Carrera 3.2 vacuum question

[Uncle914]

How much vacuum should I get off the venturi tube that comes off the intake? I am having an issue with the car now stopping as it should (turbo brakes on a Turbo Look) and I am methodically checking booster system from rear to front. I have read in other threads that the hoses can develop leaks between the engine and brake booster so i wanted to start at the source. I am getting 11 Hg on a cold motor at the Venturi. I have read in those other threads that I should be gettin about 20 Hg at booster.
Thanks,
Bob

[KTL]

Check the hose that the venturi is connected to, which goes down the firewall and connects to the center tunnel. This hose can develop cracks with age. That said I recently checked mine on my '86 Carrera and I was lucky to find it's in excellent soft condition.

vacuum leak you probably didn't ever think of

[steely]

I've never seen vac ratings spec'ed, although it would be interesting. I assume you're talking inches Hg.

Frankly, I went through a couple of years ago and replaced most of my vac lines (in conjunction with a tune up dizzy cap, rotor, plugs and plug wires) and intake gaskets & insulators, and the improvement was noticeable (by virtue of the idle and overall response).

I did not touch the brake booster line from the venturi manifold (whatever it is called) to the rear of the tunnel b/c I wasn't seeing any problems (I did the unscientific 'pop' test in the frunk - run engine to pull a vacuum, shut it down, and listen when you pull the booster line in frunk). Although it is overdue.
I did find and fix other small leaks at the pipeline behind the TB, and the PCV valve adapter by the oil fill.
With vac leaks, it's sometimes death by a thousand cuts, so I recommend just replacing them as opposed to searching for 'the' one.

[Uncle914]

Kevin L. and Dan, I did read that thread about the vacuum leak which got me thinking about this whole vacuum thing. The car doesn't stop great and I am trying to just get a baseline vacuum at motor where there should be the highest inch Hg. I've been looking at the intake schematics for any O=rings etc where Venturi goes into intake manifold. You can't see back there w/o a mirror, even with heater blower removed. Tomorrow I will try to remove and replace all 12mm vac lines as Dan advises. Then I work my way up to booster. Do either of you know if vacuum rises as engine heats up?

[steely]

Bob, sorry but no, I don't.
I do not have a clear understanding of the entire system, so I just took a brute force approach (unfortunately for you). I am trying to learn it myself.
I did read in one thread once that the vacuum is highest at the intake, and I think that would be downstream of the throttle body.

I did save the link to this thread showing some issues with the venturi (namely its O-ring could leak). Vacuum leak - check your brake booster line

Personally, I think the vacuum might increase with RPM, but I don't know what effects this has on vacuum 'consumers', or if it all gets balanced out by a corresponding increase (if any) of positive pressure being vented from the crankcase breather to the oil tank (which has a line to the TB pulling a vacuum on it as well).

There should be a vacuum line schematic sticker on the underside of your engine deck lid fyi.

edit - one more thing maybe you knew about, the rubber adapter (crimped with a ferrule on the coppery pipeline) cracked on mine, so I wound up replacing it with some new vac line pieces.

[spuggy]

Originally Posted by Uncle914

Kevin L. and Dan, I did read that thread about the vacuum leak which got me thinking about this whole vacuum thing. The car doesn't stop great and I am trying to just get a baseline vacuum at motor where there should be the highest inch Hg. I've been looking at the intake schematics for any O=rings etc where Venturi goes into intake manifold. You can't see back there w/o a mirror, even with heater blower removed. Tomorrow I will try to remove and replace all 12mm vac lines as Dan advises. Then I work my way up to booster. Do either of you know if vacuum rises as engine heats up?

My 930 reads 18 in/Hg at idle (1050) at the boost gauge, or about 47 kPa on thr MAP.

Vacuum doesn't usually change noticeably as the motor warms up.

More compression might make slightly more, more cam overlap can make (much) less.

11 in/Hg does sound low for a stock motor in good condition... My 3.2 throttle body has 3 ports below the throttle plate for vacuum; in your place, I'd probably start capping stuff off/checking vacuum lines.

[Uncle914]

Thanks all. Steely that last link is very helpful showing the connections. I will dig into it this morning and start swapping hoses. The O-ring in the Venturi tube may be suspect as my Venturi pulls out easily.

[Flat Six]

Originally Posted by Uncle914

my Venturi pulls out easily.

It shouldn't . . .

[Missed Approach]

Hi Uncle914- If you go to 6:28 on Heidi and Franny's video, they show their splice point and their vac results. I used the same gauge and also was in the same range of 12+ inches... https://www.youtube.com/watch?v=MRZG-s_55CU&t=594s

[kyngfish]

Originally Posted by steely

Personally, I think the vacuum might increase with RPM

It was my understanding that vacuum was highest at idle, when the throttle is closed. Not correct?

[Uncle914]

Missed approach, that's a great video. Have never seen her before. I replaced all of the 12mm vac lines today but I will check the others and also clean out that valve. I am waiting for my fan and shroud to come back from the blaster so I cannot yes vacuum yet but I will post results.

[Uncle914]

Kyngfish, I don't know the answer to that either. I tested mine at idle, cold, at the end of the Venturi tube with a Mityvac. I was getting not quite 11 inches/Hg. I did not increase throttle.

[spuggy]

Originally Posted by kyngfish

It was my understanding that vacuum was highest at idle, when the throttle is closed. Not correct?

If you lower the idle, motor will make more vacuum. Mine has SC cams and seems happier with a higher idle; but dropping down to 850-900 RPM makes the MAP read 37 kPa or thereabouts.

Vacuum is highest when driving; under the throttle plate, when you transition from WOT to fully-closed, it'll drop to ~20 kPa.

As you increase throttle on an N/A car, vacuum will decrease, getting close to, but never quite reaching 100 kpa at WOT, which is 0 in/hg of vacuum (atmospheric).

[spuggy]

Originally Posted by Missed Approach

Hi Uncle914- If you go to 6:28 on Heidi and Franny's video, they show their splice point and their vac results. I used the same gauge and also was in the same range of 12+ inches... https://www.youtube.com/watch?v=MRZG-s_55CU&t=594s

Ahhhhh. I watched that video, and the penny dropped.

I know where you guys are getting this "12 in/Hg" number from now

The inner scale read around 12. The outer scale read around 18-20 - which is what I read at idle (almost all cars idle around the same, unless they've got some wild cams with huge overlap)...

kPA is always quoted as an absolute, and so there's never any possibility of confusion; 0 kPa is a perfect vacuum, 100 kPa is atmospheric, 200 kPa is 1 bar of boost etc.

in/Hg mercury @ 0C/32F = ~ 3.386 kPa.

Vacuum gauges fitted in vehicles conventionally show manifold pressure relative to (always below, for an N/A car) atmospheric. And if you're citing a "vacuum" reading, it's usual to define/quote it relative to atmospheric.

But if you use "in/Hg" as a unit of pressure, but don't define the vector (starting point, direction), then the numbers come out different.

If you read vacuum:

18 in/Hg of vacuum (below atmospheric, or 100 kPa) at idle is 100-(18*3.386) = 40 kPa.

If you read pressure:

12 in/Hg of pressure (above absolute vacuum) is 12*3.386 = 40 kPa.

[Uncle914]

So according to my Mityvac, my car was reading 36kpa at the end of the Venturi tube at idle, cold, before I changed the hoses and o-ring. which if I can figure out what you are saying is pressure.

[spuggy]

Originally Posted by Uncle914

So according to my Mityvac, my car was reading 36kpa at the end of the Venturi tube at idle, cold, before I changed the hoses and o-ring. which if I can figure out what you are saying is pressure.

Looks entirely correct to me; we're both using the same cams (little difference between SC/3.2 cams) and your car came with higher static compression than my 8:1 JEs - so you make an inch or two more vacuum.

Doesn't look to me as though manifold vacuum changes with the motor hot - it might if there were some part that needed heat expansion to fill a gap. Idle speed makes more difference; for each 100 RPM you drop the idle it will pull measurably more vacuum.

[Uncle914]

Ok, so if the vacuum is good at the engine I can begin to move forward to brake booster. Check vacuum there and try to figure out why car don't stop well!

[spuggy]

Originally Posted by Uncle914

Ok, so if the vacuum is good at the engine I can begin to move forward to brake booster. Check vacuum there and try to figure out why car don't stop well!

Pump the brake pedal up until it is hard. Keep pressure on the pedal. Start the car. If the pedal noticeably drops an inch or so it's not the booster or the vacuum system.

Hmm - is this car "new" to you? Why do you think it's "not stopping as it should"? You would'nt check the booster for a spongy pedal, so maybe you think the pedal is harder than it should be, or you have to push it harder than you expect?

You do have to be prepared to push the pedal pretty hard - much more so than any modern car. 930 brakes are basically 917 race car brakes - designed towards the top end of the "acceptable effort" range for fit, motivated young men (e.g. pro race car drivers). Because a high, hard pedal is great for modulation, feel and heel'n'toeing..

i was astonished at the pedal when I upgraded my brakes. Moved about an inch and then felt like there was a block of wood under it in comparison to 3.2 brakes...

[Uncle914]

Spuggy, this car is a friend's car, but we both have had SC's and they stopped 100% better than this car does with the turbo brakes. I would think with the turbo brakes it should pretty much make your head almost snap forward. I have always worked on this car and it definitely stopped better when he first got it which was several years ago. Now you really have to think about stopping ahead of time, even incorporating downshifts while not even going fast. I have changed fluid and bled brakes. Checked for foreign objects under the pedal board, everything looks good. It's mystifying. I will do the brake booster test this weekend when I get the fan and shroud back on. I am hoping it was just the vacuum. If booster tests ok then I guess I will check master cylinder and if I have to rebuild the calipers.