Rear End Gears

[crebo]

I have a 77. I'm currently running 3.08. I'm thinking more of 3.55 or 3.73. I really want it to pin me to the seat.
Car spec's
461 professionaly built-edelbrock heads-headers-750 holley carb-beefed up 700r4 trans. It's estimated to be at least 350 hp on the ground.

What do you guys would be the best option for me? Any advice will be much apprieciated

 

[406shark]

With having an overdrive transmission as you do, I would go with the 3.70's.

You will love the acceleration compared to your current 3.08's, and because of the overdrive tranny you will have NO downside.

Good luck,
Jeff

 

[81 Corvette]

just got done upgrading from 2.87 to 3.54 and love the difference. IMO, I think 3.54 is the best gear ratio out there. The first thing I would do, is build an excel table to see where the gear ratio will put your rpm's at for each gear. Next look at the table and pick the gear that will best match your driving style and habits.

One of my concerns was RPM on the highway. I don't have an overdrive, so a 3.7 would have completly killed my cruising speed. As is, I'm pulling 3300-3500 rpm on the highway. Accelleration was important as well for me, which was the first reason for the swap.

Plan on having to buy a lot more then what you think you will need. Mine took side yokes, carrier, and strut rods to finish the job.

 

[Glensgages]

Please verify "461 professionally-built.....estimated to be at least 350 hp on the ground".....
Are you talking 350 RWHP: i.e., in-excess of 450 crank-shaft HP?
:confused

I'd think a 4"-stroke 450+ HP Rat, with 3.06:1 First Gear, and 3.08 rear-gears, would provide neck-snapping acceleration, capable of smoking almost any street tire:
that's the equivalent of a car with a THM400 and 3.80 gears in Low, and the gobs of torque, normally-associated with 454" BBC, shouldn't need much more gear to get rolling.....

What kind, and size, tire are you sticking out-back; are you sure it'll hook???

 

[GaryS]

Personally, I would go with 3.55's for the final ratio. Great all around ratio. If I read your post right, 350 horses ought to be enough to excite most of us even with 3.08's.
Gary

 

[fc3]

My 72 sb vert is putting about 350hp (TH400) to the ground with 3.08's... All I can tell you is you better be on flat straight pavement... unless you like seeing where you have been...talk about exciting!!! I personally like 2800 RPM at 70mph with a top in that will stand the hair up on your neck. It will run the qtr at 13.1 with NO traction for the first 50 feet. After 120mph I run outta guts, but not pedal.

 

[crebo]

Guys thanks for all of the info. I was out training National Guardsmen so I was offline for a minute.

Glensgages..What I meant was I took it to a Rod shop in Southern Cali and had it done professionally. It's a punched out 454. After running the #'s on the computer it came out to 461... I haven't dynoed it but the computer software and his knowledge and expertise say that it's about 500 at the crank...I'm just giving a rough est. of 350 rwhp being that I haven't actually put it on a machine...

I'm running 245/60/15...I can burn the tires pretty good. It def moves out esp around 2500-4500. I just figured having a overdrive tranny I could get away with a lower gear.....

I was just thinking going from 3.08 to 3.36 wouldn't make a big difference...3.55 sounds good. Then there is 3.73. I so want it to stay in my power band. So My dilema is 3.55 or 3.73.

The guy that built the engine recommends 3.55 but he also gave me a formula to calculate the rpms....ensuring that I use the overdrive tranny correctly...

I guess to sum it up my car pulls very hard. But my cousin has 4.11's in his RoadRunner...

I hope that I didn't confuse anyone because I'm not that computer brite.

Thanks again for all of your advice and I take some more..
Chris

 

[Glensgages]

Guys thanks for all of the info. I was out training National Guardsmen so I was offline for a minute.

Thanks for your service.....

I haven't dynoed it but the computer software and his knowledge and expertise say that it's about 500 at the crank...I'm just giving a rough est. of 350 rwhp being that I haven't actually put it on a machine...

Sounds stout!

I'm running 245/60/15...I can burn the tires pretty good. It def moves out esp around 2500-4500. I just figured having a overdrive tranny I could get away with a lower gear.....

That all sounds 'well-and-good', and the OD will keep your RPMs down to sane levels.....
except you are probably making gobs of torque.
The low-gear of a 700R4 (3.06:1) makes the 3.08s 'feel' like 3.73s with a THM350 in 1st gear.
You're turning the tires already, but 3.55s will only INCREASE the torque multiplication by 15% over the 3.08s (3.55 divided/by 3.08 = 1.15), so...
it'll only be 15% MORE-LIKELY to not hook-up with 3.55s.

I was just thinking going from 3.08 to 3.36 wouldn't make a big difference...3.55 sounds good. Then there is 3.73. I so want it to stay in my power band. So my dilema is 3.55 or 3.73.

The 3.73s will only magnify the problem mentioned above, even worse, correct?
You get that bad-boy to hook-up (bigger/wider/softer tires, better weight-transfer, etc.) to the point it 'pulls' the motor down, THEN you can throw more gear at it.....
unless you want to do the best burn-outs on the street!

The guy that built the engine recommends 3.55 but he also gave me a formula to calculate the rpms....ensuring that I use the overdrive tranny correctly...

Following is MPH for each 1000 crank-shaft RPM, using a lock-up converter, .7:1 OD ratio, and 27" tires, with the varying rear gear-ratios you are considering:
3.08 gears = 37.6 MPH
3.36 gears = 34.4 MPH
3.55 gears = 32.6 MPH
3.73 gears = 31.0 MPH

I run 3.73s with the 700R4 OD in my '82, but I'm only making 200 HP (advertised) with a bone-stock Cross-Fire, which is a BIG-difference from a 461" BBC making 350 RWHP...
I can 'almost' spin the tires at-will, with all this gearing, while YOU can with 'just' 3.08 gears.

Just my thoughts: hope this helps!!!!!
GLEN!!!

 

[crebo]

Glensgages

That makes sense. I'm not sure what I want to do. I really don't want to be the burn out King.....I can do that pretty good now. Not to sure what to do. I'll continue with my rear-end...I have two more parts to order....
I need to take it to the track to see what it's made of. I run it hard on the streets.

Hey thanks alot!

Maybe I'll put it on a dyno soon. There should be some pretty good deals here In Gulfport,Ms. It's almost time for Cruisin the Coast...
Chris

 

[Glensgages]

crebo:
15 years ago, a buddy had an early-'70s Camaro with a hydraulic-cammed, pump-gas 406" SBC that ran 13-flats at 102 MPH, thru the pipes, on street tires, with 3.08 gears.
Those numbers might-not be earth-shattering by 'today's' standards set by high-tech C5s & Mustangs.....
but if the C5 Z06 runs mid/high 12s, and your 461" BBC makes more grunt than my buddy's ol' 406", you'll be close-to Z06-style 1/4-mile performance, and still be long-legged in OD with the 3.08s...

 

[iron cross]

This is easy, a 4.11.