MD; I have 41s in my C&L 44, same hull (and most everything else) as the MT44
7tiger: I used to have 40s, till 2000, when I moved up. I saw a pair of TAMD40Bs in a MT47 pilot house, rating the engines at 165hp. Is that what yours are?
TMD40A, or B, is a glow plug start 145 hp (A, I don't know the B rating) engine, without the aftercooler. The injection is into a chamber in the manifold, so on start you need to grind for a while. This engine was designed well before the boat was, so not a high pressure injector, some blue smoke, especially on startup. Mine had 4500 hrs on them when I got the newer 41s. They burned just over 4 gph at 7.5 knots, which I measured over 1500 hrs of my own use, in 6 seasons. the only real problem with the design was the heat exchangers, with a rubber boot over the aft end, that has both in and out hoses and fits over a central vane on the brass part. when the HE was getting clogged, water pressure would cause the boot to let go of the vane, allowing cooling water to exit through the boot instead of going first through the HE, result: engine overheat. I fought that until after I bought the 41s, before I solved it. The cure was to keep the HE cleaned out, by shoving a rod down the 100 tubes if there was any hint of an overheat. Also had recurrent pinhole leaks in the high pressure fuel lines. Annoying, as the mist of fuel covering the top of the engine seemed like a potential fire hazard and got everything messy. Had that on 5 different lines by the time I got the new engines.
The TAMD41s, being a newer design (late 80s) have direct injection, higher injection pressure (still nothing like a truly modern engine, IIRC something like 1500 psi v 800 on the 40s, v 30000 on really new stuff) resulting in very fast starts, very little blue smoke, better fuel economy. Mine are the As, rated hp is 200 hp.
Fuel burn is 3.8 GPH at 8.2 knots, so you see I gained 10% in boat speed as well as saving another 5% in burn. I also repitched my existing props from 19x13 to 19x17. That results in getting the 8.2 knots at 2100 rpm instead of 2750, so should be less wear on the engines. I am expecting these to out last me by a large margin. I have put another 1500 hrs on the 41s since installation in 2000, so the numbers are accurate averages. Another big difference: zero maintenance in 10 years! I am not counting the oil and filter changes and I did start with rebuilding the injectors and replacing the sleeves on getting these engines at 1500 hrs.
Top speed went up a little, from just under 10 knots to about 10.5. Seems limited by the hull shape, which would probably take 800 hp to get on a plane. at 10.5 the bow is angled up quite a ways and the hole at the stern is deep, into which all that extra fuel burn is being wasted.
I didn't change the trannys, still run the BW Velvet drives that came on the 40s. No issues there either. I checked the damper plates on hte swap. They were ok then, at 4500 hrs, and seem ok still, at 6000 hrs. I had a tranny fluid drip from the seal at the back of one, got that fixed years ago, so no other issues.
All calculations are in Canadian Gals, nautical miles.
Sorry for the longish post. Hope the info helps.
