Universal M18 - larger heat exchanger?

[Skipd]

Am new to the CD30 and am working on the M18 engine this winter. Have removed the original 2” x 12” heat exchanger (HX) which was pretty clean. It was a fresh water boat. Am contemplating upgrading to a larger HX as cooling issues seem prevalent on the Universal diesels, especially where hot water tanks come into play. Previous owner had no history to convey as the boat was very lightly used.
The engine is forward facing (B layout) and the HX sits low in relation to the mounts. A 2” x 16” HX looks to require bracket modification or relocation. A 3” x 12” HX would fit better on the engine, but it too looks to be a very tight fit under the exhaust. Either option does not look to be a simple swap.
Has anyone with a M18 done this upgrade? Or on a similar model? Is it worth what might be the extra work?

Thanks

[Jim Walsh]

I have an M-25 so I don’t have first hand experience but the 3” HX is what the Catalina owners have been upgrading to. Just in case you are not aware a HX does not have to be engine mounted. Anywhere convenient in close proximity if perfectly fine. I know that is what Steve Laume did on Raven several years ago. I was unable to do so easily just because of how my engine compartment is laid out. It is still possible but I have to fab a custom bracket which I have not been sufficiently motivated to do…..yet.

This link is an example https://c34.org/bbs/index.php/topic,102 ... l#msg78826

[Kailua Kid]

Unless the engine is running hot, there is something to be said for not changing the heat exchanger to a larger unit. I suspect most overheating issues with that engine are due to lack of heat exchanger maintenance (resulting in reduced water flow from debris and or corrosion) and or lack of exhaust elbow maintenance (resulting in excessive back pressure from carbon build up) rather than due to bad engineering of the original cooling system. If you do switch to a larger heat exchanger, you might want to keep an eye on the temperature to make sure the thermostat keeps the engine at the designed operating temperature (not below), as running it cooler than what it was designed to run at would prevent it from operating efficiently and (counter intuitively perhaps) shorten its lifespan. Of course, overheating a marine diesel can also result from excessive load (running hard with a fouled bottom and or fouled wheel), or from running hard while over-propped (either too much pitch or too much diameter to allow the engine to run at or just above its rated speed).