ok, so i like to tinker with stuff.... maybe that's why i am building small planes

anyway, i have this MVVS .91 that will go into my Cermark f-16

yes i could get a Jett... but where would the fun be in that? the f-16 is already an ARF so i have decided to put some work into the engine: porting

preliminary research shows the MVVS exhaust port should be raised 1.5 mm (no more, no less)

and i have found an article by Robert Thacker "Easy ducted-fan engine hop-ups" MAN March 2001

although the MVVS is not a DF, most of the mods are applicable (i think)... that's where i need some help

the questions:

1) in the article, Robert test-runs the engine & then makes some mods (porting etc) and then he test-runs again.... and so on
i thought the mods have to be made BEFORE the break in (?) obviously i was wrong, can someone explain pls?

2) the raising of the exhaust port is self-explanatory, but the mods on the intake side port he "contours so that it shoots the air to the front and top of the sleeve" am not sure what he means looking at the pic.... (?) can someone explain in more detail?

3) he lightens the piston by shaping-out a part of the piston skirt in a "arch" manner, he states:
"remember that the piston modification is on the front of the piston where the boost ports are, not in the rear of the engine where the exhaust ports are"
in my case, i have a side exhaust... would that affect the location of the lightning "arch" in the piston?

see pics

any advice would be great,
Vasek

Check out Vincent Crabtree.
[link=http://www-staff.lboro.ac.uk/~elvpc/bikes.html?http://www-staff.lboro.ac.uk/~elvpc/...vpc/intro.html[/link]
Also try to get hold of a copy of Blair's design and simulation of two stroke engines. It is worth the while

If you are planning this work out of interest: okay. There is a high chance, however, that you will trash the engine since you are having to ask these questions. If you are prepared to replace the cylinder set if you go a bit far with the cutting stone, or even the whole engine: okay, go ahead.

Whether or not the engine is run-in should make little difference. If it is run in and you are accustomed to its performance you will then see the benefits (or otherwise) of your tuning.

If you raise the exhaust port you may also need to raise the compression - the port shuts later on the compression stroke. This mod raises the power peak up the rev band at the expense of mid-range power. Do this too much and your engine will not rev on anything larger than an 8 x 4 prop.

Directing the gas flows by shaping the transfers and inlet ports is the killer modification. This allows you to maintain or even improve your midrange power and keep the gains made by raising the exhaust. Directing the gas flows assists scavenging and is the aim of the Schneurle layout commonly found in modern engines. You are aiming to improve on the standard layout and blast the exhaust gases out of your enlarged exhaust port with the incoming charge. The art is to get rid of most of the burnt exhaust without losing half of the fresh charge before the exhaust port shuts. This is probably area where the most tuning skill and experience is required. This is because you need to know how to profile the ports to target the gases and also where the gases should be aimed for optimum effect.

Shaping the skirt does not just lighten the piston. It allows more cooling inlet gases under the piston crown (to deal with the new higher running temps) and can clear the skirt away reducing obstruction to the inlet charge. It will also alter inlet timing and here you can wreck the engine. You will need to allow for the cylinder orientation of the MVVS 91(which is optional when new) to work out which ports are which. Presumably you are looking at lightening the inlet side of the piston: that is opposite the exhaust port. You should also cut an equal amount from the exhaust side - this does not affect any timing but does restore the balance of the piston.

You may also gain revs by matching the crankshaft inlet to the carb throat and by enlarging the crankcase transfer passages as much as possible without cutting through the side of the engine. Smooth and polish inside until everything is shiny. You may then benefit from a larger carb to exploit the higher gas speeds (and revs) through the engine.

If your engine still runs after all this, then you have a basis for improvement.

If it doen't then you have still learned.

Good luck: you will need it.

Vasek;
Be carefull cutting the skirt on the ex. side. If you go up to high, when the piston is at top dead centre, the ex port will be exposed to the crankcase. Put a mark on the piston at the bottom of the ex. port and don't cut above that line. Use something like a nylon tip marker, don't scratch the piston. I'd practice on an old LA motor. They are a mild mannered engine and will show an improvement when modded if done properly. If it doesn't work you havn't lost much

I agree with Mick....you'll need to learn a whole lot more about the engines
before you take the grinder to it, that's for sure. Another thing to watch out
for....if the liner is ABC....with a chrome bore....and you grind through the
chrome....you run the risk of peeling the liner and such, because the chrome
is applied after all the machining is done on the cylinder. Once you grind the
chrome off the champher, bad things can happen.

On the other hand, if the engine is ringed with a steel liner you can grind away
on it till your hearts content, but you must re-champher the ports.

Generally speaking, you don't want to mess with the transfer ports and mess
with the direction of flow. That has been carefully worked out ay the factory
and you won't improve on that.

FBD.

I agree i have learn more... and after thinking this over, here is the plan:

since the engine is barely run-in, i will try to tune the minipipe by shortening the header to see how much rpm i can get from the setup... can anyone tell me what i can expect with this engine on the f-16?

then i will try to raise the ex port a bit and see what happens....

if i screw up or if the plane is not fast enough, i'll get a Jet

thanks to all for your replies,
cheers Vasek

I once asked Duke Fox about porting design and modifications. He suggested I get a dozen engines and start cutting and testing until I ruined all of them, then "You will have a pretty good understanding of how they work".

....and then there is the physical skill of cutting ports and such, that isn't
learned overnight. It takes many hours with the carbide cutters to hone
this skill and become a good "port and polish man".

FBD.

First thing is to get a 360 degree protractor and make your self a degree wheel to bolt on the crank so you can measure how much before and after TDC your ports open and close. Then you can rationally increase or decrease timing rather than just cutting off a 1/16th here and there.

The sleeve mod as shown in the middle pic of the first post is an easy one to do, and it works...
Also, a guide channel is even better. If you have seen the sleeves out of some of the high perf. R/C car engines, you know what I mean.
I have done it on many engines, and have seen at least a 300 r.p.m. increase and as much as 500 with all else being the same.

I did an O.S. .61 FX because I thought it needed some help, and saw almost 500 r.p.m. after just working the outside of the sleeve.

Also, I agree with Dave, in that you don't want to break the mechanical bond of the chrome inside the sleeve, or you're just asking for trouble (peeling)...

The best Dremel cutters for that job are the flame shaped carbide, and the 1/8 and 3/32 dia. cylinder shape.
If you can hold your breath for long periods, that helps too (helps keep a steady hand)

Vasek:

You might slso consider getting some fuel with 50% nitro or higher, pour it into an Omega bottle marked "Five percent" then when people ask how you got the engine to be so powerful just smile at them.

Don't let anyome else use fuel from your bottle, of course.

Haw.

Bill.