exhaust hose size

[jr438234606]

How do i know what size my exhaust hose should be? I currently have a 2" hose that was connected to my 50 horse perkins 4.108. I have a vernalift muffler which has 2" ports. The thru hull at the stern is 2". The new engine I'm putting in is a yanmar 54 horse diesel. The mixing elbow on this new engine seems rather large, certainly more than 2". Could I adapt this to my existing exhaust hose?

[Cruisingdad]

My guess is that it is probably a 3", but it is more complicated than that (isn't it always??). It depends on the length of run, elbow, number of turns, the muffeler, the rise, and the ehaust T-Hull. It is complicated.

It is a matter of back pressure. Too small of a hose will cause excessive back pressure, causing heat in the rear cylinders, causing carbon build up, then scoring, compression loss, then failure. That is the process.

You can contact one of a couple of marine exhaust comapnies (there is a good one in FL... but do not remember their name) to discuss their suggestions. Or, you can try running with what you have a taking a measurement. The exhaust pressure should probably be around 2.5 psi at WOT (Wide Open Throttle). Check with MFG on their specs. THey will know this. If it exceeds, say, 3... you will have problems if you run the engine for long periods.

Checking the pressure is easy and can be done with simple parts bought from a auto parts store. Effectively, you get a needle with a presure gauge and jam it in at idle, cruise, and WOT. Take the measurement. You then will need to plug the hole. If memory serves correct, I used a HT Silicon, covered that with a piece of HT Exhaust hose, AND clamped that with a hose clamp.

Buy a CO indicator afterwards to make sure you do not have any leaks (you need one anyways... that should be ABYC IMHO).

Hope that helps.

- CD

[negrini]

No, you shouldn't go for a smaller system. New engines, including Yanmar, does provide a higher torque figure at less rpm, so exhaust constraint will not allow you a correct SOW neither the benefit of cruising at lower rpms, and excessive back pressure will lead to further internal problems, as CD explained. Ask Yanmar as they provide good insight about this. I had to live with my 40HP and big noise as they recommended me to remove some noise reduction connection, to pose the minimum constraint on the system.

[cmdseahorse]

You definitely will want to use an Engine Manufacturer recommended ( approved) exhaust piping system or at least follow their installation manual for one of their recommended exhaust systems. Of all the issues in designing an engine exhaust system ,remember that it is a complete system, therefore absolutely critical that the total of elements connected in series must not exceed the manufactures max allowable backpressure/ exhaust flow limitations. A restrictive muffler, ( flow issue not just the size or connection sizes ) bent pipe or partly crushed hose, or too small a piping diameter for the length of piping run can all cause a problem . Your best and safest bet is to ask the mfr or a dealer for the specifications or a complete installation manual if the new engine did not include a manual for installation. Installing an unapproved exhaust system can void the engine warranty as well as cause poor performance etc