discussion of transfer, boost,exh ports,int/exh rotor timing...

hi gang,i am always looking to soak up more info on our 2 stroke engines. kind of interested in learning more about the boost and transfer ports.i see different manufactures like different #'s.why is this?also have read that some like the timing #'s the same between the 2 and some like the transfer higher,roof angles,wider and higher,etc.rotor/opening and closing #'s.real world testing of your results are better than theory.thanks,mike.

Funny thing is OS21 outboard has **** for numbers and its kicking azz at every race

Transfer tunnel shape is as important as timing. Nearly all model engines have poorly shaped transfers. Below are drawings of very poor (YAM 12) and very good (YAM 14) transfers as tested by Gordon Blair a long time ago. Lots of later testing has confirmed this. The latest Quickdraw Pioneers got a good power gain from improved transfer shapes.

Lohring Miller

Have to agree with Lohring;

When the ports are well designed, we can concentrate on FLOW improvements and management. Properly managed flow can produce a somewhat moderately timed assembly that can pull like a freight train from almost dead stall, to well over 20K ( on gas ). And it will do that with a very short pipe that "hits" at several ranges.

I agree with Lohring. In fact, when you consider comparing the "tea cup handle shape" of Professor Blair to the radaii shape used by the Quick Draw Pioneer & the 125 Aprillia racing motor cycle engines there is still improvements to be made. This would also apply to what is being done in the exhaust windows & transfer tracts of production engines. Quick draw again seems to be leading the way in both areas. It would also appear that almost any radius on the internal walls of a transfer can make an improvement in the mass flow through this area.

Various timing numbers have little effect on what amount of HP is generated, but they have a pronounced effect on where the maximum HP, RPM point will be found.

Jim Allen

In regards to rotor timings for a two cycle engine, there is one induction valve design that produces more HP than any standard drum, inverted drum, rotary valve types or reeds. The name of this induction valve is called a "BELL VALVE". It's design allows the greatest time area number possible without the early openings & the late closings found in the other types of valves.

Jim Allen

Jim Allen said:

The name of this induction valve is called a "BELL VALVE". It's design allows the greatest time area number possible without the early openings & the late closings found in the other types of valves

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are both these two carb/induction-valve samples "Bell" types?

It's not as simple as looking at just the durations and angles of the ports. You need to consider the volume of the ports, the volume of the crankcase the length of travel from the induction valve to the ports, the length and volume of the induction tract, fuel, rpm range, tuned pipe and how you load the engine. It would be nice if we could simply say cut the transfers to 130 deg and exhaust to 190 and you'll get max power for an engine, but it's not that simple.

No. The induction valve on the left is a bell valve. The valve on the right is a standard drum valve. If you look carefully at the bell valve, you can see that it has a wide open period of rotation (45*). This happens because the valve is located on the circumference of the back end. The valve is fully balanced, with an opening of (34* ABDC) & a closing of (63* ATDC). Even though the valve's total duration is only 209*, it's time area number is hugh compared to any other type of valve with the same numbers. Numbers such as these make it easy to pipe an engine up! The valve is not affected by the pumping action in the crankcase because it's axial movement is controlled with a ceramic ball bearing. Since the valve is totally balanced it's radial load on the backplate's center bushing is minimal. Notice the circular oil groove which drives any lubricant present towards the center bushing as the valve rotates. No part of the valve's ID touches any part of the back plate's OD, which means that the valve runs without any rubing friction! All parts of the valve are fully hardened!

Jim Allen

Time area is king in a two stroke eng. Now how you get the most is limited to what is available in a production eng. It is easy to look at what is best in as far as different designs. But this dose not mater if you can not buy it.

So why not just discus what is available to the masses. Dream are nice but that is what thy are dreams.

so what can be done to available RC Boat nitro eng?

Not one off hand made dream eng but what you can buy.

So on that note what do you think?

How do you get time area?

dwilfong said:

Time area is king in a two stroke eng. Now how you get the most is limited to what is available in a production eng. It is easy to look at what is best in as far as different designs. But this dose not mater if you can not buy it.

So why not just discus what is available to the masses. Dream are nice but that is what thy are dreams.

so what can be done to available RC Boat nitro eng?

Not one off hand made dream eng but what you can buy.

So on that note what do you think?

How do you get time area?

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dave,i have know jim since we lived in ny and i was blown away the 1st time i saw his hand made engines run at South Pond and he ran the motor on the bench for a LONG time with NO WATER and revving to the moon! was amazed to see it lived!.he explained to me about the crazy tight tolerences and the hand crafting was amazing!! but yes i am a realistic person and racer weather it is drag racing or modeling and would like to learn more about the engines we can buy and work with our selfs. this thread was started by me not to set any records, but to learn more about motors that i have played with for over 35 years and what really makes them tick.i have looked in a lot of ,plane,car,boat engines that i have raced over the years and have tried to look in all off them to see what works and what makes a good one and a great one.i do understand that no one motors timing and other specs will work on another motor and that is what interest me how CMB goes in one direction while N/R goes another way.thanks to all who contribute,mike.

SayMikey said:

Funny thing is OS21 outboard has **** for numbers and its kicking azz at every race

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mike, i ran OS Max in planes (u-control carrier class 1) back in the 80's and we kicked everyone's ass and guys were pissed that there Rossi's,Nelsons,and every other top of the line engine were getting beat.the guy who used to do my engines was Tom Roman and he just knew what to do and how to go fast. also took that same motor years later and converted it to a boat engine and put it in a Muck Sport 40 and kick ass with it in Namba D1 for quite a few years and should have won D1 but car broke down on hwy with dad and we missed the race i think that if a motor finds the right combo,along with the pipe and prop,it could be competitive anywhere.i know the pipe is a big thing and that has always interested me also.have read lots of papers on pipe design but i am the first to admit i do not understand the whole deal.kind of like when i tune guy's drag cars i am the 1st to say i am lost when it comes to 4 link cars...guess that's why i still run a ladder bar and go really fast!! PS,some of the fastest guy's in R/C Off-Road cars still run OS Max and are REALLY FAST!!

also wanted to ask about case vol and such.have seen on some engines that the case area (transfer) are opened up.is there any gain to this? i know in 4 stroke engines that more fuel and more air = power..is there a limit to this?

Jim,

On the Bell Valve, how close of tolerance do you have to use between the bell valve and the backplate?

Might be a dumb question but why can't a drum induction have its opening area increased like the bell valve?

Mike

Jim Allen said:

No. The induction valve on the left is a bell valve. The valve on the right is a standard drum valve. If you look carefully at the bell valve, you can see that it has a wide open period of rotation (45*). This happens because the valve is located on the circumference of the back end. The valve is fully balanced, with an opening of (34* ABDC) & a closing of (63* ATDC). Even though the valve's total duration is only 209*, it's time area number is hugh compared to any other type of valve with the same numbers. Numbers such as these make it easy to pipe an engine up! The valve is not affected by the pumping action in the crankcase because it's axial movement is controlled with a ceramic ball bearing. Since the valve is totally balanced it's radial load on the backplate's center bushing is minimal. Notice the circular oil groove which drives any lubricant present towards the center bushing as the valve rotates. No part of the valve's ID touches any part of the back plate's OD, which means that the valve runs without any rubing friction! All parts of the valve are fully hardened!

Jim Allen

Click to expand...

I see, thanks Jim

So what happens when you try and push 60% nitro threw a case?

Dose volume in the case and ports matter more than say running 30%?

dwilfong said:

So what happens when you try and push 60% nitro threw a case?

Dose volume in the case and ports matter more than say running 30%?

Click to expand...

i can't see there being a difference in nitro %.kind of like 100 octane and 110,depends more on comp,cam,combustion chamber,etc.i see like N/R there trans #'s are higher than the boost and was wondering what the be the beneifit?

Mike Rappold said:

Jim,

On the Bell Valve, how close of tolerance do you have to use between the bell valve and the backplate?

Might be a dumb question but why can't a drum induction have its opening area increased like the bell valve?

Mike

Click to expand...

Mike,

.0001" to .0002" between the insides surfaces of the bell & the outside surfaces of the backplate. The bell's ID & OD plus the hole for the center shaft are ground in one setup. The center shaft is pressed into the bells center. The center shaft has a shoulder & a threaded end which fastens it to the inner race of the ceramic ball bearing mounted in the backplate.

Not a dumb question at all Mike. You wil find the answer if you look carefully at the design of a typical drum valve. The window of a typical drum valve for a .90 size engine is located on an OD of .786" with an ID of .622". The window of the bell valve is located on an OD of 1.386" with an ID of 1.227". The "maximum" width of the window in this drum valve would be .6152". The total duration of the window in this drum valve is 103*. The width of the window in the case is .6972" or 125*. The drive pin is positioned so that the valve opens 15* after BDC & closes 63* after TDC. This gives a total open duration of 228*. The valves wide open duration is 125* minus 103*, which equals 22* of wide open timing. This valve cannot be totally balanced & it will be affected by the crankcase's pumping actions. It also will need a bronze bushing in the case for the valve to run against.

Since the bell valve is on the outside OD of the backplate, I'll start with the width of the window in the backplate. It is .8048" or 82* wide. The width of the window in the valve is 1.1416" or 137* wide. This means the total duration of the valve is 219*. The drive pin is positioned so that the valve opens 24* after BDC & closes 63* after TDC. However, the valves wide open duration is 137* minus 82*, which equals 55* of wide open timing. This is where the bell valve is superior to any other type of induction valve. It has a very large time area number & the bell valve is totally balanced plus being totally unaffected by the crankcase's pumping actions!

I have posted some photos of a bell valve used in a .15 size engine. The valve is made of aluminum & it is mounted directly to the engines crankpin.

Jim Allen

Why such a small window in the back plate? Is this based on the small engine size? Is the case bronze bushed with the valve bolted to the crank? Very good information and can only imagine what the cost would be if you farmed it out!!

Brad

I wanted to keep the venturi shape of the induction passage way. I do not know if a larger case window would have improved the engines performance. No. there is no bushing of any type used with any bell valve.

JA