engines. This street/strip build, which was mostly in the low-buck category, targeted peak power at 6,800 rpm so the maximum RPM figure (at 200 over that) would be 7,000. The CF for a Comp Cams street roller (248 degrees at 0.050) with the basic race-ported Dart Iron Eagle heads (using the green curve in Figure 6.4) came out to 1.065. Putting this data into the equation, you get:CFM = 482 x 7,000 x 1.065 / 2 x 1,728=1,039.7
    The answer rounds up to 1,040. The carb selected was a 1050 Holley Dominator, which worked out very well. Hereís another example: a Ford small-block 5.0 built for my road race Mustang. This 306-ci engine featured race-ported Dart heads, a Comp Cams solid street roller cam with 258 degrees duration at 0.050, and peak power at 7,600 rpm. The correction factor (using the green curve in Figure 6.4) came out to 1.07. Putting the numbers into the equation you get:CFM = 306 x 7,800 x 1.07 / 2 x 1,728 = 738.96875
    The answer rounds up to 740 cfm. The carb used was a 750 Street HP and this pump-gas 306 turned out 525 hp and 396 ft-lbs of torque.This example targets just about the biggest carb you should use. However, it makes no allowance for the fact that a tricked-out carb with high-gain boosters can successfully use greater CFM.Letís say you took a stock 750 and spent time streamlining the throttle shafts and butterflies. This can increase the airflow by about 35 cfm if you do a halfway respectable job. This allows a little more power to be developed without sacrifice in the lower RPM range. Going this route means you have to know your carbs or work with a carb specialist.


    [img]https://i.ibb.co/Ykzn9mf/F3-F931-B4-D0-AB-4-F6-F-8-EF9-DE1699-F61-EB1.jpg[/img]
    Fig. 6.15. Dyno tests support my ďthink biggerĒ philosophy when using a flow-efficient, dualĖplane intake. The relatively basic 383 test engine has a set of Gil MinkĖported World Products Sportsman iron heads with a 10.5:1 CR. The cam is one of my hot street-spec hydraulic flat-tappet grinds. As you can see, it produces 536 hp and thatís pretty respectable output for an engine like this. Most engines with this sort of spec donít make that high an output with a single-plane race-style intake.The point, however, is that if the typically recommended 750 carb had been used, the output peaks would have been 476 ft-lbs of torque and 511 hp. Although thatís hardly an output that anyone would complain about, it is not the 487 ft-lbs and 536 hp seen with the big carb. A couple of points to note to validate the results here are that the intake had no plenum cutout and the torque curves of all three carbs were virtually identical up to 4,000 rpm.

    Discuss.
    Last edited by BasicQ; 04-01-2021 at 10:33 PM.

  2. #2
    been here .......too long Smitty2's Avatar
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    .. one thing missing a bit from all that

    the bit that connects the carb to the engine aka the inlet manifold.

    I learnt that as a junior revhead trying to make in-line Holden red motors go faster

    Stuck a 500 Holley on a CAIN manifold, bolted that to the engine and despite
    dyno runs, rejetting and even a different camshaft... it ran like shit

    Sold the 500 back to the mate who sold it to me and stuck a 350 Holley on
    and wooheee... now I had an engine that half performed as I wanted.
    But i knew that manifold was holding it back


    ps.... said engine got an overbore, angle cut head and 3 side draft webers
    and made another 30% more power
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  3. #3
    been here .......too long Smitty2's Avatar
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    .. and re 2)

    Quaddies work on just about anything above 200ci (up to 560ci). Why so?
    as their design feeds the engine the amount of air/fuel mix the engine asks for.

    Yeah.. you still have to 'tune' it like any carb, right jets needles etc but that
    big secondary only opens when and as much as the engine asks

    It actually, in concept, is simplicity at work. Feed the engine only the amount
    of fuel it needs,... at idle, off-idle, power and WOT.
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  4. #4
    casual poster BasicQ's Avatar
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    Quote Originally Posted by Smitty2 View Post
    .. one thing missing a bit from all that

    the bit that connects the carb to the engine aka the inlet manifold.

    I learnt that as a junior revhead trying to make in-line Holden red motors go faster

    Stuck a 500 Holley on a CAIN manifold, bolted that to the engine and despite
    dyno runs, rejetting and even a different camshaft... it ran like shit

    Sold the 500 back to the mate who sold it to me and stuck a 350 Holley on
    and wooheee... now I had an engine that half performed as I wanted.
    But i knew that manifold was holding it back


    ps.... said engine got an overbore, angle cut head and 3 side draft webers
    and made another 30% more power
    Yes itís all about complete induction system. Note the last chart is based on using dual plane. Very interesting if single plane results would reflect dual plane results in that cfm test.

    I donít think Weberís are rated in cfm but how much more than 350 would they have been?


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  5. #5
    casual poster BasicQ's Avatar
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    Default Holley CFM selection

    Quote Originally Posted by Smitty2 View Post
    .. and re 2)

    Quaddies work on just about anything above 200ci (up to 560ci). Why so?
    as their design feeds the engine the amount of air/fuel mix the engine asks for.

    Yeah.. you still have to 'tune' it like any carb, right jets needles etc but that
    big secondary only opens when and as much as the engine asks

    It actually, in concept, is simplicity at work. Feed the engine only the amount
    of fuel it needs,... at idle, off-idle, power and WOT.
    So like a Holley vac sec?

    Playing devils advocate to the accepted norms, conventions, practices and theories.


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    Last edited by BasicQ; 04-01-2021 at 10:49 PM.

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    HI
    An engine carb --- inlet manifold---cyl head --headers. These all have to work together to flow air. Common misconception is only need to be bothered with the cyl head.
    Holley recommend 100% VE at XXXXX rpm for street .Upto and around 110% VE for race .

    Port EFI can have high flow TB because the injectors are not entirely engine vacuum dependant for fuel metering .Injectors give great throttle response vs carb. Injectors sit behind valve therefore some throttle response can be traded for more CFM

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    Do you ever leave? EH179's Avatar
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    Vizard is using the old school formula for calculating his cfm ...CID X RPM X V.E / 3456 (2 X 1,728) =
    The newer school thinking looks more like this... CID X RPM X V.E /2820 =

    Try it on yours and see.

  8. #8
    casual poster BasicQ's Avatar
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    Default Holley CFM selection

    Quote Originally Posted by EH179 View Post
    Vizard is using the old school formula for calculating his cfm ...CID X RPM X V.E / 3456 (2 X 1,728) =
    The newer school thinking looks more like this... CID X RPM X V.E /2820 =

    Try it on yours and see.
    1) Vizard with correction factor comes out at:

    388 x 6500 x 1.055 / 3456 = 769.88

    2) New school

    388 x 6500 x 0.95 / 2820 = 849.6

    So around 80cfm difference.

    An online calculator punching in just cubic inch and rpm gives:

    Street: 620cfm
    Race: 802cfm

    None of these calculators get near 1030cfm that the 383 in the last graph I posted performed best with, a similar engine to mine. Hypothetical: should I go with a formula or what that real world testing proved?

    (388 is .060Ē over 383 and conservative 95% V.E used)


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    Last edited by BasicQ; 05-01-2021 at 01:36 PM.

  9. #9
    been here .......too long Smitty2's Avatar
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    Quote Originally Posted by BasicQ View Post
    Yes it’s all about complete induction system. Note the last chart is based on using dual plane. Very interesting if single plane results would reflect dual plane results in that cfm test.

    I don’t think Weber’s are rated in cfm but how much more than 350 would they have been?


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    the 45 DCOEs that I used are rated 300cfm each
    ... thats is 150cfm per throttle body. But.... and there is a but

    Do I choose Weber 45s or 48s? coupled with the reply "Surely the 48s will give more power". This shows a basic misunderstanding of the construction
    and principles of operation of the Weber DCO series. It is not the carby barrel size (40, 45 or 48) which determines the airflow and therefore potential horsepower;
    it is the size of the main venturi or choke. And boy is there a complicated mapping that goes with that.... Why?

    biggest is NOT best when selecting a main venturi size, Why? the purpose of the main venturi is to increase the vacuum acting on each main jet in order to
    draw in and effectively atomise the fuel mixture. The smaller the main venturi, the better this is, but a smaller venturi can also inhibit airflow obviously.
    A large venturi may give more power right at the top end of the engine revs, but will give this at the expense of lower RPM tractability

    I remember all this shit from the days of playing with them ... in the end I gave the car to Mark Andre* and told him to stick it on his dyno and fix it
    He did... that Torrie made just over 230rwhp (think the actual number was 232 or 233) and when he gave it back, he said DO NOT TOUCH those carbs
    I didn't touch, the Torrie ran 12s at Calder and did so until the day the then GF had a fight with a horsefloat when she was driving it to work.


    So with Webers, cfm is only one of several things you need to consider


    * Andre ran Andre Performance Tuning as well as the My Corvette Stingray drag racer in the 70s and 80s
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  10. #10
    casual poster BasicQ's Avatar
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    Quote Originally Posted by Smitty2 View Post
    biggest is NOT best when selecting a main venturi size, Why? the purpose of the main venturi is to increase the vacuum acting on each main jet in order to
    draw in and effectively atomise the fuel mixture. The smaller the main venturi, the better this is, but a smaller venturi can also inhibit airflow obviously.
    A large venturi may give more power right at the top end of the engine revs, but will give this at the expense of lower RPM tractability
    Fundamental carburetor operation and what those formulas and charts are trying to nail down, coming up with a cfm number not inhibiting airflow or affecting tractability.

    Playing devils advocate again. Loss of tractability can only be an assumption unless real world testing is done with complete tuning. Your Webers would have flowed a lot more than the 500cfm Holley and gave better peak numbers but also gave lower rpm tractability due to a better overall fuel curve with careful wholistic tuning. The single carb manifold would definitely have flow problems compared to a Weber set up and we know that with Holley type carbies idle and transition takes care of the fuel for most of the low speed operation until mains come on. Was the idle and transition circuits on the 500cfm addressed before declaring it was too large? You had a large two barrel, swapped for a smaller two barrel and had improvement but then when going to an overall higher flowing induction had better overall results. The carbies we are talking about have fixed venturi dimensions but not fixed metering.
    All functions of Holley carbs can be tailored to alter fuel delivery, MJ, IFR, IAB, MAB, emulsion make up, pvcrís even booster design. Metering systems in Quaddies are changed up to suit the engine they were installed on and cfm rating is rarely raised. Are we too readily tossing aside the Holley type carb that we believe is too big for our combos and substituting it for smaller without wholistic tuning? Tuning starting point is often easier on the smaller cfm but for performance applications what could we be giving up for less time tuning. Remember I am talking performance oriented applications that the double pumper chart is guiding us for in post 1. Also refer to last graph post 1.


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    casual poster BasicQ's Avatar
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    Default Holley CFM selection

    Maybe I will paint a scenario or two to illustrate.

    1) Your rebuilding a 253, nothing special. It came with a tired old two barrel and you decide to go with a quaddie. You managed to source one but it was originally of a +400 cubic inch engine. Could run better once installed so you consult a quaddie guru who adjust the overall calibration and it now purrs. Cfm never enters the discussion.

    2) As above replacing two barrel on 253 rebuild. You buy a cheap 750cfm spreadbore vac sec Holley. Runs like a dog and a few jet changes doesn't do much to help. Conclusion is drawn carby too large cfm as that was the predominant indicator of the carburetor. Go out and get a 465 Holley and it runs better reinforcing it must have been a cfm problem all along. Could addressing idle and transition circuit made the 750 Holley work? Probably yes.

    In the second scenario it may not be worth the effort and stick with the easier to tune 465 as performance is not a priority. But in performance situations scenario 2 is quite often played out to find a tune and not know how much power is given up.



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    Last edited by BasicQ; 05-01-2021 at 12:36 PM.

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    casual poster BasicQ's Avatar
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    A bit of context to not only the discussion but my replies.

    I am always looking to challenge accepted norms and conventions. Not to be rebellious or argumentative but to be thought provoking and always find out the fundamental question ďIs better possibleĒ? I can afford to take all aspects of my car and engine to experts and have them do it all for me but it wouldnít be long and I would be dissecting it in search of that fundamental question, is better possible? Thatís the challenge that gets me up in the morning, thatís what has me endlessly working on my car as much as driving it. Thatís why I am tuning three different carbies at the moment. Good, better, best.

    That mechanical secondary selection chart seems to question conventions and can be a catalyst for discussion as to its validity and the validity of status quo. So I put it and other methods of cfm selection up here for discussion. When we throw out there something that disrupts our way of thinking it can ask us to redress and analyze that way of thinking or confirm itís validity. Discussion can tease out and come up with improvements on the way things were done. After all, how did we get to our accepted norms or status quo in the first place?


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    Last edited by BasicQ; 05-01-2021 at 03:57 PM.

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    Hi
    Quadie has smaller primaries and very large secondaries . Good carb for smallish engine or better throttle response to a larger engine when vs a large sq bore .Quadie better suited to automatic over geared heavier with an interest in fuel economy and midrange response.. A sq bore is also a compromise . EFI does not have these issues .

  14. #14
    casual poster BasicQ's Avatar
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    Default Holley CFM selection

    Quote Originally Posted by swampy View Post
    Hi
    Quadie has smaller primaries and very large secondaries . Good carb for smallish engine or better throttle response to a larger engine when vs a large sq bore .Quadie better suited to automatic over geared heavier with an interest in fuel economy and midrange response.. A sq bore is also a compromise . EFI does not have these issues .
    In the cfm discussion ratings are all 4 barrels open despite barrel sizes. In the performance arena we anticipate WOT use and the engine sees 750cfm no matter the size of throttle bores.

    However:

    308 x 7000rpm x 1.0 VE / 3456 = 623.84 cfm

    Thatís a high strung efficient 308 that shouldnít be using a Quaddie because itís too big, or

    EH179 newer formula

    308 x 7000 x 1 / 2820 = 764.53cfm

    Closer but quite the modified 5lt

    Closer to factory

    308 x 5500 x .8 / 2820 = 480cfm

    So primary size makes Quaddies a better all round versatile carby but why was a carby capable of 750cfm made to work as opposed to using a 480cfm carby?

    Cost is one answer, the other is because it can. The 750cfm size was very successful on an engine that only required around 480. They didnít just have to be tuned right for drivability but meet tight emissions standards, thatís fine tuning. Again, part of their success for versatility was small primaries but it doesnít seem the huge secondaries were a problem that couldnít be tuned for on the smaller engines otherwise they wouldnít have went to the public en masse.

    But where have the spreadbore 4160 carbies gone? Using the logic of Quaddies function with small primaries large secondaries would I be able to use a 1000cfm spreadbore if there were one out there for best of both worlds? Why has the aftermarket industry gravitated to square bore and why does no high performance and race Holley style line have spreadbore?

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    Last edited by BasicQ; 05-01-2021 at 02:45 PM.

  15. #15
    Not the Kingswood! hq308's Avatar
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    To answer the throttle body vs carb question, carbs rely on a good signal to be able to pull fuel into the air stream and carb size plays a part in how good the signal is. EFI uses fuel pressure (captain obvious I know) so having extra cfm through the TB generally has no adverse effects.

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