1. swisherred

    swisherred Well-Known Member

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    So what would be the optimum RPM to be running at on a 383/727 at say 70 mph....staying in the torque enough to pass with ease and a little get up and go but not kill the gas? I'm trying to determine the correct gearing/tire size for what I want. If I know what RPM to be at then I can figure out the rest. I'm looking at multiple scenarios, gears and wheels compared to my desired driving habits. No OD hurts on these 727s, but I cannot see spending the money on adding it via aftermarket.
     
  2. 68PK21 440.6bbl

    68PK21 440.6bbl Senior Member

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  3. BigblueC

    BigblueC Well-Known Member

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    My opinion, 2000-2500 is the preferred range running 70.
     
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  4. stubs300

    stubs300 Senior Member

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  5. 68PK21 440.6bbl

    68PK21 440.6bbl Senior Member

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    I remember that 3200 rpm was the sweet spot on my 68 440's whether it was the 6bbl or a 4bbl, but I also seem to remember with 3:23's and tall 15 inch Michelin X radials of 70's vintage that the speed was more up around the 80'ish range so maybe 2500 is better for 70mph.

    My 77 PK41 didn't have a tach and by then (in my 30's) I had many other things on my mind.
    :p
     
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  6. swisherred

    swisherred Well-Known Member

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    I know how to figure what rpm I'm running....and the engine specs....I'm asking what is optimum in 3rd gear....so that when I drop down for passing it puts me in a good rpm for the engine. The calculators just tell me what its at ....not what's good for it. Every motor has a spot its happy at....that's what I'm asking about for the 383.
     
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  7. LocuMob

    LocuMob Fluid Technician with a hat FCBO Gold Member

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    383/727 3.23 26" tall tire I'm running @2600 at 70.
     
  8. CBODY67

    CBODY67 Senior Member

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    Key question . . . which cam? What rpm for rated-peak torque?

    With a 2.76 axle ratio, the effective ratio in 2nd is 4.000. Plug that gear ratio into the calculators and see what it comes to for 70mph. See how that plays into the mph/1000rpm calculator.

    With a 2.76 axle ratio, it's comparatively got the 'OD" built-in, rather than having an extra OD gear and a 3.91 in 3rd and 2.76 effective ratio in OD.

    Even IF you put an OD automatic in, you'd need about a 3.73 axle ratio in order to be able to use the OD on the highway, at normal speeds. End result, you're pretty much where you'd be with the deeper gear and OD, now. At NO additional cost.

    With the H78-15/3.23 combination on my '70 383 "N" Monaco, I noticed that until it hit 62mph, the throttle response (using that famous "rch" measure), the response was "soggy", but got really "tight" at 62mph. Which would equate to about 2400rpm on the highway. The same rpm in the '66 Newport H78-14/2.76 is 68mph. Possibly that was the rpm where the converter dynamics had a greater degree of efficiency? On the '66, everything was the same at any highway speed.

    Understand, too, that the '66 had the factory 2bbl, and the '70 had the factory AVS. The '66 had the "large" torque converter and the '70 had the "small" torque converter, so probably not an exact comparison per se, but is an indicator of the rpm levels on each car on the highway.

    Given the dynamics of a normal street car, especially a heavier one with a stock stroke motor, that "passing gear" performance is more important to me than stop light grand prix performance. Certainly, off-idle response and power ARE important, but my priorities are more oriented toward "2 lane blacktop road" passing performance. Which allows 2nd gear to run out to about 90mph or more, quickly. With your 4bbl set-up, it would get there quicker than our '66 Newport 383 2bbl did.

    Enjoy!
    CBODY67
     
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  9. Snotty

    Snotty Senior Member FCBO Gold Member

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    These cars are never going to get good MPG. I got 12.8 a few weeks ago when I drove to Dunsmuir to meet Carrman for lunch. That's with a 440/727/2.76 SG combo. Would I like better? Of course. AM I willing to spend a lot of money trying to find a few MPG? No.
     
  10. swisherred

    swisherred Well-Known Member

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    Best I ever got in my coronet with a 2bbl 383 2.94 was 11mpg. I'm not exactly worried about the mileage nearly as much as I am about running the engine at the most efficient rpm while cruising the highway, and still dropping into a good rpm for passing. Maybe I should be asking what is the best rpm for passing at 70mph? With the 1.45 trans ratio i can figure the rest from there.
     
  11. stubs300

    stubs300 Senior Member

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    I'm having a hard trying to figure why you are so infatuated with trying to get a answer to a impossible question? Why is this so important to you?
     
  12. twostick

    twostick Senior Member

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    There are a lot of variables that can affect what you're asking.

    Assuming a stock engine in tip top tune, that is carb with a proper fuel curve and the same with ignition timing, plus a functioning vacuum advance, a vacuum gauge is your friend here.

    Take the car out on a nice calm day on a level highway and hold it at a steady speed, 70 being your choice here.
    Take a vacuum reading and it should be rock steady if you aren't moving the throttle. What ever that reading is will be pretty close to your MPG.

    2.76 with a 28"ish tire (235/75/15) will be around 2300 RPM and I suspect your vacuum reading will be 12-13" maybe a tick higher.

    A 3.23 will up your cruise RPM by 400 RPM but I think you might find it will take less throttle angle to maintain it and your vacuum should be 2 or 3 inches higher because of it. No way to be sure without trying it. If you have a 235/15 tire on it now and you can find a set of 14's to put on it, that would give you an idea if it would like a shorter gear or not.

    You can take this for what it's worth but I used to have a 79 F100 with a 300 stick and 2.75 rear and it was 10 MPG 10" vacuum on the highway. I swapped in a 3.50 and it ran 600 RPM higher but the vacuum increased to 15-16 and so did the MPG. It seemed like it was pretty busy there but it took very little throttle to hold it there unlike the 2.75 that you had to lean on it to maintain.

    Kevin
     
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  13. swisherred

    swisherred Well-Known Member

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    See....now that is exactly what I'm talking about. Even though the rpm is higher...the engine seems happier there ...I'm looking for where these engines run best but also the most efficient. I want to figure out what gears and tires I need to combine to get that happy medium...by numbers...not just what people run and are happy with themselves...but what the car seems happiest with. Some of these answers have been very helpful.
     
  14. 65Fury440

    65Fury440 Senior Member FCBO Gold Member

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    A stock motor is probably going to be happier closer to 2500 rpm than 3000 rpms.
    Back in the 70s and early 80s, it seemed like every 3/4 and 1 ton truck, before overdrive was big, was being driven on the freeway singing along at 3500 plus rpms, They just didn't last long.
    Having a 3 speed is just a compromise, I'd go for the taller gears , keep the rpms down until you need passing gear.
    You could look at charts and figures till you are blue in the face. Seems like the same engine, installed in a different car, can have a lil bit different sweet spot.
     
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  15. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    I'm having a hard time understanding why you don't understand the question.

    For a car to operate efficiently with good performance, it's important to know where the "sweet spot" is for going down the road. Some cars like a little more gear than others. If this was a stroked 440, you'd gear it differently than you would a 273, wouldn't you? That 273 might get better mileage with some 3.23 gears over a set of 2.76 gears because it doesn't work as hard. That stroked 440 ain't gonna like 4.30 gears out on the street.

    It's an intelligent question. Having never owned a big car with a 383, I don't have a good answer, so I stayed out, but if I had to make a guess, I'd say that the 3.23 gear would be a decent compromise between highway and performance. Having run 440 cars/trucks with 2.76, 3.23 and even 4.10 (my A12 car) gears, the 3.23 was always my favorite for a stock engine in a heavy vehicle. Good for towing and all around performance. My 300 has a 2.76 gear in it. It's not great for around town, but on the highway, it works well. Of course, I'm not towing and not being a lead foot in it either.
     
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  16. stubs300

    stubs300 Senior Member

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    Like you said in the rest of your reply. If you read what he first wrote, he left out so many variables, that it's impossible to give a honest answer. I give him a calculator to figure out what he's looking for with what he said, but he's to lazy to use it! Read it again!
    I don't know what rear gear he's using, tire size, etc., do you? Then he starts talking about the kickdown. Well, at what speed are you starting at to use the kickdown, how fast you want to get up to? The RPM's change with each scenario and speed, It's like asking, what the best speed on the freeway? It's impossible to answer.

    Swishered- I'll say it again, run some numbers with what you know with some of these sites and tell us what you come up with, as were not fucking mind readers and don't know WTF your looking for!
    rpm calculator gear ratio at DuckDuckGo
     
  17. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    He's trying to figure that out. I got it from his first post.
     
  18. stubs300

    stubs300 Senior Member

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    So then why you askin me why I'm having a hard time about it? You know your gears and your right on about the difference's. We don't have that info from him, so how are we to help him if we don't know or have all the answers? I'm not trying to start a pissin match with anybody, if y'all want to feed him garbage input without all the facts of what he's asking, go ahead! It just makes the board that more stupid spewing answers without all the facts. But go ahead and gangbang for it!
     
  19. Turboomni

    Turboomni Senior Member

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    Would it be possible to go to a dyno with a few different sized rear tires to try and see the readout or you can't use a dyno that way.?
     
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  20. 65Fury440

    65Fury440 Senior Member FCBO Gold Member

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    That's probably getting closer to the answer.
    If there was a dyno torque curve for OP's engine (or one similar), a guy could see where the torque starts picking up and where it's peak was. I'd think you'd any gear would put you in that area cruising at 70, then, when you kick in passing gear, you'd be somewhat close to peak hp.
    Probably not a whole lotta difference from what we already know, taller gear will hold passing gear to a higher speed, shorter gear will get up speed faster.
    Hey OP what gear do you have now? And do you have a factory 4 barrel car?
     
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