[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked)

Jan or Frits:
"The radius of the exhaust duct does NOT follow the piston radius!
Instead the volume of the exhaust duct was reduced as much as possible.
This improved flow, as seen on the flowbench.
And above all, it improved blowdown!
Improving blowdown without raising the port height is, of course, a very good thing!
Reducing the exhaust duct volume also gives stronger pressure waves."

When my son and I were racing go-karts years ago we used a reduction of about 5% between the cylinder port window area and the exit port window area and then jumped up to the pipe size at the pipe manifold. Is this about the right percent reduction? Thank you,Jeff

jfn2 a écrit:

When my son and I were racing go-karts years ago we used a reduction of about 5% between the cylinder port window area and the exit port window area and then jumped up to the pipe size at the pipe manifold. Is this about the right percent reduction?

A simple question Jeff, but impossible to answer. Which percentage of your port window area was blowdown area? Wat was 'the pipe size'? Etc...

This drum throttle body is the same length as the carb it replaces. It is approx 100 Grams lighter when finished and smaller in all other dimensions, part of the bellmouth is machined into the drum. It is important to have the TPS linked to the throttle mechanisim as accurately as possible. I have had trouble with " in cable type" systems. Any linkage will add some inaccuracy. Butterflies restrict flow and disrupt pulse waves, I don't like butterflies. When the drum valve is open there is no restriction, needles, slots in the walls, just a nice big smooth hole! This is to replace a carb on a YZ250 ( 39mm ). This drum is finished ID at 40mm and could be made out to 42mm. If bigger units are needed I'll just make a bigger pattern. The small threaded lug is there if we need to run an oiller in the future. We have other plans for a different injection system again that will require an oiller for the crankcase, but thats later.

That makes good sense, thanks. I wonder how important pressure pulses are in the inlet tract?
A butterfly story to confirm your dislike: I have Ducati 916 50mm tb's on my SV650. With the bypass screws fully in
and the idle stop fully out, it idles at 2000 cold and 2500 once warm! I suspect Ducati deliberately mis-tune to pull the idle speed down.

husaberger a écrit:

As it was i who asked the Question of Jan and Frits regading the Twin Disks. I asked it only because Jan himself had previously brought it up and i hoped he would expand on it. From What i understand Jan was involved in the RSA.................
I really sincerly apreciate the response recieved from all the people who contribute both questions and answers but especially the time devoted by both Jan and Frits

I wasnt saying about anyone specific,it aplies to myself too you know(like now...)

XpTpSMTT a écrit:

Jan Thiel a écrit:

...................
If I had to make a new engine today I would certainly NOT make another RSA!
And it would have a straight exhaust..............

So what path would you take,generaly?two rotary?
belt primary drive as mentioned before?Am i asking too much?

I know what was brought up previously very well,and how privelaged we are from their involvement here.

Is there any rule of thumb, on where the optimal piston pin position is? I'm referring to the distance from the top border of the piston to the center of the piston pin, the so called "compression height".
I'm making a 52mm bore (stroke 46mm, conrod 95) cylinder and I have a few piston options; one has the compression height 24,5 and the other 28,5mm.
So I wonder which one would be the optimal choice (disregarding the crankcase volume difference).

Thank you for any answers.

roost a écrit:

Is there any rule of thumb, on where the optimal piston pin position is? I'm referring to the distance from the top border of the piston to the center of the piston pin, the so called "compression height".
I'm making a 52mm bore (stroke 46mm, conrod 95) cylinder and I have a few piston options; one has the compression height 24,5 and the other 28,5mm.
So I wonder which one would be the optimal choice (disregarding the crankcase volume difference).

Thank you for any answers.

24,5 should be the best.

hi everyone.
I'd like to ask Jan or Frits:
About the part throttle pre-ign.,was this happening at high rpm,and the exhaust valve open?

could an electric controlled enrichner fuel circuit,like a solen.valve that would choke the main air jet be used to kill this knocking or this would be eneffective or bring adicional problems?

could you please explain how was this controlled and what was tried?

I'm still catching up on this topic I'm about 20 pages back so if this was mentioned please excuse me,I'll get to it eventualy.

Thank you so much

Your explanations have been like a revelation,I've been reading about 2 strokes for years including the known and the obscure authors including a bunch of (mostly)Japanese S.A.E. papers,academic publications and experiments etc,everything I could get my hands on.
But this pages have prove more worthy than alll that,when I look at an engine now I can imidiatly understand most of it's limitations
Thank you

motoholic71 a écrit:

hi everyone.
I'd like to ask Jan or Frits:
About the part throttle pre-ign.,was this happening at high rpm,and the exhaust valve open?

could an electric controlled enrichner fuel circuit,like a solen.valve that would choke the main air jet be used to kill this knocking or this would be eneffective or bring adicional problems?

could you please explain how was this controlled and what was tried?

I'm still catching up on this topic I'm about 20 pages back so if this was mentioned please excuse me,I'll get to it eventualy.

Thank you so much

Your explanations have been like a revelation,I've been reading about 2 strokes for years including the known and the obscure authors including a bunch of (mostly)Japanese S.A.E. papers,academic publications and experiments etc,everything I could get my hands on.
But this pages have prove more worthy than alll that,when I look at an engine now I can imidiatly understand most of it's limitations
Thank you

The auto-ignition occurred at high revs and part throttle.
So the exhaust valve was open.
We used a powerjet to weaken mixture at high revs.
The only solution on-track was to weaken the mixture less than was necessary for max power.
So this caused a certain power loss!

XpTpSMTT a écrit:

XpTpSMTT a écrit:

Jan Thiel a écrit:

...................
If I had to make a new engine today I would certainly NOT make another RSA!
And it would have a straight exhaust..............

So what path would you take,generaly?two rotary?
belt primary drive as mentioned before?Am i asking too much?

I know what was brought up previously very well,and how privelaged we are from their involvement here.

I would make a twin-disc engine if I had to make a new engine now.
And it would have a straight exhaust.
A belt for the primary drive would not be necessary.
That would be good for a kart engine that has the inlet flow blowing against the crankshaft rotation!

Jan Thiel a écrit:

The auto-ignition occurred at high revs and part throttle.
So the exhaust valve was open.
We used a powerjet to weaken mixture at high revs.
The only solution on-track was to weaken the mixture less than was necessary for max power.
So this caused a certain power loss!

Thank you!
I'm thinking maybe it was possible to minimize this with T.p.S. mapping,I see you had this progressive servo mechanism on the carburettor.

The belt primary would make a very simple,light and probably cheaper engine design,it also crossed my mind,H.D. choppers have been using it for years.
Alloy pulleys instead of heavy steel gears,no backlash accuracy,single shaft through the side case,less oil,silent running,maybe the clutch basket could be the pully and the rubber belt itself cushion the mechanical shock,exhaust facing back....I'm just saying it sounds good to me!

Otoh, Ducati needed quite substantial belts just to drive the cams... which they've now abandoned them for chain drive on their production bike. On their motoGP they use the same as Yamaha and Honda: gears!

I'm with you, gears are where it's at!

The belt primary drive and twin disk configuration where mentioned for two different reasons-objectives.
Belt for reversion of rotation of crankshaft in relation to the location of the carburetor and exhaust.
The ''twin disk* dream"to achieve the benefits of the rsa engine with the chassis(balance)advantage of the rsw.
All together meaning
straight rear facing exhaust,
symetrical mixture flow in crankcase
(NO counter rotation needed)
Better wheight distribution of bike.
I guess this could be our imaginary RSJT!!!!
And fuel injection for the cherry on top!
edit:*and I thought it was a new idea until I stumbled on this:
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GrahamB a écrit:

Otoh, Ducati needed quite substantial belts just to drive the cams... which they've now abandoned them for chain drive on their production bike. On their motoGP they use the same as Yamaha and Honda: gears!

I hear you,but that is camshafts where synchro accuracy is imperative.

At 17K r.p.m. the strength demand should be massive.I feel that's not a suitable comparison.

For a primary transmission the belt elasticity could actualy help us.

Harley Davidson has toothed belts on most if not all their engines,and though low r.p.m.,they have high torque.

I'm pretty sure some R&D with design and material would yield positive results for small power loss and reliability.

For instance:Big pulleys will have fast perimetral speed,but reduced torque and wide bending angle.

Think the possibilities:Less 2 expensive and heavy gears,no primary cover necessary,no built in cushion shock device,since the belt is rubber and so possible direct drive on clutch basket.

Just a single input shaft with an oil seal and grease packed 2RS bearings for the basket.
Possibly a 3rd pulley for water pump.

Crank cases without side covers!simple, cheap!

I think it's very possible a suitable belt exist already on some industrial or automotive catalogue.

Only possible drawbacks I can think of is bad power efficiency and/or reliability but who has serious development done on this?Beside H.Davidson?

motoholic71 a écrit:

I hear you,but that is camshafts where synchro accuracy is imperative.

At 17K r.p.m. the strength demand should be massive.I feel that's not a suitable comparison.

It was dropped as you say by ducati as it was not as acurate (due to cyclic variations) for cam timing.
Bordi wanted rid of it in the early 90's along with the roller mains but wasn't allowed due to tooling costs.

motoholic71 a écrit:

I'm pretty sure some R&D with design and material would yield positive results for small power loss and reliability.

Only possible drawbacks I can think of is bad power efficiency and/or reliability but who has serious development done on this?Beside H.Davidson?

a suitable belt exist already on some industrial or automotive catalogue.

Yes as used by almost all modern classic racers G50 Manx Goldie Triumph comando etc
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Forget those wide things you see on Choppers 20-25mm would be more than ample for a 50hp 2stroke
Newby Hayward etc. Most can be run dry so no adding heat either.
A belt is actually supposed to be a couple of percent more efficient than chain or gears.

I understand the efficiency rates rate like this
Belt then Chain (assuming it is it great condition) then straight cut gears.

A belt was proposed from frits (with the CVT) but it may also offer a way to tidy up a 4 disk valve twin.


Of course i would be more interested in the major porting changes between RSV and RSW. What was the changes? Was this when the deck was first closed

Howard:
How did your epoxy hold up? Did you post cure it as was suggested? I'm about to do the same thing to some bike motors.
How's the racing going? What is the name of your driver?
Jeff

husaberger a écrit:

motoholic71 a écrit:

I hear you,but that is camshafts where synchro accuracy is imperative.

At 17K r.p.m. the strength demand should be massive.I feel that's not a suitable comparison.

It was dropped as you say by ducati as it was not as acurate (due to cyclic variations) for cam timing.
Bordi wanted rid of it in the early 90's along with the roller mains but wasn't allowed due to tooling costs.

motoholic71 a écrit:

I'm pretty sure some R&D with design and material would yield positive results for small power loss and reliability.

Only possible drawbacks I can think of is bad power efficiency and/or reliability but who has serious development done on this?Beside H.Davidson?

a suitable belt exist already on some industrial or automotive catalogue.

Yes as used by almost all modern classic racers G50 Manx Goldie Triumph comando etc
[Vous devez être inscrit et connecté pour voir cette image]<img src="http://www.britcycle.com/products/465/465_01_C_001_small.jpg" width="370px"/>
Forget those wide things you see on Choppers 20-25mm would be more than ample for a 50hp 2stroke
Newby Hayward etc. Most can be run dry so no adding heat either.
A belt is actually supposed to be a couple of percent more efficient than chain or gears.

I understand the efficiency rates rate like this
Belt then Chain (assuming it is it great condition) then straight cut gears.

A belt was proposed from frits (with the CVT) but it may also offer a way to tidy up a 4 disk valve twin.


Of course i would be more interested in the major porting changes between RSV and RSW. What was the changes? Was this when the deck was first closed

The ports and ducts of the RSA and RSW were exactly the same.
Cooling was improved.

Thanks for the reply and information Jan
From a look in the parts book the RSA seems to have a larger disk than the RSW(asumption based on the o-ring on the intake cover)

Is this correct?

husaberger a écrit:

Thanks for the reply and information Jan
From a look in the parts book the RSA seems to have a larger disk than the RSW(asumption based on the o-ring on the intake cover)

Is this correct?

No, it had the same diameter.

Hello

Jan or Frits can you tell me what is the better shape for the exhaust if the only structural restrictions is to make a 90° angle.

either as picture 1 that the entire exhaust makes a 90° arc.

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or something like picture 2 (better like this). first on straight piece then a very finely segmented pieces that makes the 90° arc and the rest straight.

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thanks Manuel

I would not not know without trying!

Frits and Jan,

RE: the RSA400 upscaled cylinder

The 400cc RSA cylinder is underway, After struggling with the design of the cylinder for some time, I found that a friend of mine in Italy had better 3D CAD skills than me, had previously designed a world 50cc GP cylinder and has an actual Aprilia RSA cylinder to copy.

So this 400cc upscaled experiment (retain the stock 370cc capacity, 78x78mm) should be ready in a couple months, and I have faith that my friend will be able to get it right, or at least much closer than I could have!
Trying to take on a task like designing a GP-spec cylinder from scratch really showed me how much I still have to learn.

This thread is again close to 50 pages long, so perhaps in part-3 of this amazing thread I hope to be able to post up the results.

Regards,

I am looking forward to it, Jeram .