91formulabird

I have some questions about two transmissions both with the MD8 transmission code.

One trans is from a 91 formula 350 and the other is from a 91 firebird 305.

What are the internal differences between these two transmission?
Stall speeds of TC, governer psi, gear ratio's etc...... or anything else??

What would one have to do to make the 305 trans interchangeable with the 350 trans, just bolt it up and not care what the differances are or what effects the differances will have on performance? Take the trans to a trans shop for modification is not an option for me. I'm looking for specifics such as part number and component changes etc.

I have the factory service manual and currently reviewing for differances.

When I take the VIN's from the two cars in question to the GM parts dealer, they tell me that the trans are not interchangable because they have differant build codes. Can anyone enlightening me?

The factory manual shows that the trans gear ratios for 1st, 2nd, 3rd, 4th and reverse are all the same. So far I found line pressures are different between models and controlled by a pressure regulator. Along with this there is a shift speed chart with 13 different shift speeds from 1-2, 2-3, 3-4,wide open throttle and coast down. How is the press reg adjusted? Does it have a manual adjustment or require a spring replacement? Still in the factory manual searching. Any help from anyone will be appreciated.

Ok found that there are five "type" wiring diagrams for the TCC solenoid. Believe there is one part number for the TCC, you just have to ensure the three wires are properly connected. Not all wires are used though. There is 2 two wire types and 3 three wire type hook-ups.

So far we have a differance in the pressure regulator outputs and the wiring of the TCC soleniod. This information was found in the Diagnosis section of the factory manual. Now on to the unit section to find further differances.

I'm just loging info as I go here. Valve body checkballs are the same size and no differances noted in unit repair. Unable to deterime if the valve bodies are infact all the same for the five "type" variations of this the MD8 transmission.

Another major differance just found is in the 1-2, 3-4 accumulator spring tensions. There is a color coding chart giving the differant colored springs to be inserted into the 1-2, 3-4, accumulator for the 13 different MD8 transmission configurations.

Some other differances are with the speedo drive gear fitting to a case extension and some transmissions have a temperature switch installed. The temp switch is non-electric and if fails will cause the "No convert clutch apply" fault, torque converter does not lockup. Does a trans cooler replace the internal trans temp switch? My car (350) came with a trans cooler and no temp switch. Why would GM not pair the 350 trans with an internal trans temp switch?

I'm asking all these question, because I want to have a spare tranny that I can go through with my 18 year old son. I'm trying to teach him to respect our old bird and help him to be able to maintain the vehicle after I am long gone. He has all ready told me he will never let this car go.

StealthESW

The differences will be in the valvebody and governor for the most part. As far as I recall the TCC wiring should be the same at the connector.

The valve body, wiring, and governor is interchangable for the most part. The valvebody and governor are usually a matched set, fyi. Just as an example, the trans from the 350 will likely have a different RPM shift point than the 305 at WOT to take advantage of the 350s difference in powerband.

There are many modifications that can be done on these transmissions and there have been many books on the subject. Sometimes it is easier to just give advice on the changes needed based on your use and engines output.

There are some weak points on the 700R4/4L60 that can be fixed with a rebuild, like the sunshell, valvebody, and plate mods.

The key thing to the 700R4/4L60 is that it can be built to work with quite a lot of horsepower/torque attached to it, as long as you get the right parts. I would say stick with a beastshell with up to 400HP or Sonnax reaction shell if you have more than 400HP output. The Sonnax shell is a nice upgrade and matched with the OEM 5 planetary front and rear gears, can make a real wheel spinner of a transmission with the correct modifications to your servos, clutches, valvebody, governor, and pump. Also, you will want to match the torque convertor to your intended use and powerband. There is a long list of options for this transmission.

BTW, if you make any modifications it is always a good idea to add an external filter and cooler to keep your 700R4/4L60 running long and strong.

As for the pressure, it is set by a number of internal things, like the boost valve in your pump for example. The pressure is also regulated with the TV cable, so proper adjustment of the TV cable is critical to the life of your transmission.

jmd

iTrader: (4)

You're going to extremes to know every last detail. And if that's your thing, I'm not here to talk you out of it. But you'll spend a lot of time researching whereas tearing into the core and documenting the parts would be the first step so you can take notes against the resources you've already tapped. If it's had a part serviced that's now different, you'll better know.

Bottom line: If you want the buildable core to behave exactly like the one in the car when you're all done, re-use the parts that might make a difference. Valvebody, pressure regulation related pieces (into the new pump,) servo, governor. Along the way, you'll be able to compare pieces side by side. By doing no more work than an average shift kit involves, you could get a start on seeing what's in your core, then go from there. The clutches, shafts, bearings, seals and drive parts are going to be something you can proceed with doing without using the 350 unit for reference.

Sounds like a fun project. I did my first shift kit at 20.

91formulabird

StealthESW thanks for the intuitive responce. We will heed you advice on the upgrades.

A little about our intent for the car. My son wants to build a 300 plus hp stroker in the distant future. We started in the rear with the weakness of the rear axle. Moser did the build up. The intent of the car is a little street fun and a little drag strip

91formulabird

jmd, yes it was a little extreme but I wanted to know as much info before hand so I can teach my son. we are looking forward to this project together. I found someone parting out a 305 car for some quick cash and getting a good deal on a tranny to rebuild. I did not see this exspence coming but it will be fun! We are planning on a master rebuild kit.

91formulabird

jmd, you were right about being a little extreme. This being my first 700r4 rebuild, Iwanted to be well informed. your advise about just trearing into the tranny was a good idea. This will defiinately give me an idea if anyone else has modified the tranny. I found the link below in an earleir posting on thirdgen and gives more details about future concerns about this build and regulating shift speeds. Again appreciate you advise. Good info on govening the governor at the link below.

http://www.dragracingonline.com/tech...vi_9-bm-1.html

John in RI

iTrader: (167)

In Short - you can bolt up any 83-92 tranny and drive away. ( using the original TV cable that matches the cars existing induction type ! ) Because your 2 trannys are of the same year the changes needed to make it FULLY compatible are - very minor.

Each TC should have a code on it,... you can look up that code to find out the rated stall speed. I've found that TPI cars usually have 'beefier' accumulator springs than TBI/Carb trannys. ( firmer shifts ) TC lock-up occurs a couple different ways from year to year and you can research the tranny ID code to get specifics on each tranny. Governors are usually related to the rear end gearing ( to reach optimal RPM during WOT operation ) so I'd be sure to re-use the original or at least verify the replacement governor is the same unit. And - as always - verify/ swap your speedo gears !

You'll find that you've spent more time reading / learning about the tranny than it took to pull it and re-install. Just be sure to always use the original tranny 'stuff' like speedo gears, springs, gov, TV cable, TC, ect......

StealthESW

I would tune the governor to your engine more than anything. You want your shift points to be in the powerband of the engine and not too far into the redline. The rear gear doesn't usually matter, however there are exceptions. As an example, in '84 there were 2 V8 engine options for the Z28, the LG4 and L69. There were also 2 different part numbers for the governor based on the engine used 8642730 or 8642731. Part number 8642730 was used on the 305 H.O. (G) 700R4. Part number 8642731 was used on all 305 (H) 700R4 models. Those part numbers were used on the prospective engines with the 700R4 regardless of rear gear ratio.

As you will see from the link below, the part number stays the same until about '86 when other part numbers were added. There are some allowances for rear gears with the V8, but only from '86 to '88 in the Z28 and IROC or if you had the package that included the 2.73 rear. I have included a link with the engine and governor part numbers for '82-'92, hope this helps. These governors can be used as a baseline when dialing in a governor for your engine setup.

http://www.gmpartswiki.com/getpage?pageid=129757

John in RI

iTrader: (167)

I TOTALLY agree that engine power band is important, but believe the gearing was AT LEAST as significant, if not more important.

For the sake of argument I'll point out that the HO engine/700R4 combo got 3.42 gears The HO combination was the only Thirdgen V8 that came with a gear higher than 3.23 - all other 82-92 V8 autos were 3.23 or lower.

Was it the increase HP or the special gearing that required a different governor in 84? I'd argue that both were factors but that the gearing was more significant. Why; Because all 87-92 305TPI cars used the same gov as the 350 in 87-88. They all had similar gearing (2.73-3.23) but the 350 yellow lines @ 5K while the 305 falls flat at 4500; yet they use the same gov.

I'll also add that I've opened up more than my fair share of automatics and the only time I've ever found a governor that was 'different' was the gov's I pull from V6 700R4's. ( they almost always have higher gearing than V8's )

On left; the gov I pull from almost every V8 700R4 (most I've been in are 87 and older),.... on right, the gov pulled from a V6 with 3.42s.

91formulabird

Guys, this is the type of info I was hoping to find here. John in RI, really appreciate the pictures. I have not tore into the trans yet, but will definately be looking at the goveners.

I have saved and printed the information posted here and will definately be using it as a guide for this rebuild. Can't wait to get started.


As far as a performance rebuild book, I found ATSG has rebuild and update books for these trans. Has anyone used these books for their rebuild or is there another recommended book?

I've called several trans parts supply companies and these seems to be the book a lot of them carry (ATSG).

91formulabird

John in RI, I'm currently doing some minor cosmetic work on the car and hope to have the car back on the road soon, at which time my son and I will record all of the current shift points under slight acceleration and then at WOT. This will give us some idea of what our USED factory base line is,and allow us to aleast get close to the origional setting if we go to far in one direction or the other with shift points.

It's always nice to be able to get back to where you started from.

As we proceed through our rebuild, which I hope is with in the next couple of weeks. I'll post further question as we go along and will keep checking this post for other comments and tech bits. Thanks everyone!!

But man, I can't tell you how much I want to get started on this. This is GOING TO BE FUN!