can I use my merc 2.5 120hp without exhaust relief ports?

hi!

I have bought a new transom plate for my boat, since the old one was in a really bad shape. the old plate has two small holes inside the big oval exhaust hole, which leads out on each side of the transom plate. the new one has two small holes underneath the big oval exhaust hole, which my exhaust pipe does not cover. my plan is to simply weld these holes and run without exhaust relief ports..what problems can this cause?

regards,

tomzern

Re: can I use my merc 2.5 120hp without exhaust relief ports?

Ayuh,... Welcome Aboard,...

What are the Vintages of the transom assemblies,..??

I'm guessing there might be other differences...
No chance of gettin' the downpipe that belongs to the 1 yer using, eh,..??

Re: can I use my merc 2.5 120hp without exhaust relief ports?

I can post some pictures later to show you better what I mean, but the transom plates both fit in the same hole, so the only difference is those holes, which are closed on the old, and open on the new, because it was used with a V8 before..I have not found any exhaust pipes that cover the small holes, that would fit directly onto my engine..

thank you for your answer, and for welcoming me btw

Re: can I use my merc 2.5 120hp without exhaust relief ports?

Having the relief ports allows for a perfect "balance" of just the right amount of exhaust exiting via the prop hub, due to the negative pressure that's created directly behind the hub. Effectively, the prop hub "pulls" the amount of exhaust it wants, and any additional exits via exhaust ports.
That's my take on it, so I wouldn't mess with it.

Side note: Even the lack of a diffuser ring causes major problems of allowing exhaust gases to enter prop blade area (a type of ventilation).......so, I wonder what would happen if ALL exhaust is routed that way.

Re: can I use my merc 2.5 120hp without exhaust relief ports?

Having the relief ports allows for a perfect "balance" of just the right amount of exhaust exiting via the prop hub, due to the negative pressure that's created directly behind the hub.

Click to expand...

Sounds good, but not what they are for.
They are idle relief ports. At idle, there would be to much back pressure to go through the prop to idle correctly, thus the ports.

Re: can I use my merc 2.5 120hp without exhaust relief ports?

Don S said:

Sounds good, but not what they are for.
They are idle relief ports. At idle, there would be to much back pressure to go through the prop to idle correctly, thus the ports.

Click to expand...

Yes, you do save an additional foot of back pressure (approx .5 psi), but I'd bet that you'll have issues if either not enough or too much exhaust exits via the hub. I've always like to watch how as soon as you shift in gear, the exhaust immediately gets drawn through the prop hub.

Re: can I use my merc 2.5 120hp without exhaust relief ports?

Hi again, and thanks for all the answers!

This is a picture of my old transom:

It looks the same as the new one at first glance, but the small ports under the big oval port are actually not leading anywhere.

Instead there are these small holes that leads out on each side of the transom plate (you can see one of the holes inside the oval hole here):


I apologize if my english is poor, I am from Norway:redface:

I have also thought about buying an exhaust pipe from a 3.0 litre ( which covers the small ports as well) and get a friend of mine to cut and weld together a custom pipe for me, but that will probably be expensive

Any other ideas?

Regards,

Tomzern

Re: can I use my merc 2.5 120hp without exhaust relief ports?

bump