Custom Exhaust Elbow Design

[svcattales]

Originally Posted by barnakiel

Experience will be gained by using this specific design too. We might yet hear from the designer/owner.

My experience taught me that if engine designers designed something in a specific way, we have only very slight chances of improving on their design.

Slight, not nill.

Now if the design could be single wall with injection point close up and high, WHY IS IT NOT on Volvos, Yanmars and the rest of the crowd?

b.

I disagree with your high opinion of Volvo design engineers and not just on exhaust elbows, but I'll keep to the theme of my own thread.

My reasons for replacing the existing elbows:

1. The water cooled elbows are made of cast iron and they are rusting away. Not only do they rust, but they develop cracks and the hose barbs split. Why cast iron rather than stainless?

2. The water passages clog with rust and the water injection area gets loaded with carbon and neither can be directly cleaned. I have used muriatic acid to clean them, but the water flow is still restricted. Design is not suited to a cruising boat since I can't maintain them. My single injection port can be cleaned easily without removing the elbow.

3. Each original equipment elbow costs $650 and i have two of them. My new stainless single wall elbows cost $162 ea plus an extra $20 for heat insulation on hot part of elbow.

Do engineers really design the "best product" or do they design the most "profitable product?" I suspect Volvo makes a large part of their profits on replacement parts with limited life spans. I'm sure other companies do the same thing.

I just got the new elbows back today so I'll take some pictures of the installation and report back on how well it works. Stay tuned.

[barnakiel]

I share most of svcattales attitudes and concerns but must admit that my exhaust elbow (cast, double wall) was in the boat when I got her and last at least 7 years (I imagine totall life span of at least 10 years. Not bad. The part is 200 USD new.

Now I have seen a couple of well designed SS elbows badly corroded in less than 5 years. All of them at welds.

So maybe the cast iron thing is too about avoiding welds?

Perhaps if we could have a 'cast' (one piece) SS thing this would work best?

How much would it be to have a 'one piece' SS elbow built?

barnie

[svcattales]

I finished installing a new custom stainless steel elbow on the port side. My local shop really did a nice job with the welding and copied my PVC mock-up precisely. The new elbow fit just like the PVC elbow and mounting/plumbing didn't have any surprises for me. I bought 2 inch fiberglass insulation from NAPA and wrapped it with at least 1/4" overlap over the hot section of elbow. This product claims a 70% heat reduction radiating from the pipe. (On the other engine, I plan to wrap the pipe before installation to make it a little easier.)

When I started the engine I noticed a better sea water flow immediately. After 45 minutes of warm up at 1200 RPM, I checked the center of the elbow temp with a laser thermometer and it was 174 degrees F. When I measured elbow closer to the engine, the temps gradually decreased. Likewise, when I measured the temps closer to the muffler, those temps were lower due to proximity of cooling water. When I increased RPM gradually to 2800 and checked temp of elbow again, it reached a max temp of 201 in the center. The 3/4" water injection pipe stayed cool to the touch even at high RPM... no problem overheating the inlet hose. I also checked temps on large pipe downstream of the water injection site. Just opposite the water injection site the pipe was hot, but within one inch below the water injection site the pipe was cool to the touch so there was no problem overheating the exhaust hose to the muffler.

The only part of the mod that worries me is the length of the pipe supported on one end by 4 bolts. Although the new elbow is lighter than the original cast iron elbow, it extends out further and has greater leverage. I think the muffler hose gives it a little support on the other end and dampens vibration, but only time will tell if this is really a problem. The only real test is going somewhere "off the beaten path" where all weaknesses fail on a regular basis.

[sailingaway221]

Never Monday made a good point...the hose barb is technically not pointing out and you may find some pressure forced back into the heat exchanger......you may get yourself some grey oil if the angle of that barb is not pointing away from the engine!

[svcattales]

Originally Posted by sailingaway221

Never Monday made a good point...the hose barb is technically not pointing out and you may find some pressure forced back into the heat exchanger......you may get yourself some grey oil if the angle of that barb is not pointing away from the engine!

I believe Never Monday was referring to a different design in the first picture. He seemed to like the current design.

The water injection pipe angles down at a 45 degree angle into a downward angling main pipe. I fail to see how the injection water can go back uphill, particularly when testing showed increased water flow out the exhaust and no "grey oil."

[sailingaway221]

SV Cattles : We moved the injection pipe on my exhaust elbow from the base to the 45 also....ran it for about 45 minutes checked the oil and it was gray. We did it for space reasons.....but changing the spot where the injection pipe was from the downward tube to the 45 for whatever reason changed the pressure and water-flow... pressurizing the heat exchanger.... changed it back changed the oil ... no problem. Good luck with yours.