A number of people have asked to ride in my W116 (1978 300 SD) lately and everyone comments on the smooth, seamless power of my 617.950 engine.

At the request of one local 617 enthusiast, we hooked up a boost pressure gauge so he could observe the boost pressure curve over the rpm range, as opposed to just the maximum boost pressure output.

I found the results interesting and decided to post them here in the forum.

Keep in mind I have modified this engine with increased fuel delivery pressure, '05' cmashaft, advanced camshaft/injektion timing, fully polished turbo/intake tract, 6 angle valve and seat faces, port matched cylinder head and a modified T3 Airesearch turbocharger.

The turbine housing was machined to accept the larger Buick Grand National V6 Turbine which allows faster spool-up at lower exhaust gas velocities. The housing was fully polished and 'Jet-Hot' Thermal barrier Coated.

The intake turbine is also special. It has twin vanes (blades) instead of one and they have a unique pitch. This was provided by Advanced Diesel Systems in Fresno, CA.

We used a Snap-On Vacuum/Pressure Gauge for this test. We attached a fitting into the plug at the front of the intake manifold. This allowed us to retain the Overboost Protection Circuit. The air filter was left in place.

At an idle speed of 875 rpm, the needle fluctuates slightly, which gives you an idea of how sensitive this instrument is.

Snapping the throttle wide open (and quickly releasing it) indicated 7-8 psi on the gauge. Rpm peaked at about 3200 during this portion of the test.

As we took the car out for a drive, we noticed the boost gauge imitated the movement of the tachometer. We were registering about 3-4 psi by 1200 rpm and it continued to climb in relation to engine rpm very rapidly.

At a 65 mph cruise, we were developing 6.5 psi at an indicated engine rpm of 3000. Opening the throttle fully, instantly swings the needle past 10 psi of boost.

We then conducted the WOT test, holding the engine against the brakes at 4000 rpm in the second speed range. We recorded 14.5 psi of boost pressure.

I blocked the wastegate hose to test the overboost circuit. I connected a test light to verify the circuit was operating. The boost pressure would 'flash' to 15.5-16.0 psi followed by a noticeable decrease in acceleration and a subsequent reduction in boost pressure to about 10-12 psi.

The interesting part of the test was how quickly I could produce 10 psi of boost pressure. In the second speed range, at a vehicle speed of about 20 mph, I can sweep the throttle open (without downshifting) and swing the needle instantly to 10 psi.

At 70 mph, I can sweep open the throttle quickly and swing the needle up to 14 psi as quickly as the throttle is opened.

Most people who ride in the car comment on the fact it doesn't feel like a turbo diesel because there is no acceleration surge. The power is immediate and constant. I am sure adjustement to the Aneroid Cell (ALDA) on the injektion pump is associated with this sensation.

It is night-and-day compared to my Dad's stock 1982 300 SD. We are planning a similar modification to his car in the near future.

Fuel economy is fantastic and visible exhaust smoke is present only during high boost operation. The car doesn't 'chalk' the back bumper with soot either.

One of the reasons I think my fuel economy is so good (over 30 mpg so far) is due to the increased boost pressure. I have to open my throttle very little to get big performance from the engine.

I wanted to post this in hopes you might find it an interesting read. Best of luck on your projects...Bert