[v12-engine] vacuum reading on a V12

Can anyone tell me what their stock V12 has in regards to
vacuum at idle? I still have not resolved my orange headers
on the left side of my engine at 2500+ rpm. My vacuum gauge
indicates advanced cam timing due to the fact that I only
have about 12in of vacuum. I eliminated my balance tube, and
have the same vacuum on both sides, as it was suggested that
my booster may be leaking. I don’t think it is because I
have the same on both sides. Shouldn’t I have about 19-22in
of vacuum?–
Gene Holtzclaw
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At 01:44 AM 1/08/2010, you wrote:>Can anyone tell me what their stock V12 has in regards to

vacuum at idle?

Shouldn’t I have about 19-22in
of vacuum?

Gene Holtzclaw


The aftermarket EFI system on the HE engine in the '79 coupe reports around 40% at idle and 600RPM.

In this case 100% is WOT, no vacuum = zero inches.

So I guess at idle vacuum is 60% of 30 inches. ( 30 inches ? We are metric and use bars or something ).

Richard Dowling, Melbourne, Australia.
1979 XJ-S coupe + HE V12 + 5 speed, 1988 XJ-S V12 convertible, 2003 XJ350

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In reply to a message from Richard Dowling sent Sat 31 Jul 2010:

Gene I have a carburetred V-12 that’s resonably recently
rebuilt (5000 kms now). One of my vacuum gauges gives 21’’ at
idle, the other says 19’’.–
The original message included these comments:

Can anyone tell me what their stock V12 has in regards to
vacuum at idle?
Shouldn’t I have about 19-22in
of vacuum?


Neville S1 XJ12, Megasquirt ECU
Christchurch, New Zealand
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In reply to a message from Nev.W sent Mon 2 Aug 2010:

Thats exactly what I needed to know…Thank you! My vacuum
gauge says that between 19 and 22 in is normal. I guess its
back to the drawing board on retiming my cam timing.

I was advised that advancing the cam timing would really
help the low end performance. What I have found, is that
when you advance the timing, the vacuum drops, and the carbs
do not work right at all. It takes the vacuum to lift the
pistons properly in the carbs. Not only that, at higher
rpms, the exhaust valve is also opening early, so the engine
actually blows the unspent exhaust back through the carbs!
If anything, I now see that ‘‘retarding’’ the cam timing may
be what causes these engines to turn on more than the
factory allowed them to.–
Gene Holtzclaw
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In reply to a message from Gene Holtzclaw sent Tue 3 Aug 2010:

Hi Gene, I had the same problem when I had some hot cams in
the engine. No vacuum at idle. It was very hard to get the
car to pull away from rest, but once it was moving it flew.
One must remember that Jaguar’s D type cam grind, which goes
well in V-12’s came from an engine that only ever ran fuel
injection, or maybe Webers with throttle pumps, and had a
mechanical brake servo. I’m guessing they have no vacuum to
speak of at low revs either.

Just to clear up a matter from some of your previous posts,
remember that once you have found TDC and the ‘‘A’’ mark on
your damper, BOTH banks are timed of that one mark. The
crank DOESN’T turn 60 degrees for the B bank.

Good luck, I’m sure you will get it sorted.–
The original message included these comments:

gauge says that between 19 and 22 in is normal. I guess its
back to the drawing board on retiming my cam timing.


Neville S1 XJ12, Megasquirt ECU
Christchurch, New Zealand
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In reply to a message from Nev.W sent Tue 3 Aug 2010:

The problem still persists of my left side header turning
orange above 2500 rpm. I did time both cams from the ‘‘A’’
timing mark. The first time, I used a dial indicator, and
set them at 6 degrees advanced. When I retimed it, I set the
crank at 2 degrees BTDC, and then set both cams straight up.
My vacuum is low. And these are my stock factory cams. I
guess I am going to have to tear it down one more time, and
as Kirby suggested way back, install the cams with the cam
tool as the factory intended. Appears that there is very
little to no room to wring out any more performance out of a
stock V12 than what it came with.–
Gene Holtzclaw
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