My particular question is, of course, Diesel related but maybe this applies to all turbos. I'm fixin' to order my boost gage and can order either an absolute or gage pressure sensor. On the dyno we used gage pressure but we were at a fixed elevation and the engineers could do the math either way (I don't know how they ciphered the data once they had it).

I don't know how Edge handles this in my tuner on the Dodge as it came pre-set-up. I also don't know how mechanical boost gages are built.

EDIT: I'm guessing most of you know this but I'll post a reminder - Gage (or "gauge") pressure is referenced to the surrounding ambient pressure and Absolute is referenced to a standard pressure, generally atmospheric at sea level. Here in Wilmington there's no practical difference as our house is about 18' above sea level but at the Ohio mile we're about (IIRC) 900 feet above sea level.

So -
- Do I want absolute or gage pressure - and why?

Thanks

Dan

Let me preface this with "That's a very interesting question, but I don't really know" however, I'll hazard a guess and then be prepared to be educated.

I believe that what you want is Absolute Pressure. I have a hunch that the gauge manufacturers often reference gauge pressure, because with the engine off it will always register 0 rather than something else that might make it appear that the gauge is broken... particularly in Denver or Albuquerque. Let me also suggest that the engine really doesn't care (or have any way of knowing) what the ambient pressure is. Which is likely why all the manufacturers that measure pressure for ECU systems use absolute pressure (think Manifold Absolute Pressure (MAP) sensors). The power that the engine is making is going to be directly proportional to the absolute intake pressure as that determines the amount of fuel that can be injected.

So, I think that what you really want to know is what is the actual pressure that the turbo is producing in the intake that the engine is able to use and not the actual pressure plus a fudge factor for the altitude the car happens to be at.

I'm prepared to be schooled...

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

Thanks for the input. It'll be interesting to hear what others have to say. I'm decidedly indecisive on this.......

Dan

Most people run a gauge that reads gauge pressure. This is fine if you are aware of the ambient pressure for your elevation and tend to stay around the same elevation. A gauge with absolute pressure would be better. It just isn't popular for some reason.

__________________
Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

I would go with Gauge.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I'm thinking (not sayin' I'm right) that what you REALLY want is differential pressure across the turbo - that is, the delta from inlet to outlet. The inlet pressure would be basically atmospheric so basically we're talking gage. I may need MORE INPUT!!!!!

Dan

I'm confused by why the question would even come up. Maybe I'm simple minded but I don't see why you would want anything other than gauge pressure.

On my truck, all I use the gauge for is limits like "no higher than 25 PSI on the stock turbo" and "no more than 12 PSI below 1800 RPM or it will chatter".

To me it's just a number and all relative, unlike a gas engine where you have to calculate injection quantities based on boost pressure to keep the A/F ratio within certain limits.

That said, my '98 measures manifold pressure in absolute terms. If I hook my scangage up, it always reads about 14 PSI when the engine is idling and up to 30 PSI when under a load. In my head I'm always kind of subtracting about 15 PSI from the reading.

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

Remember that this is a race-only application. I have a set of 315 injectors so there will be quite a bit more fuel going in and I don't know what that'll do to turbo output. Also, I'm running a T04 compressor so all bets are off from a stock configuration. But you may be right - it may not matter much in the real world. Just that I have to order something so I might as well make the best choice.

BTW - The guy who built the turbo expects about twice the volume but only about 1 ATM pressure (obviously gage pressure). Again, I'm not sure how that will play in the real world.

Dan

... only 1bar? odd for a diesel... as it needs PRESSURE to increase HP... most race turbo applications for diesel is around 25psi... gauge... some sled trucks generate 50ish...

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I have a buddy who is an engineer at Isspro and I can ask him if you like. I still don't get why a standard gauge won't do it.

After all the engineering is done - RPM, turbo volume, boost pressure, etc... the gauge is just a indicator.

When I design an induction system, I work from the air side - RPM, boost, turbo map, desired HP and then adjust limits of the fuel to keep EGT's under control.

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

Shoot, my truck hits 25 PSI with no problem and it's far from "race"...

Triple stage compounds can run 300 PSI!

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

I think you guys are missing the VOLUME part! What we need is cylinder filling and the pressure is not as important as volume (think standard cubic feet per minute). You can force it in (at a resulting higher temp) with lots of pressure or you can supply a LOT of air at a lower pressure and therefore lower temp. You do need some of each, of course, to convince the volume of air to enter the cylinders.

It's easy to make pressure but it takes a bigger compressor to make volume. Most aftermarket turbo companies push pressure as it's easier (just spin the compressor faster) and shows a bigger number on the gage (whether PSIA or PSIG) so guys get instant bragging rights. I'm sure my T04 can make more pressure if there's a reason to in the future.

I'm getting about 50/50 opinions on the type of pressure sensor between here and the other forum. I'm leaning toward just ordering up a standard boost gage on the Interweb and calling it good, even though I don't know if it's PSIA or PSIG. At least I'll be speaking a common language with other turbo folks. Once I have it I'll know if it's referenced to atmospheric by just reading it - once I drive away from the coast.

Dan

A bit surprised you are racing a M-B diesel. Most of us are happy if we can merge smoothly on the freeway.

Regardless, I'll answer as a Mech Engr with only minor work experience with IC engines. The cylinders only care about the absolute pressure and temperature, so for you app, an absolute gage is best. For most people, gage pressure makes more sense since that shows what the turbo is adding. For power, the main limiter is the mass of air per cylinder charge. Mass is linearly dependent on air density and cylinder displacement. Density is a function of pressure and temperature.

Simplistically (ideal gas) the relation is PV=RT, but better to reference a thermodynamic table, especially at >400 psi pressure. However, why bother since the cylinder pressure is much different than the intake manifold pressure. It is very complicated, with pressure drops thru the ports and intake valves, left-over exhaust gas, cam timing, pulsing, etc. The temperature also varies from the manifold due to pressure drops, heat conduction to the walls, etc. That is why even very smart engineers tune on a dyno. Trying to calculate from "first principles" is extremely complicated and requires a Phd spending a career in Computational Fluid Dynamics.

The reason for racing the M-B Diesel is that:
1) It meets the displacement for the class I want to run (F/D(iesel) T(ruck) in Land Speed Racing)
2) It was cheap
3) I could hear it run - I actually bought a 300 SD and pulled the engine
4) It was cheap.

I was an engineer's worst nightmare - I was a crusty old technician. Part of my job was telling young engineers "no, we're NOT going to do that and here's why". But I also know that engineers can be very useful when you need to know - for example - what kind of pressure gage you need.

I still can't find what measurement protocol they use with the Ebay style boost gages. The sales write-ups make no mention that I can find. I'm guessing that most buyers don't know that there are options.

Thank y'all for your input!

Dan

Anytime you question as to why, you need to only watch this... in fact, watch it anyways... I bet I have at least 100 times... Turn up your volume....

W124 OM606 Superturbo - YouTube