DCNF progression tuning | Page 5 | FerrariChat

DCNF progression tuning

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

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  1. bill308

    bill308 Formula 3
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    May 13, 2001
    1,160
    Windsor, CT
    Full Name:
    Bill Sebestyen
    #101 bill308, Jun 18, 2016
    Last edited by a moderator: Sep 7, 2017
    Jmaienza,

    Even the LM-1 readings bounce around a bit when measuring exhaust gas. It is steady when looking at clean air. At a 100 mph cruise I saw A/F ratios vary between 14.8-15. There may be other things going on too, but carb cars just do not meter as well as FI cars.

    I posted about the vacuum taps I installed and the soft plumbing I used to tie back into the charcoal canister. Below are photos of these items, the vacuum taps on cylinders 5 and 6 and the soft plumbing I put together to re-enable the charcoal canister.

    Also attached is a plot of my existing ignition curve9, what it would be with vacuum advance enabled, and how they compare to the R1 and R2 family of curves for a 1978 308.

    Bill
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  2. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    I was surprised how well they transitioned from the idle circuit. Very drivable.Full throttle from 4k felt like something epic was about to occur …. then it just didn't happen.
    Base line acceleration time with 34 chokes was 5.2 seconds 60-120 kph 3rd gear.

    145 main/175 AC AFR 10.5-11.2 accel time 5.5 : sounds dull & rich.
    145 main/200 AC AFR 10.8-11.8 accel time 5.8
    145 main/220 AC AFR 11.2- 12.8 accel time 6.0 : funky induction/exhaust note & flat spot at 6.5K despite AFR at 12.8 ( poor fuel delivery and burn with the large AC )

    I will try some 135-140 main combinations with AFR data logs before returning to a 32 choke comparison

    Cheers Rob
     
  3. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    #103 911308, Nov 16, 2016
    Last edited by a moderator: Sep 7, 2017
    I reworked the bowl and SSR similar to the image below to suit the larger intake valves installed 10 years ago. This has been largest gain I have ever made to this car with no more than 1cc of metal removal per port! As I mentioned else where, I gained power every where but freed up power over 7K which in retrospect was always elusive. The only other change was a conservative thinning of the DCNF throttle shafts.
    A big supprise was when I retarded the intake/exhaust cams to 100/100 (from 97/100) degrees LCA my elapsed time slowed to 5.4 from around 5.1 in cooler weather. Re installing the wideband AFR showed the previously jetted carbs now running at 11.2 – 11.4 at WOT every where through the rev range. With the 34 chokes I ended up with 130 main 200 AC (from 130/170) to gain 12.5 – 12.9 AFR WOT with near 15 cruise mixtures. Elapsed time back down to 5.2, but such a great 5-7K+

    It appears that if you improve the flow capacity of the cylinder head then the increased vacuum signal is transferred upstream to the carburettor. This “improved signal” requires less main jet to perform its fuel delivery. I wouldn't have believed it could have made such a large change.

    Cheers Rob
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  4. magnesium

    magnesium Karting

    Nov 15, 2003
    51
    #104 magnesium, Apr 3, 2017
    Last edited by a moderator: Sep 7, 2017
    At last back to the carbs and some real-world testing - it's been too long but the results are promising and it's starting to look like this particular carb adventure might yield the results I'm looking for. Just as a reminder, this is a near-standard euro-spec 308 carburettored motor with factory marelli electronic ignition. The only significant departures from factory are a de-restricted airbox and a (possibly) more free-flowing exhaust silencer.

    Apart from life and other projects, one major barrier to progress over the past year was the final failure of my old Camic in-car gas analysis kit. I have relied on this for many years and it has paid dividends in terms of sorting out tricky real-world problems that can be hard to reproduce on a rolling road dyno or a simple lambda sensor. Very long full power runs, vibrations and cornering effects are a struggle on a rolling road and so is transition tuning, no matter how well you think you can simulate it. So I'm including a couple of farewell photos to this old kit which didn't quite live to see the 308 transition sorted. Anyone into this kind of carb tuning should be able to see the attraction of driving with two big additional gauges in front of you and I had got used to the delay (compared to say a wideband lambda), turning it to good use by replaying the previous 10 seconds of driving in my head, watching the fuelling play out on the instruments.

    I replaced the Camic with a Sykes Pickavant 4-gas analyser with an electrochemical oxygen sensor and the improved accuracy this gives has been directly translated into a better running engine. It has taken some getting used to and for serious work, although this tool has primitive data-logging, it is essential to have an assistant. 4 gasses instead of 2 and a smaller display means that operating it safely while driving is a struggle single-handed.

    Photos here, results in the following posts...........
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  5. magnesium

    magnesium Karting

    Nov 15, 2003
    51
    32mm chokes
    130M
    170AC
    55 Idle
    155 IAC
    F36 ET
    45 PUMP

    A good drive and shake-down of the car, taking in a variety of roads - long straights, very twisty mountains, rough and smooth surfaces and varied driving from gentle cruise to extended periods of full throttle in 4th and 5th gears.

    Main circuit - I'd describe as getting close to perfect. Very wide power band and, when on the main circuit, from 4k - 7.7k , at no point did it drop below 5.3% or go above 6.2% CO. There was a very slight tendency to richen with high rpm so I'm going to try 175AC and see what that does. When I last changed AC's, a single step of 5 made little difference so I may need to try 180. I'm putting this need for increased AC down to the better signal from the aux venturi mods but it could be down to better measuring kit........ or even atmospheric conditions. These are carbs after all.

    Idle circuit - a very steady 960-970 rpm idle, a stable and clean-running off-idle transition. The car seems to idle best and give lowest O2 near to 3.5% CO, even slightly below. One cylinder on the rear bank is a bit less efficient than the rest but it's not too bad and I'll follow this up as it gets some more miles on it. From the transition point of view, the car has never been better. Mixture screws do seem to be more sensitive than before. Not too bad but it's an observation that's worthwhile. Often when I've encountered over-sensitivity in adjusting idle mixture, it is an example of oversize idle jets. That might not be surprising as the top end of the idle circuit is now quite lean, between 1.5 to 2% CO. i.e. I may have over-sized idle jets a bit to compensate for too much idle air correction. Although the transition hesitation is very much diminished and the off-idle stumble is non-existent, it is very clear that the car runs lean at very light throttle cruise before the main circuit is fully in. In fact, with the blocked-off third transition hole, everything has become much easier to follow. Previously, an off-idle stumble was followed by off-idle richness leading to a high idle circuit leanness....... or at least, that's what I think. New test equipment coming after the idle hole modification means I'll never know for sure what I'd have seen if I had used it prior to the mod. I do know that the car runs very much better though. One thing about this hand-held sykes gas analyser, it does seem to get through a lot of paper filters when driving on the road. I guess there aren't many modern instruments designed for testing dirty old carbed cars......... I might need to modify that for a bigger filter now.

    Next step is clear - smaller idle air correctors and test again. I'm feeling confident enough that this is the way to go so I'm going to pull out the idle air correctors (currently 155, press fitted into the carb body) and replace them with 137 which I'll make from brass stock in the coming days. If I need to enlarge them, I can drill them in-situ without risk.

    More tests soon......
     
  6. derekw

    derekw Formula 3
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    Sep 7, 2010
    1,520
    London, UK
    Full Name:
    Derek W
    Great work Rob. That step in the SSR you removed must have been causing some separation and turbulence. I will check my ports again and spend a bit more time near the seats as it seems to be worthwhile. I don't have a lot of jets (unless some of my DCOE stuff fits) so would welcome some suggestions for my 348 cranked 81.25mm bore with the cat cams and 10.5:1.

    cheers, derek
     
  7. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    Thanks for your follow up information. I remember I couldn't get the car up the driveway with the 55 idles so I went from 60 to 65 in quick succession. I could then trim back the idle/off idle mixture with the idle screws while the transition & cruise mixtures fell into the near 14 AFRs. I really didn't want to change the GAM air corrector sizes and found external adjustments satisfactory.

    I would venture a guess that the DCNFs on the larger engine/cam combination would run well with the 34 chokes and standard jetting (130/220 F24 combination or 140/200 F36) as a starting point,
    but you would have to creep up to the WOT jetting on the dyno/wide band etc.

    The more cam overlap and reversion, the fewer transition holes/size or smaller the idle jet requirement because of all the pulses and pumping effect in the transition circuit which enriches the mixtures. This may explain some of the wide variation in the idle jetting solutions which people have used including 45-50 idles on P6 cammed engines!

    To muddy the water further the modified transition circuit with the larger 65-70 idle jet seems to add the equivalent of 5 to the main jetting ie. 130 acts like 135 at higher rpm.

    Cheers Rob
     
  8. magnesium

    magnesium Karting

    Nov 15, 2003
    51
    #108 magnesium, Apr 11, 2017
    Last edited by a moderator: Sep 7, 2017
    There appear to be several sizes of idle air corrector for 308. The standard ones for 1978 euro spec are 155. Following on from my last post, I have reduced these in the hope of getting a less lean upper range of the idle circuit. It is a revelation and I'm very happy with it.

    I feel that I need to add a reminder each time I post to this thread because what I'm saying is both very specific and also part of a general contribution, started by Rob which can get you a much better running carb'd car. While others are working on modified engines, this one is near standard and original and this rather extreme pursuit of perfect running is not a case of fixing a non-existant problem. Petrol has changed, jetting needs to change and very fortunately some obsessive amateurs are better equipped than the factory was in the early seventies.

    Back to the idle air corrector jets...... Sadly these are not removable jets but are pressed in to the carb body meaning replacement is not just a simple task of unscrewing the old jet and replacing it with a different size as you would on, say, a DCOE.

    I looked at various ways of threading the carb body and using different jets and I carried out such modification on my spare (scrap) dcnf's but nothing was satisfactory. I ended up with very small diameter OD jets which had nowhere to seat or larger ones which meant significant modification of the carb lid. In the end I opted to stick with the factory system which implies, 1. an almighty groan if ever you need to reduce the jet size, 2. some very careful drilling and swarf removal if you decide to increase the idle air corrector size in-situ. But the good news is that it's possible and the results are absolutely worthwhile.

    Since I'd like to avoid repeating the process of making new idle air correctors, then pulling the old ones and press fitting the new ones into the carb body, I have opted to go for the smallest IAC size (what Rob calls the GAM) that I think I might need. If it's too rich, I'll enlarge it in-situ.

    @rob - What is GAM / what does it stand for? For me it's Idle Air Corrector. :)

    First, removal of the old IAC jets. This is rather like removing a very small roller bearing from a blind hole and I'd have liked a miniature version of a slide hammer / blind hole bearing extractor..... but in the end, I took advantage of the 1.55 mm bore size and threaded the old IAC's directly with a M2 tap. The tap was suitably greased and a lightly greased steel rod placed in the idle jet bore catches swarf that might fall inside. Once the tap was completed, an M2 screw was wound through the IAC, onto the steel rod (or a 4.5mm drill bit) and simply pushed the old IAC jet out of its bore. Once completed, blasts of cleaner and compressed air got everything clean...... In the end this was not as nerve racking as I thought it might have been and all the old IAC came out rather easily. I made an installation tool and the last photo shows one of the new IAC in place. 137 down from 155. That's a pretty big area reduction........ results in a following post.
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  9. magnesium

    magnesium Karting

    Nov 15, 2003
    51
    My last post on this thread was back in April when I first installed the smaller IACs. I promised a proper report of how the car drives.....

    It's so easy to say exaggerated phrases such as "it's like it's fuel injected" or something along those lines but really, it's getting close to that quality of running even if there is still a little improvement to be made. Maybe I'll always be looking for improvements however well it drives.

    Since April I've made a few more extended test runs and some little tweaks to jetting but mostly I've just been driving and enjoying. My overriding impression is that it is so damned good and that perfection is frustratingly within sight.

    Enough emotion, here's the latest jetting and observations:


    32mm chokes
    130M
    175AC
    56 Idle
    137 IAC
    F36 ET
    45 PUMP

    Spot the difference?

    The really big difference is the 137 IAC, down from 155. I made that change in isolation and wow, what a difference it's been. To anyone thinking of doing this without the transition hole modifications, I doubt very much that it would work although I'd be really intrigued to find out what it does. But of all the other things I've done to the carbs and jetting, this has made the biggest single difference. When I started the engine for the first time with the smaller IAC and revved it progressively to 3.5k, there was an immediate exchange of glances between myself and my assistant: it was so obvious that this engine was grateful for the extra fuelling at the high end of the idle circuit. Never had it run this well stationary. Having now driven and tested on the road with the 4 gas analyser, I can say that I'd actually go a little smaller if I were to do it again. Maybe 135 or 133. Those are small differences and of course depend on engine spec and what you're doing with your ignition but the ballpark for a standard euro engine must be in the 132-137 range.


    The observant will notice the increase of AC from 170 to 175. I agree entirely with Rob's observation that "the modified transition circuit with the larger 65-70 idle jet seems to add the equivalent of 5 to the main jetting ie. 130 acts like 135 at higher rpm". In fact, I'd say that a particularity of the dcnf compared to other performance Webers is the degree of interdependence of one circuit on another. A change of main jet has a very significant effect on the idle circuit (fuel passing via the main to feed the idle jet) and the idle, progression circuits still have a noticeable effect very high in the rpm range. In any case, going to 175 with this spec really puts the high end mixture back into an ideal range.

    Even more observant? No, it's not a typo going from 55 to 56 idles. That's what I'm on right now although that part is still subject to small change. Think that such small changes are not noticeable? Try it, back to back with 55, 56, 57 (and I'd previously tried 52 and 60 too). None of these idle jet options is perfect, in part because the influence of the idle circuit covers such a large rpm range. I can choose where the perfect mixture and perfect running lies and in this case 56 puts it in a place which fits very well with the transition and main fuelling. Could I get that much closer to perfection with say a 54 idle and 133 IAC? Maybe but because what we're talking about here is light throttle running and transient driving characteristics, there's also scope to change other things, not least tweaks to the ignition. When those are finished, I might well come back and say that "driving with ear defenders, you'd think it was injected". In the meantime, I have to content myself with a friend telling me how impressed he was with the low down torque of the 308 engine. It's not a compliment you hear often and it's not at the expense of the top end.

    For now I'm happy and, if I'm going to make further changes to the idle jetting, it's not this year. I might come back to this thread and post a bit of video at some point though. In case I don't, I should say thanks again to Rob for being the final catalyst in making me attack the idle circuit in this way. It has been very rewarding, worthwhile and absolutely necessary. It's not the first time that I heard about successful transition circuit modification on 308s though. In 1989 I met a woman who owned a 308 and whose boyfriend had carried out similar modifications to her car. As anyone who has followed this thread knows, it's a passionate thing.
     

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