3.55 or 3.73

jimynick · October 26, 2020, 06:24:29 PM

I did a 6 hour run once in my Valiant 340 4 spd 3:91s and would never do it again! I think we saw most of the gas stations on the way and the engine roar was considerable. While 340's do like to rev, I don't think a diet of high rpm is good for any engine for more than a mile or two. My 72 Duster 340 4 spd 3:55s was better, but the Challenger has 3:23s and like Goldilocks, they seem to be just right. You want more acceleration? Drop a gear and get on it, but running in the mountains and freeways, IMHO, isn't the place for a short gear. Just my

Dakota · October 27, 2020, 02:25:53 PM

I just did a gear swap from 3.91 to 3.55 with an A833 OD and 27" tall rear tires. I'm very happy with the swap. I can't tell you the exact speed/rpm combo I get because I still need to change the speedo gear, but based on sound and how I drive this is the right spot for me.

Rdchallenger · December 28, 2020, 04:45:21 PM

So I pulled the trigger.... On 3.91s

after much thought, I realized I can just take the scenic route around here to head to cruise ins. The most highway this car will see will be on the yearly trip to Carlisle which since I got the crush can eliminator I will switch to 2.76's when that time comes.

anlauto · December 28, 2020, 04:57:30 PM

I just got 3:73's from Dr. Diff today for the project I have in the garage...he didn't have 3:91's for a 741 case...

Rdchallenger · December 28, 2020, 05:04:34 PM

Quote from: anlauto on December 28, 2020, 04:57:30 PM
I just got 3:73's from Dr. Diff today for the project I have in the garage...he didn't have 3:91's for a 741 case...

I was in luck, ordered them last week and got them today from Dr. Diff... 489 case here.

dodj · December 28, 2020, 05:44:26 PM

Quote from: Rdchallenger on December 28, 2020, 04:45:21 PM
So I pulled the trigger.... On 3.91s after much thought, I realized I can just take the scenic route around here to head to cruise ins. The most highway this car will see will be on the yearly trip to Carlisle which since I got the crush can eliminator I will switch to 2.76's when that time comes.

3.91's will be a blast in town and the 2.76's will make for a nice hwy drive.

Scooter · December 28, 2020, 06:04:04 PM

^^^ I've got a 2.76 open in mine... mostly highway driving and I really like it.

ek3 · December 28, 2020, 06:44:40 PM

no mention of the gear set in the automatic trans. doesnt a car with a factory installed 2:73 used a completely different set of trans gears ?... is the top rpm all you are looking at ? hi hp = more rpm . hi torque = lower power rpms.... best acceleration depends on that

anlauto · December 28, 2020, 07:03:48 PM

Quote from: Rdchallenger on December 28, 2020, 05:04:34 PM
Quote from: anlauto on December 28, 2020, 04:57:30 PM
I just got 3:73's from Dr. Diff today for the project I have in the garage...he didn't have 3:91's for a 741 case...

I was in luck, ordered them last week and got them today from Dr. Diff... 489 case here.

Super great guy to deal with

dodj · December 29, 2020, 05:53:46 AM

Quote from: ek3 on December 28, 2020, 06:44:40 PM
no mention of the gear set in the automatic trans. doesnt a car with a factory installed 2:73 used a completely different set of trans gears ?... is the top rpm all you are looking at ? hi hp = more rpm . hi torque = lower power rpms.... best acceleration depends on that

I thought all the tflites gearsets were the same?

ek3 · December 29, 2020, 05:19:57 PM

yep now that i think about it they were all the same... must be thinking of the race sets... or gm stuff....

chargerdon · January 09, 2021, 07:48:41 AM

904 auto has a slightly lower 2.74 first gear as opposed to 2.45 in a 727. Other than that i think their all the same. Your perception of different gears for 6's is that they used the 904's for 6cyl and base 318, while the 727 was used for the big blocks and i think also the 340's.

Look, if you want the best of both worlds, put in your 3.91 for drag racing and great around town performance, and then add to the 904 a gear vendors electric overdrive for around $2,500 bucks. They will set the overdrive ratio to what you want, typically around 65%.

A less expensive option is to replace your 904 with an A518 or A500 from a Dodge van or pickup. The A518 is basically a 727 with an overdrive tail section, and the A500 is a 904 front with the overdrive tail section. Depending on the year of the transmission it will have a locking torque converter... i think 93 and newer have the locking torque converter...92 and older do not. Its easy to tell them apart as the one with lockup will have a three pin connector (OD and lockup are electrically engaged) and the non-lockup will have a two pin connector. There is a long thread on here for the A518 conversion. I have done it. It basically is just a bolt in replacement, with the right Torque Convertor.

Modifications for A518 needed are:
1) a shorter driveshaft as the a518 is 4" longer than a 727 or 7" longer than a 904.
2) The A518 requires a little bit of "web" trimming to fit it in around the gear shift lever... You will also have to make a different holder for the bottom of the floor gear shift. No biggee..
3) A new crossmember as while it still goes in the same place the thicker tail section rquires a deeper crossmember. I fabbed my own for about $15 worth of 12 gauge sheet metal and a lot of measuring, cutting, and welding.... You can buy one premade from a guy on here for $300.
4) A new Torque Convertor...lockup for newer ones
5) A pair of toggle switches and a pigtail to plug into the control port... and/or a 40 or 50 PSI switch to control the OD and LU. (note: on the A518 there is a port that puts out pressure at 1 psi per mph...so if you use the 50 psi switch you would shift into OD and LU at about 50 mph. I just use toggle switches to manually control both...tho i plan on putting inline the 40 psi switch as well, as if i forget to turn off the Locking Torque Converter bad things will happen when i come to a stop. Ill leave my toggle switches in place as well to manually control, while the 40mph switch is just a safety feature.

My total cost was: $300 for the used A518 (stronger than a 904), then $150 for a rebuild kit ( i installed myself to insure that all of the seals and clutches were new), $150 for locking Torque convertor, and $200 to have a driveshaft cut to proper length and rebalanced, then about $40 for new U-Joints...$5 for two toggle switches. Total: $845

Results, With my 14" (25.1" height tires/wheels) and 3.23 sure grip rear axle, i have enough torque in my 360LA/408 with stroker kit to burn the tires off anytime i want...I cant floor it in first until im doing at least 10 mph or both tires will break loose. Steady highway driving at 60 mph is around 2800 and 70 is around 3300-3400. Its noisy and burns gas at about 8-9 mpg. Flip the switch at 60 for the OD and revs drop to 2,000 then flip the switch for LU and it drops another 200-300 to around 1750. Sounds so much quieter at that rpm and my gas mileage locally goes up to 11 mpg, and i would guess on a trip to Myrtle Beach (130 miles) would probably be in the 14-16 mpg range...and on premium thats a huge difference...not to mention the lower noise level...PS...im running a pair of Flowmaster 40's...big mistake !! I have had zero problems with it so far in about 1,000 miles of driving. My only reserve is that if i knew for a fact i was going to put the overdrive in, i would have picked 3.55 or 3.71 sure grip instead...with 14's 3.71 or higher numerically would be a waste as the tires cant take it. But someday i want to replace my 14's with newer 17's as there aren;t many choices in 14" available anymore. ps...my LU TC is a 2500 rpm stall...it works well with my tires, and the Lunati Vodoo 703 cam.