355's to 391's how much does it help?

-
j par

j par

Well-hung Member
Joined
Jul 2, 2014
Messages
33,156
Reaction score
20,808
Location
Portland Oregon
I like to do my own work, but a local guy parts and labor $250
Will do the job. I know this depends on my motor/ transmission set up,
but what have the results been for you guys out there? (quarter mile wise?)

sorry so many threads I just like to learn:)
 
i used to street race a 68 barracuda 340 s 4sp with 3:91's. 340s with a 4sp like 3:91s because the ideal shift rpm (6000) for a street 340 comes quickly in first and second with third and fourth being matched well to the combination of 3:91 and the 340's power band. a 340 with headers, a holley 750 dp and 3:91s will outrun any "old school" GTO, Ford, Chevy or big block mopar in the 1/4. however, you DO NOT want to install 3:91s if you are going to be doing a lot of highway driving. 14-15" tires with 3:91s will turn your 340 at right around 3000 rpm at 60 mph. you're probably going to blow up your 340 if you try and drive 70 mph with 3:91s for any length of time. if you are going to do a lot of highway driving, i would stick with the 3:55s. but if you want to smoke the tires in first or second on a regular basis or run your car like you stole it a 1/4 or a mile at a time, you just can't beat (literally) 3:91s.
 
I really would have thought there would be some racers out here that would have some direct comparisons time slips for changing gears?
 
My daily is stock 73 340 dart sport, 3.23 2800 rpm 70 mph 1/4 14s spinning, fun car and drive it anywhere. 3.91s in my other dart, local driver and drags TNT, runs 12.5 and a blast to drive!
Pick your rear gear and tire diameter to match your cams upper power band crossing the stripe.
 
mderoy340 said:
My daily is stock 73 340 dart sport, 3.23 2800 rpm 70 mph 1/4 14s spinning, fun car and drive it anywhere. 3.91s in my other dart, local driver and drags TNT, runs 12.5 and a blast to drive!
Pick your rear gear and tire diameter to match your cams upper power band crossing the stripe.
Click to expand...

First and second seem good, third is where I can use more.
I have to hit forth just before the finish line. And if that's the case, could 391's, 411's, or 456's hurt?
 
j par said:
First and second seem good, third is where I can use more.
I have to hit forth just before the finish line. And if that's the case, could 391's, 411's, or 456's hurt?
Click to expand...

If this is the case then 3.91's will put you closer to peak rpm in 4th as you cross the stripe, which is what you want. That being said I'm not sure 3.91's will make THAT much of a difference, or atleast get you to that peak point. I'd say 4.10 or 4.30's would probably get you there but those are steep gears for the street. I'm switched from 3.23's to 3.91's but have yet to run it yet, I'll be curious to see what the difference is too.
 
On my old mild 360 combo, going from 3.55's to 4.10's picked me up half a second (14.0 to 13.5). Optimal gearing is combo dependent...every one is a little different.
 
Beatnik said:
If this is the case then 3.91's will put you closer to peak rpm in 4th as you cross the stripe, which is what you want. That being said I'm not sure 3.91's will make THAT much of a difference, or atleast get you to that peak point. I'd say 4.10 or 4.30's would probably get you there but those are steep gears for the street. I'm switched from 3.23's to 3.91's but have yet to run it yet, I'll be curious to see what the difference is too.
Click to expand...

I'll be curious also, please let me know?
 
flyfish said:
On my old mild 360 combo, going from 3.55's to 4.10's picked me up half a second (14.0 to 13.5). Optimal gearing is combo dependent...every one is a little different.
Click to expand...

Flyfish that's the real world stuff I'm looking for! I'm thinking 410's also
 
j par said:
Flyfish that's the real world stuff I'm looking for! I'm thinking 410's also
Click to expand...

Every car/combo is different though as far as what works best. I have seen guys go from 4.10 to 4.56 and gain nothing too.....just food for thought.
 
I was thinking the higher gears will help 3rd gear pull a little harder since I'm barely using forth,
maybe a little more 4th gear?
 
I switched from 3:23 to 3:91 in my GTX and didn't pick up a single .001!! Now with the new converter, it really woke up, although I haven't run it this year! Every combo is different as stated above, and the gear ratio should be considered in your entire package! If your peak HP is not in the RPM range of a higher numerical gear, it will do no good to change! If it is a mostly street driven car, 3:91's will do more harm than good in most cases!! Yea, they're fun on takeoff and burnouts, but not too practical! 4:10's on the street are even worse, and forget 4:56 and above!! Geof
 
I have no trouble with any burn off's !! 4 speed and all. Mostly strip, some street, and very little high way.
 
flyfish said:
On my old mild 360 combo, going from 3.55's to 4.10's picked me up half a second (14.0 to 13.5). Optimal gearing is combo dependent...every one is a little different.
Click to expand...
Sorry to bring an old trend back but I love this one totally agree a good pulling motor to 6 k the 355-410 will be in that range most the time
 
j par said:
I have no trouble with any burn off's !! 4 speed and all. Mostly strip, some street, and very little high way.
Click to expand...
Did you ever change gear and any et change my firend had a 340 727 71 dart ran 12.7 with 355 and swapped to 391 ran 12.35 ish. Really depends on if your motor makes good hp through 5500-6000, 391 gears and 4 speed is the ideal combo imo.
 
Moparhead said:
Did you ever change gear and any et change my firend had a 340 727 71 dart ran 12.7 with 355 and swapped to 391 ran 12.35 ish. Really depends on if your motor makes good hp through 5500-6000, 391 gears and 4 speed is the ideal combo imo.
Click to expand...
I made farts made changes at once that year to know what changed but I can tell you I would probably rather have the 3:55 now then the 4:10.
 
j par said:
I made farts made changes at once that year to know what changed but I can tell you I would probably rather have the 3:55 now then the 4:10.
Click to expand...


That says you are getting old. That's why I still run 4.56's. When I do the Dana I may go 4.88's for even more fun.
 
yellow rose said:
That says you are getting old. That's why I still run 4.56's. When I do the Dana I may go 4.88's for even more fun.
Click to expand...
Especially now that I have the Stroker motor there's a lot of torque low down and next winter not this winter I'm going to probably put a cam in that takes more advantage of that.
 
j par said:
Especially now that I have the Stroker motor there's a lot of torque low down and next winter not this winter I'm going to probably put a cam in that takes more advantage of that.
Click to expand...


Probably not a bad idea.


Edit: hell...you can swap cams in a few hours. Why wait?
 
yellow rose said:
Probably not a bad idea.


Edit: hell...you can swap cams in a few hours. Why wait?
Click to expand...
I've worked on that car and truck all winter! I need to start getting some projects done around the house for the wife? That really cool wife? The one I want to keep around!
 
You must log in or register to reply here.
-

Back
Top