#21  
Old 12-04-2016, 09:47 PM
73T/A 73T/A is offline
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Quote:
Originally Posted by John V. View Post
Thanks HFR, still a lot I don't know about the '73 TA.

I spent a fair bit of time searching the archives. Not a whole lot of help either way.

Found nobody that has claimed to have an original drivetrain base '73 TA with the M20.

I've been contemplating why PMD may have offered both but haven't come up with any good ideas. And why put the M20 in the SD if the base TA was getting the M21?

I'm 98% sure mine had the original trans in it when I owned it in '78. But there is no way I believe I could have sensed the difference between an M20 and an M21 so that is a dead end.

dadsformula, you stated that "nearly all of them were M21 Muncies", referring to the 227 '73 Formulas with 4 spd and L75 455. Does this mean you know of documented '73 Formulas that got the M20 with the 455?

Based on what I know about the usage of the M21 in the '64 GTO and the fact that a LOT of '64 guys thought they had an M21 in their original drivetrain '64s that invariably proved to be an M20 (when the M20 was a 2.56 1st to boot) and I'm convinced very few could detect the difference from just driving.

So unless somebody out there steps up with proof of a base '73 TA with the M20, I'm resigned to the idea that the M21 was the likely base '73 TA 4 spd.

Makes the '73 M20 I picked up not quite the prize I thought it was.
My 73 TA L75 4 speed still retains it's original drivetrain.Transmission is a M21 VIN stamped on trans also date code with the B designation.A=M20,B=M21,C=M22.One thing I thought of,my TA has the 3.42 rear end ratio could the 3.08 rear cars have had the M20?Or was 3.42 the only rear available with the 4 speed?

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1970 GTO
  #22  
Old 12-04-2016, 11:10 PM
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The 350/2 BBL would have used the Saginaw 4 speed.

Excerpt from the Pontiac Service manual...

And, from the Accessorizer, Trans Am Std. Equipment...
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Last edited by Held for Ransom; 12-04-2016 at 11:27 PM.
  #23  
Old 12-05-2016, 07:51 AM
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Rocky Rotella Rocky Rotella is offline
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Undoubtedly in 1973 the Trans Am and Formula 455, when equipped with the L75 D-port engine, were equipped with the Muncie-built M21 close-ratio 4-speed.

Formula 400 buyers had to choose between Muncie's M20 and M21 if they wanted a 4-speed. I have PHS invoices for examples with either trans. For years I have been trying to determine if it truly was order-taker selection because it doesn't appear that inclusion was limited to ancillary equipment (such as performance axle or A/C).

  #24  
Old 12-05-2016, 01:12 PM
John V. John V. is offline
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Rocky, just to confirm, are you saying that no base '73 Trans Am (standard L75) got the M20 and no '73 Formula with the L75 could be had with the M20?

Do you know if a later revised version of the '73 Firebird Order Form exists, the one I have is revised 6-12-72? Perhaps the trans option choices were corrected at some later date?

I am convinced that the M21 could be had in the TA and Formula with the L75, but trying to determine if the M20 was possible for either or both.

The Accessorizer clearly states that the M20 was Standard for the Trans Am. And the early Order Form not only implies that, it also shows the M21 as Not Available for the TA. But no such prohibition was indicated for the Formula which doesn't seem at all logical.

The verbiage from the Service Manual excerpt seems to align with what you and HFR have indicated, that is, the M20 was available for the 4 bbl 400, an engine that was only offered with the Formula.

And the M21 was ALSO offered for the 4 bbl 400, a choice that seemingly was readily made on the Order Form. But the only 4 spd allowed for the L75 according to the Service Manual description was the M21.

If the Service Manual was correct and the evidence seems to agree, the Order Form and Accessorizer were wrong. That would not be earth-shaking news.

The Service Manual and the Order Form would both seem to suggest the 4 bbl 400 (Formula 400) could be had with the M20 or the M21 as a buyer's choice, no other ordering restrictions seem likely and you have found no evidence to suggest otherwise.

And while the Order Form did not indicate a prohibition for ordering the M20 with the L75, perhaps any such attempt to order it that way was over-ridden at order entry.

My early GTO experience makes all of this a bit strange to me. When the M21 was made available very late year for the '64 and continued for the '65 GTO, you could only get it with the 3.90 axle among other mandatory options (was not required with the 3.90 but couldn't be had with lower numeric gear ratios).

But for the '73 Firebird, they were allowing the M21 with some pretty low numeric gear ratios although I am not certain what ratios were available in the Formula or Trans Am.

And I also think it is bizarre that they put the M21 behind the D port 455 but put the M20 behind the 455 SD. Was that intentional to give the SD a "quickness" advantage? Or was it in recognition that the SD could stand higher revs than the D port 455?

HFR, do you have an Axle Ratio availability chart for '73? Was the 3.73 axle available? Were the axle ratio choices same for the D port 455 as for the SD?

The Order Form seems to suggest a G92 Performance Axle option was available for the Trans Am. Relying on my early GTO experience again, no performance or economy axle regular option was released for the '64 GTO, but it was possible to special order a limited variety of approved numeric axle ratios for the '64 GTO while other ratios were only available over the counter from the dealer. Curious if special order ratios were allowed for the '73 TA?

  #25  
Old 12-05-2016, 06:53 PM
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John,

That is the way that I understand it.

Based on the powertrain usage chart in the 1973 Firebird Assembly Manual, the L75 455 was only available with the M21. Both the M20 and M21 were available with the L78 400. The M20 was limited to the SD-455 only, and that’s due to emissions certification testing.

I’ve been trying to clarify what occurred for a while. I haven’t located any bulletins that outline transmission usage. I do have PHS invoices that show Formula 400s invoiced for M20 and M21 transmissions. None of the invoices includes AC, so that couldn’t be a factor. And neither included G92.

I haven’t come across any L75 Trans Am invoices with M20.

  #26  
Old 12-05-2016, 07:53 PM
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One of the documents would have been the mid-year bulletin dated 5-10-73 from the Service Bulletins.

I didn't even think about what was listed in the Assembly Manual.
Here is the early (AMA) and later mid-year sheets (I don't have the Powertrain Sheets from the Service Bulletin)...
And, the Assembly Manual sheets.
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Last edited by Held for Ransom; 12-05-2016 at 08:37 PM.
  #27  
Old 12-05-2016, 08:46 PM
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1973 Power Train:



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  #28  
Old 12-05-2016, 10:31 PM
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John, thanks. Was there a date attached to it?

  #29  
Old 12-05-2016, 10:49 PM
John V. John V. is offline
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Interesting stuff guys.

Couple thoughts. Rocky, why would the trans affect emissions? I thought the cam was changed for that reason, didn't realize the trans had an impact. The higher gear multiplication provided by the M20 reduction gears was better for emissions? Seems a bit counter-intuitive to me.

The Excerpt with the Fig. 3 was interesting. I knew the trans changed with the mid March EGR snafu.

So no surprise that the Chart gives the old and new trans p/ns and codes.

I thought I was on to something when I saw the new p/n 6271518 code WK was tagged as a Close Ratio.

But neither of the two "Old" Muncie 4 spds was tagged as a Close Ratio.

But when I checked p/n 332578, code FC, I found it listed in the '77 MPC and described as Close Ratio. So just another error in the documentation.

Curiously, the 6271516 and 6271518 were not listed in that '77 MPC.

But I found a forum post elsewhere that indicated the 6271516 and 6271518 were used in the '72-'74 Corvette (at least until the Muncie was discontinued and the B-W T10 superseded it). In '72, the Corvette also made use of the 6271517 which was the M22 version. Same Manifest codes as used in the late '73 TA too. So looks like at mid year, PMD was able to make use of the same Muncie 4 spds that the Corvette was employing.

So now that I know my TA would have been built with the Code FC M21 (mine was built in January), I'm only a little torn as I think I might prefer the gearing of the M20 that I have acquired anyway.

I'm pretty certain the rear axle housing is original (center casting has a Jan '73 cast date) but I'm not sure it still has the original gears in it. I'll need to check on that.

Thanks for all the info on this. I love this kind of detail even though I am not planning a very correct resto for this TA. I still like knowing what it was born with.

  #30  
Old 12-06-2016, 01:35 PM
dadsformula dadsformula is offline
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No I have no evidence of an M20 used on a Trans-Am or Formy with the L75
455. The guys have this correct. No M22 in 73, and the only Trans for the
the L75 was the M21 Big in Big out Muncie. I think there was a reason for this
or multiple reasons. This Combo L75/M21 used only a 40 pound Flywheel and
an 11 inch Clutch. The max Axle ratio was 3:42. This extra inertia I think was
to compensate for the reduced axle choices, and keep the MPG's in a reasonable
range. Remember this was 1973 (Oil embargo Ect) in 71 and 72, you could
get the axle ratios, with the M22, and this was the major difference in 73. The
reasons were predictable, it was a way to continue the 455 without the HO features
for marketing, and the reduction in axle ratios with the big flywheel kept the RPM's
down for MPG. Pretty much a requirement of the times. The SD however is the
aberration, and everyone knows that story. Really kind of amazing the thing was
even offered during that time. I had a job at the Gas station at the end of my street
in 1973, and when I arrived the line was already a mile long. It sucked even owning
a Car then, and these days very few realize how easy it is for something to happen to
return us to those days or worse. Living in that time was crazy as MPG became the
phrase known in every house for the first time. Know one used that term before the
embargo.

  #31  
Old 12-06-2016, 07:25 PM
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Quote:
Was there a date attached to it?
Sorry, Mike, haven't been able to find the paperwork yet?

I had pic of it.
(and page 2)

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  #32  
Old 12-06-2016, 11:21 PM
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It's probably from the 1973 Pontiac Fleet Buyers Guide.

  #33  
Old 01-12-2017, 04:19 PM
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unruhjonny unruhjonny is offline
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Quote:
Originally Posted by John V. View Post
... My understanding is that in '71/'72, the M21 was not offered in any Firebird ...
My understanding, and the Corvette always being the exception to the rule, is that divisions only got one close ratio per year/model line.

1970 Firebird = M20 or M21
1971 Firebird = M20 or M22
1972 Firebird = M20 or M22
1973 Firebird = M20 or M21

Quote:
Originally Posted by dadsformula View Post
... The reasons were predictable, it was a way to continue the 455 without the HO features for marketing...
That is a very interesting take on the matter - I never looked at it this way(!);

1972 Trans Am = 455(HO) + Close Ratio 4 spd (M22)
1973 Trans Am = 455(L75) + Close Ratio 4 spd (M21)

Effectively they both have the same displacement engines, backed by a close ratio four speed.

I wonder if the M22 was dropped for the same reason as the shaker being closed - the additional euphoric symphony, err, noise from the M22 may have contributed negatively towards the car's newly EPA(?) regulated noise levels as a whole...

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1970 Formula 400
Carousel Red paint on Black standard interior
A no-engine, no-transmission, no-wheel option car.
Quite likely one of few '70 Muncie three speed Formula 400's left.


1991 Grand Am: 14.4 @ 93.7mph (DA corrected) (retired DD, stock appearing)
2009 Cobalt SS: 13.9 @ 103mph (current DD; makes something north of 300hp & 350ft/lbs)
  #34  
Old 01-12-2017, 06:15 PM
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As far as '73 goes, what I find Amazing, ACROSS the GM line up, is that by 1939 Gm had a Reliable 4 speed automatic...( They put them into TANKS in WW2) .Not an OD, but how much work is that really? It took 42 years ( and 8 since the '73 oil embargo, to get a "C" on the back side of a 1981 Firebird ( TH350-C) for economy...And Why did they not build Over drive 5 speed Borg Warner Trannys? I stood in line at gas stations holding a spot for my Mom as a teen , to get a mere 5 gallons of fuel. Pontiac/GM waited till The 1973 formed EPA "CAFE" standards for '81 came along to actually do anything.( TH350-C and 6X15 Rally Wheels) OVERDRIVE in Big engine Pontiacs could have saved many models from extinction. Its all water under the bridge, but it makes one wonder why they were NOT thinking about the future markets as the imports came rushing in...

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  #35  
Old 01-12-2017, 07:09 PM
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/\ /\
ON a related note, I was interested in doing one of the custom gear sets for a muncie which was nick-named a marriage-saver;
The 3rd & 4th gear positions got swapped, but what you were left with was a four speed with 3rd gear being 1:1, and 4th gear was OD;
Considering the gear spread on a TH350 is seen as about perfect, why didn't the manufacturers do more of these OD sets... more because in my searches, I was finding stuff used in the 1950's that was overdriven...
But nothing which would be a direct bolt-in for my 'bird.

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1970 Formula 400
Carousel Red paint on Black standard interior
A no-engine, no-transmission, no-wheel option car.
Quite likely one of few '70 Muncie three speed Formula 400's left.


1991 Grand Am: 14.4 @ 93.7mph (DA corrected) (retired DD, stock appearing)
2009 Cobalt SS: 13.9 @ 103mph (current DD; makes something north of 300hp & 350ft/lbs)
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