i have a 8.75 323 rear plan on putting in my 65 cuda thinking about changing to 3.55 gears will i really feel much of a difference? car wont see the race track just cruising and car shows and toying around town. oh and one more thing has a healthy 904 trans plan on keeping in the car was told have to change drive shaft or shorten mine? any one know if one will go right in from another mopar?

 

Changing the gear wont make a "dramatic" difference. Only time I would make a change that small is if the new axle came with a locker that you needed and it was already setup or the new axle had discs..

There are some free speed ration calculators on the web that can do speed / rpm for you.

As for the drive shaft, I haven't done it in your ride. I have done it in other applications and occasionally you can get lucky and find a yoke that has an extra inch to make the difference if you are off. It worked with converting old 3 speed mannies to Top Loaders.. Good Luck..

 

Stay with the 323.

 

If it currently has a 8 3/4 drive shaft would be the same. If it has a 7 1/4 yes you will need changes. Like bremeric I would keep the 3.23 But that could change depending on what engine mods are done.
If you are putting a complete 8 3/4 in is it from an A body

 

65 -

It all depends on what's in the motor and what you want it to do.

3.23s are great highway gears, for more street / strip action you may want to a higher ratio, and just going to 3.55s ain't gonna do it.

If you want an idea of what your rpm / mph will be with various gear ratios / tire sizes, there are a few calculators on the web that will let you know. There's a pretty easy one on Summit's website.

Archer

 

I consider something in the 270 to 290 range to be for sure highway gears. 323 is just about prefect and the 355 is the first step up to semi-performance. I run them in my Dart and turn 3500 at 70 on the freeway.

 

Keep the 3.23 gear ratio and throw in a sure grip (pois, limited slip, etc.), you'll see a difference with just that.

What motor w/upgrades are you going to be running?

 

thanks for the input guys. the 340 is being built will have j 2o2/i60 heads steel crank forged pistons .030 comp cams thumper 280 same as 280 purple cam decided to go with newer tech on that one. ld340 intake eld 750 carb stock 340 exhaust manifolds 2500 stall converter 904 trans with shift kit 8.75 rear from a 69 dart. plan on keeping it for street play not the track. any guess on how it should perform? again thanks guys

 

65 -

Sounds pretty decent. Think you have the right mix on converter / gears. Only concern would be the 904. While a lot of guys use it on the track because of it's lighter weight, not sure how well it will hold up on the street. Make sure you have a tranny cooler at the very least.

HP numbers are questimates at best. You didn't mention any head work and that can be a major factor. Either way, should be a fun car.

Archer

 

Thats an excellent build! VERY dependable and she should make good power and rev real easy. Good call on cam and stall. It will be fun for sure.

Gears are fine. Take the gear money add headers.. (airgap would be a bump too).

Are heads stock beside the valve work?

 

i plan on keeping the 904 in it will be street only im sure ill be ripping around with the car but wont be hammering it to hard and wont see the track. the heads have been worked so they should be fine with the whole combo. just not sure what way to go with the drive shaft? when the 8.75 rear is put in i know ill need a shorter shaft was hoping some one would know what would fit right in in not ill have mine cut or a new one made. thanks for the feed back

 

Heads will work very well.

A good compression 340 with the 280 cam, headers and the airgap makes 380-400HP any day of the week. (Airgap and headers would account for at least 50HP of that though).

Let us know how it runs when its done..

 

i had thought of the airgap manifold but have been told by two others [not knowing each other] get my hands on a LD340 intake and claim it was the best designed for the 340. or is that old tech to? feels like im bein schooled lol but hey thats fine always good to learn more thanks guys

 

Nothing wrong with what you have. Good intake. It flows on line with the rpm. Just slightly under. Folks who live in colder climates really love the ld340 cause it makes good heat and you don't have any frost over issues for drivers with the gap.

When you post above the motor "will" have xxx.. I unfortunately make the assumption you are parts gathering.... I wouldn't go out and buy one but if you have it, run it for sure.. You will be happy. I was just trying to get you best bang for the buck..

Only other thing you need to watch is make sure who ever sets up your heads gets you real close on spring pressures for the cam.

Let us know how she runs..

 

i got a great deal on the intake the guy was just getting out of the hobby and as far as the valve springs im told im good on that and heads should handle 450 hp no problem. would like to find a posi unit to go into my 8.75 rear 741 small bolt pattern really dont want to spend more than the rears worth for it though hey if not ill just keep it the way it is.

 

And, will roast the tires with the 3.23 Suregrip!

 

Amen to that! Having an open diff would not be advisable

 

if you cant ported the heads by your self, clean up the intake for grater flow..stock look like ****, i'm a weekend porter that clean up or go for full out porting, i have seen all heads get hp with just to polish them..

 

if i cant find a posi unit would a mini spool due? or would that really suck for around turns and curves?

 

We don't know what a POSI is.

You mean sure grip??


Find a 3.23 SG. You'll be set.


Oh and for you guys saying 3.55's arn't a performance type gear.... I went 11.60s with mine, 26" tire, driving to the track an hour each way. 2900 @ 60 mph.

 

yes i did mean sure grip i figured every one was used to hearing posi

 

Not on a mopar site..

 

You will feel a great difference at the gas pumps, a loss of gas mileage. A727 Trans is ok with bigger gear ratio's the 904 not so much. All you are gaining is a bigger burnout and an increase in gas money. Yes you should be able to feel a little more torque on a step down from stopped. But cruising the interstate at speed limit for example a jump from say 2000 rpms to 2400 rpms for the same speed. burning gas my friend.

 

Joe -

You know this is a 10 year old thread, right?
And sorry, 3.23 to 3.55 ain't gonna be that noticeable - anywhere.

Other than that. welcome.

Archer

 

this lol...

 

The 3.55 is a nice street gear , it just depends on how much motor you have. It should wake it up at stop lights and still give you a nice cruise gear…