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Thread: 68S - how rich / lean should the fuel air mix be?

  1. #1
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    68S - how rich / lean should the fuel air mix be?

    Had my car dyno’d today, power was 152.5 torque 127; fuel mix was 9.3 .

    Tester suggested mix was too rich, is he right? What adjustments can I make? It’s a stock 911S with K&N filters (in case that matters). Running Esso super which I understand is 0% ethanol 99 octane (am uk based)

    Thanks Rich
    Rich
    68 911S
    mbr. 2393

  2. #2
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    14.1 to 1 is the theoretical best air / fuel ratio . For older 911's mid 13's to 1 is good . 9.3 to 1 is too rich . If this is true , the carburetors need smaller main jets and possibly bigger air correction jets .

  3. #3
    Senior Member NorthernThrux's Avatar
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    Quote Originally Posted by Richy View Post
    14.1 to 1 is the theoretical best air / fuel ratio . For older 911's mid 13's to 1 is good . 9.3 to 1 is too rich . If this is true , the carburetors need smaller main jets and possibly bigger air correction jets .
    14.1 is for perfect combustion but it is rarely best for horsepower or torque. If you really want to optimize you need to do it on a dyno. My ‘73S with MFI runs from 12.8 to 13.1 accelerating across the rev range. Typically you want a little rich at WOT and can tolerate a leaner mixture at steady state cruise with low throttle openings. Regardless, 9.3 is way rich and you will foul the plugs eventually.

    Ravi
    Early 911S Registry # 2395
    1973 Porsche 911S in ivory white 5sp MT
    2015 Porsche Macan S in agate grey 7sp PDK

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    MFI engines need to be a little richer than carbureted engines , so I think mid 13's at steady state and lower throttle opening is a good number for engines with carburetors .

  5. #5
    Senior Member NorthernThrux's Avatar
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    Quote Originally Posted by Richy View Post
    MFI engines need to be a little richer than carbureted engines , so I think mid 13's at steady state and lower throttle opening is a good number for engines with carburetors .
    Off topic, but I'm curious if anyone knows why. I have heard repeatedly that MFI needs to run richer than stoichiometric and the dyno tuning on my car (an many others) also bears this out. But I've never understood why. MFI should atomize way better than carbs and my experience with EFI on motorcycles is that good atomization allows one to run leaner.

    Ravi
    Early 911S Registry # 2395
    1973 Porsche 911S in ivory white 5sp MT
    2015 Porsche Macan S in agate grey 7sp PDK

  6. #6
    Ravi. It Has absolutely no bearing if it’s mfi or carb, beetle motor or a 356 1600.
    They all tune the same. Max full throttle power peaks between 12.8/13.1 and partial throttle cruising for best response @ 13.5 zone. I usually find the max torque zone likes a slightly leaner 13.4/5 than max HP.
    I usually try too tune the idle around 13.7/8
    Regards
    [SIGPIC][/SIGPIC]Robert D. Groß

  7. #7
    Senior Member frederik's Avatar
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    I believe that the "MFI should run rich" mantra is generally to hide other problems. My car ran way better when it was leaned out to a normal A/F ratio exactly like Mr. Gross describes above. And you get less fuel dilution of the engine oil. The thing is that MFI will still work when set way too rich because of the high fuel pressure, but this is far from ideal.
    1972 911 2.4T Targa Aubergine (MFI)
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  8. #8
    Quote Originally Posted by Richsmith901 View Post
    Had my car dyno’d today, power was 152.5 torque 127; fuel mix was 9.3 .

    Tester suggested mix was too rich, is he right? What adjustments can I make? It’s a stock 911S with K&N filters (in case that matters). Running Esso super which I understand is 0% ethanol 99 octane (am uk based)

    Thanks Rich
    Most wide bands will not register below 10 and if its true that it read 9.3/1 it would have been smoking black like a diesel and I highly doubt it would have pulled the power it did or even maxd out for high rpm .
    Id sooner suspect a sensor or gauge error.
    Regards
    [SIGPIC][/SIGPIC]Robert D. Groß

  9. #9
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    Cannot get the picture of the dyno test to upload, summary of results are 13.5 @8k ; 12.75@7k; 13@6k; 11@5k; 13.5@4k; at 3000 rpm the mix drops from 11 to 9.3 when the throttle is applied.
    Rich
    68 911S
    mbr. 2393

  10. #10
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    Looking at the diagram on page f36 in book one of the workshop manual, Is the solution to tighten the idle mixture control screw / and /or open the air adjustment screw?

    Thanks
    Rich
    68 911S
    mbr. 2393

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