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Discussion Starter #1
My father just purchased a 1985 Mercedes 300SD. It's the 3 liter straight 5 that's turbocharged. Well i'm his partner in crime when it comes to fiddling with these cars. Since the purchase, we've had three goals, to improve the car's responsiveness, give it more torque, and lower it's running temps in this hot weather. So far the intake we installed (looks like a ricers cold air intake with all the hoses for conversion and a cone filter on the end, purchased from some performance diesel guys) really improved it's response time. Then we converted the coolant to phosphorus free antifreeze (dex cool), with a touch of water wetter additive, and then blocked off the egr. It all translated into a motor than used to run during 100 degrees days from 105 C to 90 C. Now we're looking for ways to reduce it's turbo lag and increase the torque a bit. We are very familiar with tweaking turbo gas engines but not much with diesels.
We have two ideas, but i thought i'd run it by some experts because i have no idea how this car's injection system works. First was to remove the cats or replace with high flowing cats to reduce lag and achieve quicker spool. But i'm not familiar with this car and don't know if this will create a problem, or if the design of the motor and turbo rely on a certain amount of backpressure? The next idea would be to raise the boost to acheive more torque. But there's no intercooler and i don't know how safe that would be and if i were to add an intercooler, i don't know if the system measures for air temp before entering engine and will compensate the dense air with more diesel fuel?
 

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Congrats on your purchase, and welcome to the board! It sounds like you have a good idea of how engines work, so let me give a quick orientation to your OM617 engine.

First off, no cats. Some models (primarily CA models) had a "trap oxidizer" that caused problems and was later recalled for removal. It is not likely that you have one. There is a possibility that a "pre-muffler" (looks kind of like a catalytic converter) can clog. I recently replaced mine due to a clog.

Second, there are no computer controls or intake temp sensors for this engine. Your TDi is very similar to most gas fuel rail injection engines and depends on its computer to help its running and does so very efficiently. The OM617 needs no computer, and therefore no electricity to run. The fuel metering is accomplished by an "ALDA" device that is basically a calibrated diaphragm that has manifold pressure sent to it and plunges a fuel metering rod on top of the injection pump. Many people benefit from cleaning the gunk out of the banjo fitting on the intake manifold as well as the line that connects the manifold to the ALDA.

Before you go any further, make sure that your valves are adjusted and your timing chain stretch is within spec, as well as your injection timing. This site should help:
http://skinnerbox.steaky.org/

It is a different car, but the same engine. Of course, make sure you are running clean filters as well.

Once that is all done, if you still are not happy with the performance, you can check the turbo boost with a boost guage while driving. This can be connected, via a "T" fitting to the previously mentioned intake-ALDA line. Spec, I believe, is .8 bar and is often set to less than that. If you are more adventurous and want to play with added boost and intercoolers, then that is a different game entirely. Doing the work described above should give you a nicely performing vehicle.
 

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Discussion Starter #3
So then the new adventure is to check for clogs in the exhaust. It just seems in comparison to the tdi that it has a whole lot of lag. Judging by the sound of the turbo spooling, it doesn't start spooling till about 2.4k rpm, and then the "kick" which i'll assume is full boost isn't till about 2.8k. Where as my tdi has full boost by 1.8-1.9k rpm depending on the gear i'm in. We already ran one of those heavy duty diesel injector cleaners through it. The type that is a pressurized can and plugs inline before the fuel pump and the engine runs off the can (i think it's like a mix of ether and detergent of some sort). If we one day were to add an intercooler to the party, i'm assuming that the alda thing that plunges based on pressure in the intake, variates the fuel in a linear fassion. Therefore, if the charging were to be intercooled and more dense, is there a way to increase the amount a fuel the engine recieves per atmosphere of pressure in the intake?
Oh, and btw, is that 928 an s4?
 

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Just because the car is not as quick as the TDi, it is not reason to be looking for clogs. Compare 0-60 times with literature, and try all the other tricks first.

There area couple of advantages that your TDi will have over the benz. One is weight, the other is gearing. The jetta is significantly lighter than any S-class benz, and the gearing is so short on a TDi that it is hardly in first or second. With the benz, it usually starts in second gear and lasts until 30 or 40 mph. (Those little dots you may have noticed on the speedometer are actually redline shiftpoints.)

There are also a couple of tricks that can be done to make the transmission always start off in first. I personally choose not to do anything permanent (ish) like a parallel kickdown switch. This basically makes sure that the transmission thinks the car is always floored and can burn out the kick-down solenoid. The easier, more reliable way is to just downshift and stomp on it. On my '93 diesel, I have the bowden cable, which modulates the shift points, a little tighter than most would like.

As for the fuel mix if an intercooler were installed, that would involve the ALDA thing that I described. Its purpose is to adjust fuel mix for the atmospheric pressure, as it can vary by altitude or boost. This is not something that should be messed with under normal circumstance, so I decided not to mention it previously. Turning the screw on top couter clockwise will give a richer mix. The manual suggests only 1/4 turns, since it is that sensitive. It is also a real PITA, as mine is running rich (still) since I last played with it.

Yes, the 928 is an S4, the first year of the S4 actually.
 

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Discussion Starter #5
S4's are quick, like riding an ultra stable rocket, because it always wants to go faster no matter how fast it's already going, and it does it like it's nothing. Just a real PIA to work on.
Wasn't comparing to the TDI in terms of acceleration, that's what the 911 is for, I was comparing lag, the turbo spools up much faster in the TDI than it does in the mercedes. But yet the TDI's engine has 19:1 compression, and the mercedes has 21:1. Typically in the gas engine world, that would mean that you can't put as much boost through the mercedes as you can the tdi (every increment of pressure put through the motor increases the static compression just before combustion occurs). BUT, the mercedes with the higher compression should spool the turbo up much faster. That's all dude, i'm not looking to rice out this mercedes and embarras the local civic group, just trying to see if there's a problem to solve here. There's no reason for this much turbo lag.
 

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OK, now I'm sure I understand what you are after. There is often discussion of getting more power from these things, and it is often tempting. I (obviously) suscribe to the same thought with regard to a different vehicle for different purposes. The the benzes are our commuters, the P-car is for fun, and the truck is for the home-cheapo runs.

Yes, the S4 is a real chore to work on, and I just got done giving it a major service (any even a little powdercoat). It all becomes worthwhile when I get a chance to drive it hard (is there any other way to drive a P-car?). I would really like to get a 356 some day, but the wife is already very patient with all of our vehicles AND my addiction to flying.

As for the turbo, compression is not the only factor for how quickly a turbo spools, but also compressor and turbine sizes. I don't think that the turbos in the two cars are really comparable. All things equal, the turbo will spool up more quickly on the higher compression engine. It has been a while since I have driven a TDi, but as I recall it does spool up very quickly.

Of course, I have not seen you benz and it could be that something is very wrong. My concern was just that you may have been using the TDi as a basis for comparison. Maybe somebody out there can chime in and tell when the turbo spools up on their 617 engine? I'll have to check mine (603 and 602 engines) out of curiousity.
 

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Discussion Starter #7
If someone could check and confirm i'd really appreciate it. The guesstamates stated above are from sound of turbo and the pull of the engine. There's no boost gauge in the car, so i can't be exact but i'm sure that i'm pretty close with my current assumptions. Thanks for all the help, and if you ever wanna talk porsches, throw me a line, i've never had the opportunity to dive into a 928, but 911, 930, 944, and 951 are no stranger to me.
 
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