New engine question

[Lawrence Snow]

I’m in the process of changing out my old Volvo MD47 6 cylinder diesel with a new Volvo D2-75. My existing exhaust system is 3” hose and the D2-75 with a turbo calls for a 3.5” exhaust. I intend to change out the old 3” system with a new 3.5” system. My question is can I transition from the new 3,5” to the existing 3” exhaust thru hull fitting without causing excess back pressure? It would only be approximately 6” of 3” ID. Obviously I would save the cost and labor of changing the thru hull exhaust pipe.

[Scubaseas]

If operating less than near wide open throttle all the time you'll be fine. For normal cruising RPMs having the last 6-7" of the exhaust reduced to 3" will have negligible impact.

[SailingHarmonie]

I am afraid that the answer really has to be "maybe/maybe not."

The pressure drop in any single section of exhaust doesn't matter, what counts is the TOTAL pressure drop from exhaust elbow to thru-hull. This is a function of the length of hoses, the changes in levels and the about of water injected and carried along with the exhaust gases. Necking down the exhaust diameter does add significant backpressure. Is it too much? I have no idea. And neither does anybody else who hasn't seen your detailed system design.

I don't know how anybody can say Yea or Nay without knowing all that...

[Scubaseas]

I would bet good money that if you put a manometer anywhere on the system and measured the same RPM with the last 6" at 3" and the whole system at 3.5" you would not be able to detect a difference in readings unless your gauge was calibrated out three decimal places. Some exhaust systems actually put a restrictor in to improve flow. Unless you're operating at very high RPMS the amount of necking down back pressure will be negligible. At wide open throttle for long periods? Sure, maybe but for normal sailboat use I doubt it.

What is the definition of "excess" or "significant"?

[DougR]

All of the answers you have received are more or less correct, but the reality is that you will have to measure the exhaust back pressure during your seatrial.

I would suggest you make the step down piece between the 3.5” hose and the 3” thru hull fitting a gentle taper to minimize resistance. Gentle sweeping turns are better than tight 90 deg. bends to keep backpressure down. The backpressure needs to be measured just downstream of the exhaust elbow at full RPM and full load.

Max allowable back pressure is 15 kPa or 2.2 psi.

[skipperpete]

I’ve struggled with the yanmar 75mm exhaust requirement when installing JH series engines for years, sometimes it takes longer to fit a new 75mm exhaust than to actually install the engine.
Volvo are not great at honouring warranty under normal circumstances so I suggest that a non spec exhaust would give them a good exit point if the engine failed during the warranty period. The maximum allowable back pressure is 2.2 psi.

[Scubaseas]

The Volvo Installation Manual shows a D2-75 needs a minimum back pressure of 15Kpa or 2.18psi and a maximum back pressure of 20Kpa or 2.9psi.

https://www.lundskov.eu/bavaria/manu...ion_manual.pdf See page 24

I'm sure the minimum is there because it's a turbo.

I agree 100% with DougR on measuring the back pressure.

I'll put good money up the engine under normal speed and load falls between the min and max pressures. I'd even bet that at WOT and load the back pressure is OK. Please let us know.

[Lawrence Snow]

Thanks to all for your replies. I pretty much thought the same things. I know that all systems are different. Mine has all large radius tube turns (1 @ 90 degrees and a 180 degree above WL. Stiff wall hose @ 10’ (eng. midship). Water flow is adjustable with a diverter valve. I will ponder my options and then share my decision.