As some of you might know I'm installing a V12 in a Series III sedan.....replacing the old 4.2 6-cylinder. I post here at times because there is more V12-related experience.

https://www.jaguarforums.com/forum/x...roject-142259/

Anyhow.....

I wondering about the vacuum tank for the power brake system. I have the tank, pipes etc from the donor car (also a Series III V12). This vacuum reservoir system was not used with the 4.2 6-cylinder although the braking systems are the same on both cars

Vacuum Tank Assembly-Lhd - Parts For Series III Saloon | Jaguar Classic Parts UK

Is there something different about V12 vacuum characteristics than would require a vacuum reservoir for the power brakes....where the same car with a 4.2 6-cylinder did perfectly well without it?

The XJS V12 used the same vacuum reservoir set-up, by the way

It wouldn't be too hard to install. I could take it or leave it. For the sake of simplicity, though, I think I'd rather leave it

Any thoughts?

Cheers and thanks,
Doug

 

The reason for it, Doug, as you probably know anyway, was that if a failure of engine vac occurred, or you needed brakes when the engine was not running, there was a "reserve" of vacuum to help. Basically an extra safety feature. Up to you therefore, but FWIW, I would be inclined to keep it...
Greg

 

Actually, no, I hadn't thought of it as a reserve supply. I thought perhaps the V12 didn't typically develop as much vacuum....although I have no recollection of that ever being as issue

Hmmmm. It might be the first time I've ever seen a reserve vac tank on a vacuum assist system.

Jaguar thought the V12 owners deserved the extra safety measure but the 6-cylinder owners were less worthy ?


Cheers
DD

 

Interesting:


1. I wonder if there are spec's to compare V12 vs I6 vacum numbers.
probably between 15 and 20 hg's ???


2. I do know that '"souped up " engines with cams that have a ot f overlap suffer from low vacuum at lower rpm's. Cure? vacuum tanks and even electric powered vacuum pumps.


3. A lot if not most cars do well sans a vacuum reserve tank! Each of my cars have none and do well.


So, inasmuch as you have one, I vote for it's use.


Carl

 

Simple marketing Doug, V12 owners were paying more, so worth protecting more so they would be around to buy another! pip pip
Greg

 

I think you probably know that all conventional vacuum assisted braking
systems carry a reserve. At least you should

Thinking back to your GM days, you will recall that the reserve
was integral to the booster.

Perhaps the particular booster used on the V12 did not have
an integral reserve, or it was deemed to be inadequate.
Maybe physical packaging requirements of shoehorning the
V12 into the space forced using an external reserve.

It's not the only car with an external reserve tank. I just
can't remember a particular example to point at right now.

 

The old reserve tanks on some of my older NA cars looked like a black tomatoes juice can.
I'm thinking about a 12v vacuum pump for my brakes and accessories. I'm going to up grade my cam again but need to change tranny first.

 

They did? No, I didn't know. Which models? Was it some sort of separate chamber or ?



No, the V12 and 6-cylinder cars used identical systems, including the booster



Let me know when the bells ring

The only external vacuum reserve tanks for power brakes I've seen were add-ons....for hot rods with lumpy cams where there just wasn't much vacuum to work with.


Cheers
DD

 

What size can? Single serving, or family size? If single serve, that sounds more like a vacuum reservoir for heating/air condition systems.

The Jag vacuum reserve is about the size of a small watermelon ! Maybe 8" diameter and 16" long or so

Cheers
DD

 

Family sized. This is old style Dodge

 

BC:


Sure looks like a tomato can sans paper label!!!


Carl

 

I'm sure it is too. We called them tomato juice cans

 

Also, never by me, used for exhaust tips!!! Termed "blooey" cans, back
in the day. At times to simulate "dual pipes". Swap to blacken the inside!!!


Exhaust tips seldom appealed to me.


I only recall a couple. One used in 58 on my T speedster project.
straight single pipe. No muffler. About 12" of 8" pipe. Open on each end. T exhaust pipe suspended in the center. Theory, In motion, air would enter the front of the 8" pipe and create a scavenge for the exhaust??? worked?? But, sounded much better. It could and did run off and hide from friend's restored T three door "phaeton"!!!


Carl

 

Just a thought while I'm at it, and I'm happy to be proven wrong...

The V12 can only be cranked when in Park, so unless you're stationery, there's no swinging a V12 over... The Straight 6, if I'm correct*, can be cranked in Neutral and Park. If you were to lose engine power suddenly at a low speed for any reason with the V12 you have about 3 strokes of vacuum assisted power, there after you're on your own. The Straight 6 would still be able to be swung over, generating a little vacuum to help with the braking.

Just a thought, some may disagree. I'd keep the reservoir though, no harm done.

 

The V12 cars can be cranked/started in neutral as well!

Cheers
DD

 

Something's up with mine then, I've never started her in neutral...

Just tried now, refusing to be defeated, first time I've heard her swing in Neutral, had to fiddle with the shifter though, otherwise no crank, hell, I learn something new every day.

Ignore my previous hogwash...

 

The little switch at the gear likely needs a little cleaning or adjusting



As we all do, hopefully.

Sometimes I get lucky and learn my 'new thing' early on, before I'm even done with morning coffee. Then I can take the rest of the day off

Cheers
DD

 

The shifter likes to be budged forward when neutral, not that it bothers me, it works!

I can relate to learning my "new thing" early in the day, many days I've spent with my head under the hood, sure don't forget afterwards though!

 

Hi Doug ,

Under Jags " vacuum verification " test procedure , the vacuum gauge should read
at 800 rpm , 14.5 (in. Hg) .

Most cars have a safe reading of 16 to 21 (in. Hg) , at best your XJ6 would be running somewhere between 20 and 24 (in. Hg) .

At only 14.5 (in. Hg) at idle , it won't take much to evacuate the booster , 2 may be 3 pumps , I can see why the V12 would need to run a vacuum tank .

Cheers ,

Sid

 

My suspicion is it has to do with the injection. The XJ6 uses an air flow meter, the V12 does not, it determines engine load from the vacuum signal. There is a vacuum pipe that runs the length of the car to the ECU in the boot.

I suspect the extra vacuum reservoir is a way to separate the braking vacuum and the signal vacuum so that when you step on the brakes there isn't a drop in vacuum that would be read as extra load and the ECU then adding more fuel. In extreme cases that could give you a rich stall.