400 RPM is worth going after.

Yeah, I think it's well worth it too! The most time I've spent doing the work is only about half an hour, or maybe 45 minutes if I really want to make it purty (polish it a bit more )

I think the idea of experimenting with the length of the intake tract is also a good one...

Hmmm...how could we try that, and keep the carb out of the prop...

A shallow "elbow"?

proptop.thanks for backing me here I have run the 12-4 and the 12.25-3.75 and I get 13500 and 13100 on them. I have a mini tuned pipe on the mvvs and the tower muffler won`t fit, so my next option here is a portjob to squeese out even more power although as Dar Zeelon pointed out my engine will wake up more after some more runtime. I wrote that I never had ruined an engine before..well thats not entirely true. I modded a Dynamite mach 26 car engine this way and added a 9mm carb as opposed to the 7mm stocker.that engine ran like a scalded cat and the rod snapped after a gallon of fuel.way more rpm than it could handle, so I call that modding successful

the above goes for my mvvs 49. my now ported asp 52 gives 13400 peak on the 12.25-3.75 on 15% fuel

I was wondering about a velocity stack made of some flexible polymeric material that could be easily slipped over the carb inlet & could also be easily cut to to adjust the length. Kind of like the silicone rubber exhaust extensions. We would need a removable bell-mouth at the inlet.

I think that would be much easier than mounting the carb on a long inlet runner & trying to cut that back in steps (throttle linkage would be troublesome that way).

The elbow (metal) idea could be used to "tip" the carb up vertical, so that the polymeric stack wouldn't get too close to the prop.

Like a tuned pipe, this would be RPM specific, but I think that it could be quite effective, particularly if a synergistic effect could be tuned in with an exhaust pipe. Tricky stuff.

sounds a bit tricky yes, but if you go ahead and experiment with that be sure to write about it here on the forums. I think its very interesting stuff. If I`m not satisfied with power after modification on my 50 sizers I will shoehorn in my fx 91 in the 40 size plane and start modding on that one too.Already done the muffler on that 91, but the sleeve is up for a treatment[sm=lol.gif]

THAT'S exactly what I was going to say. The easiest and most practical way to get more power from a model engine is to buy a BIGGER one. Or even the same saze with a higher output like going from a 40FP to a 40FX. No chance of ruining an engine or voiding the warranty and it sure is easier. If you like hacking on engines and don't worry about wrecking one, have a ball whacking on one. Adding a tuned pipe will give you more power without damaging an engine also.

If you like hacking on engines and don't worry about wrecking one, have a ball whacking on one.... This is exacly why I do it. I also learn alot as I go. It`s just a hobby, my life will go on even If I destroy an engine

That's a no-brainer, but it is neither interesting nor challenging.

As long as you do not alter the intake or exhaust timing on a two stroke engine and do not alter the squish band or combustion chamber design, then the idle / transition properties will more than likely not be affected too much, if at all.
I have modified many engines in the past without altering the stock timing and have had some results better than others. A lot of it depends on where in the rpm band you intend to do most of your flying. So, smoothing out and opening up a lot of ports in an engine that is used mostly from off idle to mid throttle will have a lot less (if any at all) affect on power to the prop in the low throttle settings where the intake velocity is not very high. If you are flying at high throttle settings most of the time like in combat, then the extra few hundred rpms will be noticable.

Everything like Dar has pointed out will no doubt eliminate all factory warranties.

I think that the modification is popular on car engines because many manufacturers either do not warranty engines or limit warranties to 30 days.

muahaaahaaa. I`m in love! this morning I pulled the sleeve on my mvvs 49 and dremelled it. I had measured the top rpm on the 12.25 prop on faI fuel at 13000 last time I flew it.Today after my grindig I measured the rpm at 13500 It will still develop even more power as I go through a couple of gallons. It now has more power than my asp 52 and vertical pull was even better.It maintained rpm`s and just climbed and climbed. My asp looses rpm`s as it climbs straight up. You guys with all the formulas how many % increase in power is going from 13000 to 13500 on the 12.25-3.75?? It all took only 30 minutes

asmund...
I have an MVVS .49 too... I rotated the cyl. for rear exhaust.
Check the port / bypass passages for alignment to the lower part of the crankcase.

edit: got "timed out" there?! and that error message came back again?

Anyway, I think the outer finned cyl. passages are designed for a better match w/ side exhaust? They were not lined up as well with the exhaust to the rear.

Before I type any more, let's see if this gets the error message again...

How does 15,100 with an 11 X 5 mas on 10% sound? I didn't check it before the mods, because I didn't want to run it, then disturb the piston/liner wear pattern when I took it apart to modify it.

BTW that was "open faced" so I will have to try it again with the mini pipe that came with the engine the next time I run it.

I also got that same r.p.m. with a Mac's fuse. top header, un-cut...what does that mean? I don't have very much experience w/ tuned pipes. That means it's too long, right? I need to get some more experience with tuned pipes.

BTW...I don't consider it "hacking"...I consider it an attempt to optimise the stock set-up. You're basically just port matching and rolling the edges. Adding guide channels is a little more advantageous, but it's a mod that apparently works well in the car engines?

After seeing the insides of a lot of r/c car engines, and doing mods on various 1:1 car cyl. heads for many years, I know a few things about flow velocities, etc. etc. etc. but I'm always learning something, especially here talking to you guys!

Edit again: Forgot to say that the MVVS .49 I have has the older 7mm carb. and it has the natural alum. head.

All this has made me want to do some more testing on the .49...
When the weather gets to about the same temp. as it was the first time I tested it I will try some more props and the pipes.
I will have to check the "diary" for the exact conditions, but IIRC it was an early summer morning about 70 degrees.
I am at about 500' above sea level here.

Pretty expensive way to learn something. Also a shame to ruin an engine just to see if you can make it more powerful. Funny how all the big boys who are into racing including pylon racers and C/L speed guys use tuned pipes to gain more power. Think maybe they know what they are doing?? They sure aren't chopping up their engines. Guys like Dubb Jett design their engines for max power without the need to chop on them. Most R/C engine manufacturers design their engines to be as powerful as possible while still maintaining reliability and a good idle. They have to to be competitve. So far none of you have mentioned anything about the crankshaft and venturi. Back in the days when I ran Rat Race and C/L speed that was one of the ways to get more out of an engine. It was also one of the "tech tips" Hi Johnsom put out for improving the power in a Johnson engine.

Tom,


15,100 RPM on a Master Airscrew 11x5 translates to 1.36 HP.

13,600 RPM on a Bolly 11.5x6 translates is 1.57 HP, so you have gone beyond the maximum this engine can give.
I believe the actual HP peak is somewhere around 14,400 RPM, so the 11x5 MA is over the hill, so to speak.

The RPM with the 11.5x6 Bolly was taken with the new, larger 8 mm #3219 carburettor.

Dar Zeelon, how about my 500 rpm increase, how many % increase in hp is that? And what hp am I currently making at 13500 on the apc 12.25?? I don`t have the formulas to calculate this myself

Just how are you figuring these HP numbers??

Asmund,


Go to [link=http://home.wanadoo.nl/pereivers/prop-power.xls]this web site[/link] and open the PropPower within it.
You can calculate it by yourself, for any prop that you choose.

Make sure you select the correct prop factor.

I was not able to open it. I am missing some program on my pc to read this page. Well it doesn`t matter that much, the increase in power was certainly noticeable when flying the plane, so I am very happy

Thanks Dar...that was the only prop I tried on it, and was during break-in. I don't usually use mas props (APC's ) but I figured the 11 X 5 would be a light load for break-in.
That r.p.m. was only peaked for as long as it took to get a tach reading, and I just ran it a few times and put it back in the box (after adding after-run oil of course )

14,400 huh? I'll see which prop it takes to get the r.p.m. close to that.


I have a Webra .50 w/ Ultra Thrust muffler (did the sleeve mods on that one too ) that will turn an APC 10 X 8 at close to 16,000 on 15%...what would that figure out to be?

(I had a problem with the calculator too )

asmund
You had a 12% increase in HP. All you need do to work it out is take the new rev figure and cube it (multiply it by itself 3 times) then divide that number by the original revs (after you've cubed it as well).
Usually this will overload a calculator so just take the first 3 or maybe 4 numbers of your revs. For yours I used 135 and 130 so it looked like this..
135x135x135/130/130/130=1.119879 (call it 1.12). This means it's now developing 1.12 times more power than the original revs or 12%.

No we would not, at least not enough to make it profitable. The proof is in the pudding. Two automobiles come to mind. The Mecury Marauder and the Pontiac GTO. Both cars were beyond resonably priced and both cars would eat contemporary cars like Mustang GTs and most Camaros for instance. Most people wander around undereducated about what they are buying. We in the U.S. have adopted the bazarr mentality. If it's cheap we will buy it in other words.

Won't buy expensive engines? Oh yes we would. Have you checked the prices of a JETT or a NELSON a YS etc?? They are in business and their prices are out of this world. A nelso 36 Combat (C/L) goes for about $300 bucks and he sells all he can make. Dubb Jetts engines are right up there in price too. Try the Schadels if you want to see high prices. They ALL sell. YS sells like there is no tomorrow and they are the most expensive and powewrful fourcycles made. Funny how OS is still the best selling engine line in the world and they are some of the highest priced engines on the market.

Non RC related content removed.

After re-reading my previous post, it reminded me of a couple of other modifications I have done...

The crank can be modified by bell-mouthing the opening of the inlet passage (the outlet, where the mixture flows into the case )

Another is a "Turbo" cut-out from the above mentioned bell-mouthed inlet blending into the counter weight. If you don't know what this is, check out the link I posted to the Savage .25 engine modificatons.

Another is a radius on the leading edge of the counter weight. Be careful not to remove much material, so you don't throw off the balance. (too much )

JB weld in the induction passage just under the carb and blended to a nice radius works well on engines that don't have a ball milled passage.

I did the above mods to the aforementioned Webra .50

Another mod is blending the transfer passage into the port at the sides of the port -- this is in addition to radiusing the lower lip of the port.

Often enough, there is a step in the casting at the port, wherein the passage is slightly narrower than the port itself.