Transmission recommendation - 68 GTO
 

I'm in the early stages of working on my 68 GTO (automatic) and I'm curious as to what transmission some of you guys are running. I currently have Paul (gtofreek) building my motor. He's using a standard bore 455. I'm not sure yet what kind of HP or torque it's going to have. The car currently has a 700R4 transmission, Global West suspension, 12 bolt 3.55 gears. I just ordered and received my wheels and tires. (Year One Rally ii's) Front: 17x8 - 245/45/17. Rear: 17x9 - 275/40/17.

I'm pretty new to cars and just want to hear opinions on the automatic transmissions you guys are using. I've read some good and bad about the 700R4. My car will mainly be driven on local roads but occasionally on the highway for some road trips.

Any thoughts or suggestions would be appreciated!

In my opinion if you want an automatic for crusing the 700r4 can't be beat except by maybe the 4L80E. If you want to row, a 6 speed conversion is made for that car. Personally I'd keep the 700r4
Ponchjoe

Run the 700...till you break it.

4L80 requires a shortened driveshaft, all else can be managed (i think).

If I'm not mistaken, Joe wants to do this once and done. The 455 .030 build Paul is doing for him has unported (for now) KRE d ports and I believe less than 10.5 comp; and a Hurricane intake (unk carb) It will have a yet to be determined HR cam. With this in mind, he needs a trans that will survive the combo.

I still say considering the cam, a TH400 with a 308 gear would be fine with what you're trying to do, and allow for somewhat friendly highway driving.

IMO the 700 is too much of a risk, it's not if it will fail, it's when.

Thing with an OD trans, with the dual gate shifter, OD moves to where N is, and N will be between N & P. Not optimal, and is a pain/confusing. This is why I changed directions on the 442, and am going to a 455 with 308 gears in it.

For the amount of money it costs to get a built 700 or 200 that will handle the power even somewhat reliably, you will be so close to the cost of a 4L80E, it would make no sense to do the 700/200.

I'm getting ready to sell my CompuShift trans controller and TPS setup, if you want to go 4L80E. Also have a Cable-X box so you can use the factory speedo.

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A rear gear change adds to the cost of a 1500+ th400, just something to consider but may not be a deal breaker...

Yeah, but it's not that much considering. He can drive it with the 355s for now, see what it's like, and if he wants more highway, he can do a gear change. Or that's what I was thinking.

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Btw, His tire height is 25.7"

Thanks for your continued help. This is certainly overwhelming for someone that is clueless about cars. I'm learning a lot but I never would have imagined every decision is so important and can effect future decisions. As I said from the beginning, I would rather do it once rather than regret "going cheap" with something. I don't want to build a show car but I do want a well put together, bullet proof car.

4L80E
 

I've always liked the ratio spread and the durability of the 4L80E.
Considering how strong the 455 can pull with 3.08s and the right converter, having a .75 OD allows you to go with 3.42, 3.55, or even 3.73 with less converter and still knock the final drive down below the 3.08 range with full lockup in OD. A little pricier on the front end, but well worth it if it's in the budget.

I'll echo the general consensus here- if you have the budget then the 4l80e would be the way to go. A more budget friendly option is the th400 with 3.08 gears. Be sure to get a good converter as that will realty bre key to maximizing performance with that combo.

My formula with 3.23s, th400, and a Coen converter is a blast to drive though I do wish the gears were a littler taller when I driver for any distance on the freeway

From what I've leaned here, a 4L80e is the way to go as it's based on the tough old TH400. The 700R4 is based on the TH350 which was designed by GM as a medium duty trans for lighter cars/lower power cars. Hence the 4L80e is a better choice since its based on a heavy duty design to begin with. (Used with big blocks, trucks, RV's, etc.) Hope this helps.

T5 5 speed transmission. You won't have to open up the transmission tunnel. All the parts are easily available. Will have to put clutch pedal in but that only takes an hour. You will have to shorten the drive shaft and change it for a Chevy or a Ford. I've had no problems with mine and I'm putting out close to 400 horsepower. Im not very easy on it and I am constantly doggin it. If it ever does grenade, you can buy another one for around 3 to $500 and it will take an hour to swap out. They have made them now for 25 years so there's a lot of them out there.
See my post in this forum and it will show you exactly how to do it

For us old timers, and anyone who is not familiar with the electronics/computer system required to use a 4L80E trans, will somebody please explain, in detail, exactly what is involved in the COMPLETE install, set-up & maintenance of this trans. I realize that this info can probably be Googled. But, it might help some, who may be considering this trans, to make their decision.

Personally, I'd just keep it simple, and go with the tried & proven, PROPERLY BUILT TH400, with the correct rear gear & converter, to meet the owners desires. Strong, reliable, simple, and well proven. No computer, TV cable, or kickdown cable required. Just a good gate or ratchet shifter is all that's needed, for street/strip use.

I have a 700R4 in my tempest. 389 Tri-power 282/292 camshaft. The transmission was built by Gearstar out of Akron Ohio. They put all of the heavy duty guts in it. At the strip I have more torque than traction in 1st. So I must baby the gas to launch. Great street manners, and I am turning 2000 RPM at 70 MPH. I did put a manual switch in it to keep the torque converter lock-up from hunting back and forth when climbing hills.

X3. A 4L80E with your current 3,55's, or a TH400 with 3.08's or even 2.93's. I run a 2.56 posi behind my very mild 406 with a TH400 and 1900RPM stall converter in a '67 GTO ragtop, and the car moves out just fine (for me). It'll burn rubber if I want, and RPM's are 2450 at 80 MPH. Fuel mileage is killer, too.

The T5 was designed for a maximum torque load of 300 ft/lbs. It won't live long behind a STOCK 455, let alone one with any modifications. Horsepower doesn't kill transmissions as quickly torque DOES... IMO, a 4L80E is the no-brainer if you want a dependable transmission with O/D. My second choice would be the old, reliable TH400 with a 3.08 rear gear if you're running a 455, a 3.23 with a 428/400.

I've not done a 4L80 retrofit, but have done other computerized OD trannys. They are all similar, as far as what you have to do.

First thing, when using a carb, is that you will need a TPS - throttle position sensor. There are a variety to choose from, mine mounted on the passenger side of the carb, and a tab bolted to the end of the throttle shaft operated the sensor. This sensor tells the TCU (the computer for the transmission) whether you are at idle, full-throttle, or where in-between. Instead of a kick-down cable, or a TV-cable, it uses the TPS.

Then you have the computer. You'll need to wire it to a switched power source. It will have outputs for your neutral safety switch and your back-up lights, and it will have an output for a digital speedometer. Depending upon what transmission you get, who you get it from, etc. it may or may not be compatible with a cable-driven speedometer.

What I LOVE about these transmissions is the tenability. You can program it drive like a Cadillac around town, but like a full-race tranny at WOT - or however you like it. Want to drop the 1-2 shiftpoint by 200rpm, but raise the 2-3 by 500rpm??? Simply and quick, just go to the controller (or plug in your laptop, depending upon the system), change a few points on a graph (or data points in a table), hit "enter" and it is done. Line pressure can be altered at different TPS/RPM, upshifts and downshifts can be tweaked, converter lock-up characteristics tuned...

I love computer controlled transmissions. WAY easier to tune than having to mess around with governor weights/springs and such.

Awhile back when I was going deep into my Firebird I was pondering what OD transmission to use. I had a issue with going from a strong Th400 to something that had to be built up just to get in the ballpark of the Th400's strength. Yes, I could get much better gas mileage with an OD but the gas savings would never pay for the transmission. I decided to put 3.08 gears in it and have never looked back. Another way I look at it is I would never pull my Pontiac 400 for a 350 Chevy, the engine is a classic ...a Th400 is a classic also.

I would get a good converter. In my case I have a Switch Pitch converter but I realize they are not easily found.

Well, you guys have been spot on with all your other suggestions. I went ahead and got the TH400. It's being built now. I think between that trans and what gtofreek (Paul) is building me, I should be in great shape. My car does have 3.55 gears and I recently received my Year One wheels. Went with 245/45/17 for the front and 275/40/17 for the rear. I've been working on the car pretty hard lately to have it ready for the trans and the motor. All of this is very new to me and I really do appreciate all the help from you guys. I should have started messing with cars a long time ago. At least I'm learning a lot now though. Thank you all again!

The TH400 is the toughest transmission you can buy dollar for dollar.

I too wrestled with whether to go with a regular 3-speed auto behind my 421 or to step up to an overdrive auto. After weighing the pros and cons of both I decided to just use a tall rear end gear and use the fresh TH400 that I already had sitting in my garage.

The added cost and installation hassles just weren't worth it to me, not to mention the extra couple grand or so that any overdrive auto would have added to my hobby car budget that's already starting to break the bank as I get closer to retirement.

I think a little while after your new big-inch engine has been on the road you'll find that those 3.55s are only good for scooting around town, out on the open road the excessive RPMs will soon drive you to install a 3.07 gear set in your 12-bolt.

Yea, that gear ratio will be great for around town. Highway is going to be short runs only. I've got 3:42's and am moving to an OD transmission. It's going to have 2K worth of stuff from Art Carr's shop, but built locally. My guy here has a much better warranty, and if anything happens it won't have to go back to California.

Thanks b-man. Depending on when my motor is going to be ready, I may switch the gears out before I receive the motor. I have tons of work left to do on the car. Still need to order a ton of parts. I hope for it to be on the road by May. It's been an awesome experience. I've never done this much work on a car in my entire life than what I've done in the last 3 months. I've photo documented every step of the way.

You are about to go down a path that is going to result in a lot of extra money and down time on the car.

Each engine is going to have a power band. Your engine, gears, and torque convertor all have to match up or you car isn't going to drive worth a crap. Talk with your engine builder about your plan to go with a 3 speed auto. Figure out how you are going to drive it, so you can get the right cam, convertor, and rear gear so you don't hate driving it.

A hot cam with a lot of duration isn't going to work with a stock-ish convertor and tall gears. You will be calling your engine guy telling him he built a turd, when there is nothing wrong with it.

Sounds like Paul (GTOFreek) is building the engine. Doubt he'll get steered wrong on the cam. With stock, unported heads it's not likely a very big cam will be chosen.

Look at taman's car - dipping into the 10s with a very basic 455 shortblock with a cam that's good sized but not "radical", TH400 and 3.08 gears.

The keys to this combo will be the cam and the torque converter.

It seems like the TH400 with 3.08 gears is a common combination. Paul is building me a 455. I'm hoping it will have around 500+hp and 600 ft lbs. Paul is using the KRE'S heads that were on the car when I purchased it. What problems do you see coming Navy Horne 16? I really do trust Paul with my build. This is going to be mainly a street car. I may take it down the strip to see what it does. Previous to this car, I had a 68 Firebird HO, 350, 4 speed car. I had that car for 2 years and took it on the highway once. I would like to do a couple of road trips with this car so I'm pretty sure I'll be changing the gears at one point. As for the choice of cam, I will leave that up to gtofreek (Paul). He knows how I will be using this car and actually, I hope he posts the build on the Forum. I know guys on the Forum have a great deal of respect for Paul's builds and would like to see the work that he does!

The 400 Turbo can't be beat as far as toughness and value. Even the 3.55 gear may not bother you too much if most of your driving is in town and short hops on the highway. Millions of cars were built with non-overdrive transmissions and gearing like this for decades. We are just used to the lower RPM's now since OD transmissions have been the norm for 30 years now. I would try it and just see how much it bothers you running around 3000 RPM's on the highway. It's certainly not going to hurt the engine. Sounds like a great build.

Thanks mgarblik, I think that's the plan. If it becomes a problem, I'll have the gears changed. I do have the 700R4 that I may hold onto until I see what direction I'm going with. I've read some conflicting things about the 700R4. If things get too crazy, I'll invest in the 4l80e.

I don't know if anyone took your tire size into account but those things are REALLY short. I get 25.6" when I do the math. That doesn't take into consideration the roll out with car weight on it (usually comes up a pinch shorter)

With a 400 turbo and even a 3.55 gear, and with that really short tire, you'll still be buzzing 3,000 rpm just to go 65 mph. And that's not taking into consideration any torque converter slippage so that rpm could be a smidge higher. So with any trips planned for the car, you may not like the highway manors at all.

Depends on what you prefer for a highway cruise speed, but as an example, You would need to get a 27 or 28" tire on the back of that thing to make those 3.55's livable on the highway. That would knock about 400 rpm off your cruise rpm.

Or, start thinking about swapping in a 3.08 gear if you want to keep those short tires.

My opinion, when people want to run a really short tire and plan long highway trips, you almost can't take enough rear gear out of the thing to make it highway friendly, and in this case the overdrives start to look like a better option, keeping your rear gear and short tire, excellent gear multiplication up through the gears for best acceleration, and still get your comfy 2,000 rpm highway cruise rpm. It's getting your cake and eating it too.

What Lee posted is right on the money. The programmability of the electronic overdrives simply can't be beat. I can use the lap top and program shift firmness vs throttle position, making it a pleasure around town and neck snapping at full throttle. Shift points are infinitely adjustable etc.... A few key strokes and you're done.

With Paul building the engine, you're in good hands.

As an example, my Z with 3.55's, a 4 speed Muncie (no converter slippage here) and a 27" tire that actually rolls out at 26.5" takes 2900 rpm to go 65 mph on the highway.
It's not bad and doesn't bother me much as the little 302 loves rpm. I generally push 70 at about 3100-ish.
Your shorter tire, and with converter slippage, I suspect your cruise rpm at 65 will be slightly higher.

If that doesn't bother you then you're golden.

I don't see any problems at all if ask Paul what torque convertor will work best with your motor and gears. Just FYI, you are probably going to need at least a 3000 stall, maybe higher. If you are used to riding around with an OD transmission with an electronic lockup, a 3 speed with a high stall and no lockup is going to be way different. That will only be a "problem" if you know what to expect and can accept the difference in driving experience.

You are also going to need to make sure that you are ready to keep that big engine cool. More displacement with more HP and more RMPs means a lot more heat to remove.

I recently put in a 469, and I'm upgrading to an OD transmission and had to get a new radiator. One big changes causes other big changes to be necessary.

I recommend you try those 3.55s before swapping gears. I have 3.50s and a 28" tire and think nothing about driving it 3 or 4 hours on the highway. Mine is just a little baby 400 not a 455, though. If you don't like them, change them next winter.

The OP has 25.6" tires with that 3.55 gear. Makes a huge difference. That's why I recommended a 27 or 28" tire.

With a 28" tire, 3.50 gears are a piece of cake on the street and highway. I run 3.73's in one with a 28" tire and have done cross country trips with it.

A tire that is barely over 25" is a pretty good change though and makes a 3.55 gear feel a bit steep. A difference of 400 rpm at least verses a 28" tire.

I have the same size tires as you with a TH400 and 3.55 gears. It has a 3000 stall Continental converter and I had my carb and cam setup to work with the rest of the car. I run at about 2900-3000 rpm's at 60 mph. It doesn't bother me for 1-2 hour trips but some might find it a little loud. If I was doing a lot more highway trips or on roads where the speed limit is 70 I would probably go with a 3.08 gear. I do drive on highways where the speed limit is 65 but I just get in the right lane and do 60. I'm sure I piss some people off that think 65 means 75 or 80 but I don't worry about it. In VA you have to maintain a speed no lower than 15 mph of the limit so going 5 mph under is legal and if other people stress about it then let em. I had a 350 with this combo but now I am putting a 455 in it so I might make a change, just have to feel it out.

Why even drive a pontiac then. 60 in the slow lane. Disgraceful

Good decision with the TH400. Very strong transmission as everyone knows, and cheap to build or service.


Having said that your car will be a blast on the street light to light, but I suspect traction will be an issue with 3.55 gears and a 26" tire.



You will need the 3.08 gear to tame it down, but with the 455 making 500hp, the 26' tire and TH400 it will cruise nice and still be a blast on the street. I suspect traction will still be an issue.



Just my .02

The 700-R4/4L60's came in many flavors. They are tough as nails set-up with the correct internals and shift program, however they require an adapter plate and rear mount relocation and shorter driveshaft to fit into one of these vehicles.

I have a couple out there over 20 years now doing just fine behind powerful engines. One is behind a 454 in a 1970 Chevelle, another behind a 496 in another 1970 Chevelle and one behind a stout 400 Pontiac in a 1969 GTO. All have been flawless now at or close to two decades.

They are all non-LU with custom built converters that we supplied with them.

One advantage of the 700-R4 is the 3.07 low first gear. It works really well with 3.08 to about 3.42 gears and you still get .70 in OD. Many claim the gear spread from 1st to 2nd it too great which makes me chuckle every time I read that because all the ones we've done work flawlessly and you'd never know there was an issue in that area.

700's, much like 200-4R's got a bad reputation early due to "soft" internals and poor shift programs. In 1987 the factory corrected most of the issues with them and from there on up they just got better. We can even use parts from the 4L60E units, like the molded apply pistons to make the older units even better. In any case I wouldn't be afraid of one behind one of these engines if it was done correctly and a high quality converter used in it......Cliff

I am currently looking at all the options for OD. I have improved my car by increasing power and drive ability to include fuel injection. For me....an electric transmission is just too much money with all the required changes. Then you have the speedo issue using a stepper motor with sensor or GPS control, if you want to keep your car looking stock. Possibly change the tail housing for a mechanical speedo but that costs a bunch too. Its probably 1500 to 2000 above the cost of the transmission to just sort out all the required cables, sensors, speedo converters, torque converters, lock up control, TPS sensors, etc. Yeah, my TH400 shifts too early and isn't programmable, by that's why I shift it manually at times. I am warming up to the gear vendors unit.....I don't need to change the shifter detents, benzel, wiring etc either and it gives me an overdrive. I "think" I can get this done for a little over 3000.00 total. A 700R4 and the TV cable for a Fitech Unit is an unknown for me, there maybe a cheap way around it, but the only items I seen specifically made for it is $250.00, then you still have the shifter detent etc. shortened driveshaft and possible yoke change.

Many options, all cost more than most want to spend. Sure looks like the gear vendors could be one of the more cost effective and easy install option. A shortened driveshaft and likely an exhaust change. My 2 cents.

You need to recheck your math if you think there's a big gap between a 700/200 and a 4L80e. Why not list out the parts with prices of both and post it?

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chuckies76ta,
Well, it's been over 2 years since I posted this thread and my car just got on the road a few months ago. I had a 2 year knocking noise that was finally resolved. The car drives great from light to light but I haven't driven it on the highway yet. I have gotten it up to 60-65 mph and it's a little over 3,000 rpm's. I was considering doing the 4L80E but after this nightmare of an engine, I may just have to switch to 3.08 gears.

If you have over 100 miles on it it's time for WOT run through the gears on an open road. 3.08 gears will be fine with your power level, look at TAMan.

Maybe wait to decide until you get a ride in my car, so you have a 1st hand experience.


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AG,
This trans has over miles on it by now (maybe 300 miles). Since the engine was repaired, it has about additional 60 miles. I plan on doing some work to the car this winter...just not too sure what. Thinking about doing some suspension.

HWYSTR455,
Yeah, I'm looking forward to your car being on the road. Did you get the new sensor yet?

I decided to try the one I got, think someone used it to test and returned it, so it either works or not. If it doesn't heat up or read properly, I will send it back for another. Summit is on board with that approach. I should know this weekend.

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