455 swap now TH350 shifts too early

This is a 1971 GS350. I swapped the 350 for a 455. The 455 is mild, Stage 1 cam, headers, rejet carb, but really no major modifications. I kept the TH350 trans and I installed a mild street torque converter with stall of 2000 or so. The part throttle 1-2 shift is about right but the 2-3 shift is almost immediately after it, like 20 mph. Can you guys please tell me where to start looking? Is this likely due to the increased torque, the new converter, or something I overlooked during the swap. Thanks.

Before I always had nice crisp shifts that I could feel. In fact if I had to guess I would say one of the previous owners may have installed a shift kit. Now I can't hardly feels the shifts. Especially the 2-3 shift, can't feel it at all, I have to listen to the engine speed to know when it is shifting.

 

The mushier torque converter is going to soak up some of the harshness/firmness. I believe a 350 turbo shifts based on vacuum and the governor. Does the 455 pull more vacuum than the 350? How about the kickdown? Does the 350 kick down when you stomp it in 3rd gear?

Here's what I would do. Hold it in first and run it up to 2500 RPM, above the stall speed, and shift into second. Does it feel firmer at that RPM? Then do the same thing from second to third. Did that feel firmer?

After that, if you can feel all three gears and they are firm enough, I'd move on to the vacuum modulator at the tail end of the trans. Turn it about 1/2 turn in/clockwise to make it shift a bit later and firmer.

If that helps but doesn't cure it, you may need to check into a governor kit.

Report back the difference.

 

Make sure the detent ("kickdown") cable is adjusted properly. A different engine may have a slightly different bracket to carb distance. Check the vacuum modulator, as has been stated already. The 455 is going to have to work less to move the car, so your vacuum signal to the modulator will be higher, which is an apparent lower throttle value.

 

I didn't have to disconnect the detent cable because it runs from the gas pedal to the trans. I changed the trans oil and filter, when I had the pan off the cable seemed to be adjusted properly, the linkage is pushing the button down all the way when I get to WOT. I assume the detent cable was for WOT only.

I modified the throttle cable bracket at the carb (cut and weld) to lengthen it for the 455.

I'll check for correct down shifts, etc and report back. Is my modulator adjustable??

 

Update:
Manual upshifts are nice and firm. WOT 2-3 shift occurs at 65 mph. 3-2 downshift from 40 mph is good. So I guess there are no problems.

The only thing is the part throttle 2-3 shift happens at like 22 mph, I can live with it and the car pulls fine, but if it's an easy fix I would like to raise it. Even my Centurion has a part throttle 2-3 shift at 30+ mph.

 

I think what is happening is the 455 works so little, as previously noted by SteeveeDee, that is is shifting prematurely due to the higher RPM and higher vacuum signal. You probably want to increase the vacuum required to allow part throttle shifts.

There are two types of adjustable modulators. One is huge, can't be covered with your closed fist and the vacuum line is on the side with an adjustable screw at the end. The other is much smaller, will just about fit in a closed fist with the vacuum line at the end and has the screw inside the vacuum nipple. Go clockwise to raise the vacuum signal. I may have it wrong but it should make a difference. Go about a turn clockwise and if it is too much go a 1/4 counterclockwise.

Good luck!

Vacuum Modulators

 

I gotta get used to that Buick setup. I'm used to Chevy 350s for the detent cable adjustment, which is up on the intake manifold. Sorry if I steered you wrong. Also, unless you have the right type of vacuum modulator, it may not be adjustable. Not sure on this one, it's been a couple of decades since I've had to adjust one.

As for a 2-3 upshift at 65 MPH, I'd say that sounds OK, depending on your rear axle ratio. I've a 2.73 axle behind the 350 and it kicks down at 75 if I punch it.

 

Rear axle is 3.08.

Thanks guys, I'll look at the vacuum modulator tomorrow.

 

Update: I have one of the small modulators. I turned the screw in (clockwise) 360 degrees and no difference. In fact at light throttle it's in 3rd gear by 15 mph. If I am driving 35 mph and give it half throttle, just enough to get a 3-2 downshift it shifts back to 3rd pretty quick. Again WOT is fine. Do I need to pull the governor?

Not sure if it matters but my vacuum at idle is 18 in..

 

OK, if one turn in didn't make any or negative difference, go two turns counter clockwise to see what difference you get. I'm never sure which way is which.

If that doesn't work, you will most likely want to work on the governor weights.

 

Update: I turned the screw two full turns clockwise. It's better. The 2-3 shift is at 20 mph if I am just off idle which is how I am accustomed to driving it. If I give it any gas the 2-3 shift is at 25 mph. And obviously the more gas I give it the higher it shifts. I'm happy with it. I may want to play with a governor kit in the future after I've run out of other things to do. Thanks everyone for your help.

 

How does it shift at WOT? That is what the governor will do for you. What ever your shift point is desired to be, that's what I'd shoot for on the governor. Mine would shift right at 5200, which was perfect for me.

 

When I did my 350 to 455 swap, I also had the "shifting into third way to soon. I am running the TA212 cam and pulling about 16-17 inches of vacuum. It would shift almost immediately to third after second. Is this adjustable with the stock modulator, or do I have to buy a different one? Did you buy one? You posted that you turned in the screw 2-3 full turns, is this with the stock modulator? Did you have to take the cover off? Any details?

 

Yes, you are correct. Part throttle shifts. Like just starting out normal from a stop sign or something like that. The harder I get on the gas the longer it takes to shift into second and third. They seem like normal shifts when the pedal is like half way down, but at say a quarter down it shifts pretty early. It's hard to explain really unless youre driving the car lol. I am happy with the full throttle shifts though.