Outboard 2 stroke piston port holes? Please, HELP ME! 3

[robert1612]

In rebuilding a Mercury Sportjet with Force 90hp stroke I used Wiesco forged pistons but they do not have the 2 large diameter (approx 1") holes on the pistons intake skirt as the originals had. Wiesco claimed they are not needed in reguard to power and were intended for reducing piston weight. Wiesco also stated they weaken the piston skirt (which I beleive may be correct) and for this reason they do not use them.
At this time the engine is together and running however the boat speed will decrease 2-3 mph when moving throttle from 3/4 to full throttle.
I have checked the relationship of the intake ports and find they would allow passage of a fair amount of air/fuel from the crankcase to the intake ports (or at times in the opposite direction) during engine operation. At the time the pistons were installed I decided to machine holes of a much lesser area as a compromise in case they were important to airflow. Just to certain I have tried stepping up jetting .003" and feel this did not make any improvement. Spark plug coloration was normal to now just slightly rich. You should also know I have confused the issue a little more by installed Boysen reed valves at the same time of the rebuild. Cylinder port timing was left basicly stock to retain the same rpm band but was smoothed throughout intake passages and crankcase. In the transfer ports I removed the sharp edges on the top and bottom. 'Core mismatch?' The 4 intake ports holes (into the cylinder were radiused rather than the original sharp drilled entry.
The original shape of the 2 piston skirt holes was not 'matched' to other ports but did pass somewhat in alignment with the 4 intake ports. I used 4 smaller holes (approx 1/2" diam) each roughly centered with the factory intake passages.
I feel the harmonics or critical flow path of the induction system may have been changed by me not duplicating the 2 large piston skirt holes, can anyone add comments to this thread as to the operation of these holes? I intend to diassemble the engine soon to modify the pistons into a larger and hopefully better than original (updating the outdated design). Thanks.

 

[MikeHalloran]

It appears that either Wiseco was wrong about the purpose of the skirt holes, or the Boyesen reeds are not an improvement, or your porting efforts have screwed up something.

Here's what I would do: Remove the Boyesen reeds, re-test, and then change one and only one thing at a time.



Mike Halloran
Pembroke Pines, FL, USA

 

[Fabrico]

Holes in 2-stroke pistons are usually not for lightening. The larger the holes in the intake-side skirt, the closer the engine is to being reed induction.

 

[robert1612]

Yes, the engine has reed valve induction. Does the size and/or location of the holes determine power band or power output?

 

[FoMoCoMoFo]

If you want the engine to behave the way it did before you need to use the same type piston or CAREFULLY reproduce those holes.I dont think you should attempt to enlarge or reshape them. proceed with caution !

 

[kenre]

Love the 2 stroke engines!!

4 reasons for the holes in the pistons,

1, To lighten them, to aid in balancing,
2, Air fuel mixture flows through the holes to help cool the piston crown,
3, Breathing, if the transfer ports are small, or the engine was designed to use these holes they can contribute to ensure adequate transfer flow,
4, Reed valve engines may use these holes to help intake flow.
depending on where there located.

Real engines Burn there oil !!!

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[Fabrico]

Yes, the engine has reed valve induction. Does the size and/or location of the holes determine power band or power output?

Having reed valves only means it has reed valves. Some 4-stroke dirtbikes have reed valves. Some 2-strokes will run quite well with the reeds removed.

A true reed (or rotary valve) induction 2-stroke engine does not use the piston to open or close the intake tract. The reeds and carburetor can mount anywhere on the crankcase.

Even with a standard carburetor mounting location, it is quite possible to have the piston holes and intake port arranged so the crankcase is never closed. This is usually done to achieve a specific power band.

 

[robert1612]

Sounds like I should try to duplicate the same x-sectional area open when the ports and skirt holes aline in reguard to piston position or travel. I can measure this with a degree wheel indexed to TDC and calculate the piston position at a few different points such as when initial port to skirt holes has opened to expose 1/2 square inch, again at the maximum open point, then when it closes to the same 1/2" x-sectional area. (Thinking of it as blueprinting or degreeing a cam).
Maybe if I see an easy set-up to use my Quick-Flow bench I'll find the point that say 10 or 20cfm flows thru the intake/shirt hole opening and the stock max flow. From this maybe I can enhance the rate of x-sectional opening to act similar to a 'high energy' camshaft. Do you agree this may be on the correct track to maximize performance without changing RPM power characteristics?
I usually don't quit a project 1/2 way done but I do not what to disassemble the engine more than this next one time. At the same time, I have spent MANY HOURS tweaking small gains from engines so I don't mind spending the time or sharing the results. I do, however, plan to accept the results as they turn out or sell the boat if there are no significant gains soon.

 

[MikeHalloran]

I think you need to first duplicate the original round holes wrt to size and location. That will get you back to where you were.

Then you might consider 'squaring' them somewhat, i.e. drawing a superposed square (with rounded corners) and filing to that line, which would give you more area with the same timing. Only after that would I consider moving the edges a bit.





Mike Halloran
Pembroke Pines, FL, USA

 

[Fabrico]

The degree method will work as long as you make no mistakes.

A more fail-safe way is to put the new and old pistons side by side with the pin halfway in both and carefully transfer the marks. The power band is usually much more sensitive to the top and bottom of the holes (timing) than to the sides or shape.

I would avoid getting creative. Not only is bigger not always better, but 2-stokes don't like many of the things that 4-strokes do.

 

[Metalguy]

You might try going a bit leaner on the A/F ratio.

"When the eagles are silent, the parrots begin to jabber."
Winston Churchill