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Standard rear differential gear ratio question

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WCKAMX390 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote WCKAMX390 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/11/2013 at 11:09am
Best gear for street use is 3:54 or 3:73.   3:91 works , but you lose some of your 4th gear hiway driveabilty
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FuzzFace2 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/11/2013 at 1:44pm
Originally posted by WCKAMX390 WCKAMX390 wrote:

Best gear for street use is 3:54 or 3:73.   3:91 works , but you lose some of your 4th gear hiway driveabilty
You really cant say any of them are best or not with out knowing what trany they are using. Take a 3.xx first gear and the 3.54 is too low and using a 3.91 gear with over drive could be a good combo.
You have to look at the whole package, trany gears & rear gear, power out put & weight of car.
I think a 2.95 first, .68 over drive and hoping 3.54 rear in my 360 power Javelin will be a good driver.
Dave ----
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Plan B Quote  Post ReplyReply Direct Link To This Post Posted: Jan/11/2013 at 7:26pm
Just to share I am going with a new set of 3.73 that should be at my door any day now. I will be using a 700r4 reverse manual valve body.Here are the trans ratios 700r4 3.06 1.62 1.00 0.70 2.29 Running 2800 stall. My 68 AMX 390 was running 3.15s? I think
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Jan/11/2013 at 8:49pm
You have to understand that the engine needs to turn around 2000 rpm to be producing enough power to pull an overdrive gear effectively at cruising speed. 3.23-3.91 gears are needed, depending on tire size and what your average high speed cruise speed will be. Even with 3.55 gears, a 26" diameter tire (215/65R15 in my case), and a 0.70 OD (typical, but varies from 0.83-0.65, with the bigger number being "less" OD... subtract from 100, that's the percent OD, 0.70 = 30% OD) my car is traveling 62.3 mph @ 2000 rpm. Cruising at 55 mph it's only turning 1766 rpm. I've got a good torque cam that "comes in" at 1800 rpm, but 55 mph works the motor a bit. Dropping to 3rd gear brings engine rpm up to 2523, which is more like it, but the 4.0L computer starts to richen the fuel mixture right at 2500 rpm. So I cruise at 60 in OD, and usually don't travel at 55 at all -- it's either around 50 or it's up to 60. Best for economy! It's geared about right to cruise in the 70-75 rpm range (2250-2410 rpm). That's turning fast enough to produce good power to not fast enough for the computer to start throwing a bit more gas in the mix. I'd gain 5 mph in cruise speed at 2410 rpm with 3.31 gears, 3 mph with a 1" taller tire (27" diameter -- 215/70R15). I'm going to change to 70 series tires when I need new tires. 

There's a lot more to selecting gears for an OD trans than just reducing rpm! Another thing to consider -- a "lumpy" cam is going to want more than 2000 rpm to be in the torque band of the engine, so you will need more gear. Your lowest cruising speed in OD should be no less than the lowest rpm in the rpm range of the cam, and you really need to be a couple hundred rpm over that. The Isky 280-HL has a "lopey" idle and an rpm range of 2500-6500. I would want to cruise at 70 mph in the 2600-2700 range. That would require a 4.11 rear axle, which would give 2600 rpm with the 26" tires and 0.70 OD. Less OD might be the way to go, but you're kind of limited there.

Oh, one thing about the GV OD.... They correctly state that you can get six forward gears from it. It's just not practical. 1+OD is only 100-200 rpm from 2nd, 2+OD only 100-200 rpm from 3rd. Not enough difference to warrant shifting gears, not in a street car anyway. It MIGHT be useful in a heavy tow vehicle, but even that is "iffy". AMC did it with the Twin-Stick, which split-shifts an OD unit to achieve five forward gears. OD isn't used with 1st, just 2nd and 3rd. There is a big "jump" between 2nd and 3rd gear in the three speed trans to make this practical. If you lock OD out (or just remove the fuse to disable it) and shift it will feel like using a four speed and skipping 3rd gear (1-2-4 shift).
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amx39068 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/12/2013 at 8:16am
My favorite combo for a 343, 360, 390 or a 401 is the 3:15 rear gear with the wide ratio 2.64 first gear 4 spd that was in 67 and earlier cars with a T-10. You end up with the equivalent of a 3.73 rear gear in 1st (when comparing ratios against the more common 2.23 first gear 4 spd) but still maintain lower RPMs on the freeway with 1000 RPM per 25 MPH on stock diameter tires in 4th gear with the 3:15 rear end gear.

Our speed limits outside of the major Phoenix area are 75 MPH which can be somewhat annoyingly buzzy even with 3.54 gears. I would imagine it would be downright unpleasant with 3.73 gears and even awful with 3.91s at those speeds. I once had 4:10 gears in 68 AMX that I used to drive to work from time to time when I lived in the Boston area in the late 80s and really hated driving that car on the freeway even at just 65 MPH.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amcenthusiast Quote  Post ReplyReply Direct Link To This Post Posted: Jul/08/2013 at 11:37pm
Just found this article about 727 automatic conversion to OD;  http://www.hemmings.com/mus/stories/2006/07/01/hmn_feature15. - http://www.hemmings.com/mus/stories/2006/07/01/hmn_feature15.html


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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2013 at 7:06am
I think they are just stating that you can swap the entire A-518 in. That won't work for an AMC because the bell bolt pattern is different from Chrysler. Many people don't know that, just assume that since the rest of the trans is the same as Chrysler so is the bolt pattern. The article does state that the OD unit is in the tailshaft housing, so it may be possible to rebuild a 727 with the A-518 output shaft and tailshaft housing to add OD to an AMC 727. The statement about "other internal changes" (between the 727 and A-518/46RH) and that they say you can't use the 727 converter makes me think that's NOT what the article writer did though. Still might be possible, you'd have to tear both apart and see. If that will work the 727 converter could be used. 
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote uncljohn Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2013 at 7:35am
Originally posted by amx39068 amx39068 wrote:

My favorite combo for a 343, 360, 390 or a 401 is the 3:15 rear gear with the wide ratio 2.64 first gear 4 spd that was in 67 and earlier cars with a T-10. You end up with the equivalent of a 3.73 rear gear in 1st (when comparing ratios against the more common 2.23 first gear 4 spd) but still maintain lower RPMs on the freeway with 1000 RPM per 25 MPH on stock diameter tires in 4th gear with the 3:15 rear end gear.

Our speed limits outside of the major Phoenix area are 75 MPH which can be somewhat annoyingly buzzy even with 3.54 gears. I would imagine it would be downright unpleasant with 3.73 gears and even awful with 3.91s at those speeds. I once had 4:10 gears in 68 AMX that I used to drive to work from time to time when I lived in the Boston area in the late 80s and really hated driving that car on the freeway even at just 65 MPH.


I agree with DAN, I got rid of the 3.54:1 final drive on my Donohue some time shortly after I got the car. Growling down the freeway at 75 mph became incredibly boring.
It did cause a problem in heavy stop and go city traffic though as 1st gear was rather tall and you had to slip the clutch a lot when driving in those conditions and I went through a couple of clutches. No complaints though, replacing a clutch was a small price to pay to not put up with the growling at 75mph. 
However I lost the 4 speed on a trip to Sacramento and replaced it with a World Class 5 speed in the late 80's which gave me a wide ratio 4 speed plus a highway gathering overdrive and it was the best dang thing since sliced bread with the 3.15 rear axle.

The question asked about whether 6 cylinder rear axles were the same ratio as V8's and the answer is no they are not.  Close maybe but no cigar.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote uncljohn Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2013 at 7:44am
Originally posted by farna farna wrote:

I think they are just stating that you can swap the entire A-518 in. That won't work for an AMC because the bell bolt pattern is different from Chrysler. Many people don't know that, just assume that since the rest of the trans is the same as Chrysler so is the bolt pattern. The article does state that the OD unit is in the tailshaft housing, so it may be possible to rebuild a 727 with the A-518 output shaft and tailshaft housing to add OD to an AMC 727. The statement about "other internal changes" (between the 727 and A-518/46RH) and that they say you can't use the 727 converter makes me think that's NOT what the article writer did though. Still might be possible, you'd have to tear both apart and see. If that will work the 727 converter could be used. 


One thing you cannot do for sure is use a lock up torque converter on a non-lock up transmission and visa versa. 
So if what you want to do is build a overdrive transmission from an A-518 to work on an AMC V8 and do so by using an AMC V8 torque converter,  it CANNOT be done using a NON-lock up torque converter.
If it can be done and I don't know, you are going to have to start with a lock up torque converter transmission that was used in the late stages of AMC life.
OR
Say the heck with it and use a 700R4
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Post Options Post Options   Thanks (0) Thanks(0)   Quote carnuck Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2013 at 6:31pm
If the trans is new enough ('80 up in FSJs for sure) the passages for the lockup are there, but they chose to make them non-lockup due to issues with clutches in the converters and fluid (Dex II) not cooling/lubing enough. You need a lockup trans for parts (can be Mopar) to do the conversion (valve body, input shaft and clutch drums, etc) , but the rear of a 518 case is different from a 727 so that swap is not easily doable.
  AW4 from an XJ or MJ (more than 1/2 of Comanches are 2wd, like the one I'm selling for parts) is a direct bolt up and stronger than the 42RE used in most Grand Cherokees with 4.0L except early '93.
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