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View Full Version : OMG, another guy who wants to convert his 15hp to a 10XS


Full Speed
08-08-2017, 09:49 AM
Yes, I want a faster Merc 15hp
and am very curious about the modifications that Mercury did to their little 10XS.

What's "simple" possible without losing reliability ?

During the overhauling we did some blueprinting and 'basic port work'...
The engine is further reasonably original :rolleyes:
GheenoeRacer
08-08-2017, 03:00 PM
I'm interested as well, so far I've cleaned and polished castings on the exhaust. Not sure if I want to dry up exhaust. What did you do during blueprinting? In the 10xs pics, the small oil line goes into the intake cover. Why is this?
Full Speed
08-09-2017, 12:59 AM
That's for the vacuum needed for the carb.
(I guess)
At the normal block they found another solution.


Image before engine revision:
62619
GheenoeRacer
08-09-2017, 05:12 AM
I think those lines are to pull oil to the bearings.
I was reading about merc 25 mods and they said these 2 holes under the exhaust plate are for compression release and should be plugged to raise compression. I noticed our 15s have them 2 they go into the combustion chamber. Looks like they will bleed off compression. No wonder these have such low compression. Does anyone know of this? Plug or not? Trying tooo upload pic. Here's the link to the thread
http://www.screamandfly.com/showthread.php?152867-Mercury-25-Hp-mods
Full Speed
08-09-2017, 05:40 AM
Ok. You mean the two small holes (red accented)

62620


If I'm right, the driled cylinderwall can also be seen at the 10XS
(If this is at least an original 10XS)

62621

62622
GheenoeRacer
08-09-2017, 06:21 AM
Yes notice how it won't build compression till after that, even at high rpm I would think you still loose some
Fastjeff57
08-09-2017, 06:24 AM
If you consider how long those tiny holes at open at 6,000 rpm, you'll realize how little gas could escape in that infinitesimal time.

Jeff
Full Speed
08-09-2017, 06:40 AM
Why would they have a hole in the cylinder wall?
There must have been an engineer who saw an advantage.
Who can explain this?


Upon further inspection, the hole in the 10xs seems to be closed.
But the exhaust does open a lot earlier, and are much bigger ...
GheenoeRacer
08-09-2017, 06:41 AM
Hard to quantify, yes. But lost compression if any is still lost compression, especially since these motors lack it and I don't think there's anything that can be done about that, unfortunately. I believe these motors have a lot of untapped potential. There's also a lot of interest in these motors because so many people have them. Could a piston be designed/ modified to increase compression?
GheenoeRacer
08-09-2017, 06:43 AM
Why would they have a hole in the cylinder wall?
There must have been an engineer who saw an advantage.
Who can explain this?


Upon further inspection, the hole in the 10xs seems to be closed.
But the exhaust does open a lot earlier, and are much bigger ...

That says it all doesn't it!
GheenoeRacer
08-09-2017, 07:37 AM
Upon further inspection the oil line going to the carburetor has some sort of check valve in the reed plate. Connecting to the intake cover may allow more oil to the bearings for the increased rpm and 25:1 oil ratio the 10xs calls for
MODIFIEDOUTBOARD
08-09-2017, 07:53 AM
I'm interested as well, so far I've cleaned and polished castings on the exhaust. Not sure if I want to dry up exhaust. What did you do during blueprinting? In the 10xs pics, the small oil line goes into the intake cover. Why is this?
Compression relief to make starting easier, has miniscule effect while engine is operating..........
MODIFIEDOUTBOARD
08-09-2017, 07:57 AM
Upon further inspection the oil line going to the carburetor has some sort of check valve in the reed plate. Connecting to the intake cover may allow more oil to the bearings for the increased rpm and 25:1 oil ratio the 10xs calls for
yes it is very important especially when "making" a intake to keep the bearing oil feature...........
LittleCharger
08-09-2017, 04:45 PM
compression relief to make starting easier, has miniscule effect while engine is operating..........

x2...
LittleCharger
08-09-2017, 04:47 PM
Hard to quantify, yes. But lost compression if any is still lost compression, especially since these motors lack it and I don't think there's anything that can be done about that, unfortunately. I believe these motors have a lot of untapped potential. There's also a lot of interest in these motors because so many people have them. Could a piston be designed/ modified to increase compression?

Custom Pistons or welding up top of cylinder to increase compression, same thing the guys do on the 3 cylinder merc's
Fastjeff57
08-10-2017, 08:15 AM
One other trick is to line bore the block closer to the head, then take a cut off the block to make the crank fit again. That raises compression but DE-creases port timing, so you have to bump that back to stock (which lowers the damn compression again!)

Be nice if the head came off, but... At least there's no head gasket to leak.

Jeff