67 posi - Impala Tech
Performance Our High Performance Area

 
LinkBack Thread Tools
post #1 of 14 (permalink) Old 12-01-2015, 03:54 PM Thread Starter
Member
 
Join Date: Mar 2015
Location: Rochester, NY
Posts: 60
 
Garage
67 posi

I want to install a Posi unit in my 67 Impala (10 bolt). What gear ratio would give me faster starts, without having too high RPM's at 60 mph?
whons is offline  
Sponsored Links
Advertisement
 
post #2 of 14 (permalink) Old 12-01-2015, 04:31 PM
Senior Member
 
Join Date: Apr 2014
Location: North Dakota
Posts: 130
 
Garage
That's a loaded question, need more info. What size engine, horse power, type of transmission, tire size, I would guess 3.08 to 3.73, probably in between there. What do you have now?
6366 is offline  
post #3 of 14 (permalink) Old 12-01-2015, 06:30 PM
Moderator
 
Join Date: Jan 2006
Posts: 12,361
 
3.55 was considered a towing gear for a pick-up, though I ran a 4.88 or 4.56 in most of my BBC street cars I also ran an OD tranny. 3.73 is about as high as any one is willing to go today without a an OD tranny. Though when these cars were new the Z/28 came optioned with a four speed Muncie and a 4.10 gear in the 12 bolt rear as the most common gear ratio ;and the Pontiac GTO used a 3.90 in their 12 bolts as it's most frequently installed gear.

In newer Police packages (mid to late eighties) a 3.08 was as steep a gear as could be ordered without an OD while the rest of the cars on the road had 2.55 or 2.73 rear gears. It gave the police a car a better acceleration to chase someone down.

Big Dave
Big Dave is online now  
 
post #4 of 14 (permalink) Old 12-01-2015, 08:25 PM Thread Starter
Member
 
Join Date: Mar 2015
Location: Rochester, NY
Posts: 60
 
Garage
My engine is a 350, bored out .030 over and the transmission is a Turbo 350. I'm not sure what the gears are. I will check it out this weekend. Thanks for the replies. Any info is helpful.
whons is offline  
post #5 of 14 (permalink) Old 12-02-2015, 12:07 AM
Senior Member
 
Join Date: Mar 2008
Location: Moneta, VA
Posts: 825
 
I'm not that familiar with the newer Ratios, but in '55 to '64, a good all around gear today would be a 3.36 or a 3.55. The 3.08 would give you the lowest RPM on the highway. A little daring would be a 3.70, but I wouldn't recommend it if you plan to drive at high highway speeds.

I believe there is actually some charts out there that will show in better accuracy.

-Parting Out over 75 '58 to '73 Full Size Chevy Cars-

Member:

-National Impala Association
-Vintage Chevrolet Club of America
62BillT is offline  
post #6 of 14 (permalink) Old 12-02-2015, 12:39 AM
Senior Member
 
Join Date: Apr 2013
Location: Canada
Posts: 324
 
You need to figure out how much driving you do at low speed (city) vs. higher speed (highway) and how much emphasis you put on fuel economy.

I am guessing your stock gear will have been 2.73, maybe 3.08 at best.

If you figure out your tire height you can plug in different numbers for gears and calculate your RPM at 60 mph. Add about an extra 250 rpm for torque converter slippage.
Darth is offline  
post #7 of 14 (permalink) Old 12-02-2015, 07:38 AM
Senior Member
 
Join Date: Mar 2015
Location: Stillwater, Minnesota
Posts: 328
 
Use this calculator. You'll need to know the gear ratio and tire height. Then plug in the speed you want to go and it will give 4 options for different transmissions. Your TH350 is a direct drive, so use the first box, Auto, for rpm.

http://www.ringpinion.com/calculators/calc_rpm.aspx

'62 Impala SS 409 TH350
'66 Chevelle SS 496 M20
'70 Chevelle SS 396 M20
'67 Camaro ss/rs 350 PG
'38 Chev coupe street rod
'54 Chev 210 2 door
'69 Chev C10
'89 Chev R3500 roll back
Lost in the 60's is offline  
post #8 of 14 (permalink) Old 12-02-2015, 08:26 PM Thread Starter
Member
 
Join Date: Mar 2015
Location: Rochester, NY
Posts: 60
 
Garage
Thanks alot guys. This gives me great info.
whons is offline  
post #9 of 14 (permalink) Old 12-02-2015, 10:56 PM
BA.
Moderator
 
Join Date: Aug 2008
Location: Miamisburg, Oh.
Posts: 3,129
Blog Entries: 15
 
All good tips above.
I do enough highway driving (well, I want to anyway) that I stayed on the mild side a little bit, or so I thought.

Even with 275x50x17 tires (27.8" tall) , I am around 3000rpm at a highway speed of 70mph with my 3.42. Not too bad really, but a little more than I wanted.
If you measure your tire height correctly, the calculators our there won't lie.

HOW A NOVICE REBUILDS A 66 IMPALA CONVERTIBLE:
http://www.impalas.net/forums/blog.php?u=1432
BA. is offline  
post #10 of 14 (permalink) Old 12-02-2015, 11:35 PM
Moderator
 
Join Date: Jan 2006
Posts: 12,361
 
I was running 3200 RPM with a 582 with a set of 4.56 gears and an OD transmission (4L80e) but that was actually a bad choice in gearing because the 582 had so much bottom end torque that there wasn't a set of tires that could hold it this side of a rubber cement coated drag strip with slicks: then I had other problems.



Big Dave
Big Dave is online now  
post #11 of 14 (permalink) Old 12-05-2015, 10:57 AM Thread Starter
Member
 
Join Date: Mar 2015
Location: Rochester, NY
Posts: 60
 
Garage
Big Dave, my gear ratio on my 67 impala is 3.36. Would I really notice that much difference if I put in a posi for a 10 bolt using the same gears? Also are the bolts on the Ring gear RH or LH thread?
whons is offline  
post #12 of 14 (permalink) Old 12-05-2015, 12:49 PM
Moderator
 
Join Date: Jan 2006
Posts: 12,361
 
In the newer AAW (American Axle Mfg.) built "GM Corporate" ten bolt 8.5 inch ring gear they are left handed threads. In your original Spicer made 8.2 inch ring gear ten bolt they are right hand threads.

If you use the same gear ratio in the rear end you will see no difference in performance. A higher numbered gear multiplies the torque the engine produces to accelerate your car faster. That also results in a higher RPM when cruising unless you go to a modern OD tranny that drops the effective rear gear ratio by 30% to 33% to as much as cutting it in half with some six speed manual gear boxes.

With a heavy car and a small displacement motor I would up your final drive ratio to a 3.55 gear which I feel would be as high as high as you would be willing to tolerate your motor's cruise RPM would end up. I grew up before the muscle car era so twisting a SBC to 10,000 RPM didn't bother me with a junk yard full of replacement motors and time on my hands to perform a motor swap. I have run as high a gear as a 6.13 with a 283 or 292 motor. With a 396 or 427 I ran a 5.38 or a 5.13 gear at the track and a 4.56 to a 4.88 was my favorite for the street in my old BBC powered Camaros and Nova bodies.

Today I only run with OD trannies, and no more than a 3.73 rear gear.

Big Dave
Big Dave is online now  
post #13 of 14 (permalink) Old 12-05-2015, 01:35 PM
Senior Member
 
Join Date: Mar 2015
Location: Stillwater, Minnesota
Posts: 328
 
Quote:
Originally Posted by whons View Post
Big Dave, my gear ratio on my 67 impala is 3.36. Would I really notice that much difference if I put in a posi for a 10 bolt using the same gears? Also are the bolts on the Ring gear RH or LH thread?
The gears are interchangeable between a posi and non-posi carrier. There is no difference in performance. Going to a 3.55 will get you virtually nothing either, as the change in ratio is too small to notice. A 3.73 will get the car moving quicker, but the engine will be screaming at 70 mph and higher.

If you still have the powerglide 2 speed transmission a change to a TH350 will get you better acceleration, as first gear has a lower ratio than the pg. An 1800 stall converter will help a LITTLE also, if the engine has enough power to over-ride the stall. That will let the engine rev higher before the converter transmits fluid to the trans and gets the car moving. The higher rpm will put the engine in a higher torque range, hence the better performance.

You've never stated what your engine/trans combination is. If you are burning the tire off from a dead stop then, yes, you would benefit from a posi in the axle. If the car is slow accelerating now and someone told you a posi would help, that is a falicy. The biggest reason for a posi is to get both tires powering in a low traction situation from a high performance engine/trans that is spinning the peg-leg hopelessly.

'62 Impala SS 409 TH350
'66 Chevelle SS 496 M20
'70 Chevelle SS 396 M20
'67 Camaro ss/rs 350 PG
'38 Chev coupe street rod
'54 Chev 210 2 door
'69 Chev C10
'89 Chev R3500 roll back
Lost in the 60's is offline  
post #14 of 14 (permalink) Old 12-05-2015, 02:47 PM Thread Starter
Member
 
Join Date: Mar 2015
Location: Rochester, NY
Posts: 60
 
Garage
My engine is a 350 bored .030 over and the transmission is aTH350. Thank you very much for all your input. It helps me to understand things clearer.
whons is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Impala Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome