Engine What's The Best Rpm For 302?

Discussion in 'Classic Mustang Specific Tech' started by 67rcks, Aug 27, 2012.

  1. 67rcks

    67rcks Member

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    Just thinking about replacing 2.79:1 rear end for a slower gearing. What do you think is the best rpm that I should keep the engine running at (to minimize fuel consumption).

    I have wideband o2 gauge; is it uselfull in finding the best rpm for my particular engine (with several mods)?
     
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  2. Smitten65

    Smitten65 Member

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    When you say 'best RRM", that can have many different interpretations. USUALLY the best mileage will be near the engine torque peak but your actual mileage can be affected dramatically by the rear axle ratio choice. I.E. the load that the engine will see at that RPM. Using a wideband A/F meter's O2 sensor to tune the engine will allow you to maximize efficiency and power at any RPM. YMMV.
     
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  3. blown65

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    I would say 2000-2400rpm at cruise would be nice. 14-15AF at cruise.

    It it was fuel injected then you could play with timing to give the least amount of TPS% at that cruise speed.
     
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  4. rbohm

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    this is correct. it really depends on so much, cam timing, compression ratio, cylinder head efficiency, intake and carb combination, exhaust system, etc. if you are looking for best fuel economy, you want the engine running just below peak torque at the speed you intend on driving at most of the time.
     
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  5. BuckeyeDemon

    BuckeyeDemon Member

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    i would question statements about RPM concerning where the engine makes peak torque. i believe peak torque occurs approximately at an RPM where you maximize cylinder filling. most people probably think of those dyno charts where an engine is pulled with the throttle blades wide open and they can identify where peak torque occurred. with cracked throttle blades, will peak torque still occur at the same RPM? i personally wouldn't think so.
     
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  6. bartl

    bartl Member

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    You probably have the best mpg ratio now. Anything higher is going to probably move the engine rpm too low in high gear to drive the 2-lanes comfortably without shifting and/or more throttle opening and anything lower is going to raise rpm at higher speeds. I have a 4-speed with 3.00:1 gears and at 70mph the engine is turning 3,000rpm. On a trip of 150-200 miles at mixed interstate/2-lane/a bit of city traffic I can squeeze 20mpg if I don't use too much of the "go" pedal. That's a '85HO, OE '66 4V intake, Tri-Y's and a 4777 Holley DP.
     
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  7. Hack

    Hack Active Member

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    About 1,500 rpm with your current gearing is better than trying to run the speed limit. You want to run between 40-50 miles per hour. That will reduce drag from wind resistance significantly. Also, the skinnier tires the better with very high air pressure to minimize rolling friction (I recommend bicycle tires). Smaller diameter is also better. Never use the brakes - avoid stop signs and lights at all costs. Look for cops and ignore stop signs whenever you can. Stopping is really bad for gas mileage.

    Getting a little more serious - you can help a lot by adding EGR to your car. EGR also has no detrimental effect on performance.

    Let me know if you want any more tips to help get the best possible gas mileage from your classic car. ;)
     
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  8. 67rcks

    67rcks Member

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    With current 2.79 gears the engine makes only 2400 rpm at 70 mph at 4th gear (1:1), and I almost never use 5th gear... I have t5 and larger sn95 wheels 27.2" total height.
    The car is lazy at acceleration too.
    So what to choose 3.55 or 3.80?
    2236 or 2394 rpm at 75 mph?
    2041 or 2185 rpm at 35 mph?
    Should look for even a slower gear(4.11?)
    The engine is nearly stock with some add-ons (~9.2 CR, mild cam, 4777 holley, Al 2-plane intake, no emission systems)
     
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  9. Smitten65

    Smitten65 Member

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    With those tire, I would go with 3.8, 4.11 might make first gear a tad useless, but 3.8 would let you start using 5th gear. You would probably even get better mileage with the something in the 3.8 range instead of the 2.79's too, if you can resist the urge to stomp on it with the new found acceleration.

    Hack, you make me laugh! Thanks!
     
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  10. bartl

    bartl Member

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    Ah, the truth comes out.....an overdrive trans. With the T-5 I'd go with 3:55's.
     
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  11. Hack

    Hack Active Member

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    Makes a subtle difference, no?
     
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  12. horse sence

    horse sence Please don't call me Mom
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    in my coupe converted to a convertible we have a 5.0 HO with a t 5 the gears were 3.00 with 235-60 -15s wouldnt pull in 5 gear just fell flat changed the gears to 3.71s
    made low gear worthless might as well take off in second, changed the gears to 3.55s
    seems to be the best of the bunch
     
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  13. Smitten65

    Smitten65 Member

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    It depends on which 1st gear set the OP has in his T5. With a WC T5, he could have either 3.35 or 2.95 1st. A 2.95 with 3,7-3.8 ratio would be fine and with a 3.35 and his 27+ tires, he should still be okay (but not great, IMHO) in that range as he still has more than 1" on horse sense's tires. If he has a 4 cyl T5, that would be a 3.97 1st and that would be WAY too much 1st gear with anything above 3.20 and just about right with 3.00. 6 cyl's work pretty good with 3.20 rears and the 4 cyl T5 but the OP likely has a V8 unless he's dropped a 8" behind a 6 cyl. He is running 2.79's now which weren't available behind the 6 cyl carrier.
     
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  14. rbohm

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    yep, as subtle as a sledgehammer.
     
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  15. 67rcks

    67rcks Member

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    Just double checking the recommended rpm (2000-2400).
    As per mpg, my wideband shows 13:1 AFR at some 1700-1800 rpm, and this AFR continuously drops to 14.5:1 at 2700 (I assuem the AFRs are accurate enough).
    This is nearly 60% more cylinders fills for some 8% less gas in the mixture (if my math is right).
    So is it really a waste of fuel to cruise at this low rpm?
     
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  16. BuckeyeDemon

    BuckeyeDemon Member

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    i don't believe you are thinking about this correctly (if i'm understanding what you're trying to infer).

    you are reading AFR not fuel useage (your mixture is leaner at higher RPM but you are consuming more fuel because you are performing more cycles).

    on a side note you can lean out the AFR at 1700-1800 by adjusting your low speed circuits (i.e. idle jets).
     
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  17. SoCalCruising

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    Using my handy-dandy RPM/tire size spreadsheet and knowing that a 4" dia cylinder typically has max efficient burn at around 2000RPM, I would agree that the 3.55 to 3.8 range is where you want to be. I have 3.70s with a 2.95 1st T-5 and am happy with it. If you are leaning toward mileage over acceleration, then 3.55s sound pretty good (about 2050RPM at 70mph).
     
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  18. Hack

    Hack Active Member

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    The real answer depends on cam, compression ratio and other variables too numerous to list. Determining the gearing for best fuel economy for one particular car and driving route requires iterative experimentation. The experimentation would have to be conducted in a laboratory setting because the effects from variables such as driver inputs and weather will change fuel economy more than swapping rear gears. Also if your commute is anything like mine, traffic will completely confound the results.
     
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  19. zookeeper

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    I've found that rpm's don't always equate to good MPG's . My '68 has 2x4 carbs, so I won't even mention what it gets for mileage. However, my '88GT has similar engine,trans and gearing and actually gets worse mileage at 65 than it does at 75 mph. I believe that due to decent aerodynamics and relatively small engine torque, that engine combo actually hits the sweet spot at about 1900-2100 rpms when done on freeway cruises. My F250 however does not follow that logic. It gets liveable mileage at 60 mph, but the horrid aero of that vehicle drops the MPG's drastically above 70 mph. I suspect you'd want to shoot for 2,000 rpm's with your engine, trans and car package.
     
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  20. j69302

    j69302 Active Member

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    To fine my sweet spot, I temp installed a vacuum guage and zip tied it around my steering column. I changed gear/gas to get the most speed with the most vacuum.

    Which oddly enough, is in 4th. In 5th(od) The load (even at the same speed as in 4th) Is so great that the vacuum goes to pretty much zero which results in the carb just spilling fuel into the engine. So right now, I dont really use overdrive until I change my rear end gears to something lower than 2.79s :p.. Keep in mind, with my combo, in overdrive, 80 mph is at 1800 rpms. I am going to aim to get 80 mph up around 2200 rpm eventually.

    In overdrive, 80 mph, 1800rpms results in about 12-14 mpg.
    In 4th, 80 mph at about 2800-3000 rpms results in about 18 mpg.
     
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