I have flown Tower 40 with and without the boost port blocked and could not tell any difference. 

Sorry guys if this topic has been covered.

Could someone explain in simple terms how blocking the boost port on Schnuerle scavenged engines improves the 'stunt' performance of the engine ?

Without resorting to 'surgery' , what other 'simple' modifications can be done to tame an engine and improve the stunt run ?

Very much obliged for any advice guys ,

Robin [ ex-pat Brit in the Charente full of ex-pat Brits ]

The boost port(s) are supposed to improve torque at high rpm- like 15,000 and up.  On a stunt motor turning 9-10,000 they don't really do anything.  Blocking the boost port is supposed to keep the speed of flow of the fuel/air up in the two main ports and help at stunt rpms.

Lots of research has  been done on 2 cycle porting.  It seems to say that many combinations of port size, shape, and timing can work equally well for a given purpose, but if you go beyond the bounds, performance goes to pot very quickly.

Robin. Blocking a boost port is something you do to make an engine trap the charge at a lower speed. Say you have an engine designed to make peak power at a relatively (to your needs) high speed. It will have a port and transfer area that "tunes" at that higher speed. A simple way to rationalize the port/transfer area of that engine to "tune" at a lower speed is to block off one of the transfer ports. Air needs a combination of area and time to move through the transfers and fill the cylinder. Interestingly the flow velocity that results in good trapping efficiency remains close to the same regardless of the engine speed. You adjust port/transfer area to keep the velocity correct. Blocking the transfer is an elegant/brute force way to make the high speed engine make more power (by trapping a denser charge) at the lower speed you require. Is it a universally correct thing to do to all boost ported engines to make them into stunt engines? Of course not. Engines can and have been designed to work with one to as many as seven transfer passages to accomplish a specific design goal. The idea is always to make the engine move air efficiently at a speed that is specific to it's use.

Dave --

Your explanation might imply that venturi size and total passage area is sort of a "package". Great explanation, that "blocking ports" is inherently neither a good or bad idea, but totally situational. 

Can you better describe what you mean by "trapping the charge"?

       Larry Fulwider

Dave --

Your explanation might imply that venturi size and total passage area is sort of a "package". Great explanation, that "blocking ports" is inherently neither a good or bad idea, but totally situational.  

Can you better describe what you mean by "trapping the charge"?

      Larry Fulwider

Trapping is a way of implying volumetric efficiency. The more charge you can trap, the bigger (higher pressure - pressure x piston crown area x stroke = torque) the explosion. Trapping is used in discussions that include issues like charge dilution (left over after combustion products from the last big bang) and is more combustion oriented.  If you look at the dyno torque curve of an engine the "functional" vol eff is displayed. I say functional because sometimes the torque curve is hindered by things like detonation. This is why combustion chamber design and glow plug temp have such an effect. You are spot on in with your venturi area idea. The engine is a pump and should be optimized as one. The venturi on a stunt engine in reality is a fixed throttle and so has an effect on vol eff. Charge velocity has an effect on vol eff in that air has mass. Once you get it moving it tends to want to keep moving. If you think about how air flows from high pressure volumes to low pressure volumes the implication is that as soon as the pressures are equal the the flow will stop. If you also remember that the air has momentum (edit add: and is compressible) you can see that there will be a period where the air would bounce back and forth until that energy is dissipated. The idea behind "correct" air flow velocity is that you can "trap it" before it bounces back out and you actually have a higher pressure on the cylinder side vs. the transfer side. If you can, you have a point where the torque curve rises due to the larger mass of the trapped charge. If you look at the torque and power curves of an engine this effect peaks at the max torque point (if you did your homework the point will be higher) and gradually dissipates as the speed goes beyond the this point. The power curve continues up (rpm=work) until the vol eff gets worse faster than rpm can fill the hole - so to speak....

One additional caveat. Filling the transfer also "packs" the case by reducing it's volume. This raises the c-case compression and increases velocity in the remaining transfers.

Randy , I always value your opinion.

Why do you believe so many guys out there block the boost ports if , as you clearly state , it doesn't improve the 'Stunt Run ' ?

Robin.

Hi Robin

I have done this with many many engines, It has NEVER as I said increase performance..I never said anything about improving a "stunt run" That is "clearly" a opinion, and there are more of those on that subject than we have engines.
Whenever I block a boost port or multi boost ports the POWER and Performance  always goes  down. Then you "have" to add a Bigger venturie, or more nitro, or more pitch, in order to get back the lost power, ..note... many people think more speed is more power, ..many people think being able to run a bigger prop that does  not run away because it is "out" of power is more power.
I have also done HP and Torque test on all of these, both HP and torque went down when the port was deleted or blocked, with all else being equal..same venturie, prop, fuel, etc.
You have some people that enjoy talking about retimed FPs being weak, I did 100s of them, and my guys could then run 12 inch props in a solid 4/2, They were not weaker, they were slightly stronger in a 4 cycle, and ssome of the 2 cycle power was killed off by the mild retime/reangle, I think this is somewhat similar to what people see when blocking boost ports on that same engine. You can also do somewhat the same by decompressing the engine to kill off 2 cycle power and this allows you to run the motor harder without the extra 2 cycle "runaway power"  that was the biggest complaint of that engine.
Then you had another  engine builder that setup his with blocked ports and would run the motors with bigger props in a 2 cycle..so again that solved the 2 cycle runaway problem, Still others would run the same blocked motors witth a lower compression head in a 4/2 with more powerful fuel, These worked but did not have more power or performance, I have to get back to work, I will write more later

Randy

It certainly works with some engines. I suspect it is a really bad idea for some, too (at least for our application). I have a Magnum 53 that was a marginal stunt engine. Worked fine in a wet 2 stroke if you kept the venturi size pretty small. But it was a bit anemic. Blocking the boost port allowed a bigger venturi and a more traditional, 4-2-4 sort of run and created a lot more power for the type of use. I suspect that with the boost port open and running it at the designed RPM (around 14000), it probably makes more power than what I am using it at, but it wouldn't be much use for this application.

As Dennis notes, it just depends on what you're trying to accomplish.

HI Randy

The Magnum 52 and 53 can be, and are, great stunt engines, depends on ,who and how, the setup is done to them.
They do not need a blocked port to be a great stunt engine as your post suggest, again depends on the know how of who setup the motor, And as people can see there are more than one way to rework a motor for what you  want it to do.
The Adamisins , for example,  have had great sucess with blocked port motors and are very happy with the results, and have pulled both large and small planes with them, and no one can argue with their record with them.  
Others will tell you all you need do, is cut a hemi head and the motor is now a great stunt engine.
There are 100s and 100s of  T Tiger, Magnum, OS, ST and many other brands that have been converted for C\L stunt that run wonderfully well.
The bottom line is to have what makes you happy, and works well for you.

Regards
Randy

Interesting stuff indeed! And this same topic popped up over at the Barton web site.

I have it on good authority that my dieseled MVVS 40 will definitely benefit from having its boost port and it vestige of the '4th' port blocked completely off.
(The '4th' port is peculiar to these engines as the cylinder jacket can rotate to any 90 degree position and it has 4 cavities cast into the lower crankcase to accommodate the 3 working ones in the upper crankcase. In other words there is always going to be an unwanted cavity no matter which way you position the exhaust port.)

The reasons that this particular engine benefits are -
1. You don't need the amount of volume of transfer with a diesel as compared to a glow, but you need the gas speed (kerosene fuel droplets are larger than methanol ones and so will greatly benefit from smaller higher speed transfer ports.)
2. Blocking off the third port reduces the crank volume and thus increases pumping efficiency.
3. It increases the flow rate or speed of the incoming fuel charge in the other ports and thus atomizes the charge all the better.
4. Blocking off the 4th port also stops the fuel from coalescing there and then being thrown into the combustion chamber during maneuvers.
5. This engine unloads in the air far less (about 1000 rpm less) then when it was in glow form and so it operates at a slower speed.

So that's for that engine.

I have always thought that the port opposite the exhaust, commonly called the boost port, aimed a small amount of the compressed fuel charge at an area of the piston crown that ran the hottest and also cleared a virtually stagnant portion of the combustion chamber but at lower revs these two factors are nowhere near as critical since time is more on our side (I am agreeing here more than anything else.)

The remaining question that I have is what can be gained by 'partially' blocking it like the Fox 35 burp cure - there is no need to always consider blocking it off completely all the time.

Cheers.

P.S. By the way the venturi size also dropped down to about 70% of the original and the prop diameter would go up by an inch as a rule (at the moment can spin an 11x7.)