Air:Fuel Ratios
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Thread: Air:Fuel Ratios

  1. #1
    DX5
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    Default Air:Fuel Ratios

    What would be the optimum air:fuel ratio in a DS - specifically a DS23 injection, though I don't think bore/ stroke, compression etc. would have too much bearing on things so it would be applicable to all (19?) 21 & 23 D models...?

    Reason being I've been playing with an oxygen ('lambda') sensor and gauge. Apparently 14.7:1 is the ideal so called stochiometric mix. Then they say no old engine will run nicely on that so go richer. Maximum power for performance junkies being obtained around the 11 or 12:1 mark...

    As it happens Cindy was running as fuel rich as the proverbial Sheikh of Oman! Around the 11 or 12:1 mark in fact . Got a decent new pressure gauge on the fuel rail and backed the pressure regulator off a whopping 20% from 33PSI down to 26PSI - 2 PSI below the 28 recommended but I'd guess still plenty to atomise the fuel OK. I didn't want to tinker with the manifold pressure sensor just yet. That's made a big difference, lean if anything now which is what I wanted.

    With that done I have jury-rigged a potentiometer in with the engine coolant temperature sensor that enables me to alter the injection rate from a bit lean to quite rich as I drive. (The D-Jetronic alters injection duration based in part on the engine temperature). It is interesting to watch the mixture, it varies from extremely lean on overrun to quite rich on heavy throttle, when left to its own unbiased devices. I can override this by tweaking the potentiometer.

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    Question is is what figures are sensible? 13.5 was one target I thought sounded good. Rich enough to burn nicely, not melt anything or cause pinking (kind to the engine) but not wasting fuel or trying to get a few extra horsepower? It seems as though a crude 'mapping' may be ideal ~ a bit richer when labouring to climb hills or a quick get away, lean as practical for flat freeway cruising, let it go as lean as it likes on the overrun when the decel valve cuts in. {I'm not sure if injected 21's have the decel valve as instead they cut injection altogether...}

    I know at least one person has chucked the D-Jet system away and gone with a 'Megasquirt' set up, which I read with interest on 123 ignition's own website forum. I have made a new ECU loom for the D-Jet and come to terms with it, it's not a bad system really and so would be good to keep it reasonably original. Ultimately I'm hoping to set it up so the oxygen sensor is installed permanently and is wired by an interface circuit to the ECU temp. sensor connection so that it adjusts on-the-fly like a modern system.

    Just need to get an idea for what are good sensible air:fuel figures?

    As an experiment, we hooked the same oxygen sensor up to a mate's DS21 carby and went for a mix of city and highway driving. Effort has been put into tuning this car and it returns consumption that matches the book figures and romps along sweetly. That settled down coincidentally to a 13.5:1 figure very quickly after warming up . Like the injection it leans completely out on the overrun. It also leaned out fractionally under heavier load whilst at part load the mixture was slightly richer. In real terms it did not go outside 13.5:1 +/- 0.5:1 practically all the time. Opening of the secondary in the carb by wellying it did not affect the mixture too badly either. The mixture actually seemed stabler than the injected engine which is quite surprising (maybe the manifold pressure sensor on the injection is a bit sticky or something? )

    Apologies if this was a bit rambling but it would be nice to know what the best ratio(s) for a DS is to get the best efficiency. It seems they can get very thirsty quite easily if out of tune! Anyone got any suggestions or figures?

    Richard.

    '74 DS23 IE "Cindy" aka "The Low-Flying Chevrons" aka " @*@*$%%__##%*%* THING! "

  2. #2
    Fellow Frogger!
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    Default

    Nice one Richard.

    Firstly where have you mounted the lambda sensor? I imagine in the bend at the bottom of the down pipe would be ideal on the Dee.
    Techedge recommend it being 1m from the exhaust valves, given how long the exhaust is on a Dee and how prone it is to leaks I don't think that accurate figures can be obtained from tailpipe sniffing.
    I had similar results to you when I fitted the lambda sensor from my 2cv to the DS, albeit running LPG, it was much happier running richer than at stochi using seat of pants tuning.
    The only real way to find the most efficient numbers is on a dyno.
    Remember more fuel will reduce pinking and is considered safer as it also cools the combustion, to lean and you get pinking and can quickly overheat and cook a head gasket. It gets tricky without a lambda because to lean and to rich give the same sensations, hesitating and backfires.!

    Interesting read.

    Harley

    Ps. My 2cv is happiest (fastest) at 13.5 and will potter around town at 14.7 but get to 15.5 and it starts to hesitate. Confirmed on Dyno. Being aircooled it needs more fuel to help cool it!

  3. #3
    1000+ Posts PeterT's Avatar
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    Being injected, you should be able to have full load/full power at 12.8-13:1, then leaning out to 14.7:1 at light loads. Idle at around 14:1, or where ever it's happy (maximum vacuum). It's difficult to get an accurate idle AFR due to lack of exhaust flow.

    '92 205 Mi16
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  4. #4
    DX5
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    Thanks for the replies. At the minute, the sensor is in a venturi type adaptor which goes up aways into the tailpipe. I was concerned about diffusion into the ambient air at low gas flow (idle) so I haven't taken much notice of those readings - they do seem too lean in fact and I wouldn't have thought either vehicle would idle at all as the gauge was up around 16 or 18 mark at rest.

    For permanent mounting, the joint between the two manifold downpipes before the flexi is the only sensible and practical place to put the sensor on the DS (still not that easy!). Less than a metre from the ports but it should be OK. They say to add a bit of a heatsink if in such a hot spot, made from thick copper sheet or similar. Brass would probably work just as well.

    Interesting about the air-cooled needing richer mixture. There is a wealth of stuff about D-Jetronic in air cooled VW/ Porsche applications on the net. The temperature sensors etc. are different and have much more effect on the mixture than water cooled applications, must be for that reason...

    BTW I was getting hesitancy under load at anything leaner than 14.3 ish.

    Thank you,

    Richard.

    '74 DS23 IE "Cindy" aka "The Low-Flying Chevrons" aka " @*@*$%%__##%*%* THING! "

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