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Author Topic: 69Z rear ratios  (Read 2887 times)
jsg1010z
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« on: September 25, 2013, 10:28:27 PM »

I have searched for threads on rear ends codes for 69 z28s and I am having trouble.  My rear is stamped BN 10 16 G2  and the casting date is J 1 9 with 3969341 casted as well.  Is the BN 3.55 Non Posi ratio possible for a Z?  (TT date 11A).  Thanks for your help!
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69Z28-RS
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« Reply #1 on: September 25, 2013, 10:31:47 PM »

Yes, it's possible..  although not the std ration (3.73 is std).   Posi was an option, and not mandatory for Z28s or any other model I don't believe...  although I'd bet 99% of Z28's had Posi...   3.55 would be good for the highway, but would likely be difficult around town given the 302 characteristics...
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Gary W.  /  69Z28-RS, 72 B 720 cowl console rosewood all tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe (2), 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
72 El Camino, '55 Nomad, '57 Nomad, '57 B/A Sedan
KurtS
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« Reply #2 on: September 26, 2013, 12:45:13 AM »

Very unusual combo. I know of only one other 69 Z with a 3.55 open.
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Kurt S
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william
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« Reply #3 on: September 28, 2013, 09:50:04 AM »

For all '69s other than Z/28 posi was required with 3.73 and lower ratios. For Z/28 posi was required with 4.10 and lower; 4.10 was dropped as an option for Z/28 June 1969.
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BULLITT65
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« Reply #4 on: September 30, 2013, 11:47:53 AM »

I had a Z with 3.55 gears and a M-21, the car was such a dog off the line, until you were rolling at about 20mph, then you were good to go. In fact I had to keep a close eye on the tach because the motor was still pulling well into the 7000 rpm range. I have have the 3.73 which is a lot better in my current Z,  and I would say a 4.10 or even 4.55 would be more ideal a set up for the 302 4 speed configuration.
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1969 garnet red Z/28 46k mile unrestored X77
Looking for 3192477 (front) spiral shocks 3192851 (rear) please
Looking for an original LOF soft ray windshield
Looking for original Delco side post negative battery cable part # 6297651AV
firstgenaddict
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« Reply #5 on: September 30, 2013, 05:44:42 PM »

A 3.55 M20 (8.946) would have a steeper effective first gear than a 3.73. M21 (8.206)
It is very close to a M21 4.10 effective ratio of (9.02)

The close ratio box was really designed for road racing, where there is more need for tighter ratios in order to keep the engine in the peak of the curves through the differing turns and elevation changes.
The availability of a slightly different gear ratio between 1st 2nd and 3rd in drag racing has little over all effect on the ET, where as when you are shifting 15 times per lap and you can pick up a slight advantage in pull coming out of the turn X every lap then tailoring ratios will pay off significantly.
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James
Collectin' Camaro's since "Only Rednecks drove them"
 
Check out the Black 69 RS/Z28 45k mile Survivor and the Lemans Blue 69 Z 10D frame off...
https://picasaweb.google.com/112392262205377424364/1969_Z28_Restoration
69Z28-RS
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« Reply #6 on: September 30, 2013, 11:14:27 PM »

Good points James!  Smiley
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Gary W.  /  69Z28-RS, 72 B 720 cowl console rosewood all tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe (2), 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
72 El Camino, '55 Nomad, '57 Nomad, '57 B/A Sedan
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« Reply #7 on: November 17, 2013, 01:26:57 AM »

I love the 4.10 M20 combo which is how the Lemans Blue Z I have now was ordered, however I believe M20-3.73 would be a great combo for the street.
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James
Collectin' Camaro's since "Only Rednecks drove them"
 
Check out the Black 69 RS/Z28 45k mile Survivor and the Lemans Blue 69 Z 10D frame off...
https://picasaweb.google.com/112392262205377424364/1969_Z28_Restoration
BULLITT65
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« Reply #8 on: November 17, 2013, 01:44:24 AM »

I wonder how often back then (when they were road racing) they would swap out their rear end gears and transmissions for better ratios fro a particular track. 
I would think the M-21 with the 4.10 may have been a good combo for some tracks, but it seems like it wouldn't have been a one size (ratios) fits all week in and week out.
Would the M-20 ratio have ever been preferred for a specific track with a special rear end ratio?
Maybe Jon Mello (trans am guru) would have some insight here ?
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1969 garnet red Z/28 46k mile unrestored X77
Looking for 3192477 (front) spiral shocks 3192851 (rear) please
Looking for an original LOF soft ray windshield
Looking for original Delco side post negative battery cable part # 6297651AV
Jon Mello
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« Reply #9 on: November 17, 2013, 11:46:07 AM »

I've not ever heard of a road racing team using the M20. I'm not saying it couldn't have happened but I'm not aware of it.
Because of the relatively small displacement, the torque range is narrower and higher up the rpm scale. If there is a larger
jump between each gear ratio, the engine speed could fall out of the optimal range and that would have an adverse effect
on lap times. You are rarely using first gear in a road race except for the start or leaving the pits. Ford did homologate a lot
of different gear ratios for the transmissions but they had a huge amount of money that they openly threw at their racing
program and Chevy was not allowed to race at that time. Ford would rent track time ahead of each Trans-Am event and dial
in a car in terms of tranny ratios and rear axle ratios (plus spring rates, etc) and Penske would do this as well but I've just
never heard of an M20 being used. In 1970, the SCCA allowed race teams to use rear axle and transmissions from rival
manufacturers. This allowed the Chevy teams to use the Ford rear axles and transmissions and of course it is much easier
to make a rear axle gear change on the Ford axle than on the Chevy. Prior to that, most independent Chevy racers were
probably just running the same rear axle ratio regardless of the track and doing the best they could. Penske could afford
to make a gear change in his rear axles or have a few spare trannys and axles on the truck but most of the independents
couldn't afford that.
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Jon Mello
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BULLITT65
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« Reply #10 on: November 17, 2013, 12:26:42 PM »

thanks for the input Jon. So if they were probably using the close ratio transmission as you say, would all of the racers and Penske been using the M-22 because of the beefier gears?
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1969 garnet red Z/28 46k mile unrestored X77
Looking for 3192477 (front) spiral shocks 3192851 (rear) please
Looking for an original LOF soft ray windshield
Looking for original Delco side post negative battery cable part # 6297651AV
Jon Mello
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« Reply #11 on: November 17, 2013, 12:58:50 PM »

I would say in '67 that the majority of guys were using the M21 as that was the only tranny that was offered in the Z-28s
that year. I know for a fact that was the case with my gold race car. As each year went by in Trans-Am racing from '67 to
'72, I would believe that the usage of the M22 went up.
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Jon Mello
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KurtS
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« Reply #12 on: November 17, 2013, 02:51:58 PM »

In 1970, the SCCA allowed race teams to use rear axle and transmissions from rival
manufacturers. This allowed the Chevy teams to use the Ford rear axles and transmissions and of course it is much easier
to make a rear axle gear change on the Ford axle than on the Chevy. Prior to that, most independent Chevy racers were
probably just running the same rear axle ratio regardless of the track and doing the best they could.
I never knew about that rule change.
My father's mentor was an old German guy that ran dirttracks back in the 50's. Guy was a mechanical whiz. He could change the axle gears in about 10-15 minutes, including setting up the lash. Yikes. I bet once you had it dialed in, you knew which shims were needed for which gear sets.
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Kurt S
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« Reply #13 on: November 19, 2013, 12:41:01 AM »

Another thing to remember when road racing is flywheel weight and center of mass... removing a few ounces from the rotating assemblies via-aluminum flywheel and dual disk clutch is worth as much in lap times as removing 100 pounds of car mass. I read the formula in a technical automotive engineering & design book I have had for years, however reciprocating assemblies in engines are one of the best ways to gain power... you are accelerating each piston from a dead stop 2 times per revolution... think about that.
I worked for Gas Rhonda's Cammer engine builder in the early 90's and may have overheard a thing or two...   
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James
Collectin' Camaro's since "Only Rednecks drove them"
 
Check out the Black 69 RS/Z28 45k mile Survivor and the Lemans Blue 69 Z 10D frame off...
https://picasaweb.google.com/112392262205377424364/1969_Z28_Restoration
BULLITT65
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« Reply #14 on: November 19, 2013, 01:00:43 AM »

you have just doubled my knowledge concerning motors  Cheesy. Honestly good point, always thought about rotating mass as far as wheels go, didn't think of the flywheel, but same idea.. Wink
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1969 garnet red Z/28 46k mile unrestored X77
Looking for 3192477 (front) spiral shocks 3192851 (rear) please
Looking for an original LOF soft ray windshield
Looking for original Delco side post negative battery cable part # 6297651AV
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