TH350 expert advice needed.

[donsrods]

My Son has a th350 behind the 455 Olds in his rpu and I am not sure we have the vacuum line for the modulator hooked up to the right source. The transmission shop told us that there should be suction at idle but it should drop off as you accelerate, and I think we have just the opposite.

The problem is that the intake manifold on the motor is a W30 Tripower intake that was used only on 4 speed 1966 442 Oldsmobiles so there is no port on the intake where you can hook up the modulator. So we took a plug out of the rear 2 barrel carb at the base and put a threaded barb in there and hooked the modulator to that. But I think that spot has more vacuum suction as the rpms increase.

The transmission works great and shifts ok, but it is pretty soft between gears, and it is a built Jackson Racing unit and the local shop said it should be brutal on the upshifts. We are also running a 3000 stall convertor which should help even more.

So my question is, should the vacuum suction for the modulator get less as you speed up, and does the spot we picked do that or not?

Thanks for any help.

Don

 

[bob w]

Engine vacuum is high at idle and part throttle steady cruise. It drops off as the go pedal is pressed.
There is an adjustment in the modulator that is reached by removing the vacuum hose. A small screw driver is inserted and can be turned to adjust shift point. Don't know that i will change the harshness of the shift.

But, there are much wiser transmission guys on RRR than me. Just wait a few minutes and you will get better advice.

 

[donsrods]

Thanks for that info, Bob. You are right about that adjustment screw, the trans shop says you turn it clockwise half a turn then go for a ride, and keep repeating that until it you get it to stay in gear for the desired amount of time. That part seems ok, it is the vacuum going to it that has me wondering. This transmission should almost chirp the tires on upshifts but it doesn't.

Don

 

[Tripper]

This transmission should almost chirp the tires on upshifts but it doesn't.

You might wanna try a lil more HP!!! [ddd[ddd

 

[donsrods]

Hehe, I think his power to weight ratio is ok now. I just got off the phone with B and M and the guy there says the optimum source for vacuum is a spot on the intake two inches below the center carb. He said the spot we are using now is backwards as far as what you want the vacuum to do. One suggestion he had was to try using the ported vacuum outlet where the distributor advance normally goes. We aren't using that anyway as Dan's distributor is mechanical advance only.

The car really runs and drives fine as is, but we just felt the trans was shifting softer than it should, given the hp and weight.

Don

 

[King Herald]

My Son has a th350 behind the 455 Olds in his rpu and I am not sure we have the vacuum line for the modulator hooked up to the right source. The transmission shop told us that there should be suction at idle but it should drop off as you accelerate, and I think we have just the opposite.

The problem is that the intake manifold on the motor is a W30 Tripower intake that was used only on 4 speed 1966 442 Oldsmobiles so there is no port on the intake where you can hook up the modulator. So we took a plug out of the rear 2 barrel carb at the base and put a threaded barb in there and hooked the modulator to that. But I think that spot has more vacuum suction as the rpms increase.

The transmission works great and shifts ok, but it is pretty soft between gears, and it is a built Jackson Racing unit and the local shop said it should be brutal on the upshifts. We are also running a 3000 stall convertor which should help even more.

So my question is, should the vacuum suction for the modulator get less as you speed up, and does the spot we picked do that or not?

Thanks for any help.

Don

You NEED to have the kick down cable connected to get max revs at upshift. If it is not connected your tranny will auto-shift up at about 3500, but with the cable in place and adjusted it will hang in there and shift at about 5000.

Lots of guys say they don't need no shift cable, but it serves more purpose than just shifting down when you stomp on it.

Didn't the tranny shop mention this?

I ran my old car without one for a couple of years, then one day I hooked it up after hearing the above info, and scared the poop outta myself when I floored it in top a6t about 30mph it kicked down from 3rd to 1st and took off like a scalded cat!


And yes, vacuum drops off as you open the throttle, as long as it is connected to a normal manifold take out. Max vacuum should be about 18" or thereabouts at idle.

 

[donsrods]

We eliminated the kickdown and blocked the port on the transmission because there is no room under there for it to clear. We tried Lokar and stock kickdowns and the floor is right there. I never had one hooked up in my 23 with the same transmission because all I had to do to accelerate was open up the carbs a little. But your point is well taken, it really should be there.

Thanks for the info on the vacuum too.

Don

 

[sgtpontiac]

On the 350's the TV valve does not regulate

upshifts....purely downshift / passing gear....the governor and modulator regulates upshifts....there is no problem with disconneting the tv cable....different story on the 700's and 200's.....

 

[RAT FINK]

With your power to weight ratio and using a 3000 stall convertor....is it possible that your convertor is still locking up to soon and is only working as a 12-1500 stall due to the light weight of the car .which is making it feel lazy???..port vac should be fine with the 350 trans

 

[donsrods]

We are going to experiment by moving the line to the ported vacuum spot and see what happens there. We topped off the fluid today, it was a quart low because we didnt check it running in park initially. Tonight we put on another 65 miles and he said it feels much better already. It runs along great, knock on wood, and isn't using any oil like the last motor. I hate to jinx ourselves but it seems that we got it right this time.

Tomorrow we are going to a car show about 40 miles north of here so he will get even more miles on the motor.

Don

 

[King Herald]

upshifts....purely downshift / passing gear....the governor and modulator regulates upshifts....there is no problem with disconneting the tv cable....different story on the 700's and 200's.....

At WOT is changes your shift points.

 

[toledo]

th350

don is this the first time with this set up? if the trans shop says brutal and they are known for building good stuff then the torque converter is to loose or to high of stall also rear gear ratio can turn a mouse into a monster,,,the th350 will shift manually without vacuum and deliver its full line pressure,unplug vacuum line from trans and drive it while manually shifting it,if the tires chirp your good,turn the screw on the modulator in untill it clicks reconnect vacuum line drive it if you like it your done if its too late of shifts light throttle back it off a turn till ur happy,,,,if the shift is still slushy with no vacuum hooked up,then your converter/rear gear ratio will have to be changed,,big stall converters really act as a cushin between the engine and trans ,,,as far as the vacuum striaght engine vacuum. use a guage highest idle vacuum ,vacuum decreases as you accel but then builds as rpms climb,,,hope this helps ,,toledo

 

[donsrods]

Toledo, thanks for that info. The rear gears are 4.11 and the trans is by Jackson Racing Transmissions. Yesterday he drove the car to a show after topping off the fluid level and he says it is much better. I think he was expecting it to slam into 2nd and 3rd on the upshift but even the one I had in my 23 (also a Jackson) would only light up the tires on the upshift when I manually held it in gears, and that car was way lighter than his.

What he doesn't realize is how hard his car pulls. When I was following him yesterday to the show, when he would leave a traffic light just driving normally I had to push my 27 harder than I normally do just to keep up. It runs just fine.

Don

 

[toledo]

high expectations

lol don trust me ive been there the expectations ur son has set in his mind were most likely of him lightly accelerating across an intersection as it shifts to second and chirp the tires to impress anyone that can hear it ,,,,and u gotta admit to this day when you hear that happen you will make that face to your buddies like oh yea thats cool lol,,,,hopefully it will seat the clutches in nicely and give him the shift he wants if not pm me i can tell you how to block the accumulators to make it happen,,,toledo

 

[sgtpontiac]

No argument about WOT....it's only function....

At WOT is changes your shift points.

but at wide open throttle the valve would be fully depressed, the vacuum would be near zero and the governor pressure would then dictate upshifts....without the kickdown cable you'd have to substitue / override the kickdown by manually down shifting and or hoping the drop in vac to the modulator would allow it to downshift and override governor pressure to affect upshifts at normal throttle.....the 350 Kickdown cable/valve operates exactly like the 400 electric kickdown....it has no bearing on the actual shifts, that is the job of the modulator and governor.....the kickdown cable/valve is strickly the kickdown for passing gear....unlike the 700 or 200 transmission whereas the TV dictates line pressures at varying thottle positions ....where adjustment of the TV cable is critical for upshift and downshifts and transmission failures from improper adjustments....not to argue ...