DCNF progression tuning | Page 2 | FerrariChat

DCNF progression tuning

Discussion in '308/328' started by 911308, Sep 28, 2013.

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  1. George Vosburgh

    George Vosburgh F1 Rookie
    Silver Subscribed

    Rob, Fantastic information, great job. Thank you so much for sharing. Pretty gutsy stuff drilling into your carbs, no I don't think I'll be doing that. I'm about to start running a similar set up Wiesco pistons, Euro cams, new valves. Your article is very timely.
     
  2. Ferraripilot

    Ferraripilot F1 World Champ
    Owner Project Master

    May 10, 2006
    17,171
    Atlanta
    Full Name:
    John!
    I found the only way to eliminate optimal cruise mixture while not having any detonation and good WOT mix is to have a rich transition phase. A larger main jet and air corrector allows the main to come on earlier by say about 400-500rpm at the expense of a slightly richer mix. Transition stumble is all but eliminated this way too.

    Great info and data logging there.
     
  3. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    Yes, you will be proven right and thank you for your input. Having used programmable efi for more than 15 years I just find it difficult to accept some of these trade offs without an attempt to improve upon them.
    It may turn out that the journey is more informative than the actual result. The progression circuit has to work with & overlap the main circuit tip in under low/ moderate loads. However the main circuit also has to work independently of the progression fuel contribution when the throttle is snapped open.
    Progression fueling is now a delight and feels so modern and eager. The rich compromise to WOT main circuit transition should now be less of an issue because so little time will be spent there, so I will now be chasing this with larger main jets and AC/ET pumping changes. This car accelerates hard from 3k whereas my efi 911 3.0 doesn't wake up until 4k despite similar overall hp ( lack of itb and cam overlap )
    I have used 140 & 150s mains previously with up to 240 AC but not with the revised progression drilling and altered ET tubes.
    I may even revert to standard F 24s and report back, along with average fuel consumption figures.
    Some more driving is required.

    Rob
     
  4. Ferraripilot

    Ferraripilot F1 World Champ
    Owner Project Master

    May 10, 2006
    17,171
    Atlanta
    Full Name:
    John!
    Interesting how the engines behave differently. This stuff just has never been discussed much in the ferrari world so it's great that it's coming out
     
  5. duck.co.za

    duck.co.za Formula Junior

    Jan 9, 2007
    980
    Cape Town South Afri
    Full Name:
    Dave
    Absolutely , great stuff
     
  6. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    #31 911308, Nov 2, 2013
    Last edited by a moderator: Sep 7, 2017
    Eureka

    40 DCNF 32 Choke 55idle with modified progression 130 Main 188 AC & Funky emulsion tube.

    Results.
    130 main jet just works. You can't beat the physics and the lack of squish & swirl in these standard heads. Cruise AFR sits 13.6 – 14. Moderate throttle tip in 12.4. Mash the pedal any where gives 10.5/11 and just acceleration. No flat spots no hesitation no complaints just perfect.
    WOT 3-5k rpm 12+
    The top end 5k rpm upwards was too rich as predicted by the previous runs 125/155 = 130/170 so I used the 220 ACs with 1.14mm wires = 188 AC.
    The upper AFRs now run to mid 11s and the engine has just come alive. Too quick to watch gauges and drive at the speeds it can pull. I will have to datalog/histogram as I pull some more fuel out with larger ACs. 1mm wires will give ~ 196 AC which will be close. Pulling fuel after peak volumetric efficiency (cylinder filling ) is what really wakes up engines, but can also destroy them. I would like to just touch 13s at high rpm.

    Reduced some of the bubbles in the ET with some wire to reduce some early tip in richness. Does it work? I don't know but it may improve emulsion characteristics by reducing bubble merging at the AV opening.
    Mixed driving mostly 125 mains now 130 = 11 litres/100km = 21.4 US mpg

    The original ferrari progression holes provide a rich off idle fuel slug to help a heavy vehicle, that has probably not been warmed up, accelerate briskly into the traffic flow. A couple of pumps of the throttle an it just idles from cold! It achieves this well and for cruising to the shops is just fine. Accelerated bore wear from fuel wash was the next owners problem. I found the changes to the progression holes move the fuel delivery closer to where it is required for style of driving I enjoy. Now I wonder if there ever was a problem because it just works so well.

    I found the original Tuners article based on Holley carburettors as food for thought :-

    "In the thread “750 Holley carb help” Klaus made this statement, “On carbs it's very important that the correct two-phase flow gets established during emulsion. Otherwise you will see RPM dependency of AFR.” Thank you Klaus, but forgive me if I see your remark as a profound understatement. Incorrect two-phase flow is at the root of all this aggravation. People who have drill bits but don’t know why to use them have been molesting innocent carburetors for a long time. Now some of them are in charge of the manufacture of new carbs and they think they have improved them by using larger drill bits to make the air bleed and “emulsion” orifices. I guess the guys that engineered the original carburetors on the old muscle cars were pretty stupid or they would have “improved the emulsion” 40 or 50 years ago when they had their chance. After all, they had the awesome power of the single-point ignition system at their disposal, they shouldn’t have been afraid of a little soot.

    It is well documented that introducing air into the main well encourages low signal flow and can encourage or discourage high signal flow. The natural characteristic of a plain jet and nozzle (no air) is to get richer as airflow increases. The purpose of the air bleed system is to modify that behavior to accomplish a constant (or the desired) air/fuel ratio over as wide a range of airflows as possible. The particular ratios for power and cruise are realized by the selection of jet and rod or jet and auxiliary jet (power valve channel). The purpose of air bleeds is not to emulsify but to accomplish the correct fuel delivery. Emulsion is just a beneficial side effect.

    What I’m going on about here is Klaus’ remark about “correct two-phase flow”. That is the description of a fluid flow that is made up of a liquid and a gas flowing together in the same conduit. As the ratio of gas to liquid increases (more gas, less liquid), at some point the gas bubbles coalesce from many small ones into a few big ones and the flow starts to “slug” and become erratic. The carburetor nozzle spits like a garden hose with air in it when there is too much “emulsion” air.


    An emulsion of air and fuel has reduced density, surface tension and viscosity compared to fuel alone. This increases the flow of fuel considerably, particularly in low-pressure difference operation, at low throttle openings or lower engine speeds. Just how much of an increase (richer) is dependant upon where and how much air is introduced into the fuel flow.

    Mainly, what must be understood is that because the fuel discharge nozzle connects the venturi to the main well, whatever the low pressure (vacuum) is in the venturi, it is also the pressure in the main well. The air bleed is in the carb air horn or somewhere else where it is exposed to essentially atmospheric pressure, which is higher than the venturi pressure. This pressure difference causes air from the air bleed to flow through the emulsion system into the main well and to the nozzle. The flow of air can have very high velocities, approaching sonic in some orifices. The airflow literally blows the fuel toward and through the nozzle. A larger main air bleed will admit more air to the emulsion system and that can increase or decrease fuel flow to the engine. The size, number and location of the other air holes in the emulsion system, the size of the main well flow area, the size of the nozzle and the specific pressure difference at the moment are the determining factors. The ratios of air volume to fuel volume to flow area, with the air volume's expansion with the venturi velocity induced pressure reduction being the key. The bubbles expand as the pressure drop increases with airflow. Suck on an empty balloon to experience the effect.

    The fuel flow through the main jet is the result of the pressure difference between the atmospheric pressure in the float bowl and the venturi air velocity induced vacuum acting on the nozzle and the main well. The venturi vacuum in the well is reduced (the pressure is raised) by the "air leak" from the air bleed. This reduces the pressure difference that causes the flow through the main jet. If the air bleed were big enough, the pressure in the well would be the same as in the float bowl and no fuel would flow. Think about drinking through a soda straw with a hole in it above liquid level. Bigger hole, less soda. Suck harder, not much more soda. Big enough hole, no soda. This is the means by which the emulsion system can "lean it out on the top end". Incidentally, the vacuum that lifts water up a soda straw is in the most sensitive operating range for emulsion systems.

    It is in the lowest range of throttle opening, at the start of main system flow, that the effect of adjusting the introduced emulsion air (and its effect in increasing the main fuel flow) is most critical. Small changes can have large and sometimes unexpected or counter-intuitive consequences. The goal is to seamlessly blend the rising main flow with the declining idle/transition system fuel delivery to accomplish smooth engine operation during opening of the throttle in all conditions, whether from curb idle or any higher engine speed. The high speed and load mixture correction is usually easily accomplished, in comparison.

    The vertical location of the bleeds entering the main well influences the fuel flow in the following ways.

    1: Orifices above float level or between the well and the nozzle allow bled air to raise the pressure (reduce the vacuum) in the nozzle and above the fuel in the well. That delays the initial start of fuel flow from the nozzle to a higher air flow through the venturi and is used to control the point in the early throttle opening where the main starts.

    2: Orifices at float level increase low range (early throttle opening) fuel flow by carrying fuel with the airflow to the nozzle.

    3: Orifices below float level increase fuel flow by the effect of lowering the level of fuel in the well to the hole(s) admitting air. This is like raising the float level a similar amount (increases the effect of gravity in the pressure difference across the main jet) and also adds to the airflow carrying fuel to the nozzle. Locating the orifices at different vertical positions influences this effect’s progression.

    4: The "emulsion holes" influence is greatest at low flows and the "main air bleed" has most influence at high flows.

    In the first three cases above, once fuel flow is established it is greater than it would be with fewer or smaller holes. Visualize wind blowing spray off of the top of water waves. It doesn’t take much pressure difference to cause the velocity of the airflow through the bleed orifices to have significant velocity in the orifice, even approaching sonic (1100 F.P.S.) if the orifices are small. The phenomenon of critical flow is what limits the total air flow through an orifice and allows tuning by changing bleed size.

    Essentially, the emulsion effect will richen the low flow and the air bleed size, main well and nozzle restrictions will control the increase or reduction of high flow. Again, the desired air/fuel ratio is the primary purpose of the bleed system. "Improved emulsion" is an oxymoron if the modification of air bleeds to "improve emulsion" results in an incorrect air/fuel ratio in some range of engine operation. Correct proportioning of all the different bleeds (and, of course, the idle, transition and power circuits) will give the correct air/fuel ratios over the total range of speeds and loads and a flat air/fuel ratio characteristic at wide open throttle."

    Cheers

    Rob
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  7. fletch62

    fletch62 Formula Junior

    Mar 8, 2004
    333
    Fairhope, AL
    Full Name:
    Larry Fletcher
    Try 60 idle jets and 120 or 125 gam plugs (idle and transition air jet) this fixed mine.

    Larry
    cisflowtech.com
     
  8. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    Thank you Larry,

    If only I had known this then I would have probably stopped at step 5 rather than skipping it and trying tubes and progression circuitry.
    It has been interesting however.
     
  9. The Kook Abides

    The Kook Abides F1 Rookie

    Jan 4, 2011
    3,459
    #34 The Kook Abides, Nov 2, 2013
    Last edited: Nov 2, 2013
    Are the gam plugs removable from the top of the carb or are they internal? I see it listed on the previous page. Pierce Manifolds would have them?
     
  10. George Vosburgh

    George Vosburgh F1 Rookie
    Silver Subscribed

    Rob, Your explanation is so clear that, even for a relative novice as myself, I'm understanding the concepts. Has anyone ever tried to install a PMO system on a 308?
     
  11. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    PMO = no. Single 3 barrel casting, good progression geometry & float bowl set up. The IDA would be the racing ticket
    But...... the DCNF is a great road/ performance carburetor. Well designed AV booster and flow support to the 300+ Hp. It was delivered in the 308 to perform a role which it did well with the octane and energy density the fuel possessed at the time.

    Tuners understates the chaotic process in a single runner induction port. The most enlightening comment for me is the “spits like a garden hose with air in it” or the puking kettle analogy. This can happen in both the transition circuit (as I witnessed with the fibreoptic scope) and in the main ET circuit. I had not heard preignition in a 308 until I tried a 140/220 jet selection. Moderate load transition from 2000 rpm was almost impossible even with 15 degrees total advance. Black sooty plugs with a weird lean yellow & green striped earth strap. I assume large incombustible droplets puked out the AV with out enough air speed shear or mash on the throttle plates or heat in the manifold etc and gave a functionally “lean” mix.
    Many have observed the effect of the AC over the whole rev range which I think is true when the reduction in the AC improves ( reduces ) over all droplet size and increases combustibility and thus “richness” at the lower rpm. The exhaust note changes and it pulls better etc.
    Up to 200AC this doesn't seem to be a problem, and this seems to be where most ACs would need to be with the lower energy density fuels available.

    Increasing the idle air bleed size in early DCOE webers with few transition holes was used to richen the upper transition area !
    Hope that clears it all up ???

    Cheers

    Rob
     
  12. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    #37 911308, Nov 9, 2013
    Last edited by a moderator: Sep 7, 2017
    A quick way to test the Gam size needed is to place ~ 1cm wire into the air correctors.

    0.8mm wire = 127
    0.6mm = 138

    I can't test how this works on a standard progression circuit but I will use it for testing 34/36 choke & ET combinations prior to some dyno runs.

    Cheers

    Rob
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  13. Ferraripilot

    Ferraripilot F1 World Champ
    Owner Project Master

    May 10, 2006
    17,171
    Atlanta
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    John!
    Great work there. If you want to try and remove some of the tip in richness such as stomping on the gas pedal then you can go the next size smaller with your pump jet. Especially with F24 tubes, the pump jets dump quite a bit of fuel.
     
  14. itiejim

    itiejim Karting

    Dec 9, 2008
    81
    UK
    Full Name:
    James Ashton
    What a fascinating thread, a real education, particularly for someone with no experience of Webers other than trying to get mine set up correctly as per the excellent birdman tutorial.

    Looking at the spec of your car it seems very similar to a standard Euro like my (78) car. Recognising that I don't have the knowledge, skills or ability to undertake the learning journey that you have been through, do you think that your final set up would transfer well on to a standard Euro car? If not which modifications or changes from standard are the ones that you would recommend in terms of maximum benefit for minimum risk?

    James.
     
  15. The Kook Abides

    The Kook Abides F1 Rookie

    Jan 4, 2011
    3,459
    Thanks, Rob. Great pics.
     
  16. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    Updates – errors – speculation.

    Firstly a disclosure.
    I am now a progression modification fan boy and am totally biased (just ask me!) End mill the alloy plugs and........

    But for most owners the Gam change would be the least confronting. A 55 idle probably has enough fuel delivery but the delivery distribution is the problem. If more could be biased to the greater throttle plate angle with smaller Gam ( the theory) it would be better.
    The re drilled progression circuit provides a more even fuel delivery so I cannot talk from first hand experience with standard progression. I tried a 0.65mm wire in my Gam circuit and the result was dramatic! My normal cruise AFR 125/200 = 14.5 , 130/200 = 13.8 , with smaller Gam =12.0 everywhere. All the sparkle now covered in soot but certainly smooth and uncomplaining even with the 34 chokes under 1500 rpm.

    Errors – AC less than 155 doesn't allow the main circuit to come in with the smaller main. With my AC 170 or greater, the 125/200 takes WOT and is still too rich with 10.5 climbing to 11.2 at 7K. I have retarded the ignition curve to standard while I pull out fuel (sorry but these are the funky Ets that have a very flat fuel delivery profile so far). Just made some 118 mains to try.

    Speculation &#8211; Given the removal of the carburetor tops to check float levels, clean bowls etc then the insertion of a ~ 0.5 mm thickness wire gives about Gam <140 (normal Gam 150). Run this with the carburetors tuned to best lean and the fuel delivery might be swung in your favour by just enough. I am amazed that a 60 idle would also be required but remember not to swap out idle jets if those wires are sitting on them in the Gam holes!
    Feedback and AFR's from those trying this would add value to this thread.

    Cheers.
     
  17. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    Updates – errors – speculation.

    Firstly a disclosure.
    I am now a progression modification fan boy and am totally biased (just ask me!) End mill the alloy plugs and........

    But for most owners the Gam change would be the least confronting. A 55 idle probably has enough fuel delivery but the delivery distribution is the problem. If more could be biased to the greater throttle plate angle with smaller Gam ( the theory) it would be better.
    The re drilled progression circuit provides a more even fuel delivery so I cannot talk from first hand experience with standard progression. I tried a 0.65mm wire in my Gam circuit and the result was dramatic! My normal cruise AFR 125/200 = 14.5 , 130/200 = 13.8 , with smaller Gam =12.0 everywhere. All the sparkle now covered in soot but certainly smooth and uncomplaining even with the 34 chokes under 1500 rpm.

    Errors – AC less than 155 doesn't allow the main circuit to come in with the smaller main. With my AC 170 or greater, the 125/200 takes WOT and is still too rich with 10.5 climbing to 11.2 at 7K. I have retarded the ignition curve to standard while I pull out fuel (sorry but these are the funky Ets that have a very flat fuel delivery profile so far). Just made some 118 mains to try.

    Speculation – Given the removal of the carburetor tops to check float levels, clean bowls etc then the insertion of a ~ 0.5 mm thickness wire gives about Gam <140 (normal Gam 150). Run this with the carburetors tuned to best lean and the fuel delivery might be swung in your favour by just enough. I am amazed that a 60 idle would also be required but remember not to swap out idle jets if those wires are sitting on them in the Gam holes!
    Feedback and AFR's from those trying this would add value to this thread.

    Cheers.
     
  18. itiejim

    itiejim Karting

    Dec 9, 2008
    81
    UK
    Full Name:
    James Ashton
    Whilst trying to improve my knowledge on these carbs I came across the link below:

    TheSamba.com :: View topic - DCNF Fix

    The suggestion therein is that the stumbling throttle response whilst cornering is due to a design fault - which the author claims to have resolved with this mod (not sure I like the JB weld though).

    Anyone care to comment?
     
  19. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    #44 911308, Nov 16, 2013
    Last edited by a moderator: Sep 7, 2017
    The DCNF was a popular off road & dune buggy carburettor which suffered when fuel sloshed from the float chamber over the main stack air correctors causing a rich stumble. This is different to the lean transition faced in normal on road use with out the use of R compound tyres. Most stumbles in normal cornering show up as lean.

    The following AFR log shows the lean spike when trying to have both a lean progression circuit as well as a smaller main jet.

    32 choke 118/200 5 hole ET

    I will increase the mains to 120 and flow check ( ?underflowing jet causing miss - only seen on AFR, not obvious driving ).

    Cheers

    Rob
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  20. itiejim

    itiejim Karting

    Dec 9, 2008
    81
    UK
    Full Name:
    James Ashton
    Thanks for the feedback Rob, I think I need to get a Weber book on my Christmas list...

    Going back through this thread, and trying to pull out what's relevent to my standard '78 UK car, am I correct in assuming that the following may be a good starting point:

    32mm chokes
    130 main jet
    188 air corrector
    57 idle jet

    - all assuming standard emulsion tubes?

    Have you (would you) consider making a batch of your modified ETs for FChat members keen to learn and imprve things?!
     
  21. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    Hi James,

    I have been offered emulsion tubes from a 208 GT4 so I can reverse engineer my carburettors to standard components to find what works in a similar way (hope they are F24). I need acceleration baselines first and then I will start.

    32mm chokes 130/200 and 57 idle would be close but I will post the AFR logs with the different combinations which work in this car including Gam changes etc.

    Possibly a full menu of options by January.
     
  22. itiejim

    itiejim Karting

    Dec 9, 2008
    81
    UK
    Full Name:
    James Ashton
    Rob, wow, that'd be great. Do Maranello offer an honours system for recognising contributions to the marque?!

    I'll make sure that everything else is spot on and hope to see your results in the new year.
     
  23. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    Warning !

    Do not copy the funky E tubes. They work but for the wrong reasons !

    The F24 tubes are hollow with a 1mm internal channel. They don't use bubbling effect at lower rpm for transition, but use rising fuel inside and around the tube to meet and be sucked out by the masses of air rushing through the upper ET holes.
    This is in stark contrast to closed weber tubes where the fuel inside the tube is displaced downwards and out by streams of bubbly flow helping elevate & initiate transition.

    I only realised this when The funky tubes showed little sensitivity to changes in AC size, and the mixture variability ( misfire ) seen on AFR traces. The AC on these tubes turns out to be the 1mm hole in the ET. When I lowered these tubes ( no spacer ) the car just couldn't transition due to the fuel level rising and swamping the one hole .

    The modified less funky closed tubes transition well with short shifting and moderate load but time will tell. At least they should behave like “proper” weber tubes and be tunable for transition as well as WOT richness.

    Cheers

    Rob
     
  24. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    Correction

    The warning was right but for the wrong reasons.

    The DCNF E tubes work NORMALLY.

    The modified E tubes behavior was due to oversizing the AC to the ET air holes cross sectional area. Less than 1:2 and the AC control on fueling and transition is rapidly lost. It is possible the misfire with the 125/155 at WOT was a rich issue which the WBO sensor sees as oxygen and reads lean.

    You just can't believe everything you read on the internet.
     
  25. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    #50 911308, Nov 24, 2013
    Last edited by a moderator: Sep 7, 2017
    Update

    The majority of light load driving is done on the progression drillings and imparts a degree of character to an individual vehicle. The emulsion tubes also have a large role to play.
    Tuner's description :-
    “ It is in the lowest range of throttle opening, at the start of main system flow, that the effect of adjusting the introduced emulsion air (and its effect in increasing the main fuel flow) is most critical. Small changes can have large and sometimes unexpected or counter-intuitive consequences. The goal is to seamlessly blend the rising main flow with the declining idle/transition system fuel delivery to accomplish smooth engine operation during opening of the throttle in all conditions, whether from curb idle or any higher engine speed. The high speed and load mixture correction is usually easily accomplished, in comparison.”

    In the 40DCNF ferrari carburettors progression is often rich initially then weakens before main jet transition resulting in hesitation at modest engine loads. Full WOT loads result in prolonged deep dips in AFR (rich) before recovery and even swings into lean high rpm operation.

    I have been working through ET combinations. The best transitioning tubes required holes at fuel level ( to blow the fuel froth from the top of the fuel well down the AV ).
    Of the least modified tubes tested the standard F24 130/200 performs very well. This was only bettered by the F24 with the elevated second row of air holes.
    Cruise AFR is 13.4 – 13.6 down from 13.8 – 14.0 with the 125 mains, showing the mains starting to transition during cruise. WOT 10.8 – 12.4
    The standard F24 has slight hesitation when provoked in high gears or cornering. This was absent with the modified tube which just transitions ( bubbles ) quicker.

    The modified F24 may be all that is required to bridge the transition leanness in a standard progression circuit. These will be the tubes I will use for further evaluation with standard carburettors .

    Cheers
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