Pontiac GTO 1967 400CID 4sp manual OVERDRIVE or Tremec 6 speed Magnum

Dedek963

The European guy
89
1
8
Prague
Because of very short rear end (about 4.11 according to the calculation)
I was considering to change the gearbox, which is Muncie M 20 with another gearbox from Tremec.
I would love the 6 speed Tremec with similar gear ratios in 1st 4 gears and have 2 more over.
But the necessity of body cutting made a clear stop to this project.
Now I am considering to buy an overdrive from GearVendors, where there is probably some minor change to the body.
Do you have some experience with the overdrive from GearVendors?
 

Dedek963

The European guy
89
1
8
Prague
Thank you for the video. It seems that the amount of changes is very similar to what somebody has to do in case of changing to Tremec gearbox.
So I finally decide to buy Tremec Magnum 6 speed with this gear ratios:
2.66, 1.78, 1.30, 1.00, 0.80, 0.63

Thank you for your help :)
 

Dedek963

The European guy
89
1
8
Prague
As shown in the engine topic, this day I made an assembly of the chassis and engine/gearbox.
The Tremec Magnum 6 speed gearbox is much bigger than original Muncie M21 wide ratio.
Because it is bigger I have to resolve several issues.

1.) Despite the fact that SST Silver Sport provided me with great service and tailor made solution the clutch housing does not match with the flywheel (btw freshly balanced), so the flywheel has to be modified. it is only a minor issue.

Much bigger issue is how to attach the gearbox to the frame and how to fit it under the floor.
SST has provided me with very detailed installation manual, which is really perfectly made step by step.
However...
According to this manual is necessary to cut the frame and cut the floor also. Sheet metal for the floor modification is a standard part of the delivery.
Lets see some pictures:

20200111_143743.jpg

Lets see the original mounting point. With the blue rectangle is approximately the point where the crossmember from SST has to be placed.

Inked20191219_153016_LI.jpg

It is necessary to cut the rectangle out with some space reserve for maneuvering, place the crossmember inside and screw it to the bottom of the frame.

I am aware of the fact, that SST sold many of this gearboxes and that the solution they have is functional. On the other hand I am doing all the modifications so, that the original soul of the car stays untouched and that all changes I do are reversible. Also I do want to improve the abilities of the vehicle and weakening of the frame does not belong to the improvements.
So what to do next?

There are more than one solution. But the base for all of them is to repeat what Pontiac did with the original frame. Welding the mounting point to the frame in an appropriate height (mind the angle of the driveshaft and positioning of the engine/gearbox assembly) and:
1.) use the original crossmember or
2.) shorten the SST crossmember to the same lenght as the original
The decision depends on how The exhaust pipes will fit into the solution.

Well > one problem solved. Now the second one. The height of the Tremec Magnum gearbox. I don´t want to cut the floor.
According to the measurement I have to increase the floor tunnel space right at the front end of the gearbox by 1.3" (hopefully I am right). (Therefore I have the metal sheet received)
The standard silent blocks between the frame and the body are 18 milimetr thick (0.7 inch). I am going to make them from a harder rubber to have the same stiffness but they will be 1.5 inch high.
(note - the suspension is now 1 inch lower than the original - so the body will be in the similar height as the original 1967 GTO). Thereafter there is only 0.5 inch in an area of 2 times 2 inch to hammer out by 0.5 inch. The area is under the dashboard so it remains to be invisible.

Cross me the fingers I am right :)
Thank you
______________________________
 

melsg5

Staff member
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Just my opinion, I do not see any measurable weakening of the frame with SST's method. The main area of movement on these types of full frame American cars is the front area where the engine is mounted. Are you familiar with spreader bars? They are a threaded tube with heim joints on each end so that their length is adjustable. Brackets are attached either by welding or to the upper control arm bolts and the bar is attached across. This prevents frame flex in this area under high turning loads. This is a link to a vendor that may carry them.
Spreader Bars | SpeedDirect
Concerning cutting the floor, isn't that the area under the console? You seem to be going through a lot of work to not do this. I'm probably telling you something you already know but the whole point of lowering the car is to lower the center of gravity. Your raising the body on the frame sort of defeats that purpose.
 

Dedek963

The European guy
89
1
8
Prague
No doubt that the SST design is working.

On the other hand I think, that my solution is less laborious in my case.
The body is off the frame and the rubber silent blocks have to be exchanged anyway.
Cutting the frame or a bit of welding is similar effort.
In addition there is no need to cut the floor and weld new sheet metal, making new paint from both sides. (If I am right)
I found a manufacturer of high quality rubber parts for acceptable price. So I can design whatever part I need.

If somebody needs an unusual rubber spare part I can help.

Lowering the car is not the goal so 0.5-0.7 inch height increment is not a problem when combined with lowering of the chassis by 1 inch.
Spreader bar might be applied only if the rigidity of the car will affect the handling so I can't stand it. Otherwise I am gonna stay with the original.
 

cammerjeff

Active member
638
30
28
Belleville MI
Just FYI that Cross member is not original to your car, Pontiac A-bodies of the 60's did not bolt the trans cross member to the frame. For some reason they decided to mount the ends in rubber pockets, and then bolt the rubber pockets onto the frame with brackets that captured the rubber. I always hated the design. On high HP cars, (manual trans cars are worse) the crossmember would start to "hop" in the rubber and could even start hitting the floor boards.

1579628268447.png 1579628310868.png

We used to replace them with cross members like the one you posted, usually from a Buick or Olds, that bolted directly to the frame rails and cured the problem.

Great Project by the way. Thanks for posting.

Also are you sure you have a M20 and not an M21? the 1st gear ratio of an M20 with the 4.11 rear gears would be good for around 12MPH! before you RPM'd out. They were usually used with rear gear ratios of 3.55 or higher, usually 3.08 or 3.23 axles. The combo makes for decent cruise RPM, good launch, but a lousy 2-3 shift as the RMP's fall out of the engines powerband.
______________________________
 

Dedek963

The European guy
89
1
8
Prague
Hi Cammerjeff,
to be honest I am not sure if this is a M20 or M21.
I guess it’s not an original gearbox, but one from the 1965 year.
The gears are very close to each other and there is no RPM drop between 1st and 2nd gear.
There are following numbers at the case:

Stamped P1217
Stamped 163536 (or 163556 - the 5th letter is not clearly visible)
Cast 3851325
US patent 3088336
below this is cast number 5 and two circles. One with the line over diameter in direction 10->4 hours and the second with the line 9->3 hours

At the rear end is cast GM 3846429 IMG-20200122-WA0010.jpg IMG-20200122-WA0004.jpg IMG-20200122-WA0001.jpg

IMG-20200122-WA0009.jpg
 

Dedek963

The European guy
89
1
8
Prague
Just FYI that Cross member is not original to your car, Pontiac A-bodies of the 60's did not bolt the trans cross member to the frame. For some reason they decided to mount the ends in rubber pockets, and then bolt the rubber pockets onto the frame with brackets that captured the rubber. I always hated the design. On high HP cars, (manual trans cars are worse) the crossmember would start to "hop" in the rubber and could even start hitting the floor boards.
Thanks a lot for this information. I was not aware of this.
 

cammerjeff

Active member
638
30
28
Belleville MI
Without pulling the side cover and looking at the gears there is no sure way to tell if it is a M20 or M21, you input shaft has the course 10 Spline (as opposed to fine 26 spline) input shaft with no ring, so this would mean it was installed in a M20 trans at the factory. But as replacement shafts do not have any rings machined into them if it has been replaced in the last 50 years that identifier is useless. An 1965 M20 will have either a 2.56 or 2.52 1st gear ratio, a 65 M21 will have a 2.20 1st gear.

So if you put the trans in 1st gear it will take around 2.5 rotations of the input shaft to turn the output shaft 1 rotation if it is a M20, it will take about 2.2 rotations if it is a M21.

From left to right, early M20, M21, later M20, and M22.

1579813350745.png
 

Dedek963

The European guy
89
1
8
Prague
The discussion above led me to creating a table of gear ratios with muncie M20, Muncie M21 and Tremec Magnum 6 speed close ratio

Gear ratios with different gearboxes - Pontiac GTO 1967
Final gear ratio 3.90 to one, standard factory tires


Muncie M20ratio to prev gearMph at 3000 rpmMuncie M21ratio to prev gearMph at 3000 rpmTremec Magnum
6speed, close ratio
ratio to prev gearMph at 3000 rpm
2,52​
24,45​
2,20​
28,00​
2,66​
23,16​
1,88​
0,75​
32,77​
1,64​
0,75​
37,57​
1,78​
0,67​
34,61​
1,46​
0,78​
42,20​
1,28​
0,78​
48,13​
1,30​
0,73​
47,39​
1,00​
0,68​
61,61​
1,00​
0,78​
61,61​
1,00​
0,77​
61,61​
0,80​
0,80​
77,01​
0,63​
0,79​
97,79​
 
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melsg5

Staff member
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Maybe i missed it, but what rear end gear ratio and tire diameter are you using for the mph calculation?
 

Dedek963

The European guy
89
1
8
Prague
For this calculation I used the standard factory 1967 values for tires and final gear ratio, published in the Car and Driver Road Test from march 1964.
Actual tire size and rear end values are not relevant for comparing the gearboxes so I forgot to mention it.
In this test the final gear ratio was 3.90 to one.
The speed is used in the table just to imagine what change of what makes which difference in speed.
(I edited the table to have everything in one place)
 
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