3.91s in my 340 now!

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I love the 3.91's in my 340 '68 Dart. 4.10's would probably be better though since I run a 275/60/15 tire, but it's just a street cruiser. 340's love a short gear and a good converter, that's for sure.
 
318willrun said:
when a torque converter is advertised as a "3200 stahl" (or any other rpm range they pick), that is in a certain situation with a certain engine. So if somebody says "3200 stahl" and that is behind a 440 making 500 ft lbs of torque, your 340 might only see 2500 rpm's of stahl out of that same converter.

... And what that means to you is with that combo, and not enough stahl and gears... it will feel lazy up to a certain point, till it builds rpm's.
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Thank you for the explanation. This car (now with the 3.91 gears) doesn’t feel lazy, at all.
 
I went the opposite way. Had 3.91's in my Dart with a 340/727. Switched to 3.23's and couldn't be happier. I do a lot of highway driving.
 
If your 340 was doggy with 3.23s and a 3200, something in the combo was not right. Either the cylinder pressure was/is quite low, or the ignition timing was late, or like Willrun suggested, the stall was off.
However,
3.91s will bring the shifts closer together , time-wise, which also seems to feel faster, and 3.91s are getting close to being perfect for most streeters; 65mph in Second being about 5100 rpm.
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I have had a lotta 340 A"s, always with a 4-speed, and usually with either 3.23s or 3.55s; and with the 340 factory cam, the only one that ever impressed me was the very first one I purchased, in fall of 1970. IIRC tho 4.10s went in at the beginning of the Second summer.
Currently, I run the Commando manual-trans, with 3.55s, and a hi-pressure 360 with a modest 276/286/110cam.(230/237@050)
The starter gear is 10.97, about the same as 4.10s with the standard 2.66 low trans. Between the mega-torque 360, the BFGs, and the super-starter gear, she's a real tirefryer.
But as others have said, the GVod makes those 3.55s to be 2.77s, for cruising 65@2240rpm. The best of both worlds.
And I haven't even mentioned gear-splitting.........
 
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AJ/FormS said:
If your 340 was doggy with 3.23s and a 3200, something in the combo was not right. Either the cylinder pressure was/is quite low, or the ignition timing was late, or like Willrun suggested, the stall was off.
However,
3.91s will bring the shifts closer together , time-wise, which also seems to feel faster, and 3.91s are getting close to being perfect for most streeters; 65mph in Second being about 5100 rpm.
*********************************
I have had a lotta 340 A"s, always with a 4-speed, and usually with either 3.23s or 3.55s; and with the 340 factory cam, the only one that ever impressed me was the very first one I purchased, in fall of 1970. IIRC tho 4.10s went in at the beginning of the Second summer.
Currently, I run the Commando manual-trans, with 3.55s, and a hi-pressure 360 with a modest 276/286/110cam.(230/237@050)
The starter gear is 10.97, about the same as 4.10s with the standard 2.66 low trans. Between the mega-torque 360, the BFGs, and the super-starter gear, she's a real tirefryer.
But as others have said, the GVod makes those 3.55s to be 2.77s, for cruising 65@2240rpm. The best of both worlds.
And I haven't even mentioned gear-splitting.........
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It was only "doggy" out of the hole. But that was unacceptable to me. If I romp on it (with the 3.91s) cruising at 60 or so, it will push you back in your seat.
 
AJ/FormS said:
I would still do a compression test, they hardly ever lie.
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Probably next Spring. I have a mechanic friend that is great with the classics. I'll have him do it. How much do you think he would charge?
 
Rude72* said:
Hey guys...

I have a '72 340 Duster with a few engine mods (Holley 750, bored .030 over, Mopar performance purple cam/P4120235, Hooker comp headers, Edlebrock Torker II intake, 2.5" exhausts with Flowmaster mufflers). It has a Mopar 727-A three speed auto trans with a 3200 RPM Stahl Speed Trans Specialties converter and B&M shift kit with Hurst Quarterstick. It originally had 3.23s in it when I bought it, but it seemed (to me at least) that it was kinda "doggin" it coming out of the hole. Well, that certainly has changed! I was wondering if anyone else out there is running 3.91s with their 340s and how you're liking/disliking it.
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Hey Rude72,
I am running a very similar combo.
1972 Duster
340 with 284/484 Purple cam
750 Edelbrock on LD340 intake
TTI shorty headers, dual 2.5” exhaust
3200 stall FTI Performance torque converter
727-A non lockup trans
8-3/4 Sure grip with 3.91 gears
255/60r15 (27” tall)

Edited to add more due to a glitch:

My car responds well to the foot with the 3.91s and requires no downshift if already going 40+ mph when mashing the gas as it is already in top gear and it’s powerband, putting me nicely back in my seat. Highway is 3500-4000 rpm depending on if uphill or downhill. And no shortage of passing power if needed because engine is good to 6000 rpm. I would not go with any less gear in this setup, I think 3.91 is the sweet spot.
 
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Just today I pulled the 4:30's out of my 340 Challenger... Bolted in a set of 3:55's... But then you need the whole story... I pulled the 518 O/D... And put in a TKO with a 3.27 1st gear.... So it's actually a deeper 1st gear & a taller high gear.... It was a lotta fun before.. Should be a blast when I'm done...
 
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Wvbuzzmaster said:
Hey Rude72,
I am running a very similar combo.
1972 Duster
340 with 284/484 Purple cam
750 Edelbrock on LD340 intake
TTI shorty headers, dual 2.5” exhaust
3200 stall FTI Performance torque converter
727-A non lockup trans
8-3/4 Sure grip with 3.91 gears
255/60r15 (27” tall)

Edited to add more due to a glitch:

My car responds well to the foot with the 3.91s and requires no downshift if already going 40+ mph when mashing the gas as it is already in top gear and it’s powerband, putting me nicely back in my seat. Highway is 3500-4000 rpm depending on if uphill or downhill. And no shortage of passing power if needed because engine is good to 6000 rpm. I would not go with any less gear in this setup, I think 3.91 is the sweet spot.
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Hello Wvbuzzmaster:

Thank you for the response! Yes, I am completely satisfied with the 3.91s! Great torque and the car has been awakened!

You mentioned 3500-4000 rpm on the highway. On a relatively flat surface, what does your car pull (rpm-wise) at 60 mph? Just curious. I'm not exactly sure my exact mph now, since the gears were changed. I need to get the speedo recalibrated. Do you know how to do that? I have a friend that's great with muscle cars, I guess I can ask him. Not sure how much labor that involves.

Again... out of curiosity, what color is your 340?
 
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I had 3:91's in my Duster back in the 80's, but they were too steep for me. I got tired of it winding up so high. 3:21's now with a 4-speed and it's perfect
 
TF360 said:
I had 3:91's in my Duster back in the 80's, but they were too steep for me. I got tired of it winding up so high. 3:21's now with a 4-speed and it's perfect
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Yeah, I hear that often from people regarding the 3.91s. To each their own, I guess. I am very happy with them. I don't do much highway driving, though.
 
I have 3.91s that’ll will go in my axle swap within the next month or so.. it’s just a stock 68 318 but I’m planning for the future of a mag 360 or something similar
 
Rude72* said:
Hello Wvbuzzmaster:

Thank you for the response! Yes, I am completely satisfied with the 3.91s! Great torque and the car has been awakened!

You mentioned 3500-4000 rpm on the highway. On a relatively flat surface, what does your car pull (rpm-wise) at 60 mph? Just curious. I'm not exactly sure my exact mph now, since the gears were changed. I need to get the speedo recalibrated. Do you know how to do that? I have a friend that's great with muscle cars, I guess I can ask him. Not sure how much labor that involves.

Again... out of curiosity, what color is your 340?
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Rude72,
My Duster should be turning around 3200 RPM on flat ground at 60 with the 27” tall tire. Non lock up converter adds a couple hundred RPM from if it were a manual. But note I am usually turning over 3500 RPM on highway which is 70 MPH, which I find quite acceptable.

Speedometer is calibrated based off of axle gear and tire diameter, plus this fancy chart from Brewer. Then you add the correct number/color speedo gear in the tail shaft housing on your transmission, pretty simple.

Your next question of what color my 340 is might or might not be correct for year of car, but my non original 1969 340 is orange in my 1972 Duster.

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IMG_0137.jpeg


IMG_4136.jpeg
 
Wvbuzzmaster said:
Rude72,
My Duster should be turning around 3200 RPM on flat ground at 60 with the 27” tall tire. Non lock up converter adds a couple hundred RPM from if it were a manual. But note I am usually turning over 3500 RPM on highway which is 70 MPH, which I find quite acceptable.

Speedometer is calibrated based off of axle gear and tire diameter, plus this fancy chart from Brewer. Then you add the correct number/color speedo gear in the tail shaft housing on your transmission, pretty simple.

Your next question of what color my 340 is might or might not be correct for year of car, but my non original 1969 340 is orange in my 1972 Duster.

View attachment 1716310639

View attachment 1716310640

View attachment 1716310641
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Thank you for all the information! Your numbers sound pretty similar to mine... no surprise there, though. A lot of people complain about the high revving rpm at highway speeds for 3.91 gears. It really doesn't bother me (I have 245/60/14 tires, which I believe are 26" tall). I was just a little concerned about the stress on the motor and other integral parts. And of course, the motor running hot.

So I'm guessing the rear-end/differential needs to be exposed to calibrate the speedo. That's definitely NOT me. I am a complete novice when it comes to working on cars... but I am slowly getting acquainted with it.

Beautiful Duster! Love the color! Thank you so much for sharing!
 
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Rude72* said:
Thank you for all the information! Your numbers sound pretty similar to mine... no surprise there, though. A lot of people complain about the high revving rpm at highway speeds for 3.91 gears. It really doesn't bother me (I have 245/60/14 tires, which I believe are 26" tall). I was just a little concerned about the stress on the motor and other integral parts. And of course, the motor running hot.

So I'm guessing the rear-end/differential needs to be exposed to calibrate the speedo. That's definitely NOT me. I am a complete novice when it comes to working on cars... but I am slowly getting acquainted with it.

Beautiful Duster! Love the color! Thank you so much for sharing!
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340 with purple cam has operating powerband of 2600-6000 rpm, so why would one want to be at 2000 rpm and still have to rev it to 4000 to pass because not in power band and you will downshift… I think (opinion) that staying a constant but higher rpm is better for an engine than wild rpm variations, especially if that rpm is in the engines powerband.

Fuel mileage I can see 12 mpg highway staying mostly out of secondaries. 6 mpg in the secondaries more as I have the secondaries, I’m using them lol.

My larger concern was never engine, but transmission temperatures, ad non lockup torque converter is not 100% efficient and that equals heat from anything less than where it quote stalls. So I did the math to put my highway cruising RPM above my 3200 stall converter. This in theory is running transmission at max efficiency possible to keep temperatures down. Also being in the powerband is nice. It’s why 3.91 on a built 340 is a great combination.

Engine temps at 3500 rpm is not an issue is you have the correct radiator and are running a 185 thermostat and 18” 7 blade 2-1/4” pitch fan correct for a 340 application. I know, at highway speed the fan spec is irrelevant, but at some point you will be slowing down and stopping again lol. Never had any issues on 90 degree days here, always stays 185-200 degree engine temps.

The speedometer gear is in the side of the transmission tail housing just ahead of crossmember on the driver side. No need to remove anything but the speedo gear housing and speedo gear, probably lose some transmission oil, but very doable. I had a lift so no biggie for me, with no lift definitely would make it trickier and messier but again, still doable I think. Either way speedo will probably read off by 5 mph like mine thru the entire sweep because 50 year old part, needle must be bent 5 mph lol.
 
27" tire, 4.10, almost 4k rpm at 70mph. I think a gear vendors od is called for. And probably some sound deadening material in the floor, doors and roof. I'll eat the weight, it's more than made up for in the aluminum intake manifold alone.
 
Ottmundr said:
27" tire, 4.10, almost 4k rpm at 70mph. I think a gear vendors od is called for. And probably some sound deadening material in the floor, doors and roof. I'll eat the weight, it's more than made up for in the aluminum intake manifold alone.
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4.10s? Wow! Funny, but at highway speeds (60mph or so for me) the "noise" factor is very tolerable. Now, if I had cut-outs, that would probably be a different story.
 
When I daily drove my 71 Challenger, I would put the 3.91 in on Friday evening, and Sunday morning put the 3.23 in for the weekly commute.
I made the brake lines a little long, so I could just slide the axles out slightly, so no need to bleed the brakes.
Did it for a long time.
 
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