I have a question for all.... I have a chance to pick-up a GM 12
bolt
rear-end for 150.00 pretty sure it's a C lock gears are said to be
3:55,
don't know for sure, with out popping the cover, is there a way to tell?
it's a one legger, but better then the 12 bolt 3:08 or 2:97 I am running
now.
any help would be great..
thanks
john orr
67 cutlass supreme 4 spd
tampa fl
======================
easy as can be
the number is merely the ratio of the gears
for those who may not well understand that phenomenon:
it is the ratio of the number of turns of the input to the number of turns
of the output.
Since the gear teeth mesh [if all goes well] and never skip... it is also
the ratio of the gear tooth counts.
Removal of cover makes tooth count easy, esp. since GM gears have the tooth
counts stamped into the ring gear OD.
Don't wish to remove the rear cover? Then simply COUNT TURNS!
It only has really one complication: posi or open. Why would that matter?
EASE of the act.
The rato is commonly expressed as the number of input turns to attain ONE
output turn- though of course you could as accurately choose 2 or 3 or 10 or
3.14159 turns of the output shaft as a standard.
Off on a Tangent: By the way, the colon [:] is for placing between numbers
of a ratio, and NOT for use as a decimal in the ratio number when using the
default "### to 1" ratio format.
Example: you have two meshing gears with tooth counts of 30 and 10
The ratio is 30:10 or 3:1... or by decimal place- 3.00:1 usually shortened
to "3.00"
Not "3:00" which denotes a time of day or an impossible gear ratio. [COLON
NOT ALLOWED HERE]
Furthermore, the example ratio above will virtually NEVER be seen in
production- they like to use a PRIME NUMBER for at least one tooth count,
like 11, 13, 41, 39, etc. With a little thinking, I am sure you will
understnd why that is a Good Idea.
So, why is posi or not a crucial point?
Because you need to attain ONE FULL TURN off the ring gear while counting
how many turns of the pinion are required to do so. That is the ratio
number. You can get close enough just by quick inspection if you can get one
or both wheels to turn.
Std issue OPEN FINAL DRIVE: Because of the way a differential works, to
get one full turn _of the ring gear_, with one wheel held still you must get
-2- turns of the one wheel that is turning.
POSI: because the wheels are [better be] tied together and turn
together... you will elevate both wheels and get one full turn of EACH
wheel.
The ring gear does not care which way you do it. You can elevate both
wheels of an open diff and try to babysit BOTH wheels to get exactly 1 turn
of each, or, you can attain 1.442 turns of one wheel and 0.558 turns of the
other wheel- as long as they ADD UP TO two full wheel turns.
By far the easiest ways are 1 turn of each.. or 0 of the one and 2 turns
of the other.
Went to pick up a rare '58 convertible MANUAL TRANS car in PA a while back.
I counted turns by elevating ONE rear wheel and found that it was indeed the
std issue 3.23 ratio for MT cars. The feller that owned the place scoffed at
me and told me that was no way to ascertain the ratio. Which told me that he
does not know as much as he thinks he does. But, as I was trying to buy an
OAI air cleaner from him, I thought it prudent not to point that out. Yet.
I generally start with the pinion at a nice handy straight up-n-down
position or sideways to that. Mark the one elevated wheel's drum and backing
plate with the paint pen. A mark on the pinion yoke does not hurt as well.
Make sure the other wheel does not turn [I rarely even see a posi]. Start
turning the pinion and count turns- quarter, half, 3/4, one, etc. Very easy
to tell a half and quarter pinion turn by the positon of the u-joint yoke.
When the wheel [brake drum] mark passes once, you know you are halfway
there, so when you approach 2x that, slow down and rest it at the final spot
where the wheel's marks align again. Generally you will find the pinion has
turned just shy of,or just past, one of the 1/4-turn obvious positions.
Now use your knowledge of common gear ratios to figure what one it is [or
close enough for Engineering purposes]. 4 & 1/3 turns = 4.33:1 or "4.33". By
far more common would be just past 3 turns or 3.08. Just over 2+1/2 turns is
the 2.56 econo-ratio. almost exactly 3+3/4 is 3.73, etc. etc. etc.
Final drive ratio is carefully chosen for a balance of economy and
performance. It is not just the SPEED variation of the two shafts, but also
the TORK AMPLIFICATION that the gearset attains. That is, with the econo
2.56 gears, your engine spins 2.56 times as fast as the rear axle [in 1:1
top gear], loafing along. However, you will sooner need to downshift when
pulling the trailer up that mountain, and quarter mile performance [maximum
acceleration] will not be stellar, because you are multiplying the tork at
the driveshaft by a mere 2.56 to the final drive axle.
WHEREAS, with the performance ratio such as oh say my car's 4.33 gears-
the driveshaft must spin at 4.33x the rear axle speed, so the engine is very
'busy' and noisy and sucks gas like you own a refinery.... but the fun part
is uphill or acceleration contests, where the tork is multiplied not by 2.5
or 3.1, but FOUR point three.... Yeehah. It's a major trade-off. Fun vs.
economy. Hillclimbing or towing vs. flatland hi-speed runs. Of course, you
SHOULD choose other parts to match the final drive ratio, like the engineers
did. Turnpike Cruiser mild cam 2-bbl engine with 2.56 ratio. High
compression, 4-bbl, close-ratio M21 or M22 go well with the performance
gearsets.
Works similarly in vice-versa too. [when coasting or using wheels to drive
the engine]
You don't try to bump-start a motor in low gear, you use high or near-high
gear, where it's harder for the engine to turn the wheels, but easier for
the wheels to turn the ENGINE.
Because they make it easier for the engine to drive the wheels,
Performance Gears make it harder for the wheels to drive the engine, so they
provide increased "engine braking" effect. Thus the use of low [trans] gears
when ascending or descending a mountain. Increased tork amplification on the
way up, increased engine braking on the way down.
yes, I know I spelled TORQUE 'wrong'
It is my lifelong quest to get it officially spelt RIGHT: tork.
Other mistakes are typo's- I can spell, just not type perfectly.
WANTED:
1968-9 2 groove Water Pump PULLEY "KA-401462" stamp . NOT the common PS Pump
Pulley. Must have 4 bolt holes + center hole for water pump use.
-----------------------
Chris Witt
*the* Rocket Scientist
1303 W. Miller Rd.
Lansing MI 48911
[EMAIL PROTECTED]
Cell 517-449-0432 weekends or short weekday calls or leave message.
Home 517-882-9747 thru 10-11pm MI time most days
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