I have an 86 Mercedes 300SDL and I wanted to install a boost gauge. I have never owned a turbo vehicle, or anything with a boost gauge installed. I read that where the vacuum lines go into the ALDA since I removed it is a good spot to put it right there the intake. I guess I don't really understand how vac and boost works.

I would think the best place to put a boost gauge would be the closest to the turbo as possible to get the best reading of what you are actually putting out. Then again I don't really understand the whole technical part of how boost works. I know exhaust turns a turbine which in turn produced air pressure pushed into the intake.

I know that an intake is called an intake because it creates a vacuum which is sucking not pushing pressure. So how exactly does a boost gauge work and does it really matter where the line is attached?

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86 Mercedes 300SDL - 400k+ miles

Diesels are different, there is no vacuum generated by the engine like a gasser - that is why we have a vacuum pump.

Just buy a gauge that will have your max reading in the center of the scale ( less stress on gauge) i.e. if you run 12 psi then a 25-30 psi gauge would be good. Hook it up to the intake manifold any where you can tee into the ALDA line and your done.

Thanks for clearing that up I had no idea a diesel didn't create vacuum. I had a buddy of mine grab a spare gauge that was laying around not getting used at work. It's a 30 psi stainless 3" gauge I just wanted to hook it up to see what my boost is. I hooked it up to the line directly in the intake above the ALDA and when revving it in park the gauge goes backwards a bit. Not much but it went backwards instead of forward. I know you hardly get boost unless the engine is under a load but I didn't understand why it went back about 1 lb under 0 instead of pressuring up 1 lb or whatever.

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86 Mercedes 300SDL - 400k+ miles

My guess is you got a vacuum line by mistake instead of a boost line. The turbo should be positive pressure, even if it's just minimal, even at idle.

Goes up to 12-14 psi under load. Either way, no vacuum is likely on a boost line.

Also, it doesn't really matter where in the stream you measure it as long as it hasn't been reduced by any devices. (The engine's consumption of air counts as such a device so obviously you have to read it before it goes through the engine). But everywhere else, reading "close to the turbo" or elsewhere doesn't matter as long as it's pre-ignition. It's like a garden hose. It doesn't matter if you have 10 feet or 10 miles of hose, if you put one gallon per minute into one end, you will get one gallon per minute out the other. It's that way with boost pressure in the system. Output pressure at the turbo is the same as the pressure you read off the intake manifold.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

if you dont understand boost and vacuum, Id put that ALDA back on and start to learn about how everything works.

Blindly doing things looking for "performance" yields destroyed equipment.

A boost gauge is a good idea, Id put it on the input end to the overboost sensor, so yo see what is going on right off the manifold. You can then tune the wastegate or boost controller to top out at 11psi, which is all you want for longevity.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

The name intake doesn't imply anything other than it's the in side of the airflow in the engine.

Yes, the engine does create vacuum, and does "suck" the air in. All is relative, the pressure in the cylinders is less than in the intake or the air wouldn't flow in. The turbo simply increases the pressure in that section above the outside air pressure. In an NA diesel, it definately creates vacuum although it is minor compared to a gasoline (throttled) engine's intake vacuum.

The alda connection is a good one however, if the line becomes open somehow to the ALDA, you will lose your boost signal to the gauge (or it will become lower/inaccurate) which can hinder troubleshooting.

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Gone to the dark side

- Jeff

You will see the pressure drop a little below atmospheric when revving with no load. Take it out and drive it, then you will see the boost come up. These turbos just won't spool up like a gasser turbo or a tdi turbo for example.

I can brake torque the passat and raise boost a few pounds but (carefully) brake torquing the 300SD will not produce any boost. A testament to the long engine life engineering that was put into these cars.

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'05 E320 CDI
'20 GLE 450