HighPlains

My Feedback: (1)

Yes, you can do it with a dial indicator and a bit of math, but much easier is to remove the head, backplate and carburator and use a degree wheel to see where the ports open and close. Duration on the sleeve for exhaust, boost and bypass ports are twice what the meter reads when you move the piston down from TDC. Pretty typical reading for non-piped engines are exhaust port from mid 150 degrees up to high 160's. As you go higher, the engine torque goes down, but it runs much faster with smaller props. On the boost and bypass ports you see numbers in the range of 120-135 degrees. But first things first, make a degree wheel and see where you are starting from.

skull1971

Your on the right track, you would have to take metal off of the forward side of the roto valve. BUT if you take to much, you'll lose crank case pressure back out the carb. I don't think I'd mess with that, I'd look to match the ports, and maybe replace the carb, one of those Perry's maybe

skull1971

Hey, that's pretty cool

combatpigg

My Feedback: (3)

If you are really ambitious, you can replace the carb with a home made venturi. I used to make venturis on a drill press, with an old screwdriver as the cutting tool. The engine will respond to the cleaner and larger intake. To make the plane safe for flying, a fuel line pinch off has to be rigged up to shut the engine down.

proptop

My Feedback: (8)

Yeah, that's what I'm talking about...getting into the actual details...

I made myself a degree wheel some time back, but haven't gotten around to using it...mainly because I didn't have an engine I was willing to take the plunge with ya might say.

I figure I can mount the engine to my test stand, and make a pointer out of 1/16" music wire or welding rod.

I figure I will need to make a piston stop first, and use it before removing the head, to find true TDC, then adjust my pointer, correct?

Thanks guys

HighPlains

My Feedback: (1)

You can just push the piston to the BDC (much easier on tight ABC or AAC engines) and add/subtract 180 degrees. When the pointer is on mark, tighten the prop nut. The reason for removing the backplate, head, and carb is so you can backlight the inside of the engine to accurately determine where the ports are uncovered on the sleeve and crank.

proptop

My Feedback: (8)

Ahhh...o.k.
That sounds logical...and easier...

That'll get me started...

I don't want to hijack this thread...but I plan on using a Macs "Pre-Tuned" muffled pipe set-up for the .46FX and TT .46 Pro.
It didn't seem optimal for either of those engines? Hoping I can get it to work better w/ the .40SF?

I'm thinking I will more than likely need to play w/ the header though, or get another and trim it.

combatpigg

My Feedback: (3)

Some guys claim they can feel for true TDC, but with even a new engine there is 5 degrees of piston dwell either side of TDC. The most sure fire way to find TDC is with a bar bolted across the cylinder, with a screw that is threaded into the center of the bar. Now play with the screws' height until you can barely stop the piston either side of TDC and take note of when this happens at the degree wheel. Keep adjusting the degree wheel until you get the love taps between the screw and the piston zeroed. Now you are ready to take some measurements.

skull1971

Details, now this is not specific to model aircraft 2c's but, 2-stroke engine theory is the same

for piston-controlled intake ports, .00014 to .00016 sec-cm2/cm3
for transfer ports .0008 to.00010 sec-cm2/cm3
for exhaust ports .00014 to.00015 sec-cm2/cm3
for rotary-disc intake valves .00018 to .00019 sec-cm2/cm3

what does this mean? well its the relationship between the volume of the cylinder (cm3) and the size of the port in question (cm2) and then multiply it by the total time in seconds that port is open.

Next we need the total rpm's of the engine to figure the time the ports are open on our test subject, a OS 40 sf? and then a whole lot of math, and this is just to figure our start point. Well,, er,, uh,,, part of it.

proptop

My Feedback: (8)

C-P...I was thinking that if I could find a 1/4 X 32 bolt, and put a nice radius on the end so it wouldn't nick the piston, I could establish true TDC before I remove the head.
I have done this on full scale aircraft engines when I went to aircraft mechanics (A&P ) school, but that was 26 years ago (already!) and I need to re-fresh my memory...
And, it was dealing with 4 stroke engines of course...but finding TDC is a relitively easy part of all this...

Now Skull...what you're talking about is calculating port / I. runner flow velocity, or the actual amount of mixture flow thru the cyl, correct?

I do most of my mods by the TLAR (That Looks About Right ) method, and after doing some 2 dozen or so model engines over the years, I have certain things that I know will work, but they haven't yet entailed changing the timing of the ports in the sleeve. (mainly because we're talking about ABC or ABN engines, and I don't want to nick the plating and start a peeling problem )

One of my favorite mods is to put either a chamfer or radius on the lower "lip" of the front and rear I. port openings on the outside of the sleeve. Adding a "guide channel" as it's called in r/c car parlance is another...

Putting a radius or "bell mouth" on the crank's induction port opening into the crankcase is another...and I have experimented with various "Turbo" cuts too...

I've done the above mods to a couple of Webra .50's and a couple of Evo's, a .61FX, and several others...
Picked up anywhere from 200 to about 400 r.p.m. with those mods...now I'm looking for more.

Edit: The biggest increase in r.p.m. I've seen is with an RJL (Kraft ) .61 and that was close to 500...
The testing was all done on the same day, to get as repeatable results as possable, w/o weather conditions entering the picture. Actually, by the time I got done working the engine, and re-tested after the mods, the temp. was a little warmer...

glowplug50

Use a Performance Specialtiels pipe. Bolt on, no clamps, great oerformance.

proptop

My Feedback: (8)

Yeah, I know...got one on a Webra .50..works great
Gonna use the full length pipe because I already have it and don't wanna spend the $$

skull1971

The amount of mixture, air/fuel throu a port at a rated rpm. The numbers above are I think for a 7000 rpm engine. Advancing the port timing will do almost nothing, compaired to changing the shape for better air flow.

glowplug50

You can tweak, you can grind, you can even cheat if you have a mind to, but practice will make you the winner.

proptop

My Feedback: (8)

You are correct sir (Ed McMahon voice here )

You'd think "pro" slot car racing is easy, wouldn't ya...hahaha...it ain't...
Driving / flying skills and consistency are key...

Being able to turn consistently fast laps without commiting a major error...

skull1971

Amen[sm=angel_smile.gif]

streetrodder

My Feedback: (4)

Thanks for the info sorry I haven't got back my power was down for a few days. High planes I understand thing better since you put it into terms I understand I can compare it to degreeing a cam in a car.Prop top I see you like webras are you looking to purchase one more for your collection?Skull you state changing the timing will do nothing compared to porting how about both just like in a car if you add more air you need to add more fuel you also need to change your timing correct?

combatpigg

My Feedback: (3)

Increasing port timing duration moves the power curve to a higher rpm. More rpm is how you make more power with these tiny engines, they weren't made to be put in earth movers or tug boats. Problem is, when you do a timing mod for higher RPM, you are stuck with it and committed to using a smaller diameter prop. Reducing friction is a big one, too. There are many places where the amount of effort it takes to turn the parts can be reduced. Take a look inside a world class .40 sometime, the ports as well as the timing is huge, just don't expect it to turn a 10x6 as well as a LA .40. The type of airframe has to be considered as well, don't modify the engine for a prop and plane combo that you'll never have.

bob27s

My Feedback: (19)

Streetrodder,

Although some of the concepts in here about polishing and "cleaning" the ports are valid.... and perhaps could gain you a few rpm, their effectiveness is limited by the exhaust system. The tuned exhaust is the 2-c equivalent of a supercharger - and it is that end that can take the best advantage of timing and porting changes within an engine. Also keep in mind, that modifications, if not fully understood, can and will lead to some level of inconsistancy in the engine performance. The last thing you want is an unreliable engine.

Having said that...... and from substantial experience in the matter....

(You were not specific on your club rules. It noted that you have a stock prop/fuel to work with. I will assume for a moment that you will be using an APC 9x6 - the standard Q-500 prop for AMA424 class sport racing)


If you are getting into club racing here, do not bother modifying the engine. Get two 40FX engines ...... break them in PROPERLY (on the bench, using an 8x6 prop, take them up to 17,500 rpm). Build the aircraft straight and light. Pay attention to details like surface gaps. Clean up the landing gear. Scrap the stock fuel tank and go with a bubble-free fuel system.

Then....
FLY - PRACTICE - FLY - PRACTICE (repeat, burn fuel). Learn to fly the race course.

To be successful in racing, you need air time - and 100% reliable equipment - and more air time. Winning comes from flying the course well, and finishing every single heat.

So my advise here: Skip the engine mods - and spend that time and effort setting up the stock engine properly, and learning to race (not learning to fly, learning to race) the airplane.

You can have the fastest engine in the world, but every time you touch the control sticks, it is like putting on the brakes. Racing means a trimmed, balanced, smooth plane, with smooth pilot and control inputs, smooth course flying, and minimal corrections.

For reference, a compeitive 424 Q-500 setup, with a Thunder Tiger 40 pro, stock, unmodified - with stock unmodified muffler, APC stock 9x6 prop, completes the 2 mile race course in appx 1:20. The engine, properly set up, will typically turn the prop 16,200 - 16,500 rpm on the ground. The OS40FX performs very similarly.

funflyerf4

My Feedback: (6)

How can I fit a nitrous bottle in my F-16?Hmmm hey maybe a center tank.

skull1971

drop tank would be cool