If I understand correctly, this is a fairly common (everyone may have it) problem with the TH700. Right now, I'm turning about 5100rpm at 120mph in 3rd gear. I can let off the pedal, and it will downshift and maintain speed. I can feather in the pedal slightly more, and it will pick up a little more speed, but invariably it downshifts into 3rd again.

About all I could find in the search was this thread: https://www.thirdgen.org/forums/tran...mming-kit.html

I looked at the instructions for their solution to the problem, but from what I understand, the trans would then NEVER downshift from 4th to 3rd (even when floorboarding it from 4th at say, 70mph).

Is there a way to modify it for the best of both worlds? Is it possible to make 4-3 and 3-4 happen like 3-2 and 2-3? My other TH700 cars were a Suburban and a 2.73 geared 305 TPI Trans Am, so I've never really had to worry about getting to a speed where I could WOT shift into 4th before. If there's a solution that's not too in depth, I'd like to try it. I'm not wanting to tear the trans all the way down though, as I should be putting my spare T56 in sometime about a year from now.

Thanks guys,

-cal30sniper

 

Yes, you can install two Allen screws in the 4-3 downshift passages in the TV bushing. Youy can do this without dropping the valve body. Call me and I will help you out here. 909 795-1876. Dana

 

Isn't this something that can be fixed with a PROM alteration? Same as the top speed limiter.

 

Only thing the ECM controls in the 700R4/4L60 trans is the TCC lockup. All shifting is done hydraulically via the valve body.

RBob.

 

Correct. The shift points on the TH700 are all set in the valve body, not the ECM.

Perhaps you're thinking of the 4L60E? Top speed on the 3rd Gen is controlled by the ECM cutting off fuel, not the trans. I wish it was that easy, I'd have burned the chip as soon as I got home yesterday.

Dana, thanks for the offer. This is what I was reading on the other thread, there's a link to a picture tutorial on how to do it from there. From what I understand though, this completely eliminates the transmissions ability to downshift into 3rd (i.e., if you are in fourth, and floor it, it won't downshift even if it should). Is that correct? I don't completely understand how the automatic transmission shift logic works. If you are doing 45 in 3rd, and you nail it, it's going to downshift to 2nd. It will automatically full throttle up-shift to 3rd when it hits its RPM set-point. Is there no way to make the 4th and 3rd gear interaction work the same way? I really don't want to create a situation where I have to manually shift into drive in order to kick down from say 60 or 70 mph.

Gah, I wish I had time to put in my T56 now. I really don't like being back in the world of automatics...

 

You are looking at a TV bushing that has the 4-3 passages as far back as possible and still get WOT 3-4. I see these very rarely, and use them for specific applications like yours. I have one in stock if you need one. The cost for these is $20.00 plus $3 or $4 shipping. The 3-4 modification that I was telling you about will still have 4-3 downshifts at low speeds but the 4-3 downshift goes away qucikly when going to higher speeds.

 

Dyno Don is going to install a shift kit in mine and he is also going to install a corvette (throttle valve or valve body) can't remember it's name off hand but it will keep my 700r4 from downshifting into 3 a WOT. Corvettes came that way but not Camaros.

 

Yes, some of the Corvette valve bodys had this option. The 4-3 downshift passage is missing on these, that is where we got the idea of how to do this.

 

I had Dana setup my Street/Strip to not allow downshift out of 4th when I bought it back around 2000. It has worked very well. I don't think there was the modification back then to allow the low speeds downshifting. That would have been a nice feature.

I've always wonder is there any reason to be concerned about stress put on the transmission when going WOT 3-4 then continuing WOT while in 4th?

Wouldn't this allow us to run a numerically higher differential gear for the drag strip and use 4th to continue to accelerate before the line since we have WOT 3-4 and no 4-3 (either at high speeds or at all depending on modifications used)? Course we'd be past our 1:1 ratio in the transmission, but really isn't any different then the guys wanting to use a Gear Vendor setup.

 

Yeah, I knew the ECM shut off fuel to limit speed, I was just using it as an example of something that can be changed. I don't know why I was thinking that the ECM controlled the 3-4 shift. It is my understanding the TCC won't lock at WOT either and that is changeable thru reprogramming.

 

You guys pretty much have it right. As I understand it, the 1987 and earlier cars would do the 3-4 shift. While the 88-91'ish cars would not. Somewhere in 91/92, GM started making the cars do the WOT 3-4 shift again. The 1988 cars had upgraded transmissions over the 1987 cars, but oddly enough the WOT 3-4 was discontinued starting in 1988.

Now as far as driving a car with a modified plunger sleeve. You can still get 4-3 downshifts at normal speeds. Around town at 40 mph, the car will stay in 4th, rather than doing the 3-4-3-4 constant back and forth. It just takes more throttle to get the downshift. You'll like this. Especially with our torque.

On the highway at 60 mph, you're pretty much in 4th unless you give it about 80% throttle instead of 50% (percentages are rough estimates)

At 85 mph and above (already cruising in O/D), the car will stay in 4th gear even with WOT. Basically the faster you're driving, the more throttle is needed to get a downshift into 3rd....Until you get to 85 mph at which point even full throttle won't get you a downshift. It's only in this area that you'd need to downshift manually to 3rd in order to get the downshift into 3rd. 9 times out of 10, I just keep it in O/D and gradually accelerate faster. Once you're doing 100 mph you really don't want a downshift anyways. My car is loud enough at this speed and unless you're racing somebody, you just don't need nor want a downshift at this kind of speed.

When you slow down around town, the car will still downshift on it's own into 3rd like normal. No throttle, 35-30 mph etc.

There is a B&M valve that ups the mph that the automatic 4-3 shift occurs. I don't like this. Because while it's nice for keeping the car in 4th at 50-80 mph, it means that you'll still be getting the automatic downshift at 130-135 mph instead of 120 mph. This is a terrible idea.

Dana's (Pro Built Automatics) modified sleeve idea works. Sonnax sells a WOT 3-4 shift plunger sleeve with no holes on the sides. Same principle as Dana's blocked holes. It's about $40.

At one time, GM sold a valve like the Sonnax, since the B4C Camaros needed WOT 3-4 to catch the bad guys. But that valve was discontinued something like 7 years ago.

 

I think it's primarily a towing issue. People who are hauling campers down the highway don't want their transmissions to stay in O/D when they get to "x"% or more throttle. They want their transmissions to downshift to 3rd even with the shifter in O/D.

I suspect GM also didn't want to repair 3-4 clutch packs from people beating on their IROC's and Trans Ams back in the day. But then again, the Corvettes were good for 150-160 mph with the same transmissions. Lighter cars though.

 

Hang on, you're saying that with 3.27s and a TH700, you're seeing a WOT 3-4 shift at 85 mph? That means you are going from 3500 rpm in 3rd to a very low 2450 rpm in 4th. That rpm is going to turn you into a snail at 85mph with a warmed over L98.

You really want your WOT 3-4 shift to happen as high as possible due to the long leap between 3 and 4 ratio-wise. If you can push up to about 5200 rpm in 3rd (126mph), that'll drop you to 3650 rpm in 4th. The engine should be making enough power to continue accelerating at this point, resulting in a smooth 3-4 transition. If you shift much lower in the RPM band, the engine won't be making enough power at the RPM you end up at in 4th to keep pulling. If you do it any higher in the RPM band, you're going to run out of hp up top due to the limitations of TPI. I would say you wouldn't want a wide open 3-4 shift to happen at anything less than about 4800rpm/115mph.



You've lost me here. Are you talking about upping the maximum speed where a 4-3 downshift can occur? From the factory, there is no maximum speed for a 4-3 downshift to occur, it happens at any speed. How could you up that? I also don't see why you would want to keep the car in 4th if you're running WOT at 50-80 mph? How does keeping the car in 4th at 50-80 correspond to not being in 4th at 130-135 mph? And where does the 120 mph figure come from? Can you explain that a little clearer?

 

From what I understand, there's two reasons you want to be gentle on a TH700 in 4th: Heat and Strength.

Transmission fluid pump speed is controlled by the torque converter side, not the driveshaft side. When you drop into 4th, you get less fluid flow in the transmission due to decreased converter RPM. That results more heat build up in the transmission, which is one of the main things that kills them while towing in 4th gear. I'm sure a prolonged run at high speed in 4th gear would have the same effect without a very good transmission cooler.

Also, from what I've read and heard in the towing forums I frequent (read up a whole lot before I put together the TH700 in my suburban), the 4th gear is just generally a weak gear in the transmission due to its overdrive ratio. Putting the heavy strain of a heavy trailer on this weak link in the drivetrain is just not a good idea, and often breaks things or severely shortens the life of the transmission. Again, with a large cooler and a light trailer, you should be perfectly okay. I figure if the transmission held up behind 100,000+miles in a heavy suburban with no trans cooler, that it should take quite a bit of abuse from a much lighter 3rd gen at factory power levels.

-cal30sniper

 

Dana, when using this special TV bushing, above about what engine RPM in 4th would you no longer see a 4-3 downshift, and what RPM would you expect to see a WOT 3-4 upshift?

 

The 4-3 depends on what rpm you are at. I do not know the formula for when the 4-3 downshift starts to go away. But there are subtle differences in each valve body, and the rear end gear ratio has a lot to do with it. The WOT 3-4 upshift is determined by the governor, just like the 1-2 & 2-3 WOT shifts.

 

1) No, no. I'm saying that if you're cruising down the highway in 4th gear at less than 85 mph, you can get a downshift to 3rd with stock or modified sleeve....With the modified sleeve it will take more throttle at any speed to get a downshift to 3rd than it would with the stock sleeve at the same respective mph.

So at 70 mph, it would take about 50% throttle to get the car to downshift into 3rd gear. With the Sonnax/Pro Built type sleeve, it would take about 80% to get a downshift......Now let's speed the the car up a bit (still in 4th gear). A real gradual increase in speed to 85 mph. At this point to get a downshift, you'd need about 75% throttle with the stock sleeve to get a downshift into 3rd gear. With the Sonnax/Pro Built type sleeve, you could put the throttle to the floor and it would stay in 4th gear. Even at 100 mph, you can still get a downshift with the stock sleeve. About 90% throttle is needed. The Sonnax/Pro Built stays in 4th.

2) This is specifically in regards to the B&M sleeve. It doesn't work as well as the Sonnax/Pro Built types. It works "somewhat" better than stock, in that it takes more throttle to get the car to downshift to 3rd. It's handy in regards to light acceleration in the 50-80 mph range. (You're more likely to stay in O/D.) If you really want it to downshift, it will. Just takes more gas pedal. The problem with the B&M though is that it will still downshift from 4th to 3rd when you're doing 135 mph. There is no way around this.

Just as with the stock sleeve, it doesn't matter if you're full throttle at 120 mph (stuck in 3rd gear, no WOT 3-4) or cruising along in 4th gear from 110-120 mph when even though you're feathering the gas pedal, it will downshift into back into 3rd at 120 mph. The bottom line is that with the stock sleeve, you can't get past 120 mph (Unless you have 2.77 gears or are going past the red line).......The B&M sleeve just raises the area that the downshift to 3rd occurs from 120 mph to a much worse 135 mph. A speed you REALLY don't want to get a downshift at.

With the Sonnax/Pro Built sleeve, there is no way to get a downshift at 85 mph-top speed speed unless you physically move the shift lever. There are no issues happening at 120 mph or 135 mph. You're in 4th.

Now going from stopped to top speed, the Sonnax/Pro Built will work just like a Corvette. 5000 RPM shifts into 2nd, 3rd, and 4th.

I have Dana's TV bushing. For me, 85 mph is the point that the 4-3 shift ceases to exist.

WOT 3-4 happens at 5000 RPM. Roughly 120 mph, the car will shift into 4th gear.

 

Ah, that makes much better sense. Thanks for clarifying. I may end up doing exactly that then. It's not too terrible of a problem with 3.27s, but I could see this playing heck with the guys running 3.73s or higher.

Thanks again!

-cal30sniper

 

So between 85 mph and 120 mph, you have a few different options with Dana's sleeve. If you put you foot to the floor before you got to 85 mph, it would redline 3rd and then shift into 4th at 120 mph. The WOT 3-4.

If you were between 85 mph and 120 mph and then put your foot to the floor, you'd stay in 4th gear for as long as you held your foot to the floor......Or you could manually pull the lever from O/D to D and get that shift into 3rd. Then move the lever back into O/D at get the shift at 120 mph.

Before I had Dana's sleeve, I remember driving through some winding empty mountain roads. Doing 80-100 mph the whole time. The car would shift 4-3-4-3 depending on accelerating vs coasting......With Dana's sleeve, I could have kept the car in 4th the entire time. Much more preferable this way.