Hello, thank you all for the helpful advise in the thousands of posts.
I have an '85 300D in immaculate condition with 132k on it. I was fortunate enough to purchase it for only 3,200 bucks in NC about three weeks ago, and i just finished installing an entirely new a/c system on it. I am pleased with it after having done all the requisite MX any newly bought car should get, but have a question regarding converting the existing restrictive exhaust with the trap oxidizer and turbo to be swapped out from a 1979 617 motor 300SD version of the exhaust manifold and turbo. I have heard this motor run in the rear-ended car and it has 212k on it. My primary concern was improving the fuel mileage to better than the 22 on the highway that it is getting now. 22 is the best the 85 300d california car will do according to many sources since it has the excessive emmission controls on it. I have heard that the 300 sd 1979 model gets about 28 or so, whish would make it worth the 500 bucks for the complete motor with all the trimmings. What else should be converted in addition to the exhaust manifold and trap? Thank you all for the excellent, professional advise. Adam Bush

I have both 300D and 300SD, 1985 Calif model. They run 25 and 24 mpg, respectively. I wish I can get them up to 28 mpg. 22 mpg is a little low. In general, 24 mpg is what I expected from these late (1981 to 85) 617 engines. I know some people are playing with the ALDA screw (enriching the fuel mixture) to get a little more power out of the car resulting in poor mpg. I do not know what else I can tell you to increase the mpg besides the normal stuff: new air and fuel filters, correct fuel timing, proper function of turbo boot, working fuel injectors, etc.

As for the conversion, the 300D uses mechanical oil presure gauge. I know the fact that my 300SD uses electrical oil pressure gauge. The RPM sensing device may be different too (I could be wrong about this). I do not know the rear end ratio between the two. Find out the ratio first. You may not get 28 mpg after the switch over. The tire size may be different too that changes your mpg.


David