Another question....mixing elbow exhaust hose

[Ralph Johnstone]

Couldn't Agree More ........................

I have always understood that if an engine has a 2" exhaust inlet it shouldn't be reduced but should remain 2" throughout. That is why waterlifts are available in all sizes.

The previous owner used the incorrect size. It has to evacuate the exhaust as well as the cooling water that the engine's cooling pump passes through the system. I certainly wouldn't want to restrict it.

But if you've ever seen the inside of a mixing elbow, it makes you wonder how ANYTHING gets through there. The below elbow brings to mind the process of giving birth (let's see if this gets censored). This mixing elbow was removed as a matter of routine maintenance when the engine was running perfectly. Shows how much these things have to plug up before they DO cause problems.

But back to the 1-1/2" inlet to the muffler ................... not the perfect design of course, but I don't think, IMHO, that it'll increase the back pressure significantly. So probably not worth the price of a correctly sized (2") replacement muffler.

 

[jibes138]

I keep reading about exhaust size and 1.5 inches versus 2.0 inches etc. Isn't the real restriction in exhaust gas flow rate the size of the exhaust valve in the engine head? I find it hard to believe the exhaust valve will be the same as an open 1.5 inch hose. Also as the exhaust gas is cooled it will take up less volume. Am I out in left field here?

 

[kito]

I installed my new mixing elbow tonight. I noticed on the new one that the 2" outlet nipple also has a 1-1/2" internal thread so me reducing down to 1-1/2 at my muffler should not be a problem. You can actually see what is left of those threads in Ralph's pic.

 

[Ralph Johnstone]

I installed my new mixing elbow tonight. I noticed on the new one that the 2" outlet nipple also has a 1-1/2" internal thread so me reducing down to 1-1/2 at my muffler should not be a problem. You can actually see what is left of those threads in Ralph's pic.

It may be worth taking a close look at that internal 1-1/2" thread in the outlet of the mixing elbow. I've always used 2" hose for the outside fit but I've also imagined a few problems with screwing an 1-1/2' nipple in there.

The wet exhaust gasses are nasty, nasty stuff ................ warm salt water (with a few hot excursions) and somewhat acidic with burned sulphur from the fuel (H2SO4). The cast iron body of the mixing elbow is much more resistant to this witch's brew than a mild steel nipple screwed into the outlet. As far as using corrosion resistant materials for the nipple ? ? ? ............... could be looking at problems if the nipple is more noble than the cast iron (galvanic corrosion). Don't bother with a galvanized nipple as the zinc would be gone in a few months.

If you do go with an internal nipple, consider removing and inspecting every two years. My main concern would be a) corrosion through the nipple wall at some time and b) corrosion and loss of the internal 1-1/2" threads in the cast iron. Be sure to use lots of teflon tape and teflon paste, more for protecting the threads rather that sealing the gasses inside.

I know I'm being anal here but I have this thing about CO.

BTW ............... how much was the elbow ? I told the dealer I wanted a cast iron elbow but he insisted on selling me a solid gold one.

 

[mitiempo]

If you put a 1 1/2" nipple into the 2" elbow your inside diameter becomes about 1 1/4"

 

[Ralph Johnstone]

If you put a 1 1/2" nipple into the 2" elbow your inside diameter becomes about 1 1/4"

I knew that would be the next comment to appear.

Hopefully what happens is that as the gasses enter the short nipple with the 1-1/4" inside diameter there is a brief increase in velocity and decrease in pressure. As the gasses leave the short nipple and enter the inside of the 1-1/2" hose , the velocity will decrease and the pressure increase slightly. These changes are small, they're referred to as some law or other of conservation of energy and hopefully they don't make a bloody lick of difference in back pressure.

Might be easier for all responders here if Kito ran the hose in 2" diameter from the elbow to the muffler and use a short piece of 1-1/2" hose as a bushing so we can all go home and get some sleep.

I would expect as a MINUMUM, numerous pictures of this project when completed :neutral: .

 

[kito]

Ralph, I believe the mixing elbow is used on other Yanmars too. Maybe the internal outlet thread is used on the 8 hp version.....not that I would use it. I will take pics when I am finished. I got everything except the 2" hump hose. I found the lh/rh threaded nipple to be a real pita to get adjusted correctly with the limited space the elbow is in, plus maintaining the correct angle to clear the throttle cable. I bought all my parts which are genuine Yanmar parts from Toad marine supply in Michigan. The mixing elbow was around $130, not sure how that would convert to Canadian.

 

[petersea]

I keep reading about exhaust size and 1.5 inches versus 2.0 inches etc. Isn't the real restriction in exhaust gas flow rate the size of the exhaust valve in the engine head? I find it hard to believe the exhaust valve will be the same as an open 1.5 inch hose. Also as the exhaust gas is cooled it will take up less volume. Am I out in left field here?

I would agree with Dave (jibes138) so he would really be in "right field" !

 

[kito]

I keep reading about exhaust size and 1.5 inches versus 2.0 inches etc. Isn't the real restriction in exhaust gas flow rate the size of the exhaust valve in the engine head? I find it hard to believe the exhaust valve will be the same as an open 1.5 inch hose. Also as the exhaust gas is cooled it will take up less volume. Am I out in left field here?

Water is also a factor. Water is introduced to the exhaust at the mixing elbow. Probably why the elbow has 1-1/2" inlet and a 2" outlet. Maybe the 1-1/2" inlet is overkill but I would rather have that heavy elbow attached to a large nipple rather than a small one considering the vibration.