TH350 vacuum modulator question - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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post #1 of 8 (permalink) Old Jan 5th, 20, 11:07 PM Thread Starter
 
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TH350 vacuum modulator question

I have a 71 chevy chevelle ss350 with TH350 tranny and I had to rebuild the engine because the previous owner used it for drag racing. But now I have rebuilt the engine and installed a Quick Fuel 600CFM carb on it but I noticed that there was no kickdown cable on it. I have read a few things that people say you dont need it. You can use the vacuum line to the manifold instead. But then when I seen a video on Holly's website it said with the TH350 tranny you need the cable and brackets to hook it up. So I'm confused is it needed or will the vacuum line be enough? The engine has been bored .040 over I switched it to HEI ignition it also has a 1/4 cam with hydraulic lifters in it. I am new here and appreciate any advice you could assist me with. The last time I played with these old muscle cars in back in the late 70's when I owned a 68 charger 383 4spd.
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post #2 of 8 (permalink) Old Jan 5th, 20, 11:39 PM
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Re: TH350 vacuum modulator question

The vacuum modulator needs to be hooked up to full manifold vacuum. You can run the transmission without a kick down cable but you have to downshift manually. There are many universal cable setups or you can opt for a factory replacement and stock brackets.

Just a note, you need the other 3/4's of your cam.

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post #3 of 8 (permalink) Old Jan 5th, 20, 11:39 PM
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Re: TH350 vacuum modulator question

The vacuum line is what determines when the trans will shift. Kick down is only needed for a full throttle down shift. Hope this helps

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post #4 of 8 (permalink) Old Jan 6th, 20, 12:32 AM
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Re: TH350 vacuum modulator question

You really should hook up the kickdown cable. It has 2 functions, it forces a downshift at wide open throttle and also raises line pressure to ensure firm shifts with the increased power at WOT.
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post #5 of 8 (permalink) Old Jan 7th, 20, 10:55 AM
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Re: TH350 vacuum modulator question

Kick down cable are fine for people who are lazy to shift manually and have hwy gears. You do not need kick down cable if you have big gears (4:10 on up). I like shifting my TH350 manually with 4:10 gears. It's more fun.

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post #6 of 8 (permalink) Old Jan 30th, 20, 10:40 PM Thread Starter
 
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Thank you for your help. I will look into it because I just hooked up the Quick Fuel 600CFM carb with the brawler bracket with the springs and throttle cable holder. Right now the intake manifold has a brass plug in the back of the manifold but the plug doesn't have any T's on it. I know I will probably have a issue removing it and installing the correct T because on the front they has a solid plug in for where you would attach te coolant line. The threads got messed up upon removing it because they put something on the threads to seal it. So I had to retap the hole because whatever they put on the plug. I appreciate the comments it will help me hopefully get the issue solved.
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post #7 of 8 (permalink) Old Apr 15th, 20, 6:37 AM
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Re: TH350 vacuum modulator question

Quote:
Originally Posted by pnugene View Post
You really should hook up the kickdown cable. It has 2 functions, it forces a downshift at wide open throttle and also raises line pressure to ensure firm shifts with the increased power at WOT.
Is this true for a TH350? It's true for a TH700, but the '700 has a throttle valve, the 350 has a kickdown.

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post #8 of 8 (permalink) Old Apr 15th, 20, 10:46 AM
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Re: TH350 vacuum modulator question

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Originally Posted by Schurkey View Post
Is this true for a TH350? It's true for a TH700, but the '700 has a throttle valve, the 350 has a kickdown.
I'm not the sharpest tool in the shed, what I said is true for 700R4. After doing a little research, I learned TH350 and 400 cables are for kickdown only, the governor and vacuum modulator regulate line pressure based on speed and engine load. So, the kickdown cable can be left disconnected without any damage, you just lose the automated passing gear function.

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