cagotzmann

Here are my Install Notes when I did my TH350 conversion to a T56 Magnum 6-speed.

Car Started as

1978 corvette ZZ383 Crate motor, TH350 stock transmission. 3.08 Rear.

My First Highway mileage test came in at 27.8 MPG (23.4 MPH US)

My 1/4 mile ET isn't great I dont know how to launch a manual very well yet. Same as when I had a auto (TH350) 13.8
But I know the potential is there because my 1/4 speed is up 5 MPH. 104MPH







Parts: TUET11009 6-speed close ratio 2.66,1.78,1.3,1,.8,.63
QuickTime RM 6023 Bell Housing.
GM6-HDSS - HD/Perf Street Sprung Clutch Kit.
American PowerTrain HMGM-01201G Hydraulic Clutch Kit.
From American PowerTrain Profit kit. (didn’t buy a complete kit)
1. White Lightning Offset Shifter Mechanism (2.25” offset)- Solid billet construction, bias adjustable.
2. Seamless DOM Driveshaft Assembly - Race Balanced, Solid U-Joints
3. American Power Train flange slip yoke (IMPORTANT POINT to allow removal with diff and transmission in place)
4. Modular Reverse Light Harness
5. Modular Reverse Lock-Out Harness
6. Speedometer Conversion - Mechanical (connects to existing mechanical speedo)
T56 Transmission came with a rubber transmission mount.
Bowtie Overdrive T56 Crossmember. (needs mounting plate shortened to fit)
Additional misc parts. Custom make electrical harness to connect Reverse Light, Reverse Lock-out, Clutch Neutral safety switch.
Corvette Central PE30A 68-82 BRAKE & CLUTCH PEDAL KIT
Corvette Central Part#:592104 GM#:1993495, 1993495 Neutral safety switch on clutch pedal.

Swap was completed with the car on raceramps and a transmission jack






Here is a list of problems that needed to be solved.
1. Cut a hole in the transmission tunnel.
2. Make a custom cover plate to cover hole and seal tunnel (make sure the hole for the shifter is large enough the transmission does move during WOT )
leave about 3/8" all around. on your cover plate.
3. Mount the Hydraulic Clutch cylinder (use the factory manual 4-speed firewall location)
4. Cut the mounting plate of the Hydraulic cylinder to make room for the steering column mounting plate.
5. Mount the Hydraulic Reservoir.
6. Clutch Pedal conversion.
7. Clutch pedal ratio between 5-6:1
8. Remove clutch pedal mounting rod & drill hole
9. Reverse neutral safety switch & Hydraulic Rod connections.
10. Grind trim to allow full clutch pedal return with reversed connections. Item 7
11. Modify parking brake cable pulley wheel ( change to 3.5” dia)
12. Widen Tunnel at slip yoke (transmission is longer so factory location needs to be further back)
13. Shim transmission higher.
14. Shorten bolts for offset shifter (bolts hit the transmission frame)
15. Make wiring harness / switch for reverse lockout operation.
16. Make wiring harness for reverse lights
17. Make wiring harness for neutral safety switch. (preinstall on clutch pedal before installing pedals in dash no room for hands later)
18. Modify T56 transmission crossmember (bowtie overdrive t56 unit shorten)
19. Re-route starter positive wire location in tunnel.
20. Pre-install Transmission bolt with transmission breather bracket. No access later during install.
21. You need a offset shifter with flush fasteners or you won’t be able to get the transmission high enough to get good driveline angles.
22. Figure out how to measure your bell housing indexing.
23. Shim the throw out bearing to specs. (American powertrain procedure)
24. Get proper drive shaft lengths ( American powertrain procedure)
25. Determine proper speedo gear for wheel size and diff gears ( American powertrain )
26. Requires offset shifter to fit factory location used 2.25” left offset. ( American powertrain)
27. Purchase / make shift handle and **** I used stock hurst parts.
28. Modified stock hurst shifter handle ( shorten the length by 1 ¼”)
29. Redo the exhaust to route under new crossmember (Previous routing thur the auto factory removable crossmember)

There may be more but I think I remembered most.

First task was to cut a hole in the transmission tunnel. The complete indented recess where the original auto shifter bolted on needs to be removed to allow the transmission to mount as high as possible in the tunnel. This makes a big hole.





You will also need to relocate the battery wire cable to allow for better clearance at the rear of the transmission.
Reverse light and neutral safety connections.
Here also you can see the original auto shifter still in place.



Once the hole was cut I installed the bell housing without the flywheel or clutch to test the new hole in the tunnel. The transmission is very easy to install without the pilot bearing or clutch in place for fitting.

After the hole was cut, this shows stock T56 Shifter Installed. As you can see it doesn't come up in the stock location.
The red laser line shows where it needs to be. The shifter stick would bolt on the right side of the transmission stick. Stock location is in the middle of the bolts. This was the original auto stick location.




With offset shifter installed.








Next was to test fit the requirements for the crossmember.




The base plate is too long. So I cut a piece off and re-welding it back on the plate.




Total base length required from stock T56 crossmember is ~ 5 ¼”.
I also added a ¼” alumimum shim to move the transmission up in the tunnel.
Helps with drive line angles. This transmission is much longer than the original TH350 and creates non stock u joint angles. It was not possible to get perfect angles without additional cutting of the tunnel and changing the transmission shifter cover plates with thinner plates and counter sunk bolt heads.


Note the counter sunk bolt heads. This was perfect to allow additional clearance to allow the transmission to be installed higher in the tunnel.




Also if you look at the T56 Transmission the mid shift cover plate could also use a thinner plate with counter sunk bolts as well.




This is for the front and mid shift locations which are closed with cover plates if not used. The Corvette uses the rear shifter location of the T56 Magnum. I did not do this additional mod. I found the drive line angles ~ 4 degree’s at the transmission to drive shaft and 1.75 degree’s at the diff yoke to drive shaft. I didn’t have any vibrations from 0 – 125MPH. So I will leave it for now. I installed the drive shaft and with all weight on the wheels I measured the angles.

Now the drive shaft was ordered after I test fitted the transmission in the car and gave dimensions from the transmission tail housing to the diff yoke flange. American PowerTrain provided the correct length driveshaft which fit the custom slip yoke also from American PowerTrain perfectly. This allows for the removal of the driveshaft without any other drive train component removal (IMPORTANT POINT).

Next you will need to widen the tunnel to allow the slip yoke enough room to move in and out. The stock tunnel location is about 4 inches towards the front of the car. There is a factory indent for the space. I just used a plate and hammer to widen clearance for the slip yoke. I did not need to make any cuts to get the clearance required.






Next I installed the clutch pedal conversion and the hydraulic clutch system. First step is to remove the existing auto pedals. This is the worst part I think. Once the pedals were removed I installed the Hydraulic cylinder. This requires you to drill 4 holes in a very tight location. There is a indicator where the hole should be in a auto car. All you need to do is drill 4 holes and 1 hole for the cylinder rod.




Then you need to cut a piece off the mounting plate of the hydraulic cylinder to make room for the steering column plate.




Picture from the engine bay side. Needs to be positioned so it doesn't hit the steering column mounting plate as well.




With Steering Column installed.




Mount the hydraulic reservoir. Since I already had the best spot taken by my MSD box I created a bracket that mounted on the hood latch plate. I found this more secure than the 3 supplied self tapping screws for the fibreglass firewall. Most have installed this where my MSD box is.




Next was to convert the pedal assembly.

To do this step I removed the driver seat, gauge cluster and steering column. You need as much room as you can get.

I started by installing the natural safety switch in the stock location. In order to install the hydraulic clutch kit the supplied rod has a tie rod that connects to the clutch pedal. This requires you to cut off the existing factory rod on the clutch pedal and drill a hole in the same location to install the supplied kit.





Once I install the pedals back in the car I ran into a problem. The clutch pedal travel was greater than the cylinder allowed for and the neutral safety switch would never engage (won’t allow the started to turn). The kit likes a 5-6:1 ratio. The stock pedal has a 4.2:1 ratio. So I reversed the connections. Mounting the safety switch to the stock pedal location and moved the pedal to the safety switch location. This gave me about a 5:5 ratio. Now the clutch pedal travel didn’t bottom out and the switch activated as required. Also before you install the clutch pedals also connect the neutral switch harness. It’s almost impossible to connect later.
I made a custom harness from parts from a 1978 Malibu harness. (Friends junk car) I only needed the plastic connector ends and used new connectors from an electrical distributor. (Factory connections) I did not want to make any cuts to the original harness. All connections just plug in like the factory ones did.


Now that the pedals were install the next problem showed up. The clutch pedal would not return to the full release position. (factory rubber stopper) This was because the tie rod from the hydraulic clutch rod hits the body upper frame. Isn’t a problem if you use the stock locations for the neutral switch and the clutch pedal rod. Because I needed to reverse the connections I had to grind a little grove to make room to allow the full pedal travel. I Used the stock rubber stop position. This came with the clutch pedal conversion kit.

I test fitted everything a number of times to make sure I was happy with every step and the fit. Now its time to bolt it all together for the final fitting.

Installed the ZZ383 flywheel which came with the motor. These next steps you must look at and decide what risks you want to take.

1. Install a roller pilot bearing or use a bushing. Tremec recommends a roller bearing. I chose this option. Do this step carefully and don’t damage during install. Many video’s online on this.
2. Dial in the dowel position pins.(bell housing indexing) For a T56 magnum this is not easy step.
Youtube example. The standard bellhousing indexing doesn’t work because the T56 guide mount are not the same as the stock or TKO transmissions. I still checked the bell housing opening to see how well it was machined. Within the specs. If this isn’t done you may have shifting problems and premature wear on the input shaft or other transmission problems. I chose to trust it as is and determine the course of action on how it felt during the break in period. The transmission supplier would not warranty the install if I opened the transmission for this step. (hope for the best).
Next I installed the clutch using the supplied alignment tools and install instructions.
Now its time to install the hydraulic clutch throw out bearing from the kit. Used the supplied video’s from American powertrain.

Now we are ready to install the transmission. There are a few "it is not going to work" moments.
1. The transmission mounts to the bell housing with 8 bolts. 7 you can reach inside while in the tight corvette transmission tunnel. 1 you cannot get a hand / tool on to start the bolt. Luckly this bolt location also need to hold the breather tube of the transmission. Prestart the bolt in the hole and thru the breather bracket. You can latter get the long socket on without the bolt falling off. You need multiple u-joints / extension to get it tight. But here’s the big issue. YOU CANNOT INSTALL THE TRANSMISSION WITH THE SHIFTER INSTALLED.





I had to remove the shifter to allow the input shaft into the clutch. You cannot raise the transmission high enough and back far enough to get the shaft to align with the clutch. And you cannot preinstall the bell housing on the transmission because you then cannot get the transmission in the tunnel to align with the engine. You cannot move the transmission back enough without hitting the tunnel.
2. Once the transmission is installed I lowered the unit enough to install the offset shifter. I was able to get at all the fasteners to tighten. Also I needed to shorten 3 bolts when installing the white lightning offset shifter; they hit the frame of the transmission. Some from up top and some from underneath. I didn’t get the offset shifter in time (sent the wrong part) so I continued with the stock shifter. The offset one may fit pre-installed but I didn’t wait to see.
3. Installed the Crossmember, transmission mount, shim to raise the transmission
4. Modified the parking brake wheel. I installed a 3.5” to make up the slack in the cable. The position of the bracket is closer to the front, and using the stock wheel would mean to shorten the cable. I decided to make the path longer using a 3.5” wheel. (hardware store).

Also connected the speedo cable adapter (red circle,American powertrain) this converts the tremec ford connection to directly connect to the factory GM Cable. American powertrain part includes the proper gear / # teeth in the converter cable based on your wheel size and diff ratio. All determined by American powertrain.







5. Next was to bleed the hydraulic clutch. Their simple instructions to just push the clutch in and out for 10 minutes didn’t work. I had to use the bleeder valve. Similar the break bleeding. I then tested the clutch to make sure it worked and I could put the transmission in gear.
6. Installed the drive shaft
7. Filled the transmission with GM syncromesh fluid.(with friction modifiers) Contacted tremec on what to use. Sticker says Dexron III ATF only. Tremec recommended 3 fluids. 1 Dexron III, GM Syncromesh, Mobil 1 ATF. Never found any bad reviews for the GM syncromesh only great reviews.
8. Next I installed the custom cover plate for the hole I made for the offset shifter arm, handle, and choice of shifter ****. Note I used the existing mounting bolts for the auto shifter for the custom cover which included sound/ heat shielding and a rubber boot cover with additional bolts on the side to seal the plate. I made this removable since I cannot remove the transmission without the shifter also being removed.




9. I made 3 wiring harnesses,

Backup lights and Neutral saftey switch connected to the factory harness from the auto shifter. ( used the plactic connectors from the original shifter )






a. Connect the neutral clutch pedal switch to the factory neutral harness from the auto shifter.



b. Connect the backup lights switch to the factory backup light harness
c. Added a reverse lockout switch. This prevents you shifting into R when driving the car. 5th and reverse are in the same direction.
I will be installing a ECU controller that works similar to factory cars. Switch based on speed on the car. Connects to the VSS Speed sensor on the T56. Simple switch for now. Didn't want to connect to brakes since I heal toe down shift at the track.




Route all the wiring harnesses through the factory auto shifter hole already in the tunnel.







I have created a custom shifter console plate ( not done yet)







Redo the exhaust to fit the new crossmember.





It spend 3 months doing this conversion. I didn’t order all the parts at once and solved each problem as they came up. I spend time thinking about the best way to solve the problems as not to disturb the factory setup.

Other than the hole in the transmission tunnel, additional widening at the drive shaft, clutch pedal addition, and the hole in the firewall for the hydraulic clutch I can return this car back to auto if desired. All the factory electrical connections where left intact. I kept all the parts and removed them unmodified. I am not sure if I left out any steps since this is a few months in the doing and my memory isn’t the best.

Irocz2173

Awesome write up! Completed my T56 conversion last year. Enjoy the new found fuel mileage and sanity on the highway

cagotzmann

What year is your car ? Did you start with a auto or manual ?
Did you install a Hydraulic Clutch system ?

Irocz2173

My car is a 1981 and it was originally a manual.
I did do a hydraulic clutch setup with a parts store truck master. Your setup is much nicer.

worship79

Thanks for this great article!

Red1990VT

Did you tally up what all this cost, at least in terms of materials? And if so, would you mind sharing that? I realize the amount of your labor is pretty significant, but knowing the approximate cost of all the materials required for this would be helpful. I'm considering modifying a 78 manual.

Also, do you happen to know what the increase overall in MPG is? I'm getting around 17 overall with my 78SA and four speed. Would be interested to know what to expect, granted all the caveats involved.

Finally, what stands out in your mind as the most worthwhile aspect(s) of doing this, and knowing what you know now, would you do it again?

Thanks

cagotzmann

I never looked at cost so no didn't add it up. I am in Canada so my costs also include shipping , customs , exchange etc etc.

But here is some simple costs.

Transmission ~ $3000
Conversion Kits ~ $1100
Bell Housing ~ $627
Clutch kit ~ $ 575
Hydraulic Clutch kit ~ $600
CrossMember ~ $200
Clutch Pedal conversion ~ $195
shifter handle etc ~ $100
total ~ $6400


Engine is a ZZ383 Rear 3.08
As for gas mileage highway ~ 23.4 US
City only hard exceleration ~ 12.5 US

No combined normal driving. I didn't convert for mileage.

This conversion was for road track driving. The auto doesn't cut it.

The plus cabin is cooler, Transmission doesn't heat up like the auto.
T56 Magnum shifts great at any RPM. 700ftlbs rating, Great highway driving (6th) great on major city roads 45-50 MPH (5th). Over all the car feels like it has extra HP vs the TH350. 1/4 mile trap speed is up ~ 4 -5 MPH. Much more fun than a auto, but vs a 4 speed (stock) the major plus is it shifts so much better, can handle way more power, better highway mileage.

Down side is cost and the suppliers. The suppliers are difficult to deal with, after purchase support sucks, you will be left on your own to solve problems. The big problems will be vibration / bell housing alignment.

Vibration may come from 2 area's. Flywheel / Clutch (rpm) or drive line angles ( speed )

I was not able to do a proper bell housing index because the supplier of the transmission said it will void my warranty. (you need to remove the front of the transmssion and mount on the bell housing). I skipped this step and decided to re visit if I had shifting problems. (most common symptom of poor alignment will bell houing)

Drive line angle's will not be perfect. The T56 is too long or you need to make a new tunnel to be the same as the factory. Mine are not perfect but I dont have any speed related vibration so far. 0 - 120 MPH

The most disapointing job was the auto to clutch pedal conversion. The Hydrmax kit is not made for a corvette clutch pedal (they claim it will work only if you mod a few things ). When using a hydraulic clutch kit; pedal ratio must be addressed, or you will have 1 of 2 problems. 1 you cannot start the car Clutch neutral switch wont engauge or your hydraulic master cylinder will bottom out.

Also the clutch I have the pedal feel is way to heavy. I was looking for something similar to a C6 corvette. again the supplier problem claims.

After driving the car the past few weeks, would I do it again ???

Only if I couldn't find something better. And better for me would be a 6-speed auto with paddle shift similar to the C6 corvette's that is a complete bolt on no mods. (that will never happen)

Even with a 3.08 Rear gear the car is great to drive on the street and highway. If you are big on 2-3 second accelleration off the line you will what lower rear gears.

It was a good thing I have another Corvette to drive while I was working on this project. I spend 3 months slowly working on it. The transmission went in and out 6 - 7 times to test fit and determine the best procedure for alignment and setup.

I am glad I did the work myself. I dont think I could trust someone else to make sure all the little details were to my liking.

Red1990VT

Wow, thanks for such a thorough and detailed response. Looks like you really dedicated some time and money to your conversion, glad it worked out so well for you.

jr17127

First thank you for the incredibly detailed account of your project, second is there any project with a Corvette that isn't like pulling teeth, second I was under the impression American powertrain had this all figured out.

gkull

Really nice! Are you going to come down to the lower 48 and road race it? Our local track closed so I have to go over to now lengthened "Thunderhill" race track. It is just off I-5 in northern California. It is a very safe safe track where you work really have to work at it to hit a solid object You should look them up for one of the open two day track events. I go there for scca, speed ventures and NASA events. I've wanted to get involved in Vara because they run at Spring mountain and Willow springs where I have lots of track time

What weight fly wheel did you end up with.

I can't imagine a 3.08 rear with a 2.66 first and a .63 od. You are a good candidate for a 3.90 or even a 4.11

cagotzmann

They flywheel is the one that came with the stock GM ZZ383 It's not a light weight unit.

For gears I would like to match the C6 corvette's with a 3:42 so for me a 3:36 or a 3:55 but I will need to calculate what RPM's I will get to at our local track.

I get to about 4500 RPM at the end of most straights in 3rd. (88mph)
ZZ383 Redline = 6000 RPM.


3:36 would get me to 4900 RPM (88 MPH)
3:55 would get me to 5150 RPM (88 MPH)
3:70 would get me to 5400 RPM (88 MPH)

Assusming I can get a extra 3 MPH with better ratio's

3:36 5050 @ 91 MPH
3:55 5350 @ 91 MPH
3:70 5550 @ 91 MPH

gkull

I was wondering what tracks you run at that are limiting you to about 88 mph. I would guess some small straights to keep speeds down and you must only be using 2-3 going around the track.

I'd still go for a 3.90 or 4.11 rear. I drove around for 20 years with OD and 4.11. I just have the 5 speed but it really makes it fun to close up the gear spacing.

ianmcgee67179

I know I'm coming pretty late to this party, but I am looking at swapping my auto trans out for a TKO, cause I cant stand my 79 C3 being an auto. But I was wondering why or what reason you went with the 6 speed rather the 5 speed TKO? Also with the little research I have done, the 6 speed is much larger and makes it a lot harder to fit in the trans tunnel, is that correct? thanks for the great account of your project!

cagotzmann

First reason is torque rating of the transmission. Main use for the car is track days.

Tremec T56 Magnum rating 700 ft Ibs.
TKO units up to 500 - 600 ft lbs.

Second reason know problems 2-3 shifting high rpm unless modified correctly. 6-Speed close ratio from 2.66 1st - 0.63 6th great for overall driving, from track days to street to highway

3rd The transmission was locally available for me in Canada.

4th the T56 magnum has a great following including current production cars eg, corvette, Camaro, mustang , viper using the TR6060 production version of the transmission.

5th it fits with no problems requiring no tunnel mods (ok small denting at the tail to allow the slip yoke some extra room) to fit in my 78.

6th I think the T56 Magnum shifts much better up and down gears during track days.

The only down side for most is the cost and extra parts required / modifying the cross member or using another cross member.

71454Chevelle

I can't speak for the OP, but I choose the T-56 Magnum over the TKO in my 71 Chevelle for several reasons. First it will handle more power than the TKO (600 ft/lb vs 700ft/lb). With my engine combo (close to 700hp, 600ft/lb) I'd be at the limit of the TKO. Second, everything I've read is the Magnum is a much better shifting unit at higher rpms (I shift mine @7000 rpm). Lastly, for my setup, the extra gear makes the transmission much more versatile when driving. The beauty about the 6 speed is 5th gear is like an "in-between gear". With the camshaft I'm running, the engine is not at its "happiest" at low rpm's. I use 5th gear in what I call "back road" speeds, 45-60mph. 6th gear is for the interstate; 60+ mph. This setup gives me the best of both worlds; incredable launches with the 4.56 gears and real nice interstate cruising with fairly low rpms (2200rpm @ 70).

http://www.chevelles.com/forums/142-...m-install.html

http://s989.photobucket.com/user/714...?sort=2&page=1

ianmcgee67179

Thanks! I really appreciate it! I think I would def like the 6th gear and especially if I take it out on the interstate at all! I def like the 6th gear in my C5 I had. I was also worried on the fit as well I have a 79 as I said before, and if you said it wasn't bad that's def a relief!

Thanks again for the quick reply!

ianmcgee67179

on a complete side note, which front finder/bumper did you put on your 78 @cagotzmann? Cause I'm a big fan, is it the pace car one?! I think I am leaning towards that one my self!

jb78L-82

I like that look as well...looks like Pace Car spoiler painted?

cagotzmann

Correct pace car spoiler. Not factory but a very close replica back from the 80's

sparaski

Nice info. I am going to put a T56 in my 78 with a 502 for track.