1987 300D Turbo boost questions - PeachParts Mercedes-Benz Forum

Jeremy5848 · #1 06-08-2010, 04:43 PM

My 1987 300D Turbo (W124, OM603) had its turbocharger replaced 2-1/2 years ago as part of the trap catalyst recall "campaign" (the old turbo had a hole in it so I got a new one for free). The new turbo now has 20,000 miles on it [always with Chevron Delo-400 15W-40 oil in the crankcase] and two questions have been rattling around in my brain case:

1. Do turbochargers have a "break in" period? I would swear from watching my boost gauge that the turbo is spinning up more quickly and at lower engine RPM than when it was brand new. The boost pressure, as regulated by the waste gate, never goes over 13 psi, but it seems to get there more quickly. I also feel that the engine is a little peppier but I admit I have no numbers. [Factory paper air filter, stock exhaust, no EGR for all of those miles.]

2. Turbocharged engines are said to be more efficient than NA engines, because the turbo stuffs more air into the cylinders and the fuel burns more completely. At 45 MPH, for example, I can run in either 3rd or 4th gear, and get the following numbers:

3rd gear - 2500 RPM - 4 psi boost
4th gear - 1800 RPM - 2 psi boost

At higher road speeds, the difference should be more pronounced, I just haven't had time to take data on the freeway. [I might guess that in 4th gear, 5 psi boost will get me about 65 MPH while in 3rd gear at that speed the boost would be up to 10 psi at least.]

Since the road speed is the same, the air resistance is the same. There will be more frictional losses in the engine at greater RPMs but the higher boost should result in more efficient fuel burning. In theory, this could result in better MPG numbers but it just doesn't feel right. What says the forum?

Jeremy

Biffidum · #2 06-08-2010, 05:17 PM

To my knowledge all turbos do is get more horsepower from less displacement. Cram more air in and MORE FUEL in. Net result is actually more heat in a smaller space which typically leads to more wear and tear.

I too have an '87 Turbo so I do not despair, but the head gasket issue does play into the extra heat you might experience as opposed to the 606 engine (NA) which I am sure will prove to be a longer living engine. It already posts better fuel economy and comparable power, it just hasn't been around long enough to compare longevity.

Remember to idle your engine after highway use to cool that turbo (head gasket in the back of my mind).

Crazy_Nate · #3 06-08-2010, 07:05 PM

Here are my thoughts on turbos.

They extra excess energy in the exhaust. This is done in terms of reducing the enthalpy in the exhaust (function of two of these three quantities: temperature, pressure, specific volume).

Now, as I understand it if you were to compare two engines of identical performance (power), but one was a smaller displacement with a turbocharger. It might be reasonable to assume the smaller, turbo'd engine is able to have slightly less frictional losses on top of better thermal performance. Certain things like upping the compression ratio and temperature drop can contribute to increased thermodynamic performance, but...that's probably another discussion.

So, what you really want to know is...which is more efficient?

I'd bet on your top gear. You'd be able to use your throttle position to roughly guestimate the amount of fuel delivery...if it weren't for a certain device that modifies fuel delivery based on boost pressure

So, while the turbocharger may be doing more at higher RPMs...how do you maintain that level of exhaust-driven performance? More fuel would be my guess.

babymog · #4 06-08-2010, 09:21 PM

Higher gear is more efficient.

vstech · #5 06-08-2010, 10:18 PM

smaller displacement for HP levels assist in making better economy. the power level is not commensurate to the weight. so... not only do the pistons crank higher power numbers, with less friction/inertia losses the higher gear ratio's enable lower RPM numbers due to weight reduction and power inprovements.
the 606 has good power for it's displacement due to better breathing because of the extra valves, and it gets better economy than the older motor because of it. I bet the turbo'd 606 motors get even higher efficiencies.
the diesel motor will have great longevity because of redundancy designs, and the small turbo boost allowed isn't enough to affect that.