This makes sense..........less exhaust restriction........less work for the engine..........less boost pressure required to maintain horsepower...........

Different scenario.

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Why?

If the 4" exhaust offers up less restriction.........wouldn't we expect the engine to require less fuel for a given horsepower........and therefore require less boost?

??

There wasn't a big cork in the exhaust, simply opened it up. (at least from what I gather, truck probably has a 6.5 TD). I agree it will lose boost if you go to a larger exhaust when there is no restriction, seen this plenty of times.

The plugged cat will not even allow a vehicle to boost or at bad times, start and/or run. Like the Turbo Buick I have experience with, but that's a gas motor so it could be the difference between this.

Hope that makes sense...

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

It does make sense.

If the cat is almost completely plugged, the exhaust volume is so low that the turbo won't spool and boost is negligible.

Remove the restriction and the boost will climb to a "normal" level.

Continue to remove the restriction past a certain point and the boost will begin to fall, simply because the engine doesn't have work as hard against the restriction to maintain the output horsepower.

I suppose the question then begs to how much the cat was restricting the original exhaust flow. It appears that it's not all that much because the boost levels are close to each other.

The strange thing is that the boost after the cat removal is higher.

I figured boost was higher because the cat was quite plugged up, have not dealt with any that are sort of plugged... either working or not.

Would your L's be a good comparison as they both have 603s and one has the cat and the other still remains? I know the turbochargers are different so it may not be...

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Winter diesel has a huge effect on mileage. I can get about 28+ highway mpg on summer diesel fuel but only get about 23 mpg on winter fuel. The cold air should boost efficiency of the engine but remember that your car is plowing through denser air as well and at higher speeds this may more than offset any gains in engine efficiency.

I can state for a fact that with my 85 Cali 300SD my boost is lower than a federal emissions car under the same conditions. A federal emissions car starts seeing boost sooner and has about 1 psi more boost assuming neither car is up on the wastegate. I can also state as fact I burn more fuel than a federal emissions car hence my lower fuel economy. I will add that when I put a cherry bomb muffler on my car I did see boost drop some and the engine doesn't work as hard. So we have two different results from where the restriction is at. The only thing that can be concluded is this is not a simple relationship.

As for an explanation on the effects of excess air in the combustion chamber I will refer you to another web page. (read the blue text) http://courses.washington.edu/me341/oct22v2.htm Although this web page deals with the otto cycle this theory is applicable to the diesel cycle as well. Excess air also soaks up some of the heat that would be lost to the cooling system and converts it to power.

In short if you can get more air in the engine using the same amount of energy you will get higher efficiency and more power. Or in other words, ditching the cat will improve mileage and power.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

http://ecomodder.com/forum/fe-graphs/sig692a.png

I was out to lunch today and that should be taken into consideration. An exhaust restricted diesel can damage itself I have been told. If it is not an old wifes tale there must be a cause. When restricted the exhaust has a residual back pressure and I imagine some pressure and greater heat is retained in the cylinder . Could this result in a combustion effect simular to advancing the timing? Higher compression effect? Or should I say apparent timing change by the faster burn rate. Injection still takes place at the same moment but the residual heat and additional pressure creates a more rapid burn. Or would the byproducts of the previous combustion negate it? Would this give greater milage if an injection pump were slightly retarded from the optimum setting? Perhaps a greater knock on an engine with the pump set at recommended position? Then you would also lose milage. That may be the damage senario with restriction too.
Other than calculating your fuel milage in warm temperatures and now doing so on heavy winterized fuel. I really cannot think of anything else particularily. When you changed your injectors or installed new nozzles did you check the pump timing? Can someone out there tell me exactly how a restricted diesel hurts itself? Can someone throw me a lifejacket?
The normally excess oxygen available during the burn might make this senario possible? Is there some known interaction as something is happening to hurt the engines when restricted if true. If one had a positive indication of a higher or now lower reading on their temperature gauge it might help. More than likely though at this time of year the engines cooling system can take it in stride. If the injection pump timing had never been checked since you purchased the car it might be worth checking. Fortunatly it is pretty easy to just check it if you can borrow the tool for your pump unlike the earlier pumps.
Most of the above thoughts are of course a waste if you already know your pump is on. Wish I knew more about this area of engine operation. One thing sugests more energy release for less fuel. Another side of the same coin says less efficiency eliminates any gain. Then there are many other imponderables as well. It was well stated in a previous post that there might be a lot of complex relationships here.
I just love getting out on a limb. The problem that concerns me is I seem to take the saw with me.

Engine condition: I have changed the timing chain (at 300K miles), compression is fine at all cylinders (over 30 bar I think), timing is set at 14 degrees RIV (15 is stock) , 1 degree earlier than stock. New Nozzles all at 140 bar , though Bosch -spray pattern was fine.
I understand that the boost rates I am experiencing are in the range of 'normal' and that there is no huge draw back with higher pressures. The removal of the cat should lead to less restriction and more economy. (The cat was clogged like 50%, I can send pictures if someone is interested. I could also have tried to cleaned it out and put it back but decided to take it off. Probably a bad decision regarding NOX output)
My simple explanation for bad fuel economy is: winterfuel and other bad luck. I hope this is all right. I am waiting for summer then when i am back to 25 mpg or better. Actually I have hoped to see better milage than before , like 28. Why is winter diesel that much worse?

Other items I experienced : I wanted to change the engine mounts due to high vibration at idle. I already bought the mounts but now after the cat is out the idle is so much softer that I don't think about that any more.
Also shaking at higher rpms has stopped. Before I was worried it something at the cardan drive was bad and vibrating. It was like a very unballanced wheel.
Now this has gone and no issues any more. I didn't try but the car runs fast like 110 I guess.

The btu contained in what they cut the diesel with for winter is far less than diesel fuel. Just far less energy available for the volume. Glad to hear there was no possibility of another problem hiding in the timing... Yes I agree at this stage there is not much to be gained until spring sets in and we are off winter fuel. Milage checks would be almost meaningless right now. I also notice some brands of fuel go through my diesels like mouthwash. While some are considerably better at giving decent milage. Ask around in your area. Some of the locals should know. You may find some interesting reading in the archives about top speed if you can find the items. It was also very informative to find you lost vibrations when you got the bad cat out. I now wonder how many guys have chased vibrations without even suspecting the true source.

I don't think that the cats do anything for nox reduction as of 87. It was just there to catch black smoke.

Winter diesel is worse because it has fewer btu's of energy per volume. To make it so it doesn't gel up, they make it with a higher percentage of shorter chained hydrocarbons molecules. These contain less chemical energy than the longer chained molecules.

__________________
green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

http://ecomodder.com/forum/fe-graphs/sig692a.png