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  1. #16
    Senior Member BasicQ's Avatar
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    Quote Originally Posted by hq308 View Post
    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.
    I knew that would be the easiest of my questions to answer but what has the engineers sizing them to what they have?

    304 x 5500rpm x .85VE / 2820 = approximately 503.97 cubic feet a minute a factory 5lt injected engine would ingest at redline. Do standard throttle bodies flow more than this?
    If so why?
    Upgrading to even larger is a performance upgrade isnít it?


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    Couple of comments.

    [1] Getting the VE correct is very important if you want an accurate assessment of reqd air flow.

    [2] A big carb might generate a few extra top end horses, but lose more at the low[er] end because of poor signal/atomisation problems. Swings & roundabouts....

    [3] Carbs & FI work on the same principle, pressure differential. A carb uses the pressure of gravity, FI uses a pump to provide the pressure. End result is fuel & air entering the engine.

    [4] Disagree with post 15 that a few extra cfm through the TB has no adverse affect. Firstly, if the engine's cfm requirement is less than the TB cfm rating, then that is what gets drawn drawn through the TB, not the extra amount. Secondly, air has mass. A 100 ft cube weighs 38 tons!! Hard to imagine because we cannot see it. A bigger TB is like a big carb, a bigger slug of air has to be got moving. So throttle response is likely to be reduced when a too-big TB is used.

    [5] Weber/Dellorto style carbs. D. Vizard did extensive testing on the ratio of choke size to bore size; a narrow window works, & either side of this loses power. The range is 72-80%, with 80% giving best atomisation & air flow. I found this out the hard way when I increased choke size 3mm on my Weber 48s. Thinking it would increase top end HP, it lost power everywhere.

  3. #18
    Do you ever leave? EH179's Avatar
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    Basic, please feel free to acquire and calibrate a 1030 Holley to fit on your car.

    After some comprehensive testing on the street and track, report back with the conclusion and your thoughts.

    Anyone remember when P.Brock fitted a 1050 Holley on his race car?
    Power on the straights was fine, didn't like corners too much, running slower laps at the end of the day.

  4. #19
    Senior Member BasicQ's Avatar
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    Quote Originally Posted by EH179 View Post
    Basic, please feel free to acquire and calibrate a 1030 Holley to fit on your car.

    After some comprehensive testing on the street and track, report back with the conclusion and your thoughts.

    Anyone remember when P.Brock fitted a 1050 Holley on his race car?
    Power on the straights was fine, didn't like corners too much, running slower laps at the end of the day.
    As I have noted on another thread the Demon I have on the Engine now has the same main body venturi size, throttle bore and booster legs as the 1000cfm Race Demon. Also same dimensions as Holley HP 1000cfm and some Quickfuel. Those dimensions are marketed as 850 or 1000/1050cfm all with downleg boosters. I assume it depends on their target market but one set of buyers is being fed some manure.

    Anyway you can see the difference with the 750 and the 850 below. Drivability is barely different, acceleration, throttle response AFR observed, idle, the works. Tomorrow I intend to have both on the car so a more acute comparison will be made.

    Did Brock have that 1050cfm on a 308?

    [img]https://i.ibb.co/BT5MVCG/4-C5645-B7-BD6-C-49-C4-8808-C94-DE15-A4-D3-D.jpg[/img]


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    Last edited by BasicQ; 06-01-2021 at 09:22 AM. Reason: Change photo

  5. #20
    Senior Member LXCHEV's Avatar
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    The only thing I can add to this thread is my own recent 'experiment' of downsizing from a 750 Ultra HP to a 650 Street HP on my 381 SBC (which I would claim as probably a very mild stroker compared to what most others run) - ie. mine only makes around 440HP (at the crank), with dirty old iron heads, a dual plane, Tri-Y's and hydraulic cam. My combo has always been, and always will be about low-down and mid-range drive-ability. I spend 90% of my time cruising well under 4,000 RPM. I'd go as far as saying my setup is essentially a daily driver/grocery fetcher!

    For me, the change in carb has been positive - whilst it has it's own problems I'm yet to sort out, the car feels crisper, stronger and angrier. My experiment was based on the thinking of smaller venturi size generating high velocity/air speeds/stronger signals. My idle vacuum certainly improved too. On the dyno, car made the same power as it did with the old carb. I'm chirping wheels now and when I do punch it, I don't seem to be belting black smoke out the rear anymore. Seems to light the rears up easier too. It feels like it gets to speed faster. I look forward to hitting the 1/4 mile as that will be my ultimate comparison test.

    Having said all that - I have no doubt my old 750 (and the 825 I ran before that) are both still perfectly good and well matched carbs for my engine. I just don't have the know-how (or cash to pay someone else) to perfect the 'street tuning' side of them. For me, I rely on the 'out-of-the-box' tune, with a basic WOT dyno session to set the main jetting. I have just invested in an Innovate MTX-L Plus wideband kit, so once that's installed, hopefully I can get my hands dirty and start heading down the street tuning path as much as possible.
    Last edited by LXCHEV; 05-01-2021 at 10:03 PM.

  6. #21
    Senior Member BasicQ's Avatar
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    Default Holley CFM selection

    Quote Originally Posted by LXCHEV View Post
    The only thing I can add to this thread is my own recent 'experiment' of downsizing from a 750 Ultra HP to a 650 Street HP on my 381 SBC (which I would claim as probably a very mild stroker compared to what most others run) - ie. mine only makes around 440HP (at the crank), with dirty old iron heads, a dual plane, Tri-Y's and hydraulic cam. My combo has always been, and always will be about low-down and mid-range drive-ability. I spend 90% of my time cruising well under 4,000 RPM. I'd go as far as saying my setup is essentially a daily driver/grocery fetcher!

    For me, the change in carb has been positive - whilst it has it's own problems I'm yet to sort out, the car feels crisper, stronger and angrier. My experiment was based on the thinking of smaller venturi size generating high velocity/air speeds/stronger signals. My idle vacuum certainly improved too. On the dyno, car made the same power as it did with the old carb. I'm chirping wheels now and when I do punch it, I don't seem to be belting black smoke out the rear anymore. Seems to light the rears up easier too. It feels like it gets to speed faster. I look forward to hitting the 1/4 mile as that will be my ultimate comparison test.

    Having said all that - I have no doubt my old 750 (and the 825 I ran before that) are both still perfectly good and well matched carbs for my engine. I just don't have the know-how (or cash to pay someone else) to perfect the 'street tuning' side of them. For me, I rely on the 'out-of-the-box' tune, with a basic WOT dyno session to set the main jetting. I have just invested in an Innovate MTX-L Plus wideband kit, so once that's installed, hopefully I can get my hands dirty and start heading down the street tuning path as much as possible.
    Good feedback of stepping down from a race type carby and cfm to street type that so far shows all round improvements.

    What a carburetor needs to do effectively is supply the engine for a big as possible rpm range a homogenous air fuel mixture. The more evenly and finely sized the fuel droplets the better and more completely the fuel vaporizes. The more completely vaporized the faster and better the combustion. The 650 and its calibration must be doing a better job at that homogenous supply for your engine than the 750 was calibrated to supply.

    But how much cfm can a carburetor flow and still provide a homogenous mixture for our individual engines? Maybe that chart in post 1 is giving us some direction in answering that question.

    Then add to that question Ďand still provide great all round drivabilityí?

    A Formula that EH179 provided is probably as good a tool as anything we have for estimating what will work for our engines and provide best street ability and power.

    Cubic inch x peak rpm x VE / 2820 = cfm

    Conventional wisdom says that if we start using carburetors much larger than the formulas suggest we may make more peak hp but sacrifice low down torque and drivability. But are there situations where we can have the best of both worlds with carburetion? With a too big carb can the generic calibration of the universal carburetors we are talking about be altered enough to provide the homogenous mixture at low rpm yet still provide better peak torque and hp numbers than the ideal sized carby?

    One tip on how this is certainly possible is the annular booster.

    That chart to choose mechanical secondaries in post 1 goes against the grain of what we know. Iíll state again I am playing devils advocate and looking for reasoning in its validity. Can we successfully use bigger carburetors than we are conditioned to select. Is better possible? I am teasing out conversation to see if there is anything to discover.

    I should add LXCHEV if you are ever at Calder the same time I am you are welcome to try one or any of my three Demon carbies.
    Last edited by BasicQ; 05-01-2021 at 11:02 PM.

  7. #22
    been here .......too long Smitty2's Avatar
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    Quote Originally Posted by EH179 View Post
    Basic, please feel free to acquire and calibrate a 1030 Holley to fit on your car.

    After some comprehensive testing on the street and track, report back with the conclusion and your thoughts.

    Anyone remember when P.Brock fitted a 1050 Holley on his race car?
    Power on the straights was fine, didn't like corners too much, running slower laps at the end of the day.
    apocryphal that....
    and I can tell you the story, as told to me by Harry Firth (cornered him at one of our car club meetings maybe 15-20 years ago on this topic after he spoke about 308 race engines)

    the 1050 was never tried on the race track, only on the dyno and it was a dog. Even with jetting changes it made the 308 less powerful that the original 850
    (everyone remember the original L34 hipo engine package? came with a free Holley 850 courtesy of GMH ... in a box in the boot! )

    and the 850?
    EH is correct on that score ... Power on the straights was fine, didn't like corners too much, running slower laps at the end of the day
    true .. and acceleration out of corners is where you win races. Come out quicker than the car behind and he will never catch you (car speed being everything)

    so ?? Firth went to 2 x downdraft Webers (some racers used side draft) and even when Shepherd took over HDT (at the release of A9X) kept the webers....
    A few horsepower down on the Holley, it gave the 308 engine a response (and fuel economy) simply not available with the Holley


    this is a pic of the engine in Brock's 1978 Bathurst winning A9X ...

    1978-HDT-Torana-A9X-ENGINE.jpg
    Last edited by Smitty2; 06-01-2021 at 11:38 AM.
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  8. #23
    Senior Member BasicQ's Avatar
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    Quote Originally Posted by Smitty2 View Post
    apocryphal that....
    and I can tell you the story, as told to me by Harry Firth (cornered him at one of our car club meetings maybe 15-20 years ago on this topic after he spoke about 308 race engines)

    the 1050 was never tried on the race track, only on the dyno and it was a dog. Even with jetting changes it made the 308 less powerful that the original 850
    (everyone remember the original L34 hipo engine package? came with a free Holley 850 courtesy of GMH ... in a box in the boot! )

    and the 850?
    EH is correct on that score ... Power on the straights was fine, didn't like corners too much, running slower laps at the end of the day
    true .. and acceleration out of corners is where you win races. Come out quicker than the car behind and he will never catch you (car speed being everything)

    so ?? Firth went to 2 x downdraft Webers (some racers used side draft) and even when Shepherd took over HDT (at the release of A9X) kept the webers....
    A few horsepower down on the Holley, it gave the 308 engine a response (and fuel economy) simply not available with the Holley


    this is a pic of the engine in Brock's 1978 Bathurst winning A9X ...

    1978-HDT-Torana-A9X-ENGINE.jpg
    Weberís are a better metering device are they not?

    A Holley supplied with the car in the boot. Those were the days.

    Curious what the list number was on that 850. Looking through Holleyís Carburetor Numerical Listings I cant find a 4150 850cfm that are not 1-9/16Ē venturi and 1-3/4Ē throttle (1.562Ē x 1.75Ē).
    Anything 4165 is 800cfm.

    Anyway, agree. 1970ís hi-po 308 with 1.562Ē venturi wouldnít be the best out of the corners.

    In your circuit racing experience what cfm would be chosen for that car/engine with the mechanical secondary selection chart in post 1?

    Iíve got to ask, what cfm were those Webers?


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  9. #24
    Do you ever leave? EH179's Avatar
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    Quote Originally Posted by BasicQ View Post
    Good feedback of stepping down from a race type carby and cfm to street type that so far shows all round improvements.

    What a carburetor needs to do effectively is supply the engine for a big as possible rpm range a homogenous air fuel mixture. The more evenly and finely sized the fuel droplets the better and more completely the fuel vaporizes. The more completely vaporized the faster and better the combustion. The 650 and its calibration must be doing a better job at that homogenous supply for your engine than the 750 was calibrated to supply.

    But how much cfm can a carburetor flow and still provide a homogenous mixture for our individual engines? Maybe that chart in post 1 is giving us some direction in answering that question.

    Then add to that question ‘and still provide great all round drivability’?

    A Formula that EH179 provided is probably as good a tool as anything we have for estimating what will work for our engines and provide best street ability and power.

    Cubic inch x peak rpm x VE / 2820 = cfm

    Conventional wisdom says that if we start using carburetors much larger than the formulas suggest we may make more peak hp but sacrifice low down torque and drivability. But are there situations where we can have the best of both worlds with carburetion? With a too big carb can the generic calibration of the universal carburetors we are talking about be altered enough to provide the homogenous mixture at low rpm yet still provide better peak torque and hp numbers than the ideal sized carby?

    One tip on how this is certainly possible is the annular booster.

    That chart to choose mechanical secondaries in post 1 goes against the grain of what we know. I’ll state again I am playing devils advocate and looking for reasoning in its validity. Can we successfully use bigger carburetors than we are conditioned to select. Is better possible? I am teasing out conversation to see if there is anything to discover.

    I should add LXCHEV if you are ever at Calder the same time I am you are welcome to try one or any of my three Demon carbies.
    In my opinion, the answer is no, no yet anyway.
    Air speed or velocity is still the dictator, as it is in the entire inlet tract.
    Variable venturi?

  10. #25
    Senior Member BasicQ's Avatar
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    Quote Originally Posted by EH179 View Post
    In my opinion, the answer is no, no yet anyway.
    Air speed or velocity is still the dictator, as it is in the entire inlet tract.
    Variable venturi?
    Like the old Predator carbies? Iíve never had anything to do with them. Have you? Interesting to see what they can do. Did they die out because they were crap, ugly or business closed down? A mate at work collected them over the years, has about 8-10 of them.

    Answering my question above as well I tend to agree with you as the idle and transition system can only do so much to homogenize the fuel. The annual booster can get you closer but you are still at the mercy of the transition fuel not atomized as good while it is the dominant circuit delivering fuel.

    However doing an A/B test today with the two different sizes of down leg carbies (in the pic together in an earlier post) the bigger would be preferred. If I were to sell one now it would be the 750. But I wonít sell yet as I still have to run them up on the chassis dyno and send down the quarter. They both have equal manners on the street and same throttle response but the bigger carb just wanted to rev cleaner and quicker. Afrís are very similar. Itís like it is providing sufficient quantity and quality of atomized fuel with more airflow to increase part throttle volumetric efficiency over the 750. This isnít WOT though, itís acceleration on a highway on ramp, quick snap of the throttle without opening secondaries and spirited overtaking on the highway. The dragstrip will be the ultimate determinant of performance.

    Tomorrow Iíll compare the bigger down leg and annular.


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  11. #26
    Senior Member BasicQ's Avatar
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    A few quotes from Shrinker, jmarkaudio and others on carb sizing in the next few post. I am sure we all know it to some degree that engine ci and rpm are only two of many factors when choosing a carburetor cfm.

    From Shrinker

    The 'correct' size carby is not a set in stone thing. There will be engine combinations that benefit from a huge carby and there will be combos that need to pull 3" vacuum at full load. I have seen it all.
    Its not what size carby you run, its what the whole engine package produces.
    For instance, on our drag car its had multiple cams sized from 268 to 290 degree over the years. Its always had the same heads and same other things. Now to get the small cam to make the power the carbys were 1800 cfm total, but with the largest cam the carbys are 1500 total. The larger duration cam on the head flow we had lost dynamic compression and the engine lost power on the larger carbys, made the carbys smaller and the power went to the same as the small cam, but because the rev range is greater the diff gears could be changed etc.
    Sizing the carby is just one thing in the shuffle of factors that make the engine.
    Its all tied up with vaporization. Metering just sets the AFR, air speed assists vaporization. In the case of our drag car the heads flow 365 CFM and its 412 cubes and makes 870hp on average at any dyno session. Its peaked at 903 hp. So that's a lot of power from 365 CFM. Thats a flow factor of .309. All the cams make maximum power at approx 8200. The bigger cams just hang on longer. The engine is hampered by the head flow so it doesn't get the cylinder fill it could with better heads. What that does is it reduces the dynamic compression energy so the fuel gas content at ignition is less. Reducing the carby size increases the power by increasing atomization within the smaller bores of the carby, its still large enough carbys to give it high plenum pressure. When a carby is too small the Plenum pressure is the controlling factor and the engine will pick up with a bigger one and when its too large, atomization becomes the influencing factor.
    If we look at the influence of intake valve closing point we find that at some point when the piston is coming up the compression stroke, the pressure in the cylinder will be the same as the intake runner. If we shut the valve after that point in time we have shut it too late. Once the pressure in the cylinder exceeds the intake runner the air will go back out of the cylinder. If that happens we can do something about it without tearing the engine apart, we can install a SMALLER carby. What that does is it restricts the fill of the cylinder so that the incorrect closing point of the valve is not incorrect any more. Essentially a smaller carby makes the cam appear smaller.THATS one of the factors why sometimes engines make more power with smaller carbys.
    Our own drag car has had various cams ranging from 269 to 289 intake duration in it, the longer the intake duration the smaller the carbys had to be to get max power out of it. And with all the different cams its always made exactly the same power because we never changed the head flows. BUT i had to make the carbys smaller to get the power back with each successively larger cam. Every time we went larger in the cam it lost power on the old carby size.
    Same engine, same rpms, different cam, change in cfm required.
    Last edited by BasicQ; 30-01-2021 at 08:54 PM.

  12. #27
    Senior Member BasicQ's Avatar
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    From jmarkaudio

    In the discussion in the E85 thread about laminar, transitional, and turbulent flow in the mainwell, it has occurred to me the same thing has to play in carb size as well. It also leaves me with a few questions, as far as getting the proper carb for any given engine. In the case of the engine we have built, the 447 and 461 SB2 head engines, the design of the ports are at the higher end of the scale, resulting in higher airflow quality throughout the port. This allows for the engine to run at a higher RPM and resulting in a higher airspeed in the port before transitional and then turbulent flow, allowing for an overall higher depression below the carb. A smaller carb will allow for a higher depression below the carb, giving atmospheric pressure more force to push the air in from above, but at what point does it diminish and why? Is it the turbulence through the carb created by design limitations that limits it's ability to move more air, is it the airspeed approaching sonic velocities, or a combination of both? Both engines we built run faster with larger carbs, is this because airflow is turbulent in the smaller carbs "on our engines" in the range they make power, or is it that the combination of carb size versus depression difference at WOT "on our engines" still afford the larger carb more airflow? And would it be a fair assumption that an engine that responds to a smaller carb may have turbulence elsewhere in the induction tract, incorrectly sized tract for the RPM range, or improper camshaft for the entire combination, resulting in a slower overall airspeed? Which in turn would make the smaller carb respond better by improving velocity through the carb, improving atomization, the end result improving combustion chamber burn, or is it back to the combination of carb size versus the depression the engine creates below the carb at WOT affording the smaller carb more airflow? I wonder as well how much plenum volume plays into this, does a smaller plenum like a larger carb and larger plenum like a smaller carb? All of this seems to make choosing the correct carb for any engine a "crap shoot" depending significantly on the entire package of parts used on the engine.
    rick360
    jmarkaudio wrote:
    A smaller carb will allow for a higher depression below the carb, giving atmospheric pressure more force to push the air in from above, but at what point does it diminish and why?

    The smaller carb CAUSES the higher depression under the carb which means it is restricting airflow. The manifold pressure is what pushes the air into the cylinder. Ideally we want no carb restriction so full atmospheric pressure is available to push the air/fuel into the cylinder. Getting a carb to draw the correct amount of fuel and atomize it with very little pressure drop is difficult. Finding the balance of enough pressure drop across the carb to get a proper fuel mixture and atomized properly vs not too much pressure drop that the engine airflow is limited.

    Engines with too small carbs will want a larger plenum than an engine with a big carb. The plenum dampens the air pulses coming from the runners and it also allows the air from the carb to slow down so it can turn into the runners w/o separating the fuel/air.

    Rick
    jmarkaudio

    Choice of words, I'm aware the carb causes the difference. I'm digging a little deeper, I'm wondering if a smaller carb on a given engine combination may flow more air, not because of the carb design, but because of the design of the induction and valve events of the cam resulting in more air in the engine. I do know the carb requires some difference to meter the fuel correctly, and velocity at the booster plays a part in that as well as to how well the fuel is atomized. Too large and the fuel curve gets harder to tune, as well as the start point of the mains. It also results in lower airspeeds and lower amount of fuel atomization by the carb. A quality induction design like the SB2's will allow for higher airspeeds before turbulence, If you could match a 23˚ head in valve size and CSA the port angle and shape of the SB2 through the port you should have it still making more power. The 400+ci engines we have built over the years always seem to respond to a larger carb, as long as it has sufficient stall to put it at or over peak torque. Yet I hear of 550+ combo's that lose power with a bigger carb, just trying to figure if there are less obvious factors involved short of the big one as well as a couple of the more obvious, big one being sufficient atomization for the engine it's on.

    From previous discussions, I'm also aware that Holley's venturi shape may not be ideal, and the booster creates a restriction as well as turbulence which hurts airflow but may improve atomization. So many variables to have to ponder.... the more we can figure out the less trial and error we have to do.
    Shrinker

    Any time the cam/valve events cause pulsing in the plenum and the plenum is insufficient size to dampen the pulses the carby will flow less air. Smooth plenum air doesn't need as big a carby to fill it.

    To your comment about 550's losing power, i assume your comparing a big CFM carby on a 400 engine that doesnt work on the big engine. So were saying for instance that the 550 likes a smaller relative carby than the 400. The rules for sizing a carby are the same for either engine size, whats different is how the ports flow on either engine and the physical size of the cylinder. the larger a cylinder gets the harder it is to control it optimally. If we assume that the 400 and the 550 both have all the correct and equal values for head flow and air flow features etc then the only thing remaining is cylinder size. There was a professor dude in the 80's who worked out the optimum cylinder size to be 600cc. I cant remember his name or the rest of the maths etc for it but some manufacturers follow those guide lines. BMW for example make their engines all around the 550 to 600cc per cylinder, want a bigger engine; add more cylinders. I dont know any other manufacturers that can make the power per cube of BMW and get flat line emissions like BMW do. Those guys know their stuff. I always say to people" If you want to know how to do something, go study BMW, then you will know more than others"

    Anyway, when you have a large cylinder you are more likely to have to improve some factors to reduce detonation. The reaction speed of fuel is the same in a small cylinder as it is in a large one, so the large cylinder takes a longer time to react all products. That means the reactants that are ahead of the flame front are subjected to pressure increase prior to combustion for longer, hence they may detonate or commence uncontrolled reaction. One of the principle causes of detonation is poor atomization and homogenization. Running a smaller carby is one way to improve both those factors. Therefor the larger the cylinder gets the smaller the relative carby gets.
    So some interesting thoughts on factors that dictate what cfm an engine likes, where going smaller will benefit and where going larger will. 550+ci likely doing better with a smaller carby than a mid 400ci due to cylinder size is interesting. Same with Shrinker explaining that swapping to a bigger cam an engine responded better with a reduction in cfm.

  13. #28
    Senior Member BasicQ's Avatar
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    Quote Originally Posted by BasicQ View Post
    Tomorrow I’ll compare the bigger down leg and annular.
    Did this but didn’t report back.

    Annular 850 has the best noticeable performance and drive ability of the three.

    More airflow than the 750 and better atomization than the 850 downleg?
    Last edited by BasicQ; 30-01-2021 at 08:56 PM.

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    Couple of comments.

    [1] The Carter TQ was used by Chrysler, Ford, International. Because of the huge secondaries, they had big sec jets. In the Chrys Performance bulletins, sec jet size was increased to 0.169" [ that is close to 3/16"!! ] when used on SBs using single plane manifolds. The jet increase was without any change to the MAB. They must flow some air to require such large jets.

    [2] Also on the TQ. It is the only 4bbl carb that I know of that mechanically adds fuel on engine load demand. It has a lifting device connected to the pri t/shaft so that the instant the throttle is moved, the met rods are lifted out of the jets & more fuel flows. This saves time & you feel this as throttle response.

    [3] A brand H [ or clone ] with a p/valve can never equal a TQ for t/response due to #2 above, but also because of the torturous the fuel takes when enrichment is reqd from the PV. The fuel has to make several turns, & fill up the PV cavity until the fuel can exit via the PVCR holes; the fuel then has to make a 90* turn into the main well. This all takes time, resulting in reduced response. The annular boosters help, but do not overcome the fundamental problem. And with the ABs, you pay the price of reduced air flow.

  15. #30
    Senior Member BasicQ's Avatar
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    Screenshots from Powernation episode.

    Junkyard 305 sbc with single plane. No other specs known.

    465, 750, 950, and 1050cfm tested. Results seem to fit more with chart in post 1 than mathematical formula. Also similar to Vizards graph where the larger the more power. I have also seen other test of 650 v 750 v 850cfm where 850 makes most power but 750 is decided in theory to be the better choice due to anticipated drivability. I anticipate our discussion will also be about drivability referring to results in charts below.

    Is this thread post 1 chart best power cfm and mathematical formula best drivability cfm?

    Bench racing we would all come to the conclusion the 450cfm on a 305 sbc with single plane is a must for drivability and our mathematical formula would agree for selection. I wouldn’t even reach for the calculator in selecting a 450cfm for a 305. But look at the power and torque given up in comparison to the bigger cfm from quite low rpm to only 5,000rpm. If it’s all about airspeed it would seem to me the bigger carbies (750 up) have better airspeed at all points on the charts with an exception being the 1050cfm low down. Or are they making more power and torque with less airspeed?

    Adhering to the mathematical selection formula what power would us street/strip guys be giving up? Using the chart what drivability would we be giving up?

    What method for best et?

    Is there something inbetween to find best power cfm just before falling off the drivability issues cliff? ie how do we know for certain that the 750cfm or even 950 below has drivability issues without actually testing on the road other than gut feel and because it’s too far from mathematical selection results? There isn’t a go/no go drivability equation. If we had the kahunas to try it rewards are there if a squirter change here and pump cam there sorts out drivability?

    Would carb companies disclosing say 50% primary throttle opening cfm provide better informed selection for drivability?

    [img]https://i.ibb.co/W00PXmK/1630-FA1-C-DC86-4-C8-F-A7-B8-9-F2860155458.jpg[/img]

    [img]https://i.ibb.co/Xt6kpzk/9117-D603-B32-E-4-CEB-88-BF-BE29-A63-B0-BD2.jpg[/img]

    [img]https://i.ibb.co/XzPZfZ0/01-EBC772-2-EBE-4798-B5-B6-34-F98-A291-D08.jpg[/img]

    [img]https://i.ibb.co/qMnng45/287952-A2-5-F60-4009-8-C90-176402-F57969.jpg[/img]

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