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  1. #1
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    Default Intake and carb suggestions

    Hey all, need some suggestions for intake and carb for my 91 5.0 302. Taking efi off and going carbed. Want something for street use that would go good with stock motor. Looking at edelbrock performer series or rpm air gap series along holly 600cfm carb. Dont want top of the line expensive but something better than my stock 84 302 parts.

    Thanks
    Last edited by Sask84gt; 05-20-2018 at 07:20 PM.

  2. #2

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    What trans and rear gear are you running?
    84 Cougar, 88 HO with 700DP, Edelbrock RPM intake, 1.7 RRs, shorty's and SS exh, T-5, KC clutch, Hurst pro billet, line loc, 8.8, 4.10s, suspension mods....blah, blah,blah.

    71 Comet, 289, Liberty TL, 9", 6.00s, 11.9x @ 112.... blah, blah, blah.

  3. #3
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    Quote Originally Posted by quickshift View Post
    What trans and rear gear are you running?
    Stock trans 84gt 5 speed with a 8.8 rear and 3.73 gears

  4. #4

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    I would use a regular Performer RPM and the 600cfm carb you mentioned. The air gap is more than a stock 5.0 needs, adds more cost and height. Really with stock heads, cam, and valvesprings a regular Performer would be a good choice. But, the RPM would be more flexible if you decide to add some performance upgrades in the future.

    Jess
    Currently own;
    1979 Mustang, v6 swapped to EFI 393, custom installed m122 blower, 4r70w trans, Megasquirt II, T-top swaped in.
    Previously owned;
    1990 Mustang, 545 BBF, C-4 with brake, ladder bars.
    1983 Mustang, 1984 SVO Mustang
    1984 Mustang convertible, v6 swapped to 351
    1986 Mustang GT, 1989 Mustang GT convertible
    1992 Mustang coupe, 4 swapped to 302

  5. #5

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    Quote Originally Posted by Mustang-junky View Post
    I would use a regular Performer RPM and the 600cfm carb you mentioned. The air gap is more than a stock 5.0 needs, adds more cost and height. Really with stock heads, cam, and valvesprings a regular Performer would be a good choice. But, the RPM would be more flexible if you decide to add some performance upgrades in the future.

    Jess
    ...what he said.
    84 Cougar, 88 HO with 700DP, Edelbrock RPM intake, 1.7 RRs, shorty's and SS exh, T-5, KC clutch, Hurst pro billet, line loc, 8.8, 4.10s, suspension mods....blah, blah,blah.

    71 Comet, 289, Liberty TL, 9", 6.00s, 11.9x @ 112.... blah, blah, blah.

  6. #6
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    Quote Originally Posted by Mustang-junky View Post
    I would use a regular Performer RPM and the 600cfm carb you mentioned. The air gap is more than a stock 5.0 needs, adds more cost and height. Really with stock heads, cam, and valvesprings a regular Performer would be a good choice. But, the RPM would be more flexible if you decide to add some performance upgrades in the future.

    Jess
    Ok, well we definitely want to add some performance stuff down the line so maybe the rpm is the better buy. Will I run into clearance problems with that intake and carb combo?

  7. #7

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    The performer rpm and the air gap are same height, for a 302. They will clear the stock hood with the stock dual snorkel air cleaner with a Holley carb and no spacer
    Last edited by Lowetlx; 05-21-2018 at 03:00 PM.
    84 LX Vert. 5.0 5speed canyon red on white
    99 cobra, electric green on medium parchment, vortech s-trim

  8. #8
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    Default

    I could be mistaken but I believe the Air Gap it higher than the RPM. The RPM will fit with the stock are filter assembly with about an inch clearance.

  9. #9

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    The 302 versions are the same height. The 351 air gap is taller than the standard 351 performer rpm.
    84 LX Vert. 5.0 5speed canyon red on white
    99 cobra, electric green on medium parchment, vortech s-trim

  10. #10
    FEP Member brianj's Avatar
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    I don't want to be THAT guy, but why ditch the EFI? I'm a carb guy, but I'd probably never swap over from EFI.
    1983 Mustang G.T. No-option stripper- I like strippers.
    5.0, GT40P heads, Comp Cams XE270HR-12 on 1.6 rockers, TFI spring kit, Weiand 174 blower, Holley 750 mechanical secondarys, Mishimoto radiator, Edelbrock street performer mechanical pump, BBK shortys, T-5 conversion, 8.8 rear, 3.73 gears, carbon fiber clutches, SS Machine lowers, Maximum Motorsport XL subframes, "B" springs.

  11. #11
    FEP Supporter Broncojunkie's Avatar
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    Quote Originally Posted by brianj View Post
    I don't want to be THAT guy, but why ditch the EFI? I'm a carb guy, but I'd probably never swap over from EFI.
    I can give some good reasons. These older efi systems work, but not without a lot of problems. The older parts are more prone to failure and not always cheap to replace, in some cases (IE: ecm's). Problems are harder to diagnose with the basically obsolete obd1 system. If a carb goes bad, you can buy a $60 rebuild kit and be back in business within a day. The old efi setup also depends more on wiring, often which is old and brittle in these cars. It's true that carbs are less efficient and require retuning from time to time, but let's face it... most of our old foxes are weekend/summer drivers that don't see many miles, anyway.

    If I were daily driving an old fox, I wouldn't depend on the old crusty oem efi and I wouldn't want a primitive carb. I'd use a new stand-alone efi system.

  12. #12

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    What about the EB Torker II, with 600, gives room to play...

  13. #13

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    Quote Originally Posted by Broncojunkie View Post
    I can give some good reasons. These older efi systems work, but not without a lot of problems. The older parts are more prone to failure and not always cheap to replace, in some cases (IE: ecm's). Problems are harder to diagnose with the basically obsolete obd1 system. If a carb goes bad, you can buy a $60 rebuild kit and be back in business within a day. The old efi setup also depends more on wiring, often which is old and brittle in these cars. It's true that carbs are less efficient and require retuning from time to time, but let's face it... most of our old foxes are weekend/summer drivers that don't see many miles, anyway.

    If I were daily driving an old fox, I wouldn't depend on the old crusty oem efi and I wouldn't want a primitive carb. I'd use a new stand-alone efi system.
    Soo true. I spent so much time and $$ diagnosing and fixing my junk cfi system. I got it all sorted out, but in the end it still didn't perform as well as my factory 4v intake and 600 holley combo did.

    Sent from my SM-G955U using Tapatalk

  14. #14
    FEP Senior Member roadkill's Avatar
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    Why not go with an EB Performer RPM and one of the new Plug-N-Play EFI systems?
    1985 Mercury Marquis LTS... "The Unicorn"
    1978 Fairmont... 306 and a C4.

  15. #15

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    Quote Originally Posted by Broncojunkie View Post
    I can give some good reasons. These older efi systems work, but not without a lot of problems. The older parts are more prone to failure and not always cheap to replace, in some cases (IE: ecm's). Problems are harder to diagnose with the basically obsolete obd1 system. If a carb goes bad, you can buy a $60 rebuild kit and be back in business within a day. The old efi setup also depends more on wiring, often which is old and brittle in these cars. It's true that carbs are less efficient and require retuning from time to time, but let's face it... most of our old foxes are weekend/summer drivers that don't see many miles, anyway.

    If I were daily driving an old fox, I wouldn't depend on the old crusty oem efi and I wouldn't want a primitive carb. I'd use a new stand-alone efi system.
    Agrees with most of the above... and with lots more money, and a tune-able EFI system that actually gets tuned with some brain-in-the-head tuning for cruise and light acceleration air:fuel ratios, sure, that's do-able...

    Less efficient... Ha! Politely begs to differ, agrees to disagree. Show me another tired, ~100hp weakling 3.8L V6 vehicle with 2.73:1 rear gearing and no overdrive, and with ANY kind of fuel injection, that AVERAGES 30+ MPG approaching 3000rpm on the highway, and I'll eat my shoes. Any fuel injection system loaded as heavily as above does nothing but add fuel, and at best if not loaded as such, peaks out at a frequently reached for 14.7:1 "stoichometric" air:fuel ratio, which has nothing to do with engine or fuel efficiency, but catalytic converter efficiency. A Holley carburetor (and others) can be fairly simply and very inexpensively modified to achieve the ideal 16+:1 air:fuel ratios that an efficiently running internal combustion engine likes very much, as do wallets at the gas station, and "sniffers" at the tail pipe/s as well. Given a more powerful and efficient engine and gearing, and my kind of situation only gets better.

    I have daily driven "an old fox" (1986) for 2+ years now, converted from it's lame and fuel-guzzling 2-bbl throttle body CFI system to a 1950's-tech "primitive carb", 1970's-tech ignition system, and repaired 1980's wiring where necessary... that has required very infrequent "retuning", and will fire up any time or with any weather with a bump of the ignition key, and briskly accelerates down the street at whatever throttle angle you want to press the go pedal to, and will get out of it's own way if pushed all the way down. None of any of those good functions were present prior to the reversion of "technology". Progress often comes by stepping back to simplicity.
    Last edited by Walking-Tall; 06-06-2018 at 11:57 AM.
    Mike
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
    1974 Pontiac Luxury LeMans

  16. #16

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    If you have a carb wizard like Mike helping you, you may get great results.

    As far as longevity of the EEC-IV multiport injection setups, there are few limits. I drove mine daily for 18+ years and the car is on the last 50K to reach 1/2 million miles. I have had very few problems.

    One ignition pickup
    one ignition module
    a few coil wires
    a few sets of plug wires and cap and rotor
    one map sensor
    two throttle position sensors
    Three sets of O2 sensorsó I have started changing them now when I swap T5ís out now

    Thsts about it as it relates to EFI specific parts

    my tips are

    buy Motorcraft parts
    Use good fuel from a reputable brand
    regularly run injector cleaner ó like every 2nd or 3rd tank.
    Replace fuel filters every 30-50K miles weather it needs it or not.
    Don't run the gas tank low as gas cools the fuel pump and they do burn up.
    Donít run a crazy big fuel pump, just adequate as fuel in the tank stays cooler and there is risk of vapor lock with highway driving on hot days otherwise

    I havenít had much by way of wiring problems. A fusible link years ago. ECU ground redo

    To me if the car was injected and itís not the antichrist CFI setup Iíd leave it EFI. If you want tuning do a simple MAF conversion


    If you really want to go carb, go for it. Youíll be happier if you swap out all of the wiring. You can get headlight to tail light for an 1985 5-speed manual carbureted car and go through and recondition and rewrap everything as you do your setup. Should give many years of easily serviceable trouble free miles.

    Personally you couldnít pay me to daily drive another car with a carb. My 86ís EFI has always been SO nice from a cold start and drivability perspective.

    that being said there is a place for a carb. One is going on my 1985 Saleen project car because thatís what the 85ís had. And for the record we are swapping most of the wiring harnesses to do it because the original ones were cut up.

    im looking forward to hearing that ol Edelbrock sitting on top of my Airgap and stroker moan just a little louder than the exhaust rumbles, but not by much
    Last edited by erratic50; 06-06-2018 at 11:43 PM.

  17. #17

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    There's really no wizardry... I've made all sorts of mistakes becoming good at what I am good at today.

    Back to OP's original questions... any dual plane small block Ford intake manifold will work fine, and along the lines of the "wizard" comment above, if you outline some details about the combination you want to put together, I'd be glad to offer some suggestions as to some ideal carburetor calibration for whatever carburetor (the bigger, the better) you happen to pick up.
    Last edited by Walking-Tall; 06-07-2018 at 07:45 PM.
    Mike
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
    1974 Pontiac Luxury LeMans

  18. #18
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    Quote Originally Posted by Walking-Tall View Post
    There's really no wizardry... I've made all sorts of mistakes becoming good at what I am good at today.

    Back to OP's original questions... any dual plane small block Ford intake manifold will work fine, and along the lines of the "wizard" comment above, if you outline some details about the combination you want to put together, I'd be glad to offer some suggestions as to some ideal carburetor calibration for whatever carburetor (the bigger, the better) you happen to pick up.
    Thanks, I will be sure to do that as I need all the help I can get lol. Thanks everyone for your opinions.

  19. #19

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    Cmon now, CFI is ... interesting! and when it works it is pretty cool and super simple. It only took me like 2 months to get it working and it has been going for several years now and across massive changes to my motor.

    Not that I would suggest it to anyone else but antichrist... no, more like 'special' or something like that.

  20. #20

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    I dont drive my mustang daily anymore, but it was completely fine driving it with a carb.

    I use a performer RPM intake and street avenger 670 carb, but I have gt-40p heads and a trickflow stage 1 cam
    Tony

  21. #21

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    Quote Originally Posted by Walking-Tall View Post
    peaks out at a frequently reached for 14.7:1 "stoichometric" air:fuel ratio, which has nothing to do with engine or fuel efficiency, but catalytic converter efficiency.
    Actually, that's not correct. Stoiciometry is used to determine the amount of products and reactants that are produced or needed in a given reaction. Stoiciometric ratio of 14.7:1 refers to the resulting reduction of non-ethanol gasoline, oxygen, and nitrogen into water, carbon dioxide and nitrogen, under the Ideal Gas Law. Under the IGL of PV=nRT, where P is pressure, V is volume, n is the number of moles of energy, R is the constant, and T is temperature, the most complete release of energy, under perfect circumstances, results in no waste heat, and no waste compounds. This is the peak of combustion efficiency, and would yield the highest cylinder pressure. However, other factors must be included and compensated for. These include, but are not limited to; thermal losses via the components of the engine, resulting in inconsistent combustion temperatures, forming unwanted compounds such as HydroCarbons, Oxides of Nitrogen, and Carbon Monoxide. The physics of the intake runner and combustion chamber greatly influence charge homogenization and flame propagation and duration, which contribute to changes in combustion chemistry as well. To compensate for these inefficiencies, the charge A/F ratio is manipulated to reach an effective combustion ratio of 14.7:1, again, for non-ethanol fuel. As alcohol content increases, the ideal stoic ratio drops, with pure ethanol being around 9:1 and methanol being around 7:1. Alcohol raises the octane rating, reducing the occurrence of spark knock (taking the place of tetra ethyl lead or methyl tertiary butyl ether), as well as aiding in vaporization to improve charge homogenization. Under different loads, different ratios can be utilized to improve vehicle performance, but not necessarily improve combustion efficiency. Driving the charge somewhat lean can raise cylinder pressures slightly to maintain torque while reducing fuel consumption, at the expense of elevated combustion temps, creating oxides of nitrogen pollutants. The introduction of an inert gas (recycled exhaust gases) partially fill the cylinder during intake, reducing its volumetric efficiency. This allows a smaller fuel and air charge to expand into a larger volume, lowering its temperature, preventing the formation of the nitrogen compounds. Under high loads, the ratio can be driven rich, utilizing the excess fuel (the charge will run out of oxygen long before the fuel is fully catalyzed) to absorb some of the combustion heat via the Latent Heat of Vaporization principle. The charge needs to be slightly cooled because of the opportunity for spark knock. By slightly lowering the charge temps, the mixture is much less likely to auto-ignite. If the engine were able to operate under ideal conditions, a catalytic converter wouldn't even be required. The converter's job is to finish what was started in the combustion chamber, but wasn't fully completed due to the operating conditions of the engine. The ceramic substrate of the converter is coated with a wash of certain precious metals, which, with the added heat, ionize the hydrocarbon particles, and cause them to adhere to the substrate via static electricity. Once excess oxygen is introduced, and if converter temperatures are sufficient, the hydrocarbons catalyze with the oxygen particles, producing water and carbon dioxide. So, a vehicles fuel management system is intentionally designed to deviate from stoic under certain conditions. In fact, one of those conditions is to intentionally load the cat with HCs, and then intentionally starve it, forcing it to 'light off', initiating the catalytic reaction. So, maintaining stoic will actually starve a cat, rendering it useless.
    Jim DeAngelis
    Cornucopia of Useless Knowledge
    Connoisseur of Dearborn Ferrous Oxide
    '83 GT hatch, currently under the knife

  22. #22

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    Quote Originally Posted by FB71 View Post
    Actually, that's not correct. Stoiciometry is used to determine the amount of products and reactants that are produced or needed in a given reaction. Stoiciometric ratio of 14.7:1 refers to the resulting reduction of non-ethanol gasoline, oxygen, and nitrogen into water, carbon dioxide and nitrogen, under the Ideal Gas Law. Under the IGL of PV=nRT, where P is pressure, V is volume, n is the number of moles of energy, R is the constant, and T is temperature, the most complete release of energy, under perfect circumstances, results in no waste heat, and no waste compounds. This is the peak of combustion efficiency, and would yield the highest cylinder pressure. However, other factors must be included and compensated for. These include, but are not limited to; thermal losses via the components of the engine, resulting in inconsistent combustion temperatures, forming unwanted compounds such as HydroCarbons, Oxides of Nitrogen, and Carbon Monoxide. The physics of the intake runner and combustion chamber greatly influence charge homogenization and flame propagation and duration, which contribute to changes in combustion chemistry as well. To compensate for these inefficiencies, the charge A/F ratio is manipulated to reach an effective combustion ratio of 14.7:1, again, for non-ethanol fuel. As alcohol content increases, the ideal stoic ratio drops, with pure ethanol being around 9:1 and methanol being around 7:1. Alcohol raises the octane rating, reducing the occurrence of spark knock (taking the place of tetra ethyl lead or methyl tertiary butyl ether), as well as aiding in vaporization to improve charge homogenization. Under different loads, different ratios can be utilized to improve vehicle performance, but not necessarily improve combustion efficiency. Driving the charge somewhat lean can raise cylinder pressures slightly to maintain torque while reducing fuel consumption, at the expense of elevated combustion temps, creating oxides of nitrogen pollutants. The introduction of an inert gas (recycled exhaust gases) partially fill the cylinder during intake, reducing its volumetric efficiency. This allows a smaller fuel and air charge to expand into a larger volume, lowering its temperature, preventing the formation of the nitrogen compounds. Under high loads, the ratio can be driven rich, utilizing the excess fuel (the charge will run out of oxygen long before the fuel is fully catalyzed) to absorb some of the combustion heat via the Latent Heat of Vaporization principle. The charge needs to be slightly cooled because of the opportunity for spark knock. By slightly lowering the charge temps, the mixture is much less likely to auto-ignite. If the engine were able to operate under ideal conditions, a catalytic converter wouldn't even be required. The converter's job is to finish what was started in the combustion chamber, but wasn't fully completed due to the operating conditions of the engine. The ceramic substrate of the converter is coated with a wash of certain precious metals, which, with the added heat, ionize the hydrocarbon particles, and cause them to adhere to the substrate via static electricity. Once excess oxygen is introduced, and if converter temperatures are sufficient, the hydrocarbons catalyze with the oxygen particles, producing water and carbon dioxide. So, a vehicles fuel management system is intentionally designed to deviate from stoic under certain conditions. In fact, one of those conditions is to intentionally load the cat with HCs, and then intentionally starve it, forcing it to 'light off', initiating the catalytic reaction. So, maintaining stoic will actually starve a cat, rendering it useless.
    Wow, Look who went all xctasy on us, lol.

    Jess
    Currently own;
    1979 Mustang, v6 swapped to EFI 393, custom installed m122 blower, 4r70w trans, Megasquirt II, T-top swaped in.
    Previously owned;
    1990 Mustang, 545 BBF, C-4 with brake, ladder bars.
    1983 Mustang, 1984 SVO Mustang
    1984 Mustang convertible, v6 swapped to 351
    1986 Mustang GT, 1989 Mustang GT convertible
    1992 Mustang coupe, 4 swapped to 302

  23. #23

    Default

    Quote Originally Posted by FB71 View Post
    Actually, that's not correct. Stoiciometry is used to determine the amount of products and reactants that are produced or needed in a given reaction. Stoiciometric ratio of 14.7:1 refers to the resulting reduction of non-ethanol gasoline, oxygen, and nitrogen into water, carbon dioxide and nitrogen, under the Ideal Gas Law. Under the IGL of PV=nRT, where P is pressure, V is volume, n is the number of moles of energy, R is the constant, and T is temperature, the most complete release of energy, under perfect circumstances, results in no waste heat, and no waste compounds. This is the peak of combustion efficiency, and would yield the highest cylinder pressure. However, other factors must be included and compensated for. These include, but are not limited to; thermal losses via the components of the engine, resulting in inconsistent combustion temperatures, forming unwanted compounds such as HydroCarbons, Oxides of Nitrogen, and Carbon Monoxide. The physics of the intake runner and combustion chamber greatly influence charge homogenization and flame propagation and duration, which contribute to changes in combustion chemistry as well. To compensate for these inefficiencies, the charge A/F ratio is manipulated to reach an effective combustion ratio of 14.7:1, again, for non-ethanol fuel. As alcohol content increases, the ideal stoic ratio drops, with pure ethanol being around 9:1 and methanol being around 7:1. Alcohol raises the octane rating, reducing the occurrence of spark knock (taking the place of tetra ethyl lead or methyl tertiary butyl ether), as well as aiding in vaporization to improve charge homogenization. Under different loads, different ratios can be utilized to improve vehicle performance, but not necessarily improve combustion efficiency. Driving the charge somewhat lean can raise cylinder pressures slightly to maintain torque while reducing fuel consumption, at the expense of elevated combustion temps, creating oxides of nitrogen pollutants. The introduction of an inert gas (recycled exhaust gases) partially fill the cylinder during intake, reducing its volumetric efficiency. This allows a smaller fuel and air charge to expand into a larger volume, lowering its temperature, preventing the formation of the nitrogen compounds. Under high loads, the ratio can be driven rich, utilizing the excess fuel (the charge will run out of oxygen long before the fuel is fully catalyzed) to absorb some of the combustion heat via the Latent Heat of Vaporization principle. The charge needs to be slightly cooled because of the opportunity for spark knock. By slightly lowering the charge temps, the mixture is much less likely to auto-ignite. If the engine were able to operate under ideal conditions, a catalytic converter wouldn't even be required. The converter's job is to finish what was started in the combustion chamber, but wasn't fully completed due to the operating conditions of the engine. The ceramic substrate of the converter is coated with a wash of certain precious metals, which, with the added heat, ionize the hydrocarbon particles, and cause them to adhere to the substrate via static electricity. Once excess oxygen is introduced, and if converter temperatures are sufficient, the hydrocarbons catalyze with the oxygen particles, producing water and carbon dioxide. So, a vehicles fuel management system is intentionally designed to deviate from stoic under certain conditions. In fact, one of those conditions is to intentionally load the cat with HCs, and then intentionally starve it, forcing it to 'light off', initiating the catalytic reaction. So, maintaining stoic will actually starve a cat, rendering it useless.
    Kind Sir... "stoichometric" is a "new math" term invented to skew the truth, and has done so with downright scary precision... the "ideal gas law" approximates the real behavior of gas, and it most closely approximates the real behavior of gas at low temperatures and pressures... neither of which are the case within the internal combustion engine.

    I stand by what I stated as 'correct' with what comes from what was previously stated to what you quoted, Jim. Practicality dictates the tuning of internal combustion above and beyond what EFI offers for the wide variations of the combinations of our present-day vehicles for nearest to perfection of internal combustion, and rises far and away above anything any of the automakers have provided since the 1980's and EFI, and their excuse for carburetion just previous to it as well... the much-touted "stoich" of "gasoline"...


    I refer you to NACA (yes, the previous connotation of present-day NASA) paper #49, that lays down the law of perfect internal combustion, that thereby makes the most use of the air and fuel ingested and minimizes any such harmful "emissions":

    http://naca.central.cranfield.ac.uk/...-report-49.pdf


    ... and a summary of the above from "tuner", an incredibly practically educated voice of reason that has been dialing in internal combustion engines for a very long time, incredibly successfully:

    "The clue here is rich idle, lean cruise, leaner (leanest, actually) part-throttle acceleration and rich WOT. The leanest is at mid-load, half-throttle or so.

    The tuning goal is to find the leanest mixtures which the engine will tolerate without missing or surging in level road cruise and moderate to intermediate acceleration. As load is increased engines will tolerate leaner A/F. Flat level road load will nearly always need to be richer than ľ or Ĺ throttle acceleration. In fact, the closer to WOT, the leaner an engine will run, although approximately 15% richer than 1λ A/F (12.5/1) is necessary for best power and engine safety.

    The first graph is from Walter B. Larew, “Carburetors and Carburetion” At the time he wrote his book on carburetors he was a retired Brigadier General who taught Military Science at Cornell, among his other accomplishments. He published this carb book in 1967.

    He didn't specify an engine type for this graph but his information is in the context of engines in general. His sources were most likely military aviation research. The math in his book is from NACA TR-49 and similar publications.

    This graph is representative of a richer part-throttle that may be necessary to tolerate with an engine that has radical valve timing and perhaps not so good A/F distribution at part-throttle.

    The thing to understand is most engines respond to being leaner than stoichiometric at part-throttle because the lean exhaust gas has hot unburned oxygen, and hot oxygen improves combustion.

    Using a WBO2 and a vacuum gauge to monitor this you will see improvement as you adjust the primary main jet to find the best A/F for moderate acceleration in the load range between a level road, perhaps 14-12 in. hg. and the point where the power valve opens, perhaps 8-6 in. hg. The engine's part-throttle acceleration will noticeably improve as the AFR is adjusted to the lean side of stoichiometric.

    When the air bleeds are configured correctly the A/F will progressively become leaner (from the rich idle) as the throttle is opened, until reaching the low vacuum load point where the PV opens.

    The key thing is, at moderate to mid load, engines will run lean and like it, and burn much less gas while doing so. They must be rich at idle and very low load, lean in the middle, and rich at WOT."
    Mike
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
    1974 Pontiac Luxury LeMans

  24. #24

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    I think we're on two slightly different, but overlapping, pages. The NACA paper you've referenced (I'm actually quite familiar with NACA, having done several research projects based on their publications) does not directly address gas functions within the combustion chamber, with the exception of some information on pages 632-633. The near entirety of the paper deals with gas functions within the carburetor and manifold, prior to the intake valve. I won't dispute any of that, as I have little experience with them, and the paper greatly describes and documents the math involved in those measurements, as the bulk of the paper is dedicated to the testing and mapping of carburetor flow and vaporization performance. My reference to stoic is a general chemistry reference, not directed at any specific engine operating condition. It's not 'new math' just a revised application of existing formulas. One thing I can definitely agree with is the paper's assessment of peak cylinder pressure at a maximum dilution of 15.5:1 (actually closer to 15.35, after doing some quick calculations). However, this ratio still yields small amounts of undesirable compounds, which in an automotive application, are unacceptable, for environmental purposes. A ratio of 14.7:1 yields the minimum of undesirable compounds, but less than maximum cylinder pressure. I guess I didn't say it very clearly in my previous post. After rereading yours, we're pretty much saying the same thing; engines like ratios richer than lambda for peak acceleration performance and stable idle, and less than lambda in nearly every other condition. Stoic is a target that is ideal, but not practical in an internal combustion engine. I only disagree with your assessment that catalytic converters 'need' a ratio of 14.7:1. This ratio only minimizes the need for them. Always great to have an intelligent conversation! Thanks!
    Jim DeAngelis
    Cornucopia of Useless Knowledge
    Connoisseur of Dearborn Ferrous Oxide
    '83 GT hatch, currently under the knife

  25. #25

    Default

    I'll add something possibly useful this time. Once you add 10% ethanol the new optimum ratio becomes 14.10:1. Not sure what your gas is like where you guys are, but around here most gas is 10% ethanol.

    Jess
    Currently own;
    1979 Mustang, v6 swapped to EFI 393, custom installed m122 blower, 4r70w trans, Megasquirt II, T-top swaped in.
    Previously owned;
    1990 Mustang, 545 BBF, C-4 with brake, ladder bars.
    1983 Mustang, 1984 SVO Mustang
    1984 Mustang convertible, v6 swapped to 351
    1986 Mustang GT, 1989 Mustang GT convertible
    1992 Mustang coupe, 4 swapped to 302

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