Auto box - influence of internal diameter vacuum line

Thijs Leuven

Thijs Leuven

Active Member
Hi guys!

I'm experiencing some gearbox troubles on my not so standard P6. Therefore, the following question is more a general one... My P6 has a 4.6 V8 out of a Range Rover, and the gearbox is a much younger three speed GM box, that has a vacuum modulator.

Does the flexibility and internal diameter of a vacuum line influence the amount of vacuum the gearbox can get? I imagine so... What kind of vacuum lines are you guys using? I have zero experience with this, so I'm not quite sure what to look for in the new replacement lines.

My car has had a bad case of a splitting vacuum line, over and over again. It wouldn't shift beyond first gear because of this. I decided to remove it completely. I had some spare fuel line in my garage, so decided to put that in instead. The outer diameter is the same as the vacuum line, but the internal diameter is much smaller. It's only 4mm, compared to the 6-7mm of the old line.

The car shifts again, but it feels a little less smooth and most importantly: it only shifts at 3000-3200rpm, no matter how gently I accelerate.

Thanks in advance, any help is much appreciated! :D
 
The GM has or should have an adjustable modulator that you can vary the shift points to suit, it's a case of trial and error.
If you look at the rear of the modulator you will see the adjustment screw, make sure it's screwed in slightly before you start your adjustments.
As for the pipe itself, I would suggest using a steel pipe for the long section between transmission and engine with only small re enforced rubber pipe Sections at the modulator end and engine end connections.
 
That is very helpful, thanks a lot! I'll make sure to let you know how it worked out! :thumb:

As for using a steel pipe: the majority of the length is far from straight. Finding something suitable might prove difficult! It makes perfect sense though, so I'm keeping this in mind! :D
 
Hard nylon tubing is not easily sucked flat, but is moderately flexible. Rubber vacuum tubing usually has thick walls to prevent collapsing - a tube with 1/8" ID will probably have walls at least 1/8" thick. Small bore copper tubing is very easy to bend, but should probably be tied in place with P clips to prevent vibration leading to fatigue.
 
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