Manifold vacuum with ITB's...

[jondee86]

So I got the engine running with ST ITB's and open stacks on my 16V.
But when it's idling (900-1000 rpm) I'm only getting about 30 kPa of
inlet manifold vacuum (70 kPa absolute pressure) And I have tapped
all four inlet runners and run them to a little vacuum tank etc.

Does that sound about right, or am I missing something ??

Cheers... jondee86

[matt dunn]

And I have tapped
all four inlet runners

What size hold have you made, needs to be at around a 10mm hole to each runner to the tank.

[jondee86]

Actually, I have a small tank with skinny hoses like the standard MAP
sensor hose. This feeds the MAP sensor and the FPR. Then there is a
separate set of larger tappings (about 8mm bore I would guess) that
feeds a manifold made out of an old fuel rail. This does the ISCV (I'm
using a 20V one) and the brake hose.

I guess I just need to know how much vacuum a 20V ST makes at idle ?
If it makes 70 kPa or a lot more than I am getting, then I shall have to
do some investigating. Maybe my butterflys aren't fully closed ?

Cheers... jondee86

[sergei]

Actually, I have a small tank with skinny hoses like the standard MAP
sensor hose. This feeds the MAP sensor and the FPR. Then there is a
separate set of larger tappings (about 8mm bore I would guess) that
feeds a manifold made out of an old fuel rail. This does the ISCV (I'm
using a 20V one) and the brake hose.

I guess I just need to know how much vacuum a 20V ST makes at idle ?
If it makes 70 kPa or a lot more than I am getting, then I shall have to
do some investigating. Maybe my butterflys aren't fully closed ?

Cheers... jondee86

Where exactly do you tap vacuum from on the manifold?
I assume you are using custom lower part of the manifold which does not have >10mm holes jonining?
The stock 20V "vacuum compartment" is fairly small, but it is being joined by fairly largish galleries that tap from injector area.

[jondee86]

Its a custom tube manifold to adapt the ITB's to the 16V smallport.
I don't use the 20V manifold, just the throttle bodies. The tappings are
in the tubes, near the flange that bolts to the head.

Since there is no flow in the MAP sensor line, only pressure changes, I
figured small tubes would be fine, and provide a bit of signal damping.
The larger tubes act as a balance tube, and flow the air from the ISCV.

Cheers... jondee86

[matt dunn]

Actually, I have a small tank with skinny hoses like the standard MAP
sensor hose. This feeds the MAP sensor and the FPR.

that would probably be the problem.

The line are too small and as the engine is only sucking on each cyl for 90 deg out of 720 (roughly) the small lines are acting as too much of a dampner, and your not getting full vacuum reading.

We had the same problem when we has ST ITB's on the XR6 Focus,
and on Dads GT40 which runs 8 V8SC ITB's.
You need big lines to a tank/chamber to get a relaible vacuum source.

[ChaosAD]

your not taking the map reading from the tube used for the iscv are you?

[jondee86]

your not taking the map reading from the tube used
for the iscv are you?

Nope... completely separate set of tubes. Factory vacuum tappings are
also tiny, so I'm not convinced that small tubes is necessarily a problem.
But I will check. Interestingly enough, the only response I have been
able to get worldwide, is from a guy who gets 30 kPa of vacuum on his
ITB/T3 adapter setup. I'd like a few more readings though... stock ST
or BT 20V vacuum readings would be fine.

Cheers... jondee86

[matt dunn]

depends on what you means 30Kpa vacuum?

from what I remember on my car and just looked up my program on pclink,

my car idles at 30Kpa, but that would be absolute, so 70kpa less than atmosphere.

Depends on your cam profile and timing too.

[ChaosAD]

yea 30kpa absolute is about right. does the gauge read about 100kpa with the engine not running?

Is the engine idling sweet?

[jondee86]

Yes, 100 kPa with ignition ON, but engine not running. One possibility is
that the ISCV is bleeding in enough air to lower the manifold vacuum.
Another is that the throttle butterflys may not be shutting off tight. I can
check this by taping something over the end of the stacks

Third possibility is that the one active runner is also drawing air from the
three inactive runners, and lowering the vacuum in the collector. I found
one guy who overcame the problem of low vacuum by inserting small
check valves in three of the four tubes leading to the collector from the
intake runners. Said he saw an improvement from 5" to 18" of vacuum
(16 kPa to 60 kPa) at idle.

I had a rock steady idle at 850 rpm on open loop TPS. When I switched
to closed loop I had to up the idle speed, as it became unstable under
1000 rpm. I've got a lot to learn

Cheers... jondee86

[Loudtoy]

Just to throw another reading out there and seeing as all the fixes have been covered my silvertop runs about 35kpa under 1krpm idle and anywhere between 35-50 when at a cold idle which is hard to put an exact rpm figure on it but is between 1100 and 1300rpm. Might wind the warm idle up a fraction to just over 1k as it seems to jump around alot under that. Not really tuned for good idle tho seeing as it doesn't really spend much time idling and most of it's warm running time is spent at 80+% throttle

[jondee86]

Checked the MAP sensor tube (skinny one) and then the brake booster
tube (large one) with a decent mechanical gauge. Both reading 30 kPa
at idle, so I'm thinking the problem is elsewhere.

Going to try retarding the exhaust cam a couple of deg, and see if that
makes any difference.

EDIT: Seems I might have a fundamental cam timing problem
Had a little play with the cam timing and now I have about 45 kPa of
vacuum (55 kPa absolute). Bound to be more in it as I only went 1 deg
on each cam. Just goes to show what happens when you think you know
where your cams are set.

This weekends job will be to degree the cams !! Feckin amateurs

Cheers... jondee86