Okay guys & gals.. I have yet another challenging question. This question is aimed at 2-stroke glow engines, mainly the higher-revving ball bearing variety. Say someone wanted to spin a 10x8 at the same rpm as a 10x6. This would (as I understand) provide more thrust at a given RPM. What would one have to do to the port timings to achieve this? I have a decent understanding of how the porting in the engines works, but I'm not well-versed in what changes to port timings have on performance. Could someone give me a basic rundown?

You wont get a significant amount more thrust just changing the pitch and keeping the RPM's the same.
Diameter is related directly to thrust.

You *might* get more airspeed if the airframe is sleek enough because the pitch speed went up.
I don't think you will gain a significant amount of power just altering the port from stock.
You may make some minor improvements but...
Why not just go with a larger displacement engine?

Well, as I understand... a 10x6 will produce less thrust than a 10x8, but a 10x8 would spin slower on the same engine. Generally, the higher the pitch, the higher the speed... right? If you could spin a 10x8 at the same speed as a 10x6 on the same engine in the same craft, wouldn't it go faster? (For now, lets leave all of the nitty gritty details like friction and aerodynamics out of the equation.) I'm using the 6" pitch and 8" pitch as a simple examples. I wouldn't imagine the speed to be that much better making that small of a jump in pitch.

Nope. Speed is not thrust. Thrust is how many pounds of force trying to move the airframe forward.

Change the diameter and leave the RPM the same you change the thrust.
Change the pitch and keep the RPM the same and the possible speed changes.

Pitch is the number of inches of forward travel per revolution.
An 8 inch pitch will try to move 8" per revolution.
A 6 inch pitch prop will try to move 6" per revolution.

So you see, there is much difference in speed potential between an 8" and a 6" spinning the same RPM.

When you increase the diameter, the thrust goes up if the RPM and pitch stays the same.


Assuming you could turn both props at the same RPM....
6" per revolution at 13,000 RPM would get you about 74 MPH potential.
8" per revolution at 13,000 RPM would get you about 98.5 MPH potential.
I'd call that a significant difference.

I say potential because the airframe drag may prevent that max speed from happening.

I know speed and thrust are different. In my mind, it would make sense that: if you increase the amount of thrust (which is the amount of force to move forward) you would incidentally increase the speed potential. If a given engine can spin a 6" pitch prop at say 14,000rpm and an 8" pitch prop at 12,000rpm (hypothetical example) It doesnt sound like there would be much of an increase in speed, but if this same engine could spin that same 8" prop at 14,000rpm wouldn't the speed potential be higher?

Yes, but not because of more thrust. It is because of pitch speed.

You could have a 20" low pitch prop making 25lbs of static thrust that would only move a 5lb. plane at a top speed of 40 MPH.

Pitch is where the speed comes from and diameter is where thrust comes from.

Okay.. I understand now. (Someone had explained to me about pitch speed in the past but I don't recall getting into the thrust vs. speed correlation at that time) With all of this understood, getting an engine to perform better on the higher pitch prop would mean increasing its performance or its ability to do work. If a given engine has a given ability to do work (stock), increasing its ability to breathe would increase its ability to do work (changing port timing). What I'm trying to understand is what changes would need to be made in what areas to increase its power output. There is a long debate of modifying engines, mainly in the car engine realm. I have seen dyno graphs of two identical engines make totally different power curves (and higher power output) - modified vs. stock form. I know that many budget engines are ported conservatively for user-friendliness and fuel economy, but I know its possible to make port modifications to increase performance. I guess what I'm trying to learn is, what effect on performance would (I'm just using examples here) opening the exhaust sooner or opening the crankshaft port sooner or keeping it open longer.

It is assumed that Nitro content is maxed to what the engine can handle, and the given RPM ability is within the limits of the props being used. Also, to keep it simple I'm going to say a full-wave tuned pipe isn't an option.

A great exhaust system will have way more affect on performance than anything you can do to the port.
The manufacturers would just change the ports if it was just that simple.

I'd go for bigger displacement.
Alternately, [link=http://jettengineering.com/]http://jettengineering.com/[/link]has this figured out and maxed out but it comes with a pricetag.

With a lot of "sport" engines, the manufacturers dont go the extra "mile" to refine their engines for reasons I outlined earlier - ease of use, and fuel economy, and one I didnt mention before - cost. I've seen vast improvements in performance by modifying the port structure of an engine. I already have plans to buy a Jett engine for one of my airboats, though this thread was started to get an idea of what changes would do what to a run-of-the-mill sport engine. I know the exhaust system has a lot to do with performance, but thats not what this is about. Perhaps I'm going about learning more of the internal workings of glow engines in the wrong fashion. Also to note: I wouldn't be going about this to make a "sport" engine perform like a Jett. I know thats not reasonable nor realistic.

My shop teacher from high school always said "The easier an engine can breathe, the more power it will produce.". I was simply trying to understand how this principle could be applied to a small glow engine. Again, I am probably going about this in the wrong fashion.

A lot of the things you know about gas 4-stroke engines do not apply to 2-stroke glow engines.

Open up the exhaust too much and it won't hardly run. It needs the back-pressure to operate correctly.
That's why a pipe has the biggest affect on performance.

The matter between pitch and thrust has been subject to myth. There has been an article by leading characters that stated pitch has no influence on thrust, because the pitch did not enter prop efficiency formulae.</p>

Let me tell you this:</p>

Thrust is directly related to power (more or less) So high pitched props, because they can absorb more power, will produce more thrust if turned at the same rpm as their low pitch brother.</p>

Another myth is stalled props. That will not happen as long as the pitch is not larger than the prop diameter. Reason: The prop creates it's own vortex with prop air entry speed. This speed keeps the blade angle of attack within the angles at which blade foils will stall.

As to engine porting, this can fill books. See http://prme.nl/forum/viewtopic.php?t=443</p>

I agree with Pe that a higher pitched prop, if turned at the same revs as a lower pitch of the same diameter, will give more thrust but of course the engine will need to be developing more HP to do that. The usual statement that thrust sepends on diameter and speed depends on pitch is really a gross over-simplification. Hence the myth .

Engine breathing (as the high school shop teacher said) is the first step in getting more power and that kind of equates to a "port and polish" starting at the carb and working through the rest of the engine. In the OP's case, wanting more HP at the same revs, the goal here is to increase the torque at the same revs (which also means getting more HP at those revs).

The second step, after the "port and polish", is to make sure the combustion chamber area makes the best use of any increase in breathing. That involves things like having a close squish area, correct size squish band and combustion chamber shape (all of which add to give the highest compression the fuel can handle).

I'm not sure what was intended to be meant by a 2 stroke needs back pressure to operate correctly but the less back pressure in the exhaust the better. An open exhaust is best but way too loud (noting the OP didn't want to go to a full tuned pipe). If it referred to increasing the exhaust timing then that'd be the wrong way to go as far as increasing torque at the same revs.

And it's likely that I'm over-simplifying things here a bit too .

As Pe said the issue of port design can fill books, heck, maybe even libraries. I've been playing with the induction timing events in two and four strokers for forty years and I'm still learning. The best thing to do is buy a book such as the one by A. Graham Bell called "Two-Stroke Performance Tuning" and read up. Expect to still be learning many, many years down the road. There is also a good read available called "Two-Stroke Tuners Handbook" by Gordon Jennings.

Pe has the Design and Simulation of 2 Stroke Engines by Blair as a pdf at the link he posted. The 600+ pages is worth the download.

When I was asking these kinds of questions, Duke Fox said to me "You are about a dozen ruined engines away from from figuring out what works really good". He was right.

Well, being that I am engrossed with the radio controlled hobby, I'm always looking for ways to enjoy it better. By nature, I'm a tinkerer. When I find something I'd like to do differently, I try to do some research before I jump into anything because its better to go into something kind-of knowing what you're doing vs. not knowing what you're doing at all.

I've ported some car engines some years ago - all with better results than factory. Though I didnt change any timing, I merely opened the sleeve ports up and smoothed the runners some inside the crankcase. I had noticeable gains. I have talked to many engine "Modders" over the years and every one of them has said not to mess with the exhaust port in any way. I'm more concerned with the crank port, boost port, and transfers. Making some adjustments here is bound to have some improvement in performance. In my hypothetical scenario, a drop of 2000rpm is roughly 15% - I would think there'd be enough room for adjustment to make up this difference or come close. Maybe I'm wrong.

Thanks Barry, I had not visited the link but did after your post and downloaded the links as I had the paper versions but not the electronic versions and a special thanks to Pe for this help. I do greatly appreciate it guys.

What? Only a dozen?

I knew a guy about 25 years ago that had a performance Jet ski business. As one of his services he offered porting. He must have done hundreds and I never seen any that really cooked except for one he finally hit upon. As luck would happen, the costumer seized the engine so he got it back and I got a chance to have a peek at it too. Even thou he had a perfect example in front of him, he still couldn't emulate the performance on other engines he ported. (this guy was dangerous with a porting grinder in his hands) I took measurements and noted port angles and directions, realizing less is more. When I got home, one of my engines was pulled from the hull and ported like the one I had seen. It was much stronger but not quite as strong as that miracle motor.

On one of my 55AX's, I radiused the inlet notch in the crankshaft (no change in timing) and radiused the bottom edge of the transfer ports on the outside of the sleeve. The bar that the metering tube fits into in the carb was air foiled(thinned) as well. The engine was started and blew plugs right away, I tried a cooler plug, better but it still wasn't happy. I went to FAI fuel and #8 plug, BINGO! A bit more power and rpm as as it had on 15% but now with no nitro, it did have to idle a little higher tho. This engine was flown like this for about 1 year till it went in from servo failure. I needed to take it apart to get the mud out. At this time I raised the exhaust port with two small notches, one to either side of the ring guard/web. This cuts into the nickle plate so it's a peel risk, even if you do everything right. The engine now could be ran on 15% and gained 1000 rpm over stock when ran on the same performance muffler.

As you can see, a very, very subtle port change can have huge effects, and if you don't know why you are cutting metal here or there you will be making junk.

Altering port timing effectively increases torque at higher or lower RPM.  Thus making it a higher or lower RPM engine.  So you are not likely to make an engine stronger at a specific RPM.  To do this you would need to decrease friction, increase breathing at the specific RPM, increase the effective compression, or increase the potency of the fuel.  You are not going to make significant improvements in decreasing friction in a ball bearing engine.  You can improve breathing by increasing the carb size, improve flow to the ports, ie larger bore hole in the crank, or smoother flow from carb to port, larger ports in the case, or wider ports in the sleeve.  But these will be minor in most engines.  You can increase both the breathing and the effective compression by installing a tuned pipe.  And the best way to increase potency of the fuel is to increase the nitro percentage.

Is Blair's book out of copyright? It doesn't show up as full view on Google yet? It's a good one.

hush now, or I must remove it again.

I just did change some. The bots were at it though when I checked! I hope I fooled them.
BTW, unless patent rights, copyright never ends during the author's life time, and for some time after his death. That is what can be observed from the body snatchers in the pop music world.
What I write here is mine. MINE, MINE! and nobody elses. Just being sarcastic, but it is the way it is. No judge will change the world by defining just how much own ideas are needed to make a publication unique. Intellectual property cash flow just is too large to tamper with.

Heck,
I nearly broke all engines I laid my hands on. Better is the NME of good, but you cannot define the border if you do not cross it.
In the process you learn a lot while paying dues.
While tuning MX engines, I did not know and just fumbled in the dark. Yet my engines performed very well and did finish the race most of the time if the rider did his prepatory work. Wish I could turn back time.

another way to gain power is getting rid of as much parasitic drag as possible. for instance, see if you can find ceramic bearings to replace the cheap crappy steel ones. the difference is like going from a bushing'd engine to a ball bearing'd one. polish the crankpin and wrist pin, put a ceramic coat on top of piston and inside the head. the less energy that is soaked up by the head is more energy pushing down on the piston. its a bit of work, but like everything else its all in the details.

Well, I've heard the opposite in the bearing department. From what I've read from others' experience with bearings is Ceramics wear out much faster than steel bearings in higher rpm situations.

If You use a mac header and a tuned pipe on lets say an OS 91VR DF engine with a mac 15cc pipe and the header cut to the max RPMs for an 10x9 or 9X9 or 9x10 APC prop You'll turn 20,000 + rpms in the air, however there is no midrange throttle response.
You run the engine at idle or (on the pipe) 3/4 to full throttle only.
Zoooooooooooooooooommmmmmmmmm.

So The easiest, quickest, cheapest HP gain is to get a mac header & tuned pipe then decide what prop You want to use and cut the header to the correct length and go have fun.
Some engines respond better to tuned pipes than other's, also there are tuned pipes designed for better midrange power so You have better throttling and still get a good boost in power.
Jeff