[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4)

By the way, thank you for your advice !

Encierro a écrit:

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What goal to make these shapes ?

Let's make a quiz out of this, Encierro: what are we looking at? (personally I have no idea)

That is the skirt (part of it) just showing a part of the c transfer entrance and at the 3 o clock position (top photo) is the a/b transfer entrance.

I was reading back a few pages, pg14, about what Ken Seeber had to say about shortcircuting to the exhaust.

Citation :

2. Go with a RSA type port layout, but have a diversion valve, say in the B ports that allows this to be connected to a short (throttled?) duct from atmosphere, ie direct inlet to the cylinder, bypassing the crankcase. Even the diversion valve could be eliminated by simply dedicating the B port to external air (& fuel?) only, leaving the A & C ports to provide sufficient flow for starting and getting up to the tuned speed when the B port passage could be opened

First rule is to keep it simple. The engine I was working on is a KTM 85 SX. After some investigation,what I did was to drill a hole into the transfer duct (transfer closest to exhaust port) near the top of the port and install a hose fitting. I then installed a one-way check valve. I did this to each side of the cylinder. Then I ran a hose from the check valve around to a tee connecting it with the other side. I then installed a metering valve into the tee for both sides. The metering valve is open to the atmoshere.
Reesults.....
Increased low end a lot. Had to jet up about 3 sizes. Also increased mid range some. Did not increase top end power that I could tell,(did not have to re-jet main jet). The bike revs a lot quicker.
Total investment is about $30 US. If you don't like the results all you have to do is seal the holes.
The theory behind this is that at low throttle openings the transfer duct fills with atm air which shortcircuts first when the transfers open. At full throttle opening the velocity through the crankcase and up the transfer ducts is pretty high and not much atm  air gets mixed into the mix. Don't have to worry about bearings getting lubed.This bike is used for hill climbing and has won a few championships in it's class. I have not had this engine on a dyno yet and don't know how big I can go with the metering size opening. I'm sure there is an optinum size. I'll try to download some pics.
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Hello:
Reading back a few pages,pg 14, about what Ken Seeber said about reducing shortcircuting. After some research this is what I tried and it seems to work fairly well. The engine is a KTM 85 SX. I drilled into the transfer port duct (the one closest to the exhaust port and installed a hose fitting near the top of the port. I then installed a one-way valve into the hose. I did this on both sides of the cylinder. I then ran a hose around to a tee fitting and connected both sides together. I then fitted a metering valve to the tee and left it open to atm.
Results....  A pretty big improvement in low end performance and some in the mid range. I did not see a difference in top end. I had to increase the low end jet about 3 sizes and changed the needle position. No change in the main jet. Engine revs very quickly. This bike is used in hill climbing and has won a few championships in it's class. Some theory is that at low throttle opening and low load ( low velocity) the transfers have more time to load up with atm air and when the transfers open mostly atm air is shortcircuted out the exhaust. When the throttle is opened and load goes up the velocity increases and now the transfer ducts are full of F/A mix. So the performance gains seem to be 'under the curve' so to speak. Total cost was about $30 US. Pretty cheap and keep it simple.
I'm sure there is a optinum size for the metering valve. I have some pics but don't know how to show them. I tried but it didn't work.

That sounds quite interesting Jeff. Pictures would me most welcome. I know it is not that simple to post them here.
Not too long ago Marc posted a how-to, but I can't seem to find it.

I think it's something like that but it's in french : [Vous devez être inscrit et connecté pour voir ce lien]

jfn2, is there something on the back side of that plastic block that would explain what you're doing? Is it an air inlet port that you can throttle?

Yes there is. Actually, it's on the bottom of the block. When I built this, since I didn't know if it would work or not, I didn't want to spend much time or money so I happened to have this nylon block layin on the work bench so I used it. What I did was to drill a hole thru the block from side to side and tappped both sides for the hose fittings then I drilled a intersecting hole into the first hole and tapped it for the metering fitting which I located on the bottom. It could be located on any side but I hid it on the bottom just to kept the neighbors quessing. You could attach a third hose and run that hose up under the tank and attach a filter to it. Remember, keep it simple.

interesting. so you drill a hole completely through horizontally then drilled another hole vertically from the bottom that intersects in the center of the horizontal hole ( the holes form a T shape) ?  then what did you screw into the bottom of the vertical hole ?

oh i think i understand. you tapped a brass fitting into the bottom hole but it has a small hose barb on it to limit the amount of air that can enter ?

hopefully you get a chance to use a dyno and see exactly what rpm range it improves.

Yes brokedown you are right. What I did was to make a tee fitting from the block of nylon I had laying around. For the metering hole I tapped threads to accommodate a hex main jet from a mikuni carb. This lets both sides of the cylinder to draw from a single metered source. Very easy to change. Remember....keep it simple.

jfn2 a écrit:

What I did was to drill a hole thru the block from side to side and tappped both sides for the hose fittings then I drilled a intersecting hole into the first hole and tapped it for the metering fitting

I have no clue as to what that is (english isn't my natural language). could you explain/picture ?

and did you experiment with different diameters ? if so, what's an optimum ratio to the cylinder volume for example ?

tnx

A metering fitting can be something as simple as a main jet from a carburetor, because thats exactly what a jet is, its a metering device/fitting that flows an exact amount of fuel.

Thank you senso! Only I'm metering air instead of fuel.

JanBros:
I did try different size jets to increase the air. The results kept getting better...so far!
As I said before I know there is a optimum size but I don't know what it is yet. I started with a #320 main jet which has about a 1mm dia hole and so far I'm up to a #470 jet. Bike is running great. As far as an optimum ratio to cylinder volume, that I would assume would depend on carb, pipe, ign and other things, and lets not forget about transfer port tunning. I don't think this is a 'one size fits all' situation. Also RPM's. I would think this would work on smaller size  and lessor tuned engines such as 50cc mo-peds or bikes. But I'm just guessing here.

It's something like this

Frits and Jan i have a question about the a-port design.
As I find the time I work on my own cylinder design for CR125 and MX use.
I try to mimick the APF Y-Type as precise as I can but as I don't have one just out of all your comments and pictures that were posted. I know that MX needs a different characteristic...I try to achieve this with the lowest timings for good STA's.
In the picture I atatched below I would like to know solution had the Y-Type or what would the difference of A to B?
I found some pictures where it really looks like the Y-Type A-port was like the version B but you haven't talked about it yet.

I would asume that A is better against short circuiting and B better for effective area?

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Thank you both for all your comments so far and in advance!

Of course both types A and B were tried.
A proved to be slightly better.
But the difference was very small.

Thank you!

I have some more question about the exhaust duct.

How wide did you made the main exhaust port at the last cylinders? I read messurements of 68% of the bore,
38mm and 40mm. Mine at the moment is 38mm or 70% of the bore.

Did you applied a radius on the port roof so that it doesn't open completly at one time?
I'm also thinking about the piston ring who get's it quite hard if the roof is flat like mine.

What do you say to the overall shape? The aux port go 1mm beyond the bore center.
The minimum wall thickness between aux. ex. ports and a-port is 1,6mm. The exhaust port is raised 2mm from BDC.
The aux. are 186° high and the main 195° including die radius on roof to the bore.
The exh. bridges  are 2mm at the bore and 3,5mm at max. thickness.

I appreciate any comments!

And one more: I like to close the aux. ports below 9000 1/min.
I think it is quite difficult to find a simple solution with a really low volume between bore and valve in the duct.
I just want to know if you  would estimate that there is still be an improvement even with left volume in the duct?

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Thank you for your thoughts!

ninethirtysix i believe jan once said the aux tunell size should be eqaul to the window, so theres no pinch points in the tunell. it appears your bottom aux tunell is slightly narrower than the window but its hard to tell for sure

The radius at port top is for when piston opens into radius on power stroke it doesn't create power robbing exhaust flow edy's.

I have a 2006 CR125 running pretty well. Some things I would incorporate in your casting design. Allow it to be bored to 58MM. I'd set transfers at 127 deg and ex at 188. That gives you some room for hand porting for different applications.

I would buy one of these, if you decide to make one!

At brokedown:
I think it beguiles as the port shape changes to a rounded rectangle at duct exit, means it gains in height and looses a bit in width.
But I will check the area over the length. Thanks!

At williamsmotowerx:
Sorry but since I'm not as good as I want to be in english I don't get your first sentence.
I will build this cylinder for a 2006 model and I'm willingly to sell one to you then..
The CAD model is fully parameterized with a excel so changing them is just few mouse clicks.  
But I don't think it is a goog Idea to hand port the heights or bore this one from 54 to 58 since all the fine details were cancelled out.
I don't think I will go beyond a prototype phase so I probably will choose selective melting processes or rapid casting.  
In both procedures it doesn't make any difference if a design changes a bit.

nine-thirtysix a écrit:

At brokedown:
I think it beguiles as the port shape changes to a rounded rectangle at duct exit, means it gains in height and looses a bit in width.
But I will check the area over the length. Thanks!

.

this is great information from jan

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hi jan and frits i have a question about the height differences between A and B. im sure youve been asked 1000 times why B is higher. i thought it was said that with A slightly lower it allowed the aux exh port to be alittle bigger towards the bottom. besides the aux exh being alittle bigger was there more reasons you left A slightly lower than B ?

No, there was no other reason!

Frits Overmars a écrit:

Haufen a écrit:

would you say opening the A ports last, so that they actually flow first, because they have the longest path to travel would be beneficial in your concept?

I've seen this remark before and I can't understand why anybody would say that. The port that opens first, flows first.

Frits Overmars a écrit:

When the transfers open, the crankcase pressure is about 1 bar absolute, even without any piston pumping effect, so no problems there.

Frits, You have glossed over this before which makes me think that it is questionable thinking. The idea is that there is positive pressure in the combustion chamber when the first transfer port opens pushing the incoming charge back into the transfer port. The secondary opening port starts to flow while the first opening port is regaining its composure and then once does, enters the cylinder.

So if the crankcase has a higher pressure (1 bar) v combustion chamber (.5 bar) there is not enough pressure in the combustion chamber to overcome the crankcase pressure, and all of this is false thinking? Have I answered my own question?