Overdrive tranny in an A-body

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AJ/FormS said:
This swap has been successfully done many times. There is now even a brand new cross-member being repoped (USCarTool) so you don't have to fab one yourself. But no matter what, the floor has to be cut open, and the T-bar crossmember raised and reconstructed.
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Edit;
no matter what; the floor should, (that's should), be cut open to raise the driveline centerline back up to it's factory height. If this is not done, the front U-joint angle becomes near zero. And so then, to restore the factory freedom-from-vibration angles, and the proper U-joint cycling, the rear pinion angle has to be adjusted...... but, unless you raise the rear of the car significantly, it can never properly equalize the angles. So, I suspect that under power, you will have U-joint problems.
I don't know this to be true.
But;
I bolted a GVod onto my long-tail A833, and ran into this exact same thing; an elusive,impossible to eradicate vibration, traced to the U-joint angles.
My car is lowered about 2 inches and leveled front to rear,and I only lowered my tail mount about 1/4 inch, compared to what looks like several times that, with the USCarTool dropped Crossmember.
So Plumkrazee70 is right that when using that mount, you don't "have to" cut the floor.........
But if I was doing it, and I probably never will, I would raise the back right thru the floor cuz that U-joint angle is already from the factory, not enough for a performance driving with a lowered ride-height.
I mean, to me, the benefits of running that overdrive are well worth the work to make it vibration-free, with 3.91s to 4.10s.
Now, If you run 2.76s like Trailbeast does, this slows the driveshaft to ~67% compared to 4.10s, and if you run 65mph instead of 80, that is a further reduction in driveshaft rpm to 81%. Taken together that should reduce the vibration very significantly, to the point that you might be able to get away with the USCarTool dropped crossmember.
But that won't solve your U-joint life problem. No biggie; I just put a 1350 joint on there.
And it won't solve your vibration when the axle winds up under power and takes the front angle from zero to slightly nose down.... to nose up; no problem, I just lowered the pinion some more.
I still have some cruise vibration left.
 
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i've read all the threads i can find here and other sites and it always seems to come back to this

plumkrazee70 said:
The floor doesn't need to be cut at all. Only modification is to the upper hoop of the cross member.
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kursplat said:
for both a 500 and 518, or just a 500?
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plumkrazee70 said:
Both...
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AJ/FormS said:
no matter what; the floor should, (that's should), be cut open to raise the driveline centerline back up to it's factory height. If this is not done, the front U-joint angle becomes near zero. And so then, to restore the factory freedom-from-vibration angles, and the proper U-joint cycling, the rear pinion angle has to be adjusted......
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so,
1: no matter what you need to cut the upper support out and redo how you stiffen the floor area
2: using the US Cartool piece just means one less thing to fab
3: in general you want to keep the drive shaft angle as close to factory as possible, even if that means cutting the floor.
4:whether a 500 also needs the floor cut to fit in there is still a question
 
kursplat said:
i've read all the threads i can find here and other sites and it always seems to come back to this








so,
1: no matter what you need to cut the upper support out and redo how you stiffen the floor area
2: using the US Cartool piece just means one less thing to fab
3: in general you want to keep the drive shaft angle as close to factory as possible, even if that means cutting the floor.
4:whether a 500 also needs the floor cut to fit in there is still a question
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I can't understand how it's still a question? @MileHighDart just finished his weeks ago and didn't cut the floor. @Junior340 and @TrailBeast been driving theirs for years and they also didn't cut the floor...
 
i think all three of them did 500's? can you do a 518, not cut the floor, and still get good drive shaft angles. i don't know. maybe i'm just worrying about something that isn't a problem :eek:
 
I've only had mine up and running for a few weeks, then had to put her back on stands to address an oil leak, and then got into doing a little work on my exhaust, and then it's been snowing a lot for a couple weeks.
Point is I've only had it out for a few drives, but it seems at this point that I don't have any drive line vibrations at all, or at least not that I can feel. I've only had it up to 65, and haven't done any hard acceleration take offs yet, don't know if there will be any issues when putting more torque to the rear end and it rotates a bit. Right now it all seems fine.
But I gotta say the overdrive is awesome, and well worth the effort.

For the guy that mentioned the Gearvendors unit, you'll probably have about $3500 bucks in that setup by the time your finished. You'll have I think its a .78 overdrive instead of a .69 like the A500. I have less than $1000 in my A500, that's the trans, some clutches/steels, gaskets, seals, the USCartool crossmember, and driveshaft shortening. The A500 also gives you a lockup converter, and a lower 1st and 2nd gear (compared to a 904).
 
It's understandable that people are going to worry; when we're talking about jacking up a heavy object underneath your car; hoping it doesn't have to come back down out of there immediately. The 500 and the 518 have the same overdrive, so cutting out the rear bracing is a given on both. And on a 67 and later, that's the only mod; outside of the bolt in crossmember. The only difference between the 500 and 518 installation is on the pre 67 A's only; where the 500 fits just like the 67 ups, but the 518 has a bigger bulkier bell housing that requires mods on both sides of the car; (Same as a 727). It's the area to the right of your toes when your foot is on the gas; and the corresponding area on the passenger side. The pre 67 has these two 3" diameter bumps sticking down.
 
kursplat said:
i think all three of them did 500's? can you do a 518, not cut the floor, and still get good drive shaft angles. i don't know. maybe i'm just worrying about something that isn't a problem :eek:
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I can't say for sure on the 518, but the bell housing would be the area where there might be clearance issue's as CFHJ mentioned.

Two things that would apply to both is the hoop over the top where the trans cross member is and where you tap into the governor port for switching pressure if making the OD and lockup shifts automatic.
That area of the trans where the governor port plug is, is very close to the to the passenger side of the hump between the seats. (or under the seat if a bench)
For automatic shifting you need to put a fitting in that port, and even though I used the shortest 90 I could find right there, I still had to ding the hump about 1/2 inch to get the rear of the trans up high enough.
If using manual/switched shifting for OD and lockup you don't need to add that fitting.

You don't need to cut out the entire hoop over the trans, but just the rounded part leaving the two straps that are left over that are welded to the floor metal.
I also have 2x3 .085 wall subframe connectors that add support to that.
Make no mistake, it's tight pretty much everywhere.
It takes a lot of patience.

I modified my cross member and built a new spool mount bracket for the trans.
Basically that cross member is two pieces now that the big bolt for the spool goes through along with the mount.

It took me a couple of days on the ground with the car on blocks while I made the mods, replaced cooler lines with 3/8 like the 500 uses, and set up my linkages.
I swapped over to a cable throttle pressure setup to simplify that.
There was also a longer speedo cable involved. (80 inch)
I probably raised and lowered the trans 4-5 times during fitting and modding, and I cut off every bracket and stub on the case that wasn't going to be used just to make things a bit cleaner and as to not interfere with the hump.

I also helped a buddy put a Gear Vendors unit in his 71 Dart and it did require cutting and raising a section of the hump the length of the OD unit.
We had to raise that section about 1.5 inches and weld in some new metal to close the gaps.
Fortunately that raised section is right under his bench seat and can't be seen.
We also had to remove and re engineer his exhaust because his X pipe was right in the way where the OD unit was going to be.
That was kind of specific to his car though, and may not be a problem with others.
We spent about 20 hours total starting at removing the seat, underlayment and carpet, so my A500 and his Gear Vendors literally took about the same amount of time.
 
MileHighDart said:
You'll have I think its a .78 overdrive instead of a .69 like the A500.
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FYI to all;
The thing about the GVod is you can use it as a splitter.
You can use the combo as a regular 3+1 most of the time. But,using it with a 904, you get very close ratios of;
2.45-1.91-1.45-1.13-1.00-.78 od,and splits of
.78-.76-.78 skipping 1.00-.69od
So if you use it as a 5 speed, and with 3.73s;
your street starter is 9.14, and you cruise at 65=2350 with 27s.
Because the ratios are so tight you can run a really tight LSAcam, that is normally reserved for a manual trans car. This is ideal for a hi-torque SBM, meaning you can run less cam, and with the additional low rpm torque, that makes the 9.14 starter gear seem bigger than it actually is, which speaks to not needing a whole lotta stall, which means you can try the factory convertor (found money).
The 3.73s will get you 60@5300/zeroslip; figure up to 5750@8% slip; see there's your 228@.050 cam. Maybe even a 223@.050
With the tight ratios, that engine is gonna be sitting on the fattest part of the power curve, for the longest period of time, and the 228* cam is gonna give ET results like a bigger cam would with your wide ratio A998/999.
A KD at 32mph, will kick the rpm from 2150 in second to 3630zeroslip; figure 3900 at WOT, in first gear; just past the torque peak.
So in the end;
with the smaller cam (and 3.73s)
You get
the 9.14 starter,
the 65=2350,
the 5750rpm@60mph in 2-od,
and the kick-in-the-azz blast off at 32mph.
Also; If you take advantage of the tight splits and order your cam on say a 105LSA, Then you also get
the fat SBM torque curve,
the ~1250rpm powerband requirement,
the power bulge over the top,
the increased efficiency at low-rpm which leads to higher fuel economy under all circumstances, and
the increased power extraction at cruise rpm with a further decrease in fuel consumption.

This set-up would be ideal for a hi-compression 318 to beat up on bigger engines, without having to run the big rear gears or the big for a 318, cam. This is not to say it wouldn't be even better with a 360.
It also works with the A999 ratios
2.74-2.14-1.54-1.20-1.00-.78od. splits of
.78-.72-.78-.83-.78
shifting; 2.74-1.54-1.20-.78od your split into od would be .65 with a tight 2-3 split.( which you will grow to love)
The tight .83 split from 2nd to 2-od could come in handy in the Eighth. You can still run the 3.73s with 10.22 starter gear; but with 3.91s, 5300 =86 mph, just about perfect for an HO318. And 65=2460/zeroslip. A 360 would probably pull the 3.73s just fine in the Eighth, looking for 90mph@5300.
And best of all, it bolts right onto the back in minutes, after a couple of trial-fits and hammer-blows to the floorpan in key places. I used the 73-up spool mount with a poly bushing, so I also had to drop the crossmember about 1/4 inch.
Yeah it's pricey; but it turned my combo into an animal. Course I bought mine when they were ~$2300,lol. Plus doing the driveshaft twice, cuz it promptly blew the 7260 rear U-joint to smithereens. So I installed a 1350 there to match the front.
So, I'm just saying;
there is a whole lot more to a GVod, than just the .78; if you engineer your combo to take advantage of it.

now,lol, if you combine this with a tight-lash, fast-rate of lift,solid-lifter cam, one size bigger,in a 360 with 180psi cranking pressure, look out world!.
 
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Overdrive, fuel injection and switching to big bolt pattern axles were the best mods I've made to my 68 A-Body. They are expensive and require some work, but the transformation to the performance, usability and fun is well worth it. Once the hood is shut and you are driving down the road, how you added these items is of little concern....until something breaks.

I went with a stroked 5.9 Magnum based motor, a reflashed factory JTEK computer with Hotwire Auto's Hot Rod harness, and an Extreme Automatics Stage 2 200R4 with a Precision of New Hampton 3800 rpm lock-up torque converter, an SFI rated Reid Bell housing (the plate style adapter is just not the best way to go) and 3.91 rear end gears.

There are many ways to upgrade to the big 3 mods mentioned above. To each his own.

I did have to trim and reinforce the torsion bar cross member and make my own transmission cross-member. The 200R4 has nearly perfect gearing, is more efficiently, more compact and stronger (once modified) than either a 500 or 518. I did not have to cut the floor at all.

The Gear Vendors just doesn't have a steep enough overdrive ratio for my tastes. I tried it in a buddies car for the weekend. It was just to little of a good thing for me, and I didn't enjoy having to try and split the gears while the car was launching forward.

200R4 2.jpg


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200R4 4.jpg


200R4 5.jpg


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Just be glad you’re not putting this one in. A little more that seven inches has to be cut out. In any case overdrive is great in a street car driven on highways. 2400 rpms at 80 is pretty nice.
 
On the 727, it is possible but expensive.
The MP draw back....
 
Are you kidding? The front of a 518 is all 727; easily modified to go well over 1,000 hp. Cheap too. And the overdrive is twice what that Gm box has.
 
CudaFactHackJob said:
Are you kidding? The front of a 518 is all 727; easily modified to go well over 1,000 hp. Cheap too. And the overdrive is twice what that Gm box has.
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I can't seem to find a 518 rated for 1000+ hp for sale anywhere. They seem to max out at just over 500hp because the overdrive unit on the back of the 518 is the weak link (part of the reason it's less efficient, heavier and takes more power to spin than a 2004R), but please post a link showing 1000+ hp rated 518's for sale.
 
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Want I should google that for yas? I would just build my own. I made a special tool to take apart the overdrive pack so I'm well into them. Have you ever held a 2004R overdrive clutch pack in your hand? A toddler could throw it all the way across a room. He couldn't even pick up a 518 pack.
 
jbc426 said:
I can't seem to find a 518 rated for 1000+ hp for sale anywhere. They seem to max out at just over 500hp because the overdrive unit on the back of the 518 is the weak link (part of the reason it's less efficient, heavier and takes more power to spin than a 2004R), but please post a link showing 1000+ hp rated 518's for sale.
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I doubt someone will be selling one ready to go. You'll have to build your own. You have to remember that the diesel guys are using a version of this trans (618) I believe. The OD unit bolts up to the A500 or A518. The guy I got my trans from had it rebuilt and they put the 618 od unit on it.
 
CudaFactHackJob said:
Want I should google that for yas? I would just build my own. I made a special tool to take apart the overdrive pack so I'm well into them. Have you ever held a 2004R overdrive clutch pack in your hand? A toddler could throw it all the way across a room. He couldn't even pick up a 518 pack.
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No thanks, I'll search Google again if I ever want to cut my floor, add 40 or 50 pounds to my car, throw away 20 or 30 horsepower in increased parasitic loss and build my own transmission with no warranty.
 
:rofl: now i'm full circle on this thing.
GV, 2004r, 500a/518a.... cripes, maybe i'll go with a stick :BangHead:
 
What good is a warranty on a transmission if you have to take it out yourself? 20-30 horsepower in INCREASED parasitic loss? Sounds like somebody has been lead around by a sales pitch. 40 or 50 pounds? Not quite. A gallon of gas weighs over 6 pounds..so do you run on empty? But, yes, a stronger trans will be slightly heavier. I can't believe people still believe in massive amounts of parasitic loss. One guy tried to tell me that a C6 took 60 horsepower to drive! Hahaha, that's just sad!
 
jbc426 said:
No thanks, I'll search Google again if I ever want to cut my floor, add 40 or 50 pounds to my car, throw away 20 or 30 horsepower in increased parasitic loss and build my own transmission with no warranty.
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No offense, but I think you're in the wrong hobby... If your wanting a trans that will hold up to 1000 HP, but also a warranty? If you ever came close to building a motor with that much power, the body will twist and shear, without extensive mods, but t those mods surely wouldn't have a warranty.

Besides, you don't have to cut the floor to add OD. Lol 518 or 500.
 
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