Gear Ratio and RPM deciding on gears

Mopar General Discussions

  1. Oldmanmopar

    Oldmanmopar Going left turning right FABO Gold Member

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    Rule of thumb for street performance is multiply the tach by two. 3000 @ 60 is what many cars were for the street .. My 70 RR with a 354 Dana 4spd and 275 60's was 3000 @ 60 It was a good driver and still quite quick. I can remember the tach and speedo were both straight up at 60 in 4th



    I would say tire size should be your first choice. You can by and sell gears easier then you can sell used tires when their hanging on the quarters. LOL
     
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    • 318willrun

      318willrun Utube channel 318willrun

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      yeah, I mentioned that in post 11. To the OP, if you have an issue with your engine or tune, do NOT try to compensate the problem by throwing gears, or anything else at it. IF you just want to move the car out of the hole, put a 8" 4800 converter in it. That should help it out under 3000 rpms.
       
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      • Mattax

        Mattax Just the facts, ma'am FABO Gold Member

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        Gotta wonder about the 904. Is it there because it was available, or is there because someone wanted a lighter transmission and did the work needed for it to survive racing and hard use?

        Lots of good suggestions in this thread.
        I'm with A-bodyJoe - install the diff already owned, and run it.
        And with A/J et al to then adjust the timing and/or get better fuel.
        Drive it and see how it is.

        Honestly, if there was no time crunch, and there obviously isn't, I'd start with a simple compression check and then measure cam lift with a dial indicator and preferably a degree wheel.
         
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        • 318willrun

          318willrun Utube channel 318willrun

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          yeah, and really all we have is "it's a 340 that's lazy under 3000 rpm's". We don't know what that means. Was the expectation to pull the front wheels up 2 ft and can only pull them 4" ??? Or did he get beat in a stop light war by a '82 dodge truck with a slant six? Who knows..... LOL
           
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          • Oldmanmopar

            Oldmanmopar Going left turning right FABO Gold Member

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            I was just thinking of my post above and it took me back to the 70's. Those were the days you could take used car test drives on the new car lot's. They would hand you the keys to anything but a HEMI, LS6 or 429CJ. You could choose what ever gear ratio you wanted to test out, They were all over the lots. I myself would always look for the low geared 4spd cars to practice pulling gears.

            Today I'm 65 years old. 456's 29-31 inch tires and unlimited RPM's are what brings back the old day's. LOL

            005 (2).jpg

            008 (2).jpg
             
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            • 340inabbody

              340inabbody Well-Known Member

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              Hi AJ thanks for taking the time to do this I really appreciate it! Not sure why the curves are not linear either. I used the Spicer online calculator which seemed very simplistic. I do have a lot of critical unknowns (engine specs and converter). I do know I need to get the Sure Grip in with something other than the 2.71's. I want to have fun around town and prefer to drive more lower end torque with this more than cruise comfortably. I don't care about noise, in fact I like the roar of the 340 and would rather have it loud than quiet. My VW is quiet if I want that. I will replace the converter after I get the rear gears done and re-tune the car against the new gearing.
              I think the issues are I have a aggressive four speed motor with mechanical advance, steel heads and a high rise manifold that is not a great set up for an automatic. This set up looks like more of a drag set up with poor low end response due to what I mentioned. At some point I will change out the cam but in the mean time the 4.10's I think will help with a tight high stall converter behind it.
              Thanks again for the post I still have to read it over a few times to understand it as I am still learning about this.....
               
            • 318willrun

              318willrun Utube channel 318willrun

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              Going from 2.71's to 4.10's will 100% help the car move from a dead stop. Put the 4.10's in and smile and be done. :)
               
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              • RustyRatRod

                RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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                Or necessarily be correct. Much like all the static and dynamic compression calculators. A computer can only do "so much".
                 
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                • RustyRatRod

                  RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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                  I guess we all just need to shut the hell up. AJ's the only one who gets thanks around here.
                   
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                  • QuickDart360

                    QuickDart360 Well-Known Member

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                    Cool! Is this with a 3 speed? I was running 3.55s and 28" tire on my dart sport 360 and seemed a little wound up on the interstate. Also was a little tough keeping up with the other cars! Now have 3.91's (which I haven't run yet.) Just curious.
                     
                  • 12many

                    12many Well-Known Member

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                    Auto no overdrive. What I have seems reasonable to me is all I can say. It’s possible I may seem unreasonable to others though:eek:
                     
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                    • AJ/FormS

                      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                      Here is a great SBM street combo;
                      3.23s with a Commando 3.09 low and a .78 od splitter.
                      60Mph is 5800 in First-over, which can be powershifted into. On the shift, the rpm drops to 0.78x5800=4500, so that is a 1300rpm powerband; which you might split 35/40% over the peak and 65/60% under (just guessing) to get low-ET. The overall gear to do this with is 7.78; remember this number. and the starter gear is 9.98. Then;30 mph is 3720rpm in first.
                      BUT: the only way to get to 7.78 with a 1.45 second gear is; 7.78/1.45=5.38s..........
                      The engine doesn't care how you get the 7.78.
                      With an automatic;
                      Could be 7.78/1.45 =5.38.
                      Could be 7.78/2.45=3.23s (first gear now),
                      Could be 7.78/1.54=5.05. (A999Second) suggest 4.88s
                      Could be 7.78/2.74=2.87s,( A999 low) suggest 2.94s
                      With an A833
                      Could be 7.78/1.92=4.10s; badaBoom!
                      Could be 7.78/1.77=4.40s, (suggest 4.30s) I really liked those.
                      Could be 7.78/1.67=4.56s The overdrive box. Sounds good but low is then 4.56x3.09=14.09 which is ludicrously low for a streeter, and then you get the 1-2 rpm drop to 54%, which is just a killer; .54x5800=3130. Yur hot 340 got torque down there? Not hardly,lol.
                      The more gears, the better, but sooner or later first gear becomes useless(see above); and by experience I can tell you that with a hot 360, three gears is too many. Yes I tried it,lol.
                      Now all you gotta do is build the engine to make peak power at about 5800 less perhaps 300= 5500 ; lessee I see that as about a 235@.050 and yur done.
                      Will it be quick?
                      Not particularly , until you figure out how to keep it from spinning the tires the whole way.
                      Will it be fun?
                      Are you kidding!
                      I'd say 11, on a 1-10 scale. 15 if in a corner. 20 if you can control it at full lock, and don't wipe it out;Yahoo!
                       
                      Last edited: Jan 18, 2020
                    • 340inabbody

                      340inabbody Well-Known Member

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                      yup 3 speed auto 904A when you say wound up I assume you mean too high revving right? Do you recall any speed/RPM's?
                       
                    • GreyWizard

                      GreyWizard Well-Known Member

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                      Here is a useful calculator to play with different scenarios.
                       
                    • Oldmanmopar

                      Oldmanmopar Going left turning right FABO Gold Member

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                      Wow this was like reading a book. But I agree 3500 converter and a set of 323's and she should turn the tires for 1/2 block in low. By a converter is first on my list
                       
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                      • QuickDart360

                        QuickDart360 Well-Known Member

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                        Well I don't have a tach to know rpm's but by ear engine seems to be up there in rpm's. But I don't drive too much on interstate. I could say it's livable but my exhaust also plays a role in how it feels/sounds.
                         
                      • MopaR&D

                        MopaR&D Nerd Member

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                        If you are running an 8 3/4" rear and the 4.10-geared 3rd member is ready to go I'd just put it in and see how it runs, doesn't take more than an hour or 2 if you know what you're doing. If you don't like it then get some 3.55s.

                        Just a ramble about gearing... Around here the freeway speed limit is 75 and I like to cruise 75-80 which puts my Duster's 360 just under 3000 RPM with 2.94 gears and 28" tall tires. I plan to put in 3.55 gears because that's what my cam was custom-ground for but then I won't feel so eager to cruise at 80 mph. I also go to events at a track about 160 miles from me so my solution? I got a 1972 Dodge D200 to tow it. 17 MPG on the freeway is what I get currently with the Duster, it'll only get worse with 3.55s. No I don't think I'll save any money on gas in the long run (I doubt the truck will do better than 15 MPG when towing) but I really really don't like driving a car for 2+ hours with the engine at 3500 RPM. The biggest reason though is so I don't have to worry about breaking my Duster at the track and not having a way to get home unless I pay out the @$$ for a 160-mile tow.

                        Not sure if that helps at all lol just what I'm currently dealing with regarding gearing on my car.
                         
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                        • 340inabbody

                          340inabbody Well-Known Member

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                          I hear ya man. So your car is turning into a race car having a trailer for it. Unfortunately I am running with an 8-1/4 and having a shop install it. This is why I am making a big deal about it.
                           
                        • MopaR&D

                          MopaR&D Nerd Member

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                          Ohh, yeah that's a bit more of a pickle I get it now. Much harder to change your mind with an 8 1/4" even if you can do it yourself. I know for damn sure though, even if 3.55s aren't quite enough they will make a MASSIVE difference going from 2.71s. I bet even 3.23s would make it run a lot better.

                          I tried 4.10s in my Duster once, I didn't like it. All the car could do was launch hard in a straight line and cruise around town. I didn't even have the balls to rev it up high enough to keep up with 65 mph traffic because it was already screaming at 4000 RPM; I just held the RPMs there and let everyone pass me. It sucked. My engine didn't seem to need them either it already had massive low-end torque and I shifted at only 5500 RPM. BUT that is a very different combo than it sounds like with your 340 so take that with a grain of salt. Still even if the engine can handle it cruising at 4000 RPM sucks!
                           
                          Last edited: Jan 19, 2020
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                          • AJ/FormS

                            AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                            the math says 4000rpm at zero slip =78.58 mph with 27" tires.
                            65 shoulddabin 3320@zeroslip.
                            With an automatic and 65=4000 that would be ~21% slip. That's a lotta slip

                            With a manual trans. I'd be looking to verify stuff.
                            My 4.88s cruised at 65=4000 with 27s. Yur right, it sucked so bad I had to get a GVod, to drag it down to 3120
                             
                          • gzig5

                            gzig5 Well-Known Member

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                            Might want to stick with the 3.55s for a while if you have them. If that engine is making any decent power and you get it to hook, that 8 1/4" diff may not like it and you'll be in the market for an 8 3/4". You should be able to get an idea on your converter stall by standing on the brakes in drive and loading the engine, it will stop revving somewhere around your stall speed and give you an idea if it's 2500, 3500, or whatever. Don't do that more than a couple seconds or the trans may not like it, and check your driver side motor mount afterwards.

                            FWIW...I have a 340/727 with 3.55s on a 29" tire and I don't think that I would be comfortable at 75mph+ for more than half an hour on the highway with it. It's right smack in the power band, but it's starting to get loud and such. On the back roads, no issues and I could go 3.91 if I know I'm not going cross country on the highway.
                             
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