I'd go with the cheapest 0-30psi pressure gage you can find. If you know of anyone who works with pneumatic controls they would have plenty. All you need is a 1/4" barbed fitting and some 1/4" poly tubing. Standard stuff in the world of pneumatic controls. I'd say 0-15 psi is ok also. You probably won't see any boost pressures on your benz over 10psi or so if its original. The combination gage of Inches of Mercury and Pressure in psi is perfectly fine also. We're not building a watch, just checking turbo boost pressures here.

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Orland Park, IL
1985 300SD 215,000 miles