Block the boost ports?

There has been some talk about blocking the boost ports in a schneurle engine. What material do you use? JB Weld?

I have used red RTV sealant with much success. If you ever want to convert the engine back to original, it is much easier to dig out RTV vs. JB weld.

>I have used red RTV sealant with much success. If you ever
>want to convert the engine back to original, it is much
>easier to dig out RTV vs. JB weld.

Ah! good plan...

>I have used red RTV sealant with much success. If you ever
>want to convert the engine back to original, it is much
>easier to dig out RTV vs. JB weld.

?? What's RTV sealant, where do you buy it, and how do you tell which is the boost port(s)??

The boost port in most schnuerle's is opposite the exhaust. It opens after the transfer ports which are approximately 90 degrees on either side of the exhaust.

Note that the newest LA 25,40,and 46 do NOT have boost ports anymore, so they are already "converted".

Also realize that not everyone goes for this mod. It is typically used to convert a high reving modern engine runing a low pitched prop into a lower rpm engine running a higher pitched prop. What matters for your plane is the thrust that it needs, which depends upon the engine power AND the prop efficiency.

That's all I want to say, because I am not sure what the "correct" answer is.

Alan;)

RTV is room temp vulcanizing rubber. It comes color coded, white is chemical resistant, red is high temp and clear is what you use if you don't need either of those things. Having a hunk of it in your boost port will decrease your HP.

Hey Giffy
Yes Blocking the boost port will decrease your HP Slightly but the end results justify the means. The boost port is for high RPM running something a true 4-2-4 engine break does not need. Blocking the boost port will end high rpm run away.

Good Flying
Mike Clark

Ron B.
F4Fguy

Noooo,the boost port is to shape the flow from the intake ports to improve scavenging.It may have to be redirected or retimed or both for our use,but it can be used to broaden and boost the low end or the high end power.There is essentially no such thing as"too much flow",only max flow at the wrong point in the curve for the use intended.AIR CONSUMPTION=POWER under all conditions.To make maximum use of it requires control of the flow.This in turn means not only getting the most mixture in,but sweeping the cylinder with it in such a way as to expel the most burnt gasses from the combustion space.This is the primary job of the boost port.

>Ron B.
>F4Fguy
Noooo,the boost port is to shape the flow from the intake
ports to improve scavenging.It may have to be redirected or
retimed or both for our use,but it can be used to broaden
and boost the low end or the high end power.

Ron,
Can you provide some additional information on the redirecting/retiming of the boost port for stunt use? Thank you.
DennisV

If you are interested, I'd recommend Dave Gierke's book on the 2 stroke engine. Now I can't really argue that everything he writes is correct (I am no expert--that's for sure), but I found what he said was interesting.

His claim (D.G.) was that the object of the boost port was to help push the intake flow up to the top of the cylinder. This causes the intake to take a longer time to get to the exhaust port (keeping more of the good stuff in). I thought that was fascinating, the idea that high speed/power engine porting is designed to slow down the intake. He talks about even newer porting ideas which are coming from the 2-stroke motorcycle guys.

I certainly would like to get hold of a copy of the 2-stroke "bible" by Gordon Jennings, but everyone who has one seems to be holding onto their copy, and as far as I know, there are no plans to print up another run.

Ron B.
F4Fguy
Dennis:
In general terms,to move the power curve downward(increase low end torque)you need to:
1.Decrease time area of the exhaust,i.e.drop the sleeve or make a new one.
2.Increase time area of the intake,i.e.raise the ports,or widen them(this includes the boost port).
3.Increase the"lead"from main intake to boost opening.
This will produce an engine with a power peak at lower R.P.M. Unfortunately,it will still be "peaky",with a narrow range of power,albeit generally in the range we want.At this point,go to step, you guessed it,
4.Angle the intakes upward.As a GROSS generalization,the flatter the entry to the cylinder, the narrower the peak (and the more power at the peak).The more upwardly inclined,the broader the power band,and the more "torquey".Again as a generality,the boost will require more upward angle than the mains,since the steeper main angle makes it harder to sweep the back wall.
I'd like to say there's a magic set of numbers,but different engines present different restrictions.Some will not allow a sleeve drop with out getting into sub-piston induction,others seem to run just fine WITH sub-piston induction.Some seem to require less port angle than others.I could go on and on........................
The only answers seem to be;a) to spend the time and effort(and bucks)to develop and test your own engine,or b) to buy one of the several purpose designed,fully developed stunt engines with a known track record.They may look a lot less pricey after destroying several piston/liner sets.

Nice overview sir:

> 1.Decrease time area of the exhaust,i.e.drop the sleeve or
>make a new one.

Or welding some metal onto the existing sleeve..

Don't know anyone who's done this, but I've seen pictures of a crankshaft with the rotary valve cutout narrowed with a piece of metal brazed on. This was on the Blackhead Rossi .15 in the early 80s. These had a reputation for being radically timed, to the point where it had to be turning 28K RPM before you make any power. That's not too unusual with today's F2D engines, but in 1984..

>Ron B.
>F4Fguy
>
> Noooo,the boost port is to shape the flow from the intake
>ports to improve scavenging.It may have to be redirected or
>retimed or both for our use,but it can be used to broaden
>and boost the low end or the high end power.There is
>essentially no such thing as"too much flow",only max flow at
>the wrong point in the curve for the use intended.AIR
>CONSUMPTION=POWER under all conditions.To make maximum use
>of it requires control of the flow.This in turn means not
>only getting the most mixture in,but sweeping the cylinder
>with it in such a way as to expel the most burnt gasses from
>the combustion space.This is the primary job of the boost
>port.

Without getting into a big debate, what you are saying has to do with maximum power. We are usually not running maximum power, and, indeed, the 4-2 type of run even works on the basis of not expelling the exhaust fumes on the first try which is what brings about the misfire. This, in turn, allows a better charge (when timed properly) on the second time arounnd to make the firing strokes stronger than they would be on a 2-cycle run.

Longer exhaust strokes, as you point out in your next posting, do indeed produce more torque, but only to a point. When you lengthen the exhaust stroke, you shorten the intake stroke. Also, raising the intake ports causes a lot of overlap between tryhing to get the exhaust out and the intake in. Everything involves a compromise.

Also, the idea that "there is no such thing as too much flow" is an oversimplification of what is really happening with these engines. There are actually two compression strokes going on, one in the cylinder to compress the charge and prepare for the "big bang", and one in the crank case that is used to force the mixture up the transfer ports and into the cylinder at the proper time. The faster the engine runs, the greater the compression in the case. The slower the engine runs, the quicker it bleeds out through the ports. Having the intake ports open longer without adequate crankcase compression to force the charge up into the cylinder does nothing but cause run away (engine goes lean--bam!)

As I have often pointed out, the boost port was not a part of the original Schnurle design. It was added to boost the high rpm of the engine. But we are not running our engines in the high horsepower (rpm) curve of the engine, which is generally around 14 to 16 thousand (or even above) on these engines. And when we try to slow the engine down, we give more time for the air to bleed off in the transfer ports and loose the crankcase compression.

OK, there are several ways of doing this and making it work. But on the FP type of engine, for instance, there is too much port area in the case and cylinder to work most efficiently at our lower rpms. Blocking off the boost port in this case is like squeezing a water hose when you have lower pressure. You can shoot the water a lot further. In our engines, we can shoot the charge from the case to the cylinder a lot more efficiently sometimes with less porting, not more. Look at the Fox 40, for instance, at the size of the transfer ports. Look at how some partially block the one transfer port on a Fox 35 to make it work better. (Put a wooden stick in between the sleeve and the case.)

We did experiments blocking the boost port, one transfer port, the boost port and one transfer port, etc. Blocking the boost port alone was the clear winner. Additionally I took an engine that another "guru" had mangled the ports on. That engine was lifeless, gutless. I blocked the boost port on that engine and brought it back to life. Yup, that was an engine with the sleeve dropped and the transfer ports raised. Although it was much improved, I then replaced the sleeve with a stock sleeve and it worked better.

The moral is, if 5 thousandths is good, don't think that 10 thousandths is better. We had one of our 40's once that we had taken ten thousandths too far (raising the intake ports). Many people would have been happy with it, but we weren't. I replaced the sleeve and now use that sleeve as a slug with which to block the boost ports in the cases.

There are a lot of good engines that do not have blocked boost ports. The PA series, the RO Jetts, all retain the boost ports. But I have often wondered, "what if...?" I find it interesting that there so many acolades given to the LA series of engines. Have you ever wondered why? All it is is a cheap FP lacking a boost port. Same stroke. Same bore (well, the 46 is larger bore than the 40, but you get the idea). Same timing. All it lacks is a boost port.

Ron B.
F4Fguy

I think it's too late to avoid the "big debate".With all due respect,I must disagree with almost everything you said.
Power is torque x RPM.I'm talking about usable power in a range where we can take best advantage of it.This is best achieved by pumping the most mixture possible into the cylinder at the appropriate time,then efficiently burning it.Doing so will give the maximum TORQUE at whatever RPM the engine is tuned (timed)for.Angling the ports upward tends to make the timing less critical,giving a broader tuning range and a flatter torque curve.The boost port was a later development only because the good doctor didn't think of it.Properly used it shapes the flow (at whatever speed it's designed for)and allows more complete cylinder filling.
Again,with respect,the "4-2-4"run is well documented but,it's mechanism is not.The scenario you describe is one hypothesis.There are others.The one which makes most sense to me is that the initial mixture is too rich to ignite,and during the "missing stroke"additional air is aspirated through the exhaust port.
Re lower exh./higher int.ports,at the lower engine speeds less stroke length is required for the same time,so exh./int."overlap" isn't a problem.On the other hand,when design speed is exceeded,scavenging is limited as is the runaway tendency.
As you correctly point out,two-strokes depend on crankcase compression to transfer mixture to the combustion space.This doesn't mean more is necessarily better.Very high CC pressures can actually reduce mixture delivery,as the "stuffing" required restricts the flow space.This will cause a loss of torque,as will poor mixture delivery to the cylinder, but in no way can it cause a lean condition.The transfer medium is MIXTURE not fuel or air alone.
I reiterate the the boost port is not a high speed,high RPM device.It is a flow shaping port which can be effectively applied to any speed/power regime,given proper analysis of the requirements.
Anytime you restrict flow ,you lose flow due to friction(PHYSICS 101).I say again there's no such thing as too much transfer,only poor control of same.As to port blocking,restricting flow will certainly limit RPM by simply air limiting the engine.However this is a very poor engineering solution.Much better is to decide where you want to operate,and what shape you want in the power curve,then determine the time/area and port shapes required to produce the desired result.Note that port shape and direction are as important as timing .Note also I said time/area,not degrees or percent of stroke.
I'm not surprised you found blocking the boost port better than blocking either main intake.The latter would grossly distort the flow balance in the cylinder,I'm surprised it ran at all.Also,simply dropping the sleeve and changing the blowdown,without redirecting the ports could(depending on the engine)be counter-productive.
I don't know where the .005 or.010 came from.I've said all along that all the things in my original posts were very general in nature.I would never touch an engine without first,measuring everything,then calculating the required modifications for the intended use.Incidentally,I have seen some engines which couldn't be modified without making a new sleeve.
Re the LA not having a boost port,you said it yourself.It's a cheap FP.Cutting ports costs money,as does accurately casting bypasses to match them.
Another interesting item,the timing of the LA46 and LA40 may be the same,but the larger bore will produce a higher time/area if the ports are machined to the same angular width.I don't own any LAs so I don't know if this is so.This may be why O.S.decided they could do without the boost port on what was to be a low end market engine.
Sorry about the rant Leonard,but I spent 35yrs.designing and developing I.C.engines and components.My lab at Reynolds worked with companys as diverse as Fox(yesDuke),Porche(911,944,928,936),and McLaren(Reynolds/McLaren/Chevrolet),plus a lot of two stroke Mfgrs such Stihl.I retired as Director of I.C.Engine Development and Testing.

Ron, I respect your knowledge and your experience. I, too, measure every engine I work with and bring it to my specs. But in answer to all this stuff about boost ports, if we are looking for power, we can run the engine as fast as we can in order to achieve a higher hp curve. Here the boost port helps. But that is not what we are looking for.

If we are looking for torque, well, I told you what I believe is the theory. But now I will tell you fact. My 35 FP, with a blocked boost port, will turn a stock 12.5 inch diameter 5.5 pitch Bolly prop and pull the plane on 66 foot lines at 5.2 to 5.4 second laps all flight long without ever breaking into a full lean two-cycle. (Running it flat, all-out, we can get about 4.5 second laps)

So, maybe in theory it does't work, but I have never seen in fact any 35 size engine able to pull that prop or anyting close to it without blocking the boost port. Would you have one to offer to the challenge? (I might add here that even blocking the boost port of the Fox 40, with its narrow trahsfer ports, boosts the torque range of that engine. I know. I have tried it.)

Hey Ron and Len, I think maybe you are both arguing the same side of the argument, but from different points of view. Ron says that the transfer ports need to be properly timed and shaped(along with the exhaust)to get maximum torque and power. You do this by experimenting with port sizes, shapes, and angles. He is coming from a design/engineering point of view. You, Len, say you have experimented with some specific engines and come up with a port/sleeve modification that produces some pretty dramatic torque increases, by adjusting the port sizes(and number), heights, etc. You just seem to be coming at it more from a development/experiment standpoint.

The latest two stroke porting technology seems to favor multiple bypass ports over a fairly large portion of the cylinder all timed to come together and sweep the charge up one side and down to the exhaust. The Brodak 40 and a lot of the Rossi and OPS and car engines are doing this. The whole point of Shneurle scavenging is to increase torque and horsepower by better scavenging of the cylinder. Seems pretty plain that there is more than one way to do this effectively.

>
>The latest two stroke porting technology seems to favor
>multiple bypass ports over a fairly large portion of the
>cylinder all timed to come together and sweep the charge up
>one side and down to the exhaust. The Brodak 40 and a lot
>of the Rossi and OPS and car engines are doing this. The
>whole point of Shneurle scavenging is to increase torque and
>horsepower by better scavenging of the cylinder. Seems
>pretty plain that there is more than one way to do this
>effectively.

Ron B.
F4Fguy

Ferocious:You're absolutely right.what seems to keep getting lost in this discussion is that any time you talk about torque,your talking power(and vice versa).That we want this power at lower RPM doesn't change this fact.It's also fact that the only way to get more power at any RPM,low or high,is to get more mixture into the combustion space.The so called boost port is in fact an additional intake port.The multiple ports in the engines you cite have the same function as the single boost port i.e.,increase the intake area for better flow,and control and shape the flow for better scavenging.It is possible to "over-aspirate"an engine,and the LA may be one that is.If it is,I would expect better results by reducing the cross section of ALL the bypasses.Also keep in mind,it's not only a matter of getting mixture in,butkeeping it in,while getting the waste gasses out.

Ron B.

> Ferocious:You're absolutely right.what seems to keep
>getting lost in this discussion is that any time you talk
>about torque,your talking power(and vice versa).That we want
>this power at lower RPM doesn't change this fact.It's also
>fact that the only way to get more power at any RPM,low or
>high,is to get more mixture into the combustion space.The so
>called boost port is in fact an additional intake port.The
>multiple ports in the engines you cite have the same
>function as the single boost port i.e.,increase the intake
>area for better flow,and control and shape the flow for
>better scavenging.It is possible to "over-aspirate"an
>engine,and the LA may be one that is. If it is,I would expect
>better results by reducing the cross section of ALL the
>bypasses.Also keep in mind,it's not only a matter of getting
>mixture in,butkeeping it in,while getting the waste gasses
>out.
>
> Ron B.

Ron, I never claim to have the "perfect" answer to anything, and I know there are multiple paths to the same goal. But you have made a statement here that I am trying to make that it is possible to "over-aspirate" an engine. If we have too much intake area (port size) for the amount of crankcase compression which our engine will produce at lower speeds, or too long a duration (which also serves to decrease the crankcase compression) then we won't get as much flow as if we decreased the size and allowed the increase in case pressure to force the charge up. I, again, point to the example of a water hose. If you turn on a hose without a nozzle in the end and want to wash down a wasp next at the eve of your roof and the water won't reach that height, you can squeeze the end of the hose to increase the pressure and flow. If you increase the transfer ports too much, you need to increase the case pressure or it is all for nought.

You suggested reducing the cross-section of all bypasses as a possible other solution. Interestingly, I tried that. The 40 FP has wider port slots in the sleeve (same case internally) than the 35 FP and this causes a problem with the blocked boost port engine when running upright. Without going into this I actually tried to partially block the transfer ports in the case and it failed miserably. Just didn't work. Tried it without the blocked boost port. Tried it with the blocked boost port. It didn't work.

Now, maybe I should have tried "this" or "that" or "something else". Maybe there is an answer to this. But the simplest and most effective solution to getting that low end torque that I have found is blocking the boost port. Now I have done this on a number of engines and have not found an engine yet that did not benefit when run with a "normal" muffler. I have not tried this on a pipe engine. So what happens when the shock wave comes back, I don't know. One of these days I am going to try it and find out. The only thing is, and I tell this to anyone, you have got to be prepared to waste a couple of engines, sleeves, whatever, when you start experimenting like this. It can pay dividents, or you can just end up with a lot of scrap parts.

Rule of thumb: "To get the maximum power out of your rubber powered airplane, wind the propeller until the motor breaks, and then back it off two turns."

> Sorry about the rant Leonard,but I spent 35yrs.designing
>and developing I.C.engines and components.My lab at Reynolds
>worked with companys as diverse as
>Fox(yesDuke),Porche(911,944,928,936),and
>McLaren(Reynolds/McLaren/Chevrolet),plus a lot of two stroke
>Mfgrs such Stihl.I retired as Director of I.C.Engine
>Development and Testing.
>
> Ron B.

Ron;

We seem to have skipped over something that, to me, is vitally important. When we are talking power for CL, we need to remember the concept of usable power. The only means we have of harnessing the output of the engine is through a propeller that makes one revolution for every revolution of the engine. We have found that with existing propeller technology and within certain weight limitation, there are certain pitches and diameters that are optimal for hemispheric flight. If and engine will produce max torque at, lets say, 18,000 rpm, but that results in undesireable flight characteristics, that power is wasted.

You have forgotten more about two cycle design theory than most of us will ever learn, so you are probably right - blocking the boost port may just rob some of the peak torque or HP potential of the engine. If, however, it moves the power curve to maximize the engine's potential at a more usable RPM with respect to propeller capabilities and thrust/speed optimization required for CL Stunt, then for our application it might be desireable. The results for me certainly have been.

Again, one can not talk about power output of an engine with respect to CL without discussing not only the propeller, but also the unique characteristics required of hemispheric flight.

>
>Ron;
>
>We seem to have skipped over something that, to me, is
>vitally important. When we are talking power for CL, we
>need to remember the concept of usable power. The only
>means we have of harnessing the output of the engine is
>through a propeller that makes one revolution for every
>revolution of the engine. We have found that with existing
>propeller technology and within certain weight limitation,
>there are certain pitches and diameters that are optimal for
>hemispheric flight. If and engine will produce max torque
>at, lets say, 18,000 rpm, but that results in undesireable
>flight characteristics, that power is wasted.
>
>You have forgotten more about two cycle design theory than
>most of us will ever learn, so you are probably right -
>blocking the boost port may just rob some of the peak torque
>or HP potential of the engine. If, however, it moves the
>power curve to maximize the engine's potential at a more
>usable RPM with respect to propeller capabilities and
>thrust/speed optimization required for CL Stunt, then for
>our application it might be desireable. The results for me
>certainly have been.
>
>Again, one can not talk about power output of an engine with
>respect to CL without discussing not only the propeller, but
>also the unique characteristics required of hemispheric
>flight.

OK,one more time.I've been talking from the gitgo about useable power.I couldn't be more in agreement that we want more torque lower down.This is what is needed to turn more diameter.What I seem to be unable to get across is that to increase torque at ,let's say 5000RPM (pick a number),is to increase horsepower at that speed.The principle remains the same,the ONLY way to do that,is to burn more mixture at that RPM.Of course,there many ways to get there,(turbocharging comes to mind)but the requirement is still better breathing,and better scavenging at that RPM.
The prop diameter is dictated by the horsepower required to turn it at a given RPM.This goes up as the square of diameter,and directly with pitch.To go from 11"to 12"dia.at any given RPM requires ~18% more horsepower AT THAT SPEED.Obviously to get this HP you have to increase torque by that amount,at that speed.
I'm not sure about "optimal"diameters and pitches.In my experience the more diameter you can pull the better,within the limits of available power,and that ol'devil precession(or control thereof).My Macchi with a homegrown modded MOKI 61 used a 15"d.x 6"p.2bl.prop , ground release @7000RPM and 5.5seclaps,unfortunately it never got fully sorted out before its pipe leak made it unflyable(the pipe went through the middle of the fuselage,fully concealed).I bring this up because it illustrates what I've been trying to say.This was a piped engine designed (only the case and crank were MOKI)to operate at low RPM with large diameter props.The eventual aim was to run 16"to 18"diameters while keeping the precession under control by lower rotational speeds.I haven't pursued this avenue further because of the CG problem posed by a pipe out to 30"+ in order to tune to the low RPM,but the concept was proven sound.

> The prop diameter is dictated by the horsepower required
>to turn it at a given RPM.This goes up as the square of
>diameter,and directly with pitch.To go from 11"to 12"dia.at
>any given RPM requires ~18% more horsepower AT THAT
>SPEED.

Ron,

I have absolutely nothing to contribute regarding boost ports, but I still feel compelled to go off topic here and point out that power required does not go up with the square of propeller diameter. Instead, and in the particular instance of assuming that power goes up directly with pitch, power is proportional to the fourth power of diameter. If the Pitch/Diameter Ratio (P/D) is kept constant for different propellers, power is proportional to the fifth power of diameter. Assuming the same P/D in the case of the 11" diameter being increased to 12", and the two different props are to rotate at the same speed, power needs to increase by 54.5%, not 18%. If pitch of the two props is held constant, a power increase of about 42% would presumably be required to maintain the same speed.

BTW, Thanks for sharing your knowledge of two stroke porting. I have never been exposed to such information and find it very interesting.

Roman

Ron B.
F4Fguy

I misspoke,thanks for the correction.Your'e ,of course absolutely right.

LAST EDITED ON Jul-22-03 AT 04:41 AM (CDT)
 
Just curious. In previous incarnations of this argument, charge velocity has come into the picture. Is it possible that, when you run a modern Schneurle RC engine at 8000 RPM instead of 15000 RPM, the extra port cross section will result in charge velocities which are too low?

(Agh.. Never mind. I see Len's mentioned it already..)

The red RTV is used for the most part, on automotive applications, such as gaskets around the oil pan, rocker covers (valve covers), etc. Most auto parts stores carry it.

That's what I use. Just be sure to degrease the area, and it doesn't hurt to scuff up the port with a little sandpaper. Just be sure to clean everything out.

When I originally posted the above comment, it was #3 on the list and was obvious that the "that's what I used..." was JB Weld.

Alan

p.s. I thank everyone for the engine comments. I really soak up this stuff.

There has been lots of interesting discussion about the ports in the cylinder. What about the timing of the intake port (frequently in the crank shaft...)? Does that affect the rpm range where the torque or hp curve peaks?

>There has been lots of interesting discussion about the
>ports in the cylinder. What about the timing of the intake
>port (frequently in the crank shaft...)? Does that affect
>the rpm range where the torque or hp curve peaks?

Yes, but not as badly because there are limits that are governed by what the piston does, not the timing in the cylinder. Although a speed (or combat) engine may alter the shaft timing slightly, it is still going to be limited to opening approximately when the piston reaches the bottom and close when the piston approximately reaches the top (there will be some overlap and offset here, but you can understand what I mean.) We tried some experiments in narrowing the range by putting a little JB Weld into the intake of the case, but all it did was hurt the performance. Of course, there was noting scientific here, as in saying, "we need to narrow the opening by 3 degrees."

For everything there is an optimum, and I suppose if you knew the optimum for the rpm range you were using there could be a slight improvement made if it wasn't already there. Maybe we should ask Randy or Richard if they changed the shaft timing on the Nelson or the Jett. These were high speed performance engines in the beginning, so I would be curious as to how the shaft timing affects them.

LAST EDITED ON Jul-22-03 AT 05:19 PM (CDT)
 
Ron B.
F4Fguy

Yes theres an effect though not as pronounced as with the intake/exhaust transfer ports.A good starting point for opening the rotary valve is one or two degrees after intake transfer closing,then advance it ~ two degrees at a time till you lose torque.Do the same with rotary closing,using intake opening as the base line,and rotary closing ~ two degrees before.
You can retard closing only until the inertia of the incoming charge is greater than the pressure rise due to the down stroke.Similarly,opening can only be advanced to the point where residual exhaust pressure cannot be overcome by the pressure of the incoming charge.Of the two points opening is,generally, considerably less critical than closing.