Boast vs Transfer

[Marty Davis]

Marty,

"If we could ever get one of the engine manufacturers to design an engine with what are called "Tea Cup Ports" we would have something special. Brian Callahan told me about this design and said that if we had that type of casting in the case, we would really have some HP. Look it up by that name and post what you find. These are port runners that actually protrude out the sides of the case similar to the Japanese motor cycles."

It is well documented & proven, with at least two available engines, that transfers with an inside & outside radius, can be better than the Tea Cup shaped ports developed & tested by Professor Gordon Blair Of Belfast University. The engineers at Aprilia proved beyond any doubt that curved transfers, with the correct inside & outside radius, that use the Coanda affect, can flow better than the Tea Cup shaped ports. Two engines that presently use these type ports are the QD Pioneer engines & the MB 40 pylon racing engines. Frits Overmars of Aprilia was instrumental in the development of the 35,000 RPM MB 40 engines.

Jim Allen

Jim:

Brian Callahan sent me an image of the Aprilla ports as rubber molds of the inside of them. He called them "Tea Cup" style ports. Your description of an inside and outside radius is very graphic and exactly what I was talking about. The outside of the case was "Boulbous" sp?

I wrote to him asking if he still has the image, as I guess I have lost mine. If I get it, I will post.

 

[Jim Allen]

Okay Marty,

I have actual prints of the ports & the engine. I'll check to see if they can be posted. The prints show the radial & axial angles of the "A", "B", "C" transfer ports plus the boost port. When the correct radius is used, the incoming charge will attach itself to the inside wall, coming out at an angle which will attach itself to the domed piston crown.

JA

 

[Jeff Lutz]

Thanks for sharing Jeff,

Now where cooking with NITRO.....................................

So what do you think will work better for a heat racing eng.

Smaller ports and a higher angle diffuser on pipe?

Larger ports sallower diffuser angle on pipe?

Is the a rule of thumb as far as diffuser angle and port area?

. Small ports big pipe, large ports small pipe is some what a rule of thumb that will work for most. I generally run ports as big as I can and pipes with big diffuser angles as well. As long as you are getting the fuel burned, the larger pipes launch more prop for me and I'm able to run the pipe shorter as well. For heat racing I like big ports on most engines with lower transfer and boost timing. I still cut and try a lot, and even though it's work to fit a lot of pipes on an engine as far as mufflers, brackets, etc, I still will plug 6 different pipes on one motor to make sure I'm not in left field with my pipe.

 

[dwilfong]

LOL...... the more the merrier................

 

[Andy Brown]

Funny most marine eng coming out now are just a rehash of a aero eng case thy already have.

It would not be profitable to make a true high performance marine eng designed for HIGH nitro.

You would have to start from scratch with the whole design.

But we can dream now can't we.........................................

Andy posted a pic of a fat belly case he designed.

Would love to see it again with some input on what is what................................

"Andy posted a pic of a fat belly case he designed.

Would love to see it again with some input on what is what..."

 

[Marty Davis]

Funny most marine eng coming out now are just a rehash of a aero eng case thy already have.

It would not be profitable to make a true high performance marine eng designed for HIGH nitro.

You would have to start from scratch with the whole design.

But we can dream now can't we.........................................

Andy posted a pic of a fat belly case he designed.

Would love to see it again with some input on what is what................................

"Andy posted a pic of a fat belly case he designed.

Would love to see it again with some input on what is what..."

Now, that is what it would take.....

 

[Adam Blevins]

Sirio had a buggy engine out quite a few years back that had large transfers.. looked similar to the case Andy posted. I cant remember what they called them off top of my head.. ill have to look em up.

 

[lohring]

Below is an explanation of transfer theory by the designer of that 40 engine, Frits Overmars. A very long time ago Queens University of Belfast tested a huge variety of transfer designs. Below is a picture of two. Number 14 was the best and #12 was the worst. Model engines almost universally use the worst design. Also below are models of the Aprilia 125 cc cylinder's transfers. This engine is considered to be the most powerful small two stroke with 54 hp. My article, High Power Two Stroke Design - Postscript Quotes from two papers that show what we are trying to accomplish with scavenging as well as CFD pictures of what happens with flow in the Aprilia cylinder.

I'm not sure that the fine details of transfer angles and timing will make the same difference in engines without tea cup transfers. Tea cup transfers contribute immensely to the aim of the flow. The flow out of model transfers is probably a wide angle spray. Our attempts with transfer aiming in various gasoline engines didn't give measurable improvements.

Lohring Miller

Transfer Theory Frits Overmars.pdf

View attachment 79335

View attachment 79336

 

[lohring]

Below is an explanation of transfer theory by the designer of that 40 engine, Frits Overmars. A very long time ago Queens University of Belfast tested a huge variety of transfer designs. Below is a picture of two. Number 14 was the best and #12 was the worst. Model engines almost universally use the worst design. Also below are models of the Aprilia 125 cc cylinder's transfers. This engine is considered to be the most powerful small two stroke with 54 hp. My article, High Power Two Stroke Design - Postscript Quotes from two papers that show what we are trying to accomplish with scavenging as well as CFD pictures of what happens with flow in the Aprilia cylinder.

I'm not sure that the fine details of transfer angles and timing will make the same difference in engines without tea cup transfers. Tea cup transfers contribute immensely to the aim of the flow. The flow out of model transfers is probably a wide angle spray. Our attempts with transfer aiming in various gasoline engines didn't give measurable improvements.

Lohring Miller

Transfer Theory Frits Overmars.pdf

 

[dwilfong]

It is funny I call the port shape that the IR.21 I have a cobra port.

It looks like a cobras head ready to strike.

looks just like #14 in your diagrams....................................

Also if you look at the newest port lay out on the CMB .91 M1 tuning kit. It has the front knife port to stand the flow stream up so it dose not go out the ex.

I make a cut in the older liners at the bottom front of the transfers I call a boost stopper.

Same theory stop the flow to the ex port when the piston is at BDC.

 

[John Knight]

Curious why the angled cut on the port does not start at the bottom of the sleeve rather than in the middle. Would it not increase more flow and charge going into the cylinder?

 

[dwilfong]

Not looking to flow more air just direct it at a certain time. Just a little puff in the right place at the right time.

 

[lohring]

Also if you look at the newest port lay out on the CMB .91 M1 tuning kit. It has the front knife port to stand the flow stream up so it dose not go out the ex.

I make a cut in the older liners at the bottom front of the transfers I call a boost stopper.

Same theory stop the flow to the ex port when the piston is at BDC.

We found that that cut on the CMB 35's transfer also helped power. I think it creates a "curtain" to help block flow out the exhaust.

Lohring Miller

 

[Marty Davis]

Also if you look at the newest port lay out on the CMB .91 M1 tuning kit. It has the front knife port to stand the flow stream up so it dose not go out the ex.

I make a cut in the older liners at the bottom front of the transfers I call a boost stopper.

Same theory stop the flow to the ex port when the piston is at BDC.

We found that that cut on the CMB 35's transfer also helped power. I think it creates a "curtain" to help block flow out the exhaust.

Lohring Miller

Would you show an image of that cut?

 

[stan simpson iv]

...

 

[stan simpson iv]

Also if you look at the newest port lay out on the CMB .91 M1 tuning kit. It has the front knife port to stand the flow stream up so it dose not go out the ex.

I make a cut in the older liners at the bottom front of the transfers I call a boost stopper.

Same theory stop the flow to the ex port when the piston is at BDC.

We found that that cut on the CMB 35's transfer also helped power. I think it creates a "curtain" to help block flow out the exhaust.

Lohring Miller

Would you show an image of that cut?

 

[dwilfong]

Lohring

That cut was some thing I got from your wright up on the 35RS dyno testing article a wile back.

Also the top of the transfers being flat.

Would be grate if you had a link to it. Lots of good info there.

 

[lohring]

Thanks. I tried to post a picture, but was it was too big (?) The picture along with a lot more on our CMB 35 testing and modification program is at http://www.modelgasboats.com/magazine/tech-articles-mainmenu-608/244-cmb-35-record-engine Our fooling with transfer port angles showed flat top transfers worked best. That shouldn't be the case in a well developed engine.

The modern 5 transfer engines use a large up angle on the A (closest to the exhaust) ports with a flatter angle and a horizontal hook on the B ports. The C (boost) ports have a large up angle. None of this information was helpful with the cruder style passages on a 3 transfer port engine. However, I think the angle cuts at the front edge of the side transfers did somewhat the same thing as the large up angle on the A ports. I would post pictures that show all this better, but I seem to be restricted to tiny picture posts.

Lohring Miller

 

[Tim van Schyndel]

This is all fantastic information!!

At what point does a majority of the transfer of the fuel/air happen up from the crank.

1. Does the majority occur when the transfers first open?
2. How much is being shifted at BDC?
3. When does the draw of the pipe come into effect?

 

[lohring]

Most of the mixture that gets into the cylinder is already in the transfer passages when the ports open. There is a computerized flow simulation somewhere that shows the path of the mixture through the crankcase and into the cylinder. The mixture spends a longer time in the crankcase than you would expect. The crankcase acts as a reservoir that feeds the pulsating flow in the transfers. Most of the flow into the cylinder occurs around BDC.

Look at the pictures in the Propwash article I linked to above. Think about how the pipe is creating the pressure traces. Then look at the flow simulation pictures of the Aprilia cylinder. Read through Frits' paper a few times. I've been working on understanding all this for years. It isn't easy.and I've written articles in the past that show my ignorance.

Lohring Miller