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Thread: Tech questions lean/rich mixture

  1. #1

    Tech questions lean/rich mixture


    First I'll describe my engine then I will list a few questions.

    Car is a '69 T non sunroof coupe, 205 16s, a 2.4 case 2.7 RSR pistons and cylinders, S heads, squirters, S cams, Webers and factory heat exchangers with a Sport muffler. tall ring and pinion I think 7:31
    and short gears in the trans. The cars does 0-60mph in 5 seconds flat even though I granny shift second. [ is there any other way with a 901 box ] The motor was professionally built for me in 1974. The motor has only 20K on it.

    My questions:

    Is there any other way to determine to see if the engine is running rich or lean other than checking the spark plugs.

    What problems will occur by running the engine too lean or rich.

    I read on another forum that it is best to lean your carbs when running megaphones.

    If exhaust cross over pipes are so important why weren't they used on the 908's, 910's or 917's. Or why doesn't Ferrari use them on thier present F1 race car.

    Megaphones, Why the different lengths for 2.0, 2.2 and 2.7, 3.0 engines.

    What is the purpose for a megaphone to taper out to a wider opening at the exit end.

    Any help is greatly appreciated.

  2. #2
    Some would say an O2 sensor and gauge would help, but regular, run-of-the-mill sensors are more accurate around 14.7:1 (which is after all, what they're designed for, not performance tuning). The only way to monitor the mixture accurately while driving is with a wide band O2 sensor and accompanying controller box, both of which are very expensive. An alternative is to run your car on a chassis dynamometer with an appropriate exhaust connection to a CO meter.

    Too rich results in loss of power, potential oil dilution and poor fuel mileage.
    Too lean results in loss of power, increased cylinder/engine temperatures and potential detonation. The terms are relative.

    There is only one air/fuel ratio that is correct for maximum power and only one A/F ratio for economy. Using a megaphone exhaust pipe (no muffler) will require testing to determine the correct A/F ratio for maximum power, and since we're probably looking for max. power, the A/F ratio will be toward the richer end of the lean-rich spectrum.

    If compared to a larger numerical ring and pinion, 7:31 is considered to be a "shorter" ratio; better for acceleration, worse for mileage.

    Crossover pipes are usually installed to increase low-to-midrange torque. Not sure on current F1 engines, but what constitutes mid-range with an rpm limit of 19,000 rpm? That could be a point 2 feet out of the pit box. In other words, probably not necessary with a 6, 7 or 8-speed gearbox (I could be wrong).

    Engine tuners change the length of megaphones, i.e. exhaust system in order to maximize power in a certain rpm range. The cone shape of a megaphone, according to engineering books, accentuates the negative pressure waves (created by opening and closing valves) which helps scavenge exhaust gases out of the cylinder, thus increasing volumetric efficiency (VE). My reference book says the drawback is that the power band becomes narrower which requires the engine (via the gearbox) be kept in this rpm range for maximum effect.

    Hope this helps a little,
    Early 911S Registry web

  3. #3
    Thanks for the answers Admin. Your time and help is greatly appreciated! I'm always willing to learn.
    Regards, Calvin

  4. #4
    Sherwood, on another board an early S owner posted that the factory muffler IS, in essence, a cross over pipe. Bingo! The light then came on for me. Of course it is...and that's one of the things that makes it so hard to improve upon, performance wise, with our MFI cars...This guy mentioned playing with a lot of the aftermarket mufflers and actually gaining a flat spot when he did...same thing that happened to me when (with no tuning of the injection) I bolted on an early sport muffler back in '75....
    Paul D. Early S Registry #8 - Cyclops Minister of West Coast Affairs
    "Now, to put a water-cooled engine in the rear and to have the radiator in the front, that's not very intelligent." -Ferry Porsche (PANO, Oct. 1973)

  5. #5
    Yes, I agree. However, Calvin's question was in regards to the factory's race cars (908, 910, 917) which, as far as I know, don't use a crossover pipe. I tried to explain why they may not have been used on a race engine that spends little time at low rpm.

    The mechnically-injected S engines seem to be particularly sensitive to exhaust backpressure (or whatever it is). Do MFI Es have the same symptom?

  6. #6
    I haven't heard...and it's a darned good question. Is it the MFI alone, or could the cam grinds be the difference? I lean towards the MFI, having read that it is a system sensitive to changes in back pressure. Good point...track cars don't mind having a narrow power band as long as they are geared for the track...street cars prefer a wider power band...
    Paul D. Early S Registry #8 - Cyclops Minister of West Coast Affairs
    "Now, to put a water-cooled engine in the rear and to have the radiator in the front, that's not very intelligent." -Ferry Porsche (PANO, Oct. 1973)

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