Sport_Pilot

Seriously I think you have it backwards. Valve overlap on 4-Strokes don't affect the pipe length, though the exhaust valve opening does. But knowing that the pipe will be tuned with negative pressure on the other side of the valve may affect the overlap. A high positive pressure would cause too much exhaust to go back up the intack at low speeds, thus the lumpy idle and poor low speed response.

Sport_Pilot

I mistype more than I miss spell. I am a fairly fast touch typer.

I was trying to point out that the effect is much more dramatic with a 2 stroke. In fact about the same as a mild supercharger, which a YS is. But I'm sorry that that confused you. The effect is not nearly as dramatic on a 4 stroke, and its to create negative pressure in the exhaust system, not positive as with a 2 stroke. Inprovements are there and cam timing has an effect. However its the opening and closing timing of the exhaust valve and its duration that is key here, not the overlap. Course it's been a long time since I delved into this and if there are newer tricks to this I wouldn't mind if you enlightened me.

Hobbsy

My Feedback: (102)

Sport Pilot, if you reread my post #23 you'll notice that I made no mention of pipe length at all.

Sport_Pilot

I said it is mostly miss typing. I know I am not a good speller either. Though I think it very rude to be picking on spelling on any internet site. I almost never proofread my posts, never thought it was important. And since it bothers you so I may never proofread them.

Dugster

Oh, now this is getting good.

This one's ignorant, that one's arrogant, and that other one's rude. Keep it up guys (and girls).

But what in the Wide World of Sports does any of this have to do with HOW to get more power from my little four stroke glow engine?

Sport_Pilot

I am reviewing past posts to determine where we went off track. In reviewing the posts I see I was a bit curt with you. I apologize for that response, but I was a bit upset with someone who could not make a point, but ridiculed instead. You are right tuning a pipe is not something to recommend for the sport flyer, I was responding to ScorpionJack's response that they are asthetic only. You had the best response in that increasing nitro is the best way to increase power. I might add if you do it might be a good idea to add an extra head gasket and a colder plug. Anything else would involve regrinding the cam, something which would be beyond even the more advanced modeler.

Dugster

No, I wasn't referring to my own post. It's been 12 posts now since anyone had anything useful to add. I was just pointing out that the thread was drifting into name calling.

As for the rest of the drivel here. You all seem to continually expound theory that may or may not be true, but you never suggest any practical solution. What the original poster asked was HOW. So far there's been only one good suggestion, more nitro.


Ladyflyer,
Several posts ago you mentioned "making a little light". Are you now getting a little thin-in-the-skin?

Scorpionjack

(LF) put out a very nice and informative responce to your original question(Stating Nitro is the most effecient method of increasing HP from a 4 Stroke. I previously wrote that 4 stroke's gain nothing to speak of from Tuned pipes, that we that use them on a 4 Stroke only use them for astetics not performance. The only performance attribute from a 4 stroke exhaust is (IF!) it's restrictive then the increase would be minimal but noticable. Then someone! Won't mention names got into a tangent reference Car's!!!!! Automotive engines are not Model Aircraft engines! Let's get to more pressing issues this one's dead.!!

Taildrager-inactive

My Feedback: (11)

AA dragersters are super charged with a blower into the injector manifold, there fore only zoomy short pipes are only needed to get exhaust away from the body & aimed at the ground. Unblown injected engines use matched length of pipes from the exhaust port into a collector which is then tuned (different lengths) to adjust the torque curve in the RPM range. A longer pipe & collector length will move the toqure curve to the lower RPM's, espcially when using a auto trans which leaves the line at a lower RPM than a manual trans racer with a clutch. I have not personally used a dyno to know what these lengths are, but it does work.

Dugster

Is it me, or are there some earlier posts missing now?

Once again, I was just trying to put some water on the flames with a little humor. How was I supposed to know the cup was full of fuel.

LF,
Where did I call anyone a name? You seem to be taking this personally. Why? I have never called anyone, any name, anywhere, in this thread or any other.

Taildrager-inactive

My Feedback: (11)

Ok guys & gals, lets cool off, & get some perspective. With much to learn in so short of time, I'm 58, he he, lets keep it civil, Thanks

downunder-RCU

OK, I'll dive into this and hope I don't get flamed (and unless there's been some editing or deleting of posts I can't see what LF said that was so wrong).
Far as I can see there's only a couple of things that can be done with any simplicity. Reduce back pressure in the muffler but try running without muffler pressure first to see if the carby can handle it...it should though. More nitro (as suggested) except I'm not fond of nitro. Raise the compression but be careful that you don't run out of tappet adjustment and that the rocker angle doesn't get changed too much. Try different plugs to find the optimum heat range. Fit spark ignition but still use normal fuel.

Dugster

Well I'm glad to see the thread is getting back on topic and things are calming down. On reflection (and rereading) this morning the entire thread looks much better after someone deleted 3 or 4 posts. It also seems to make me look rather silly since my remarks are now out of context.

Once again I'm sorry to have caused any hypertension with my first attempt at sarcastically redirecting the thread back on topic.

Hobbsy

My Feedback: (102)

LF, Thanks, your explanation is excellent.

Sport_Pilot

Cannot find it for 4 stroke tuned exhaust, I used to have books on the subject but were destroyed in a storm. I could recreate it but I am not sure the sonic wave should be just past the exhaust valve when it opens or well down the pipe. But as I recall it involves only the exhaust valve opening and the RPM of the engine. I suppose LadyFlyer and Hobbsy are thinking that the improved scavaging helps transfer fuel from the intake through the overlap period. Maybe so but this is small as the piston is at the top of the stroke, though the improved scavaging and the overlap would be usefull to maintain momentum to allow a tuned intake to help push air through the intake stroke.

I thought the argument was that our 4-Strokes do not have enough overlap to make a tuned exhaust system work. I stated that it will work and overlap doesn't have anything to do with that. Perhaps I should have said little to do with it, but still I wonder if we are not confusing tuned exhaust and intake here?

Sport_Pilot

As I recall Hobbsy was saying that tuned exhaust will not work with our engines because they don't have enough overlap. Even an economy engine has overlap and can benifit from a tuned exhaust, our model 4 stroke engines have much more overlap. Enough that they have a mild lope, but not so much that it hurts low speed throttle response. Of course I know that an engine with more cam (to include overlap) is going to have more power increase from tuning, but it would likely be close to an even percentage of the power available before tuning the exhaust.

There are formulas for figuring out the pipe length, at least for a starting point. If one knows the timing of the exhaust opening it is an easy matter of figureing out where the pressure wave would be 720 degrees later. I wish you could be more helpfull, I just don't remember if the pressure wave should be reflected just past the exhaust valve or further down the pipe when the valve opens.

Sport_Pilot

Yes one would have to figure for the density of the exhaust gas. But your statement that the exhaust valve must remain open at least 45 degrees past top dead center would be something I wasn't figuring on. I thought the gain was from creating negative pressure just before the valve opens and allowing the sonic wave to keep pressures lowered through the initial part of the exhaust cycle. Am I confuesed? Or maybe I am thinking of a milder form of exhaust tuning used on lower powered engines and you are talking of something more advanced used for ultra performance vehilcles.

What I would like to do is figure out a starting point for a tuned pipe, and maybe experment a bit. I have a TT .91 that I think has a bit more cam than OS or Saito. Know where to find the cam timing? Well I guess one could get it from a degree wheel.

wildthng

If you want more power get a 2 cycle like a 50 webra speed(and save wieght)Heck,you can use 5% and it will smoke your OS.Oh no could start an arguement here,forgive me oh 4 cycle god.

ChuckN

You know, I was going to say the same thing!

Realistically, if you want more power, save yourself some money and get a 2-stroke. Sure, messing around with 4-strokes can be fun (especially motorcycles ) but unless you can grind a new cam for your model airplane engine your options are very limited.

downunder-RCU

Possibly the only 2 strokes that approach the running characteristics of a 4 stroke are the C/L stunt engines which are ported for high torque at lower revs. Unfortunately they're hard to find and still don't have that lovely sound
On the plus side though they're very light..I have a Stalker 61 which weighs about 12 ounces and is machined to accept an OS carby for R/C use.

ChuckN

Part of the reason motocrossers are going to the new 4-strokes is because the AMA allows the 4-strokes better than 200cc more displacement

Wonder how a 400cc 2-stroke would compare to a 400cc 4-stroke?

BTW - AMA also stands for American Motorcycle Association

Hobbsy

My Feedback: (102)

In the 70's Maico made a 501 two stroke that had 54 HP at the rear wheel, I wonder how that compares to those big thumpers of today. I suspect it was much lighter than they are.

wildthng

As a motocross,enduro and harescamble racer i can tell you that our 2 strokes are far from cheap.You cannot compare the model airplane engine with a motorcycle engine.There are many differances in a modern 2 cycle that are not in our airplane engines.The power valves would make our small 2 cycles have a broad powerband like a 4 cycle but would increase the cost also.The big enemy of the four stroke is the extra heft it carries.I could really get into this but this is not the correct thread for it.I like both types of engines but use them for different purposes in my airplanes.In my bike the 2 cycle is more competitive so that is what i use.

Hobbsy

My Feedback: (102)

LadyFlyer, I haven,t got my new YS .91 FZ yet, it will be here on the fifteenth. Will it have good instructions for setup and break in, also what fuel do you suggest for break in?
Thanks, Dave

Hobbsy

My Feedback: (102)

OK, I won't lean it I promise, but after it's broken in I'm going to lean on it. I'm getting an UltraStik 60 for it. It should do lots of neat stuff. Thanks again, Dave PS, say hey to your man for me.