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

brokedown a écrit:

hi frits i might as well ask a question since im here. of all the aspects of a 2t engine, what has the most influence on rpm limit ? pipe, cylinder ports, reed cage, ign timing ?  im gonna try to advance and retard the ign and see if it has any affect on max rpm. it seems my engine is hitting a wall and just cant rev any more.  the ign curve im using is the purple line. i suspect some where between 7-8K (maybe closer to 7K but im not sure) is where the wall is.

All of the above play a role but in your case the ports are the decisive factor. You can go up and down with compression ratio, ignition timing, jetting, pipe lenghts, but you just can't create sufficient blowdown angle.area and tranfer angle.area in that short-stroke Honda CR500 cylinder. I guestimate that maximum power will be found around 7000 rpm.
If you want more revs, fit a shorter pipe. But it will not give you any more power; on the contrary.

thnx frits. there probly isnt much sense trying to rev it higher anyways

Frits Overmars a écrit:

Manuel Rainer a écrit:

the only thing i can do is to grind the shape like i signed in red.

Don't do it.

thanks frits

does this hinders the exhaust flow, or what is the reason?

please Frits can you give me a short explanation.

Manuel Rainer a écrit:

Frits Overmars a écrit:

Manuel Rainer a écrit:

the only thing i can do is to grind the shape like i signed in red.

Don't do it.

does this hinders the exhaust flow, or what is the reason?

The flow is limited by the intruding bulges in the exhaust duct where the studs pass. Enlarging the port areas in the cylinder bore won't improve the flow; it will only reduce the load-carrying area where the piston pushes against the bore.

brokedown a écrit:

thnx frits. there probly isnt much sense trying to rev it higher anyways

That's right.

These big singles are for MX'ing.

From the advent of the * tuned * Maico 501, motorwise, what really improved ?

Quite a few flat track guys tried their hands at making the subsequent 500 cc motors wind-up and go.

To be sure, some did OK, but most success was found on TT tracks.

Nevertheless, tweaking the big two-stroke bangers for higher revs always hits a low ceiling.

Otherwise, since the 60's, real race bikes from 250 cc & up would have remained as singles.

You know, reciprocating components must be strong, yet light.

And, if the big single smokers could be made to hold together with sustained R's over 10 K, what about the lubrication ?

Running a hard-hitting 500 cc single with knobbies on a MX track is one thing.

Trying a similar motor with Class C tires on a smooth, hard-packed dirt oval that's drying out in 100 * F weather is quite another deal.

On paper, the two-stroke's power-to-weight ratio looks to be a winner, but on the smooth DIRT track, the rear tire is always spinning, or close to breaking loose !

That's never a good combo.

Back in the mid-70's, the Bultaco 360's were no faster than the Jappers, but the Bul's crank wheels were stout and they used a heavy, internal sprocket / flywheel off the crank and the magneto flywheel was similarly heavy.

Bottom line = those motors at only 360cc would rev-up and put the power to the ground. Nearly all of the Jap stuff had less rotational weight , felt fast when running up & down the STREET, but once on the dirt track the quickness was not an advantage. Again, I'm referring to racing HARD on smooth tracks without knobby type tires that can dig in.

As I understand, you've run your motors in drag races, so with sticky tires, traction is not a problem for you. That's why you want and need as many R's as you can get, but more than likely, you've already peaked-out due to the physical limitations of a 500 cc, two-stroke single.

If you were to start running methanol with a big nitro spike %, you'll find more power alright, but the price will be what's revving to the moon.


Hey, on adding the photos....I got it. I do have a photobucket account. Thing has been balky and therefore I was not able to do a test. It appears I need an OS newer than a Bultaco ! lol

Buen Suerte y Gracias , Amigos !

ambike a écrit:

brokedown a écrit:

thnx frits. there probly isnt much sense trying to rev it higher anyways

That's right.

These big singles are for MX'ing.

im not sure i would want to try mx on one, considering the modern day ktm300 has just as much horsepower (and a exh powervalve), only slightly less torq and its nearly 15 lbs lighter.   . 500cc is good for the sand though  

hello. I have one question... I´m building rotary valve piaggio scooter engine (already running) with ktm sx50 cylinder. But the one big problem is, when I want to install this cylinder, I need 100mm conrod.
what do you think? isn´t too much for 39,3 stroke? what complications it can bring?
thanks

some pics from building the engine: [Vous devez être inscrit et connecté pour voir ce lien]

alcatelko a écrit:

I´m building rotary valve piaggio scooter engine (already running) with ktm sx50 cylinder. But the one big problem is, when I want to install this cylinder, I need 100mm conrod.
what do you think? isn´t too much for 39,3 stroke?

I think a 100 mm rod for a 39,3 mm stroke is not too much. But if you find such a rod, it will probably have been designed for a larger engine, so it may be rather heavy for a 50 cc engine.

I´m able to make own conrod...
we have also cnc made pistons from solid material (forged Al powder which is thermal stable to 450°C) which have 40g (stock malossi mhr team piston has 53g)

Frits, also a question about engines stopping breathing with higher rev's becasue of port time-area.
I'm preparing a Honda Sky to enter Moped Wars (something like "De Zwarte Cross" in Holland). I'm not going to put much money in it, so I'll be using the standard cylinder. It has 5 transfer ports, so that is good, but it's timing is only about 107°. rougly measured the port area's are transfer A 11x11.5, transfer B 11x7 and transfer C 11x10. it's a 39 bore x 41.4 stroke and I'll be using a MTX80 91mm conrod. I've gotten the exhaust timing to +/- 182° (wasn't material for more, and with the low transfer timing, I recon higher isn't needed).

could you approximatly tell me at what rev's the engine will stop breathing ? Need it to calculate the expansion pipe. tnx in advance.

Sorry Jan, even a rough calculation would take over an hour and I'm afraid I cannot find that amount of time. But elsewhere in this forum I've tried to explain how you can calculate angle.areas and how you can compare them to the Aprilia RSA-values. That way you can find the optimum rpm for your engine.

Hi, I'd like to hear opinions on something I've been thinking about not exactly related to these subjects.
I'm an electronic engineer and I have this idea of using acoustics to determine an expansion chamber resonance. I thought I could use a sub-woofer speaker and craft a manifold like a funnel to connect the front of the speaker to the exhaust and then use a frequency generator and go through the frequency scale while analysing the reactance or the amplitude.I even thought of using an aggressive wave form like sawtooth to be similar to the explosion crackle.
I know this only helps to establish the chamber /bleed ratio, but at least it might help to make more sense of what has changed when a cut n try approach is used.
Any thoughts?

motoholic71 a écrit:

Hi, I'd like to hear opinions on something I've been thinking about not exactly related to these subjects.
I'm an electronic engineer and I have this idea of using acoustics to determine an expansion chamber resonance. I thought I could use a sub-woofer speaker and craft a manifold like a funnel to connect the front of the speaker to the exhaust and then use a frequency generator and go through the frequency scale while analysing the reactance or the amplitude.I even thought of using an aggressive wave form like sawtooth to be similar to the explosion crackle.
I know this only helps to establish the chamber /bleed ratio, but at least it might help to make more sense of what has changed when a cut n try approach is used.
Any thoughts?  

Hi Motoholic, your proposal takes me back a hundred years, when I had just about the same idea. Back then I did not have a frequency generator at my disposal, but I used a simple ventilator. The fan blades passing over the open end of an exhaust pipe excited the air column inside, and by varying the fan's rpm I could find a resonance frequency. But there was hardly any correlation with engine behaviour. Here's a little story that may explain why:
[Vous devez être inscrit et connecté pour voir ce lien]

Nevertheless, your training will be of great value when researching resonance phenomenae. I only wish I had studied electronics as well.

Ha ha I hope those were good memories, it has occurred to me already that this was too obvious and simple to not been tried by some one before, I thought it years ago as well.
That's very clever, your ventilator design, I'm sure you'd be great at electronics.
I do have a pre-historic function generator but it's out of commission, I don't have patience anymore to measure every single component to get those analogue tank circuits ticking at the right pace, so I'm using one of the readily available freeware options from the net and the sound card hooked to an amp . I have already used this base as a bench to trigger ignitions or ypvs boxes by using the amp to drive a transistor switch.

Anyway I had very nasty setback with this financial crises in my country so I had to move and put several 2 stroke projects on hold until I can set a new lab/workshop which reminds me I did some testing on some automotive oxygen probes and found the output signal from them to be very friendly so was thinking about sketching a circuit to get a reading and use it as an aid to jet carbs. Was an O2 probe used to tune the Aprilia?Better yet,do you know if these probes will survive in a 2 stroke application?

Thank you very much for your time

motoholic71 a écrit:

Was an O2 probe used to tune the Aprilia?

No, but I got some good results with them while dyno-testing other two-stroke engines.

Citation :

do you know if these probes will survive in a 2 stroke application?

They are not doing too bad. But there are three points to observe.
1: do not use leaded fuel.
2: do not fit the probe too close to the cylinder, otherwise washed-through fresh mixture will cool it and contaminate it.
3: make sure the probe's heating is turned on before starting the engine, so any oil that hits it, gets burned off.

Sonde Lambda, in french...

Marc a écrit:

Sonde Lambda, in french...

Lambda Sonde in german, Lambda sensor in dutch. O2 probe in 'merican because Lambda is too greek (culture overdose) for them  .

Thank you Frits for your precious advice.

no prob Frits, I wasn't expecting that you'd calculate anything, just hoped you could make an estimated guess with all your experience .
guess I'll have to dig in the topics to find your post (if only the search function of the forum was a bit better   )

JanBros a écrit:

...if only the search function of the forum was a bit better.

I know the problem. At the left side of the page, below "Rechercher" you can choose between "Interne" and "Google".
Always select "Google"; it works a lot better.

hi frits i have a question about time*area. i dont really understand how it could be universal if it doesnt take into account how good or bad the ducts are, or case volume, or pipe, or inlet type etc. i can see how a window of X amount of size, can pass X amount of volume, in X amount of time and that seems universal to me. but because nothing else is considered, it isnt making sense to me.

like you said some time ago, a duct designed by jan would be much superior to what most anyone else could do. so if the rsa needed X amount of time*area, it doesnt make sense to me that we could simply use the rsa time*areas in a cr125 for example and expect great results. simply because every part of the rsa is far better than what you would see in the cr125

Hello there,im newbie in the forum!Im a wannabe tuner(!) and I try to learn as much as possible for combustion chamber design,port timings,exhaust design and pressure waves etc....
I'm planning for a new project,tzr125 engine,wr200 3xp cylinder bored to 69mm with 57mm stroke without powervalve,39mm carburetor,cr500 reed valve(the air-cooled)    and handmade exhaust,I'll try to pull out about 45hp from this,for road use
I just want to tell a big thank you to mr Frits and Jan,for their knowledge publishing!
And a question to mr Frits,
I prefer to do comb chambers with wide squish-more than 50% bathtub design like Aprilia's,with big radius and make them as shallow as possible,something like 9 or 8mm depth!
What's your opinion about that?What was the depth of the rsw-rsa's heads?
Cheers!

lef maybe this is what you need [Vous devez être inscrit et connecté pour voir ce lien]

Thank you brokedown,i need to read the previous post but it needs time to reed 150+ posts
Mr Frits can also explain us how different height's work and what excactly is affected from shallow and deep chambers.

brokedown a écrit:

hi frits i have a question about time*area. i dont really understand how it could be universal if it doesnt take into account how good or bad the ducts are, or case volume, or pipe, or inlet type etc. i can see how a window of X amount of size, can pass X amount of volume, in X amount of time and that seems universal to me. but because nothing else is considered, it isnt making sense to me.
like you said some time ago, a duct designed by jan would be much superior to what most anyone else could do. so if the rsa needed X amount of time*area, it doesnt make sense to me that we could simply use the rsa time*areas in a cr125 for example and expect great results. simply because every part of the rsa is far better  than what you would see in the cr125

Brokedown, you are right in assuming that angle.areas are only part of the story. What you need to move mixture from A to B is, in order of importance, a pressure difference, adequate angle.areas, and a good flow coefficient.
If you base your engine on the angle.areas I presented elsewhere, you will be in the right time zone. It does not mean that you will have a great engine. But If you try to correct flow deficiencies by raising port timings beyond the RSA values, you will run into other problems, so I think it is best to stick with those RSA angle.areas.