1948 Buick straight 8 to TH350 - Vacuum

[cvlawson]

I am building a custom Buick, (1923 Buick fenders, cowl, running boards, seats) sitting on a 1948 Buick chassis with straight 8 engine. I have mated it to a new TH350 trans from TCI. How do I get the vacuum to the trans? Can I use the vacuum line that was originally intended for the wipers that comes off the vacuum/fuel pump? Also, any info on what needs to be hooked up to the trans? I know I am deleting the kickdown cable. I will have a trans cooler.. What about the other ports? Not used? My main concern is setting up the vacuum line, but I am new to the 350 transmissions. I appreciate any help.

[Bloo]

Needs to connect to the manifold. Don't use the boosted wiper vacuum from the pump. Connecting to the same line that feeds the intake of the pump should work.

[EmTee]

I don't believe that you can simply delete the 'kickdown cable'.  If it is a TH-350, that is actually a 'TV Cable' (Throttle Valve).  It is used to adjust pressures inside the transmission based on throttle position.  You will need to find a way to attach it to the throttle valve bell crank so that it operates from idle to WOT as it would in the original application.

[cvlawson]

So just a "T" between the intake and pump. Run a hard line, with small rubber section at trans and that is good? What about the line that ran to the wipers? Cap it or just leave it open to vent?

[Sonomatic]

The vacuum modulator is a vital component of automatic transmissions. It tells the transmission what kind of load is being put on it, allowing the transmission to react with the proper line pressures and shift points.  Because of this, you want your manifold vacuum source to be soley manifold vacuum, not a vacuum signal enhanced by the vacuum function of the fuel pump. It’s original purpose of the vacuum/ fuel pump was to “cover up” the low manifold vacuum during acceleration. This low vacuum during acceleration is exactly what tells the vacuum modulator on  your TH350 to increase line pressure for firmer shifts and also tells the transmission to raise the shift points for better acceleration. The kickdown linkage is what tells the trans to drop to a lower gear for acceleration such as when passing. Later model transmissions used a TV cable which took over the function of the vacuum modulator and kickdown linkage as EmTee stated.  As for the line for the wipers, if you’re not using it, cap it off, otherwise , it’s the same as a vacuum leak.  What adapter did you use to mate the 263 to the TH350?

[cvlawson]

I am going to cut the line running from the manifold to the pump and place a "Tee" to go to the trans. The other part will continue to the vacuum pump. I will then cap the end that use to go to the wipers. I appreciate it greatly. 

[NTX5467]

As I understand it, the kickdown cable determines large-throttle movement downshifts, as the vac modulator determines shift feel (by varying line pressure) and part-throttle downshifts, related to load and not specific to road speed.  The vac modulator also compensates for altitude, too.  Might be able to adapt an inline 6-cyl THM350 set-up from a Chevy or GMC pickup truck of the later 1960s or into the later 1970s, I suspect.

 

What intake manifold are you using?  Are there any bumgs on it for vacuum power brakes?

 

In ANY event, you DO need the downshift linkage to be there and properly adjusted.  You might use a vac tap at the base of the carb for manifold vacuum to the vac modulator.  OR put electric wipers on it and use the vac line to the pump, which would then not be needed for the wipers.

 

Just some thoughts,

NTX5467

[Sonomatic]

For some reason I thought you had a 263, sorry. What adapter you using on your 248?

[cvlawson]

https://transmissionadapters.com very beefy and installed flawlessly.