A/F measurement with high overlap cams | Page 3 | FerrariChat

A/F measurement with high overlap cams

Discussion in 'Technical Q&A' started by pma1010, Apr 11, 2004.

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  1. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
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    Philip
    #51 pma1010, Apr 27, 2004
    The Weber manual has some interesting (for those of you interested in such topics) descriptions of emulsion tubes.

    First, the emulsion tube action is most pronounced at part throttle and gentle acceleration, i.e., when modest levels of vacuum are experienced in the auxillary venturis. This to my mind is analogous to setting the "cruise" tuning parameters in a programmable fuel injection set up. Emulsion tubes are designed to enrich the mixture. The issue is therefore one of how much enrichment action takes place. Thus a "leaner" ET provides lower levels of enrichment, rather than truly leaning the mixture.

    Comparing the "standard" F36 in many 40 DCNF set ups (and also the emulsion tube I have been running in my 44 DCNFs) with the F24 tube ("favored" by Russ, for example), shows the F24 to have a narrower outside diameter and ID and to have more and bigger holes in the top of the tube (C, D, and E holes) but to have fewer holes in the bottom of the tube (F, G, H). In fact, the F24 tube doesn't have an "H" hole at the bottom most part of the emulsion tube.

    So what? Well, its complex.

    The narrower bore will lead to a higher fuel level in the emusion well, so that, other things being equal, less vacuum is needed to pull the fuel from the well and the F24 will tend to enrich the mixture more than say the F36 for any given level of vacuum, and will tend to operate earlier than the F36.

    However, the larger air bleed effect from more holes in C, D, E positions at the top of the tube will tend to cause less enrichment action than the F36 at part throttle/low rpm [when the top of the tube is operating] but the fewer/smaller holes at the bottom of the F24 will cause more enrichment action at higher levels of rpm.

    The net effect is therefore the product of the ET bore change richening the mixture and the composite effects across the RPM band from the reduced enrichment at low rpm and higher enrichment at higher RPM.

    Net, net, I believe an F24 tube would enrich my mixture further, which is directionally inconsistent with where I am trying to go from the currect selection of jetting.

    [The complexity extends with the interaction between the emulsion tube and the (size of the) air corrector jet but we'll ignore that for now.]

    By contrast, comparing the F30 to the F36 emulsion tube, the F30 has a wider bore (OD, 6.2mm versus 5.9mm, equal ID of 3.5mm) leading to a lower level of fuel in the emulsion well (so more vacuum needed to provoke it into operation/less enrichment for any given level of vacuum). Secondly, the F30 has more and larger holes throughout the tube than the F36. This will tend to reduce the enrichment action at low RPM (top of the tube) and also at higher RPMs (bottom of the tube).

    From the one or two of you following this thread, my issue has been (or is) a rich mixture above 3000 rpm at steady throttle. The F30 emulsion tube should be directionally correct (reducing the mixture enrichment across the RPM band at part throttle openings).

    More to come.
     
  2. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    Russ Turner
    #52 snj5, Apr 27, 2004
    This is a great explanation by Philip, and I am certainly still learning a lot from him and many on the board. The interesting (and entertaining to me) thing I have noticed is that it is difficult to talk about one part without the other. While certainly a beginner still learning, I look at it as a balance of choke (venturi) size (chosen by a balance of response [air velocity] verses peak flow [max hp] ) supported by additive balance of idle+main+ET+AC - 'roughly' in order determining richness in progressively higher flow (rpm) . These are all affected by float setting, but I am keeping with recommended levels until I understand it better. The ET gives a lot of mid-range personality to the carb response as well as Philip aptly describes below. The next factor I look at are levels of enrichment at tip-in ('dumping the throttle') - for example, I have a slight hesitation when I dump the throttle at low rpm before it catches. While usually explained by a flat spot caused by too large a venturi, my dyno A/F data shows it is caused by a transient super richness suggesting I decrease my accell pump shot, and helped by a leaner over all base A/F. This was verified on dynos.

    Like Philip's racing cams, a new challenge with my new cams will be possible loss of vacuum signal at low rpm, a characteristic of 'hot cams' due roughly to overlap effect at low rpm. With this lower vacuum, Philip has had to enrichen his lower end (idle) and then compensate in the mid range with his P-6 factory competition cams. I will likely have to do this as well. I have intentionally jetted the car one size richer than required (55 vs 50) to empirically compensate a bit for installation. Exhausts as well affect the vacuum seen by carbs, especially with high overlaps, so that must also go into account when exhaust changes are made on carb'd cars.

    Anyway, this is how I, a beginner in this, initially approach a jetting problem fwiw to other Weberphiles. Will be looking forward to Philip's dyno and A/F numbers to quantify the effects.
    best
    rt
     
  3. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
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    #53 pma1010, Apr 29, 2004
    ...but not perfect!

    More data. Ran the car hard in 4th today all the way to redline and more (4th) and 7000+ (5th). This is with the F30 emulsion tube. Mid range still a little rich (~12.5:1), 6000 - 6300 about 13:1 (peak torque), and leaning to 13.5:1 by 7500. 8000 is still a little lean (about 14:1).

    Net, net, the emulsion tube leaning has had the expected effect, but I need to find one with fewer holes at the bottom (for richer top end) and more holes at the top (for leaner mid range).

    SOTP feels good but great to have data from the WB.
     
  4. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    #54 snj5, Apr 29, 2004
    Looks pretty damn good to me. If my dyno experience transfers, the difference in power will be very small. Simply terrific numbers. In the medical biz, we would say that from here on you'll be doing it for you as the car won't know much difference between the numbers. :)
    Again - brilliant Weber tuning with a challenging set-up.
    best
    rt
     
  5. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
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    #55 pma1010, May 3, 2004
    Car ran very strongly at Road America over the weekend.

    I also talked to Mike at Pierce. No e-tubes that fit what we need, so we are going to try a combination of downsizing the mains to 145 and decreasing the air correctors to 150. Should be able to report on change next week with data.

    By the way, for those of you who followed another thread on my chassis settings, the car has a really great balance, particularly in the Carousel, and Turns 7 and 1.
    Philip
     
  6. pma1010

    pma1010 F1 Rookie

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    #56 pma1010, May 12, 2004
    For anyone following this thread, I ran the car at Gingerman yesterday. Pulled the ACs and Mains and put in 145 mains with 150 air correctors. Cured high end lean condition, upper mid range around 13:1. Low end still too rich. Will see if we can lean idle jets and still get it to run when back at Gingerman this weekend.
     
  7. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    Russ Turner
    #57 snj5, May 12, 2004
    Hello from an internet cafe in London (my favorite city).
    Sounds great - 13:1 is perfect. Of course I do feel somewhat vindicated by the lower idle jet selection (inside joke here everyone :) ) I think you may begin a tight balance based on your idle. Do you still plan on an 800 - 1000 idle? Was just thinking you made need on the higher side to keep the vacuum signal up with the King Kong Cams. Would also be interseting to vary the low end advance a little if you can change your curve easily ( I'm spoiled by my new Mallory ). In a 2v, would think you could carry 12 or so and run up to 36 - 39 depending on gas. The 2v like a lot more advance than the 4v due to flame front properties.
    The great (not really) Carnac predicts: down 10 on idles to 60, hold at 150 on mains and slightly richen the a/c down 5 or 10 to 170 or 165. Idle advance at 10 - 12 with max advance of 36 - 37 at 3500 - 4000 rpm.
    Could make a better guess if we had an AF dyno, but that wide band you have is pretty terrific.

    Good luck, and following with interest.
    best
    rt
     
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