rear end gears issuse

[Texas charger 73]

Ok first of I'm running a 318,with a 290 duration, 484 lift. 1.60/2.02 valves, edlebrock intake and 4 barrel with headers. Probably around 250-275,hp. I have an 8 1/4 rear end with with 2:76 or 2:91 gears. I have a guy sending me a Set of 3:91 with install kit for like $130,shipped. Brand new. I think the gears will be fine and wake up the car. Its going to be a weekend driver most likely. I keep having people trying to talk me out of it. Saying its too tall of a gear, had milage is gonna suck,but my thoughts are I'm gonna stroke the 318or run a 340/360,built later and I'll already be set. I know your gonna say I'm wasting my time with 8 1/4,to get a 8 3/4. Well send me one. I can't afford one. Too much for this guy. For now I have to use what I can afford regardless how strong. Thank you guys

 

[PurpleBeeper]

My personal opinion is that I'm surprised you haven't had the 8-1/4" blow up into a million pieces already. I think you NEED an 8-3/4". About the gears.....I'm running 3.91's in my car now and I'm pulling about 4000 rpm in the slow lane on the highway (with a 5500-6000 rpm redline). Weekends ONLY? sure. Drive to the next town for groceries? bad idea.

I ran 3.55's also and I would say they are the MAX you should run on the street if you really plan to drive the car (unless you have an overdrive transmission - which I don't have).

 

[Texas charger 73]

I'll probably just stick these gears on shelf and sale with rear end when I find an 8 3/4. I will never pay $2000-$3000. People think there gold. $400,max. As I have a budget after my work injury.

 

[rumblefish360]

Oh my God! What may sayers!

For sure run the 8-1/4! That read is a lot stronger than credit is given!
The 3.91's are a bit steep for a stock size tire. You might want to run a 27 or 28 inch tire. My '73 Cuda runs a 275/60/15 w/4.10's with Hwy. RPM's at and around 3300.

This is the area your cam becomes active. Also, if your worried about mileage, get a VW Golf or something. Once your in the cams operating rpm range, it becomes a better operating machine. Sluggish and fuel consuming bow its operating rpm is a fact. Not when its in it.

Mileage tips with a cam like this are a multi stoke ignition and a well tuned carb slightly lean at cruise. While you'll never get good mileage with a small mill in a heavy-ish car and a med. size cam, you should still be in the mid teens.

Again the rear is capable of the power handling of that engine. I ran one for years behind a 12 second 400 CID in a Duster. Never a problem. The thing that will kill the rear besides the no brained actions of beating the crap out of the car is how well it hooks vs. the cars weight and the amount of abuse it receives.
and your not making a lot of power ether!

With ANYTHING! Don't abuse it and it will last. But that is a capable rear not getting a lot of power. Use it!

 

[Texas charger 73]

I'll be running 275/60/15 in rear, there will be no racing, no tire smoking(cause in Texas you go to jail for it). Just cruising and showing my kids true muscle. Go get them ice cream,maybe a drive in. Stuff like that.

 

[Coronet 500]

I agree with Rumble 100%. The hot rod aspect of this hobby is improve what you've got with what money you have. If I had all the money to build anything over the years then I wouldn't have built half of the half assed cars and trucks I did. But boy have I had fun.

Take the time to build the 8.25" properly and you'll be fine. 3.91 on the weekends = fun, screw the gas mileage.

 

[wedge69]

As Stated Tire Size is BIG Factor !!!!

 

[Texas charger 73]

That's what i say. I might go to a few local shows,but won't be more than 50 miles.

- - - Updated - - -

I'll be running a 275/60/15 on 15x8 4"bs American racing ar23 wheels

 

[dvw]

275-60/15 isn't that bad with a 3.91. Heck I've driven from Detroit to Columbus for Mopar Nats with 4.30 and 4.56. A 27.5 tire, 3.91, 70 mph with out converter slip is 3344 rpm. I do believe you need a notched cross shaft to put a 3.91 in a 8 1/4. I ran a hopped up 360 for years with a 8 1/4 w/o issue, no slicks. Personally I would skip ove a 8 3/4 and go right to a Dana. You buy the right ratio with a locker from the junkyard out of a truck. Get one with a locker. Cut the ends off, add aftermarket ends an axles and you're bullet proof. easily done for less than 1K if you shop around and do it yourself. We just did one for my sons turbo car. we even got the Dana out of a Ford dually for free. Reused the gear and bearings. Even adding a spool it was about $600.
Doug

 

[Cranky]

I'll be running 275/60/15 in rear, there will be no racing, no tire smoking(cause in Texas you go to jail for it). Just cruising and showing my kids true muscle. Go get them ice cream,maybe a drive in. Stuff like that.

Hell, I ran 4.56's with 275-60's and it was fun and made lots of smoke with that combo. Go to jail? Got tickets for it but never went to jail for it. A friend lived in Arlington and I cut down a tree for her that was too close to the house and a neighbor came over and told me that I could get fined if I didn't consult with the city first and get a permit etc etc. WTH!!?? Didn't know there were areas in Texas like that. Anyways, my car was mostly a toy and didn't get on the freeway much but swapping out gears with the 8 3/4 takes me about 45 minutes....that is if you already have the chunks ready to swap. If you keep the 8 1/4 and might want to ever put the highway gears back in, keep the shims stored with the old gears and then swapping them back in is a slam dunk.

 

[slepr1]

Changing the stock gears and swap to a 8 3/4 with 3.91 was the first item I changed on my 318 ride. You won't notice a heck of a difference with the gear change but there will be some. After that the quest to go quicker was on. Next it will be intake and carb...that too won't be good enough. I pulled the 318 and went with a 440. I'd love to have a stroker since the low end torque is what I crave. If you want to go quicker the sky is the limit. Plan your mods accordingly so you don't have to redo the same component twice.

 

[62 Dart Convertible]

3.91 will really wake it up going from 2.76 or 2.91, especially in a 318 with a sizeable cam! It must feel quite "lazy" off the line now.. Sure you wouldn't like to interstate freeway cruise for long periods with 3.91s, but for a weekend fun car they're perfect. Gas mileage?! - who cares!!

PS, going higher numerically with diff gears isn't termed going "taller", it's going "shorter".

Good luck, and please report back once done - I'd love to hear your take on the results...