mcypert

Hi All:

A recent thread, which led to a discussion about Turbo Boost got me thinking, again, about adding a Boost Gage. So, I order an analog and digital gage with a remote transducer (a mechanical gage would be best, but, on further examination, I decided the under-the-hood transducer would work better, because, otherwise, you’d have to run a “vacuum" line inside the cabin, verses just a few thin wires. Where to mount the gage, will be the next challenge, but I’ll worry about that later. It’s a 2” round gage...)

Anyway, the gage is here now. So, my question is: Where can I tie into the Boost Pressure?

I popped off the V-12 covers and here's what I see:


The green arrows point to some rather conspicuous “+” signs…. I always thought these had something to do with "positive” battery voltage, but, as is obvious, these aren't positive posts... There is a “vacuum" line connection to both.

Does “+” mean "Positive Pressure"? Can it be that obvious?

I have no idea what the two “green arrowed contraptions” do… but, the + sign must be there for a reason????

Here’s the schematic for the turbo:


Looks like, the “vacuum" hose I want, is the one going to 14.41 (electro pneumatic transducer, frequency—whatever the hell that is; much less where it is???).

Best guess is, the "red arrow" hose in my pic, goes to 33.06 "switching valve, air"; which has some check valves. So, that might not indicate true boost pressure. Or, maybe, it will only indicate boost, with no vacuum ever shown on the gage. That’s not necessarily what I want to know, as in, the decrease in vacuum is important information too…

Any guidance highly appreciated….

Otherwise, I’ll just give it a go, and see what happens…

And, the fallback is go straight into the intake here:



I’d rather not…

Finally, the boost gage has an “ignition on” red wire. I'm sure I'll find one, but any suggestions on where one (+ ignition on) might easily be found would also be appreciated; either under the hood or under the dash?

Regards… Mark

BOTUS

12v feed end of the dash board...

I'd go careful not to trash hoses or pipes just for a junk toy you'll never use

mcypert

Botus:

I know I don’t need it, but I'd like to see the boost; a feature lacking in the V-12 TT. After all, Rolls Royce puts a 0-100 gauge in its cars for the “Power Reserve %”. Not the same thing, but cool…

In evidence of the MB's longevity, I’ve never looked for the car’s fuse boxes. Never had a reason to... I consulted the Owner’s Manual and found the one you mentioned at the end of the dash. The OM says,

“Do not use sharp objects such as a screw driver to open the fuse box cover in the dashboard, as this could damage it… [open with] your hands…”

“Your Hands,” hell, I couldn’t even get the forbidden screwdriver in there… I started with a knife…

Then I found there is No fuse diagram under the cover, or in the OM. Here’s a link for the diagram(s):

https://fuse-box.info/mercedes-benz/...uses-and-relay

BTW, left side dash fusebox is F 1/7. The left, under-hood is N 10/1 if you want to use Starfinder. I used N 10/1 (under-hood) fuse #43 (cigar lighter) for my “+ ignition on”.

Interestingly, the dash fusebox, F 1/7, doesn’t have ANY "ignition on only" fuses; they are all hot with "ignition off". The F 1/7 fusebox access is useful, however, for grounding and snaking/hiding wires to the dash…

In that regard, I couldn’t find a good "port-hole” to get wires from under the hood to the dash (recall, I opted for a boost transducer under the hood, rather than a mechanical gauge, so I had only two wires, excluding ground, to “fish", rather than a vacuum hose). I used inside the left fender as my "port-hole” for a 3mm diameter coaxial cable (heavier than the supplied 3-wire cable), and the F 1/7’s leather cover to get to the dash. Wire is well hidden, until the top of the dash.

Now, recall, I started the thread inquiring “where" to access manifold pressure. No useful response, so I dug around a little more...

The red arrow points to a hose that goes under the VT to a manifold port just under the Throttle Body.


It’s a plastic hose, and it goes, downstream, to a check valve and on to some “doohickey”. I don’t know what it's for, but, upstream, it appears to go straight to absolute manifold pressure.

So, I made my own port by drilling a hole in the upstream side of the check valve (closest to the engine) and fitting a nipple:


I was correct!

Here’s the boost gauge under construction:


When you switch on power, the gauge sweeps from –30 to +30, and then goes to zero… Cool... Note, the gauge measures vacuum in inches of mercury, and boost in psi-- roughly a 2:1 ratio; Hg to psi. Gauge is lineal, but scaled for the difference…

Engine on, I get -17 Hg at idle; which sounds about right.

Testing boost is more of a challenge… You only get boost under load…

I haven’t been off my Island yet, and in a 30 mph limit, it’s kind of hard to test… I tried a few runs in first and second gear (I had never used my paddle shifters on the steering wheel before… Cool).

It's kind of distracting to watch the gauge when your rear tires are smoking, you’re about to double the speed limit, trying to watch the road, and being propelled into the next county!!!

Also, the sweep of the dial is insane. Absolute manifold pressure goes from about +25 psi to -22 Hg in a blink of an eye. So, I’m not sure if the needle actually got to +25 psi (which seems unlikely), or it just "bounced” the needle up to 25 due to inertia. Or, maybe in the confusion, I mistook 25 psi for 20 psi; which sounds perfectly plausible… Did I mention this happens in a blink of an eye??? I'll experiment some more when I get out on the open road, but don't expect more pics under boost. Holding a camera while accelerating in the V-12 TT is never safe...

Anyway, my “absolute manifold pressure" pick up port appears to be spot on, in case anyone wants to add a boost gauge (see pic above). That’s a better choice than the contemplated "green arrow" in post #1... I’ll save that for NO2…



Also, there’s the “where to put this damn thing” issue. I used a generic 2" cup:


Hold your applause. I know that’s not satisfactory... But, trust me, you want to be able to look at the thing, and, almost anywhere else, you won’t.

I made a little clip that just slides in between the airbag pillar and the leather dash… No physical changes to anything, and it’s removable, so I can change my Texas registration sticker… barely noticeable to the untrained eye...

But, at least I can watch it-- which is what I was after... Also, you can choose from 7 dial colors, or off. The pic above is “white"; which most closely matches the original instrumentation—albeit, not exactly. “Orange” might be better for long night driving… Cones and rods in the eye and all that….

So, the fun continues…

Regards… Mark

BOTUS

the other end of the dash has another fuse panel - you add in these little extras to create additional power connections - odd about the trim panels mine come off really easily by hand - unlike a 211 where you need a crow bar and then all the metal clips do a cluster bomb impression underneath the dash

first bit above - was where where I was coming from... when will you get a chance to see it

is the gauge an odd one counting atmospheric at 14.7 PSI and then adding the 9-12 PSI of boost - I can't see it doing much more than 10 ? or the tyres would just pop at each set of lights ?

mcypert

Botus:

Yes, the fuse panel at the end of my dash was the crowbar variety... Although, it didn't break the clips... Probably, just hadn't been off since smacked on at the factory...

And, no, the gauge is vacuum to zero to psi, linearly. Theoretically, and since an NA engine can never go to zero, a "zero indicated" absolute manifold pressure also indicates Some boost... If I'm indeed getting an indicated 25 psi there probably is a calibration error in the gauge. I haven't exhaustively researched it, but I was expecting 18-21 psi at max boost. Hard to find data for my year and model, but I've seen claims of 25 psi "spikes". Bottom-line, it's not too useful to try to read the boost at WOT in 1st gear... I'll know more after a highway run...

I may also try to do a calibration test on psi... But, how??? I'm thinking maybe with an air compressor tank set at a known (and very low) pressure??? Bicycle tire???

Regards... Mark

BOTUS

18 to 21 is mega boost

with 14.7 PSI = 1 Bar

fast turbo'd cars in std trim used to add around 0.9 bar and silly ones on over boost might land 1.1 bar for a bit ....kids r us merchants trying to self destruct things come in the 1.2 to 1.4 flavour ? or did the world move on from last time I looked ?

mcypert

Botus:

I’m not saying we've moved on, but the V-12 TT may behave differently than what you are thinking about. I owned a Stealth V-6 TT. IIRC, boost was about 7-12 psi or 0.7 bar or so. Just thinking out loud, what if you doubled the single turbo feed cylinders from 3 to 6?

I don’t know, but the V-6 TT was capable of about 20 psi on those 3 cylinders at about 4000 rpm. The boost limit (7-12 psi) was the wastegate. And, a V-6 running at 4000 rpm is like a V-12 running at 2000 rpm.

I’m not saying the V-12 makes twice the boost of a V-6, but it would probably make the boost earlier… No?

Boost curves go linearly straight up, peek, and then taper off. As far as I know, they don’t plateau. MB bragged, at some point, about the V-12 TT having a 22 psi (or 1.5 bar) boost. I don’t think that was my year and model, but obviously, the V-12 can produce, and use, some boost pressures that can’t be seen in a V-6 or I-4…. That’s where I came up with 18-21 psi. As said, I haven't come up with the factory spec (with minimal looking) for my S600. Still, I didn’t think my perception of 25 psi indicated boost, may have been even higher, Could be correct; as in, the V-12 TT doesn't make 25 pounds of boost… (I’m still yet to do any real world testing until I get it out on the open road.)

Indicated boost pressure mystery solved (sort of):

As mentioned in post #5 ,curiosity got the better of me, and I tried to verify the veracity of the gauge. (BTW, my Stealth TT also Overstated the boost on the factory gauge; that allegedly measured to 14 psi, IIRC.) True to form, the newly installed V-12 boost gauge exaggerates the hell out of the boost. Let me explain…

A few lifetimes ago, I taught an engineering course called "Mechanical Measurements" at the University of Houston. I’d give me a "C" for the results I came up with, but maybe an “A” for using the stuff I had at hand to create a measuring device(s)… The methods are:

1. A tire pressure gauge, a bunch of plastic hoses, and a small air compressor (with the output pressure regulator set really low)… a real Reuben Goldberg contraption:



2. A water column. Yes, a tube full of water to measure inches of water rather than inches of mercury. You see, 7 feet of water is almost exactly 3 psi. Here's the “reservoir tank" that I lifted to 7’ after filling a plastic hose with water:



Method 1 yielded inconclusive results. In the pic, the tire gauge (known to be correct at 35 psi) showed about 8 psi with the boost gage indicating 15 psi… So, did that mean +7 psi, or a factor of two? Further tests tended toward an almost doubling of the “indicated” over real pressure...

Method 2, the water test, more or less, confirmed the "indicated" is almost doubled! I got 5 psi indicated with the 3 psi (7 feet) water column. Assuming this measurement is the most accurate (and very close to method 1 results), the conversion factor is a convenient: 3/5ths.

Indicated/Actual (psi) is:
0 = 0
5 = 3
10 = 6
15 = 9
20 = 12
25 = 15
30 = 18

Can’t wait to peg the needle!!!

So, does that mean the boost gauge is worthless? Well, yes, if you don't know what it actually means… But, no, if you know to multiply by 0.6! Kind of like converting km to mph…. We can all do that, right?

Still, the analog sweep is good for a glance. Just don’t pay attention to the digits… Or, think of it as system of measurement no one has ever heard of (or use: km to mph; a fictional European psi; 1/25th Bar; 4 kpa; or something…)… As long as you know, who cares… All I really want to know is "Zero", where boost starts, and where it peaks. “Zero” seems to be right on the money...



It’s also useful for detecting vacuum or boost leaks. At least that’s how I’ll justify it… It is removable in 10 seconds, if you don’t like it…

Of course, this begs the question: Why the hell did they make this thing sooo inaccurate and overstated??? As Botus said, Kids R Us... Show off to the boys with a Subaru that “has” 20 pounds of boost... (This is not new… I have another tire pressure gauge that shows 55 psi at 40 psi... There ought to be a law against selling wildly inaccurate measuring instruments... or, more engineers should have taken my class in college…)

Regards… Mark

BOTUS

getting boost isn't an issue in pretty much any modern turbo engine - combustion chamber design, materials, sensors and engine management electronics have all moved on a little bit - but its pump fuel that normally limits things to 1.4 bar max boost ?

my old world understanding - normal petrol pretty much goes bang adding any more boost - after which there is not much left to go round and around any longer...

mcypert

Botus, I totally agree with your old world thinking…

I can’t wrap my head around how the MB V-12 TT, with 9:1 compression, has 20+ psi boost.

For example, IIRC, my V-6 TT (1990's tech) had 8:1 compression, max 12 psi boost, and produced a little over 100 hp per liter. The MB has about the same hp per liter, but manages not to self-destruct with, presumably, a lot more effective cylinder pressures-- however it produces a boat-load of torque. I suspect the MB is using a bunch of sensors and computers, that I don't understand, to prevent the expected detonation of which you speak. Here is some more info that the new boost gage revealed to me:

I was able to get some more data when I did a highway run; as opposed to trying to read the gauge on the confines of my Island.

As expected, on the V-12, stabbing the throttle, at speed, pegs the boost gauge. Using my calculated conversion factor, that's 18 psi. The 18 psi max dial reading comes on, in the blink of an eye, at about 3500 rpm, and drops off immediately; I suspect because the MB is limiting the boost and preventing the aforementioned detonation… In other words, the 20 psi boost is momentary, at best…

Also, following boost readings is difficult, even at highway speed. Similar to my low speed tests limitations, hitting full boost at 3500 rpm on the highway, very quickly sends you into 3-digit speeds... Discretion is the better part of valor...

Here’s another observed phenomena with the boost/vacuum gauge: IMO, the turbos don't do any work in “normal” driving. The vacuum gauge stays pretty consistently at about –17 Hg indicated (probably -10 Hg actual, if I plug in the conversion factor) when maintaining a steady speed and during normal acceleration. I have three takeaways from that:

1. The V-12 is almost always “lumbering" along, with the V-12 torque, and not using much more fuel than idling…

2. Turbos don’t scavenge much power in normal driving, and probably don’t add anything to MPG... At least on a V-12, as opposed to say a turbo I-4, which actually uses boost and efficiency during normal driving to make power…

3. With 9:1 compression, you can probably use any grade of petro. Just don’t punch it! But, what’s the fun it that! (See the beginning of this post for why this is a very bad idea. Or, who knows. Maybe the MB computer knows what grade of gas you have and dials back the boost and duration. My WAG, wild-***-guess, is: the computer is severely limiting duration regardless of octane, but may set even more limitations if the gas is not up to spec.)

BTW, I’m at sea-level, literally, with super-saturated humidity, so YMMV... In other words, my turbos are pressurizing some of the most dense air available on the planet...

Regards… Mark

BOTUS

I guess u get funny numbered octane.... (coz USA / Canada stand alone using a slightly less bad test for measuring octane rating), but ROW uses RON, and thus USA at the pump says 4 less than it is to everyone else on the planet (with USA using AKI numbering)

thinking more clearly now - if based below 1000 foot of altitude, you want to be running 99 super unleaded (USA 94) in the hope of not self destructing when it lands 1.2 Bar (aka 17PSI) of boost. Only the very latest stuff (like on >= year 2021 911 turbo) try boosting more, otherwise the knock sensors will just wind it back, and more boost will likely drop power below what you had on 15PSI with the spark in the right place

Thus over boost is probably available under ideal circumstances pushing toward 1.2 Bar till it gets too hot and knocks, so normal max boost on road cars is seldom more than 1 Bar - of course on most road cars - none of that happens if you have junk petrol. Not sure how they do it, but I guess its the knock sensors - I had a BMW bike that Ricardo was asked to make it run right (coz BM Manu couldn't manage in house - page 13 http://docplayer.net/36017804-Bmw-s-...ant-engine.htm ) Ricardo changed lots alongside moving its std fuel requirement to 98 Octane (93 for you). On this bike when you run 97 (USA 92) or less the engine does nothing extra at all vs std octane fuel - but the millisecond it has 99 it runs very differently - even at first start up - the exhaust tone changes instantaneously and the power output is transformed, you can feel it as you drive 3 yards from the pump - thus I guess the feedback loop of the knock sensors and the engine management's predisposition trying to do better is always on the case, and for the sake of good words, for all intents and purposes instantaneous in its optimisational abilities...

The trouble with turbos is the volatile nature of the boost, it means we have to de-tune std engines to cope when you have large amounts of boost (so the fuel doesn't self destruct the engine) - so at low boost or no boost, they are just a less efficient slower idea - its why fast response is so important - and I guess why today's small capacity cars with turbo's run in a low boost configuration a lot of the time, with say 0.5 Bar (7PSI) boost even at low revs. These, then can't be tuned to silly money without the turbo being a very expensive type, and or working its socks off (and often going bang - See ford Eco-Boom engines, where the 180bhp 1.5 litre engines go pop all day long) - Thus its quite strange on a large capacity engine mostly expected to be driving in a sedate manner has fitted turbos... 9:1 is very high for such an old design, but then it should have high boost compromises as the fuel won't cope.

the changes now is new designs of pistons (3D printed) and combustion chambers with 140 bar (2000PSI) direct injection and this is allowing Porsche by 2021 to run 9.8:1 and 1.3 Bar with over boost to 1.5 maximum boost (18.1 to 22.5 psi) - but compared to modern NA stuff today which is hitting 14.5:1, low boost turbo operation makes for a very flat, inefficient engine

CR and other porsche stats
https://newsroom.porsche.com/dam/jcr...g_2013.pdf.PDF

3D pistons
https://newsroom.porsche.com/en/2020...-rs-21462.html

RON, short for Research Octane Number, is one way to measure octane–and it’s pretty much the world standard. American and Canadian fuels, however, are measured using the Anti-Knock Index, an average of RON and another octane rating called the Motor Octane Number. Generally, the RON figure is 4 points higher than the AKI number. https://classicmotorsports.com/articles/rom-vs-aki-fuel-facts/





,

mcypert

Good discussion, Botus…

I mentioned in another thread that I always use the highest grade petro available in my area. That’s 93, R+M/2, whatever that is… It also contains "up to” 10% ethanol.

With whatever negative characteristics ETOH may have, it does help with pre-detonation...

To recap, my newly installed boost gage pegs out at 18 psi, based on my calibration described in post # 7. This isn’t a slow rise, but goes right to "pegged" at 3500 rpm; meaning, the actually boost is probably in the 1.5 bar range. The boost needle does as expected: straight up, peeks and slowly tapers off; what is expected of a normal turbocharger. From pegged (18 psi or more), it starts to drop to 17, 16, 15 as rpm goes up. As mentioned, after that, 15 psi, observation on a public highway is problematic due to the rocket-sled speed being achieved… So, I’m not sure if boost stabilizes at 15 psi or continues to drop... I’ll check it out some day, ”in the name of science”…

Also, I installed the pick-up for the boost right under the throttle body. So, presumably, these are fairly accurate absolute manifold pressure measurements.

Who knows what evil lurks in the hearts and minds of the MB fuel management system??? But, my data indicates the 1.5 bar boost is real, with pump gas, in the V-12 TT... As said, I can't wrap my head around it either... Especially, with 9:1 CR to start with…

Regards... Mark

BOTUS

Ethanol is great at controlling knock and its dirt cheap - toluene is even better but costs lots more :-)

Sounds like it over-boosts, then trails back as the insides get hot and bothered (like all the high boost cars do these days). The idea is, as you stand on the throttle its cool enough inside, with the engine and fuel as is, it copes with large boost. This gets you passed the slow traffic or a big gain in speed - then as the insides start to melt and the fuel will just start knocking and finish off the engine, it tapers how hard it tries - so it cools down and its gets to live another day...

If its running 18 PSI a lot of the time no-wonder the ignition has a hard life