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

Frits Overmars a écrit:

granjoie a écrit:

In full design of an admission by rotary disc for engine of solex of competition, I like to know how to define the good timing opening and closing of the valve?  What did you note like profit and loss compared to a standard valve admission? There is  it a large difference of adjustment of the carburettor and it's diameter?

A rotary disc engine can produce about 10% more maximum power than a reed valve engine. The carburettor diameters can be roughly the same.
Your rotary disc should begin to open at 40° after Bottom Dead Center and should be fully closed again at 75° after Top Dead Center. Afterward you can experiment with earlier opening and later closing timings.
The above timings are milder than in the Aprilia RSA, but for Solex-racing you will need a much, much broader power band.
A trombone exhaust pipe would be ideal for a Solex. And a good clutch setup will be more important than horsepower.

You say it is necessary to open 40° after bdc, when transfert ports are always open, I read that much says to open has the closing of the transfers ports(62° for me),what do you think about that?
Which influence on the driving behavior?
Thank you very much

[quote="brokedown"]hi frits i have a question. with inline twin cylinders spaced close together it puts the transfer tunells in a poor shape. mostly straight up and sharp turn toward the cylinder, hardly any radius on the inner tunell wall either. in a situation like this i was thinking it might be better to have the A and B ports aimed slightly higher than normal, mostly to decrease the sharp turn into the cylinder.  im thinking it should also prevent the mixture from slowing down as much

if the piston top was 10* the B ports would be set between 15*-20* upward angle. the A ports would be set between 30*-35*

does all this sound logical enough to give it a try or is my thinking wrong ?

[Cramped transfer ducts are not necessarily a disadvantage eg snowmobile engines.The 600cc SnoX Polaris twin was developed by Hentges Racing to produce over 170hp at around 9000rpm,not bad for an engine with cramped transfer ducts and reed valves.Dyno test results are here [Vous devez être inscrit et connecté pour voir ce lien]
If you want some ideas have a look at a Skidoo 800XP cylinder.]

im not a expert on snomobile engines but there must be some reason they space the cylinders so close together. seems like they are typically mostly over sqaure also. the reasons why im not sure. perhaps simply to keep the engine compact ?

some snomobile engines probly run fairly well, considering the less than perfect design but we know for certain that straight up transfers dont work as well as a sweeping radius. jan and frits have done all the leg work for us and we can learn from their many years of knowledge

i dont really like working with these cylinders that have straight up transfers but these engines are cheap and plentiful around where i live. plus im all new to this type of cylinder and looking forward to the challenge of making them perform much better than what the factory had intended. ill have a look at the skidoo and see if it can give me any ideas that i can apply to my yamaha cylinders

A 600cc V6 designed using the Aprilia parameters Jan and Frits spent years perfecting would produce an expected 260 HP .Significantly more than any snomo.Oversquare,ultra wide bores,DI,2 and 3 into one chambers all decrease the potential of these motors.

The snomo motors have a different philosophy than a GP motor in part due to emissions but also due to the drive system,jetski motors as well.

Snomo motors have PVs and are tuned to narrow powerbands.

In advance,sorry if it has been written before(ive only manage to read the first 2 parts till now ).
These days I try and study Engmod2t software,especcialy the exhaust making-subprogram.It have 3 ways for calculating ehxuast.Now i consider on the third coice,witch called Hi-Performance pipe,and it's more like the Aprilia's exhaust design.The thing is that the design is different than the usual modern and blair-type exhaust.
I'm used to exhaust designs where every diffusor part has bigger angle than the previous.Here the third diffusor always has smaller angle than the second-just like aprilia's-and I try to understand how it works on the dyno,and how the pressure waves behave through that spot,and generally through all the exhaust.
I'm building an exhaust for a moped right now,build only for 0-402m 57x51.8mm and 12500rpm.This is the exhaust:
[Vous devez être inscrit et connecté pour voir cette image]
Mr Frits and Jan can tell us more for sure!
Sorry to disturb you,and sorry for my English  

CRECY a écrit:

brokedown a écrit:

hi frits i have a question. with inline twin cylinders spaced close together it puts the transfer tunells in a poor shape. mostly straight up and sharp turn toward the cylinder, hardly any radius on the inner tunell wall either. in a situation like this i was thinking it might be better to have the A and B ports aimed slightly higher than normal, mostly to decrease the sharp turn into the cylinder.  im thinking it should also prevent the mixture from slowing down as much

if the piston top was 10* the B ports would be set between 15*-20* upward angle. the A ports would be set between 30*-35*

does all this sound logical enough to give it a try or is my thinking wrong ?

[Cramped transfer ducts are not necessarily a disadvantage eg snowmobile engines.The 600cc SnoX Polaris twin was developed by Hentges Racing to produce over 170hp at around 9000rpm,not bad for an engine with cramped transfer ducts and reed valves.Dyno test results are here [Vous devez être inscrit et connecté pour voir ce lien]
If you want some ideas have a look at a Skidoo 800XP cylinder.]

No good looking peak figures from sled engines, a CVT means they can have a 500rpm wide power band, with a cylinder already coming from the factory fairly optimised for out and out peak power. Not much translates to a motorcycle engine that needs a 3-4000wide band and to be good at part throttle.

Hello, Frits (and hello to all),

First, I am concerned that you might not have seen my thank-you for the timing specs you offered for the little KTM cylinder on the Kiwi site (I had hoped to PM this part, but cannot):

(Quote myself) Good heavens, Frits, you are handing me my engine on a silver platter! I'd never have asked you for actual port timing, just general questions, figuring it's my place to cut-and-try. Very much appreciated, understanding that it has taken you many years to be able to state such figures with confidence. I'm still shaking my head at your generosity; Merry Christmas indeed!! One way or another (with some computer-savvy pal holding my hand) I'll supply photos of the build. (end quote).



Also many thanks for pointing me to this site .  .  .  and a question based on what I've seen here so far, about 25 pages into Part 1. To the rest of you gentlemen, when I puzzle out the French I will introduce myself (an old man who raced alcohol-burning outboard hydroplanes long ago and hope to again) and explain a little about outboard racing in the USA.

I've been thinking about how your 24/7 valveless intake might apply to outboard racemotors. As I think I said to you elsewhere, I have a converted (to outboard racing use) motorcycle engine ('73 Yamaha 125MX, with intake tract opened up for a bigger reedblock) that is obsolete for racing but is handy as a dyno-mule.

In alky outboard racing we can split the carburetion duties among as many carburetors per cylinder as we like. This old Yamaha has its reedblock in what was the piston-port in the side of the cylinder, as you surely know. But (long ago) I have also put a small carb-entry into the side of the crankcase, opposite the cylinder. This was sized to take a little Tillotson floatless pumper carb (converted for alcohol) atop a McCulloch kart reedblock, a practice that a few outboarders were trying long ago. The "main" carburetor (into the cylinder) might be a 32 to 34mm Konig carb (crude, but okay for us), and the auxilliary Tillotson pumper was a tiny thing.

I'm thinking that for outboards, especially for small, low-torque classes like 125 and 175cc, any 24/7 valveless intake should be this small auxiliary type. Without a gearbox, the engine has to make enough mid-range power to get the boat on-plane and going fast enough to get on the pipes. As I described elsewhere, we do slide the pipe(s) back, trombone-fashion about 4 inches, to broaden the working range a bit. but still, I'm thinking we need to make good power even before we go to the auxiliary, valveless intake.

As you have figured out already, having an auxiliary carb means we don't have to fabricate a reedblock that swings out of the way, as you do. We simply build a progressive throttle linkage which leaves the auxiliary carb butterfly entirely closed until we get up to where the pipe comes in. There are various ways this might be set up; as with the slider-pipes, it could be manually-controlled by the driver in at least three ways I can imagine.

Before setting up the engine to test the idea, besides having a variety of carbs (and jets, et al), I'd want to make up some aluminum spacers to try under the aux. carb, and under the reedblock of the main carb. When I built this engine, long ago, I mounted the ignition sensor on a moveable stator-plate that has a fine-threaded push-pull screw so I can move the spark timing back and forth with the engine on the test stand. And the pipe is adjustable on the stand, as on the boat.

Thoughts?  Anyway, I've got to give your very-cool concept a try, next spring/summer after I build my low-tech dyno cell (I have been in the process of moving 20 miles, building a new shop, etc.; what a monumental pain!!).

Thanks again for your interest, sir!!

seattle smitty a écrit:

I am concerned that you might not have seen my thank-you for the timing specs you offered for the little KTM cylinder on the Kiwi site

I've seen it Smitty. You're welcome.

Citation :

I've been thinking about how your 24/7 valveless intake might apply to outboard racemotors.... I have a converted (to outboard racing use) motorcycle engine ('73 Yamaha 125MX, with intake tract opened up for a bigger reedblock)... I have also put a small carb-entry into the side of the crankcase, opposite the cylinder.

You just made a hole in the crankcase and fitted a second carburettor? Yes, that would qualify as 24/7 allright.

Citation :

having an auxiliary carb means we don't have to fabricate a reedblock that swings out of the way, as you do. We simply build a progressive throttle linkage which leaves the auxiliary carb butterfly entirely closed until we get up to where the pipe comes in.

The idea was proposed here not too long ago.
I answered by showing a video and some pics of an engine with just that layout, albeit with a piston-controlled original carburettor instead of your reed-controlled carb. I would give you a link to it, but I am terribly bad at finding things back on forums. And searching for '24/7' won't do you much good; I just tried it and got 754 hits.

Citation :

Thoughts?

Yes, I am very curious; you should try to figure out how to post pictures here.
I know, it's a pain; every forum has a different procedure and none of those seem to be very intuitive.

G'day Smitty,

Can you please send me a pm to : [Vous devez être inscrit et connecté pour voir ce lien]

Ciao for now,,Bob van der Zijden

Frits Overmars a écrit:

seattle smitty a écrit:

I am concerned that you might not have seen my thank-you for the timing specs you offered for the little KTM cylinder on the Kiwi site

I've seen it Smitty. You're welcome.

Citation :

I've been thinking about how your 24/7 valveless intake might apply to outboard racemotors.... I have a converted (to outboard racing use) motorcycle engine ('73 Yamaha 125MX, with intake tract opened up for a bigger reedblock)... I have also put a small carb-entry into the side of the crankcase, opposite the cylinder.

You just made a hole in the crankcase and fitted a second carburettor? Yes, that would qualify as 24/7 allright.

Citation :

having an auxiliary carb means we don't have to fabricate a reedblock that swings out of the way, as you do. We simply build a progressive throttle linkage which leaves the auxiliary carb butterfly entirely closed until we get up to where the pipe comes in.

The idea was proposed here not too long ago.
I answered by showing a video and some pics of an engine with just that layout, albeit with a piston-controlled original carburettor instead of your reed-controlled carb. I would give you a link to it, but I am terribly bad at finding things back on forums. And searching for '24/7' won't do you much good; I just tried it and got 754 hits.

Citation :

Thoughts?

Yes, I am very curious; you should try to figure out how to post pictures here.
I know, it's a pain; every forum has a different procedure and none of those seem to be very intuitive.

I'm having an interestiing time trying to figure out the French so I can do anything on this site. FWIW, I have always admired the French intelligentsia who insist upon maintaining some discipline for use of the French language, and rejecting some of the more repulsive American slang that is spread around the world by our politicians, pop singers, and Hollywood. Too bad our own academics are not concerned with standards of discourse. (But then maybe we would be prohibited from employing such cyber-contractions as "FWIW"!!)
Like most of us Yanks, I've been too distracted (lazy!!) to learn languages (45 years ago I could sort of get by in Japanese, but lost it from disuse). This is changing, and we are all having to learn Spanish to talk to the Mexicans who are evidently trying to steal back the lands we stole from them, LOL.

End digression, back to 2-stroking. Frits, I think you missed seeing something. Be assured that neither I nor any of the other outboard racers of years ago ever anticipated your valveless intake idea; personally, I'm not that smart!! When I "made a hole in the crankcase" for the little auxiliary carburetor, I did put a reedblock there. Only now, thinking about your idea, am I considering discarding the reedblock, and linking the carburetor butterfly to be closed until the engine gets on the pipe. We have always used full-coverage butterflies and other carburetor valves that close entirely when the driver releases the throttle. We don't want to have the engine left idling when we happen to get thrown out of the boat .  .  .  .

Bob, I'll try the email address. (EDIT) Email message wouldn't connect with the address you gave me.

Its
bobrace@xS4all.nl

If you, Yankees, begin to let go all the lands you have stolen, I hope you are very good at swimming
ps ; please do not swim east, we are also sinking.

Dear All, what a fantastic thread this is.....congratulations to all involved.

My question(s) please. My RSW250 (2004 APC, vhsd41, ac38) is fantastic to race in all but ONE aspect, that being its sensitivity to jetting. I infer from Jan Thiel's comments that the Dellorto Carb is not the best design. Did Jan try the Keihin 39mm power jet carb? It is very forgiving to jet on a Honda 250GP, although this could be due to engine design rather than carb design..... Is there a reason why I should NOT try it on the RSW250?

Fuel: I have always run the RSW250 on super-unleaded 98RON (because it came set up for this fuel). However, I run ALL my other racing 2-strokes on Avgas 100 MON LL because I get better mid range once head volume, fueling and ignition advance have been optimised. Further, the "detonation propensity per jet reduction" is less with Avgas. In other words, being one jet out doesn't mean piston erosion! Are there any unexpected problems ahead if I try Avgas on the RSW250.

Thanks,

Paul.

Was the difference between sparkplugs tested on RSA engines?
Where surface discharge plugs tried?

Thanks!

Captain Scarlet a écrit:

Dear All, what a fantastic thread this is.....congratulations to all involved.  

My question(s) please. My RSW250 (2004 APC, vhsd41, ac38) is fantastic to race in all but ONE aspect, that being its sensitivity to jetting. I infer from Jan Thiel's comments that the Dellorto Carb is not the best design. Did Jan try the Keihin 39mm power jet carb? It is very forgiving to jet on a Honda 250GP, although this could be due to engine design rather than carb design..... Is there a reason why I should NOT try it on the RSW250?

Fuel: I have always run the RSW250 on super-unleaded 98RON (because it came set up for this fuel). However, I run ALL my other racing 2-strokes on Avgas 100 MON LL because I get better mid range once head volume, fueling and ignition advance have been optimised. Further, the "detonation propensity per jet reduction" is less with Avgas.  In other words, being one jet out doesn't mean piston erosion! Are there any unexpected problems ahead if I try Avgas on the RSW250.

Thanks,

Paul.

The sensivity to jetting is mainly due to late inlet closing.
We never tried a Keihin carb, but we used the Keihin powerjet with very good tesults.
Running your Aprilia on Avgas will give no particular problems.

Senne s a écrit:

Was the difference between sparkplugs tested on RSA engines?
Where surface discharge plugs tried?

Thanks!

I am very much in favour of surface discharge plugs!
But they need a higher tension, and therefore a better quality ignition coil.
When we tried them our usually used coil failed.

The sensivity to jetting is mainly due to late inlet closing.
We never tried a  Keihin carb, but we used the Keihin powerjet with very good tesults.
Running your Aprilia on Avgas will give no particular problems.[/quote]

Jan when does the late closing become to much of a problem with the jetting, is there a clear point at which it is detrimental or are the power gains worth the trouble.

RAW a écrit:

The sensivity to jetting is mainly due to late inlet closing.
We never tried a  Keihin carb, but we used the Keihin powerjet with very good tesults.
Running your Aprilia on Avgas will give no particular problems.

Jan when does the late closing become to much of a problem with the jetting, is there a clear point at which it is detrimental or are the power gains worth the trouble. [/quote]

The late closing works in combination with a big crankcase volume.
The gains are certainly worth the trouble!

Jan, would you please clarify, When you say it is worth the trouble do you mean the trouble in the tuning or are you saying the trouble in the work required to alter the crankcase volume so it will then work as intended

my guess is that the "trouble" is the sensitive jetting ?

JanBros a écrit:

my guess is that the "trouble" is the sensitive jetting ?

Yes!

RAW a écrit:

Jan, would you please clarify,  When you say it is worth the trouble do you mean the trouble in the tuning or are you saying the trouble in the work required to alter the crankcase volume so it will then work as intended

The trouble in tuning!

Greetings.
I have a question for Jan or Frits please with regard to flow modification.

Firstly, did you investigate using any amount of dimpling in the ports such as these treatments available by CNC, or does the tight curvature and draft angles of the two stroke transfer ports negate this as a tool woth pursuing:
[Vous devez être inscrit et connecté pour voir cette image]

And secondly, did you ever experiment with wet flow testing to observe the homogenicity and port wall interaction of the mixture in the port. Pictured is an american V8 head in translucent resin for such studies:
[Vous devez être inscrit et connecté pour voir cette image]
Image sourced from [Vous devez être inscrit et connecté pour voir ce lien]

Many thanks in advance for your consideration and expertise.

Jan, Frits, or anyone who may have already tried this.
Would the oval exit to the cylinder port work on a single exhaust port engine (tz350). From memory I think the port exit is 39.5mm in diameter.
No replies.
Was it a stupid question, or are all the TZ owners doing it already
The exit of the exhaust port looks exceptionally large compared to a RS125 H@#&da cylinder I have.
Am I
Thanks in advance
P G