vacuum

[norseman]

I recently needed to replace the small bore rubber hose that enables vacuum from the plenum of my old V8 RR to advance the distributor timing.
The replacement length of hose (now silicone) purchased is one metre, which is longer than required. It's occurred to me that it may need to be a specific length, based on the fact that the longer a vacuum pipe the less 'suck' there will be at the end.

I'm currently using the entire metre & the car runs well, but should I reduce the length? It's not practical to simply go by the length of he original hose as the ends have been trimmed back more than once due to the old rubber splitting.

 

[Burnerman]

I really wouldn't worry about it - the vacuum isn't enough to collapse the silicone hose so just trim it to a suitable length.
John

 

[Lower]

It won't be that sensitive to the exact length of hose. I'd trim it to a length similar to the original but slightly longer. With a longer length of hose, the amount of 'suck' as you call it won't change, but it will take slightly longer to create a vacuum in a longer length of hose as it has it greater volume.

 

[foxhole]

Just match to original length .

 

[Avocet]

I'd be amazed if you'd notice any difference. As Lower says, it's only the time taken to evacuate the tube that will change. The level of vacuum won't. The shorter the tube, the faster the distributor will react to changes in manifold pressure though.

 

[norseman]

Many thanks to you all for the replies, sometimes a seemingly silly question isn't ..

I've shortened the hose to a length I think will match the original spec.

 

[Burnerman]

Scratching what's left of the brain cell......
Would that be the Lucas 35D distributor originally? Just wondering if you've considered an electronic ignition upgrade or perhaps you prefer originality?
I do recall seeing these things on an original Krypton tuner (!) and the advance curve was all over the place......sometimes due to internal wear or weakened bob weight springs.
Regards
John

 

[norseman]

Scratching what's left of the brain cell......
Would that be the Lucas 35D distributor originally? Just wondering if you've considered an electronic ignition upgrade or perhaps you prefer originality?
I do recall seeing these things on an original Krypton tuner (!) and the advance curve was all over the place......sometimes due to internal wear or weakened bob weight springs.
Regards
John

RR's went 'electronic' in the dizzy department in the early 'eighties, but thank you for your contribution John.

nb. yes it was originally the 35D, mine is a 35DM8.