[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) (Locked)

In Aprilia used the same clearance for both cast and forged pistons???

Jan Thiel a écrit:

Filandro a écrit:

Hi Jan, how much was the clearance piston-barrel on Aprilia engines?

0,04mm
I tested with 0,05, the engine gave more power.
But nobody wanted to know!

Jan, when you conducted tests on the dyno lowering the running temp of the engine down to 30c, did you feel it necessary to change the piston clearance to prevent cold seizure. Where the engines pre-heated prior to starting.

What did you estimate the lowest temp the bike could run at the track considering the space available for the radiator.

Filandro a écrit:

In Aprilia used the same clearance for both cast and forged pistons???

Yes.

dutch fisher a écrit:

Jan Thiel a écrit:

Filandro a écrit:

Hi Jan, how much was the clearance piston-barrel on Aprilia engines?

0,04mm
I tested with 0,05, the engine gave more power.
But nobody wanted to know!

Jan, when you conducted tests on the dyno lowering the running temp of the engine down to 30c, did you feel it necessary to change the piston clearance to prevent cold seizure. Where the engines pre-heated prior to starting.

What did you estimate the lowest temp the bike could run at the track considering the space available for the radiator.

When doing these tests we started with 80° and went down to 30°, without ever changing the piston.
The engine was always preheated.
This depends a lot on the cubic capacity.
And on the ambient temperature of course!

When working for Bultaco I used water from the tap for cooling.
The temperature was never higher tha 25°
And I never had a seizure!

Jan, how did you preheated the engine ?
Like in F1 or you fill the engine with warm water.

bengui a écrit:

Jan, how did you preheated the engine ?
Like in F1 or you fill the engine with warm water.

We kept it running on full throttle and 10.000 rpm.
Until a certain temperature was reached.
This took about 5 minutes.

Jan Thiel a écrit:

There were cast and forged pistons.
The cast pistons were the best.

Jan, Do you remember what the aluminium alloy specification was and by what method the pistons were cast.

Hi Jan. Earlier on in this thread..(way back in fact) you said if you had to do design the RSA over you would make a twin disc engine. I assume this was to vary the inlet closing timing. I have an idea I want to try. My idea consists of a centrifugal flyweight mechanism similar to the advance mechanismin in a car distributor with flyweights that rotate with the rv shaft. The movable plate would rotate slightly on top of the second plate with the amount of movement depending on the springs and flyweights tuned to the rpm. This way the inlet port would not be restricted with retarded closing at lower RPMs like my current design. What are your thoughts? Frits (or anyone else) feel free to chime in as well.
Howard...

It has been done, Howard; see below. Two further remarks: to my knowledge Jan's primary goal was not to vary the closing timing, but to realize a symmetrical inlet flow without the complicated, vulnerable rear disk drive of the RSA.
And if you do wish to include variable closing timing, I would choose Uniflow's solution, shown earlier in this topic.

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Are you talking a disc on each end of the crank and 2 carbs?

Yes.

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Exactweld 250

Howard Gifford a écrit:

Hi Jan. Earlier on in this thread..(way back in fact) you said if you had to do design the RSA over you would make a twin disc engine. I assume this was to vary the inlet closing timing. I have an idea I want to try. My idea consists of a centrifugal flyweight mechanism similar to the advance mechanismin in a car distributor with flyweights that rotate with the rv shaft. The movable plate would rotate slightly on top of the second plate with the amount of movement depending on the springs and flyweights tuned to the rpm. This way the inlet port would not be restricted with retarded closing at lower RPMs like my current design. What are your thoughts? Frits (or anyone else) feel free to chime in as well.
Howard...

Years ago i drew up a contrarotating design with an offset disk to gain the advantage of a larger disk as well. it stayed on paper like most i my mussings.
Frits drew up a bobweight design that i didn't consider at the time (which will be i guess what you are saying)and posted in reponse to a similar question i posted elsewhere.
I hope he won't mind me posting it as finds the search function on the site disagreeable.

Citation :

Think of a centrifical advance on an old car or brit bike and quite a few old Japas they have a bob weight to advance the timming with speed

the amount of advance can be controlled by both the strength of the springs and the weight of the bob weights.
If this were added to a disk valve engine . As the whole disk would seemingly need to retard with speed on a disk valve engine.
The rotation of the valve would therefor have to be reversed? I hope that makes a little more sense?

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Citation :

Not at all, Husa. How about keeping the direction of valve rotation intact (changing it in the RSA125 would be quite complicated) and mirroring the bob weight arrangement?
Centrifugal disk adjustment has been built before; the idea is not bad (I feel you might need a little comfort ).

I really really like Uniflows varible slide also (as Frits mentioned above)
as it simply an more importantly without undue complication varies the timing with the retardation of the closing.........
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Frits said the power curve does follow the Bell graph which was my inspiration
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First - thank you very much for all the time you spend here sharing your knowledge.

I wonder if you could share your findings regarding using different types of coils. I have never had the opportunity to do back to back testing of different types of coils. Now and then, a new 'super duper' high tension coil appears in the market. Does it make any difference? If so, which type do you recommend? Thank you.

So sorry; I do not know nearly enough about electronics. I wish I did.
All I noticed was that CD Ignitions need low-resistance coils - less than 1 Ohm.

Hi Guys

Apologies if this is not strictly Aprilia GP 125, but I know many knowledgeable enthusiats visit these pages looking for inspiration and information! I'm just wondering if anyone on here can help me out.

I am just starting to work on a customers recently purchased and partially incomplete, Aprilia RSV250 engine from a 1992 Production GP Bike.

It has an ignition that has (I believe) a Nippondeso generator (marked ND).

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Obviously this is a fairly sophisticated ignition and maybe logging system, which I presume is programmable and uses inputs from (perhaps) water temp/EGT/throttle position/speed/rpms, detonation etc. to modify the ignition timing curve.
I presume that it takes the trigger from a cup that should be bolted to the back of the rotor (missing). In fact the generator looks exactly like the one from a Rotax 129
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There are no manufacturers markings on either of the boxes and I don't have a programming interface, software or any sensors.

Any information or leads to where I might find any information would be most welcome.

Also the motor has come without pistons, but from measuring the insert volume it would appear that a flat top piston was used, this giving an 11.9cc total volume to the top opf the plug hole including a 0.8mm squish clearance. It seems a little low, but with an Aprillia GP dome top piston it would give an impossibly high compression. Anyone know of the piston type used (I have yet to check rod length and the piston crown height requirement). The cylinder assemblies, I think, are Rotax 129.

Thanks for your attention.

Best Regards

Ian

I can't help you with the electrical parts, Ian. But to my knowledge no Aprilia race engine has ever left the factory with flat top pistons. The impossibly high compression ratio you mention may still be correct: before leaded fuel was banned, this ratio has been as high as 19.5.

Ian was this the one sold on ebay recently? I have Tony Murphy checking with Rotax about those very trigger cups. They are mounted 180deg apart for the 250 to 125.Has the rotor got "258" embossed on it(I can't see in your pics)? To the best of my knowledge the ND ignition has 4 coloured plugs (2 pin) that change the curves similar to the old Rotax digital(same curves to I think).

Hi Guys

Frits, that would indeed put it at around 19.5:1, so it may be OK with Avgas which the customer uses in his other bikes. It also has narrow squish bands that perhaps point to leaded fuel being used. Otherwise I will machine the inserts to suit our 102 unleaded race fuel. The inserts are brand new and unrun, so it could also be the case that they were considered too "tight" to run and that the set which were run in the engine were retained by the previous owner.

Lozza, No it's not the Ebay one which I also saw, but that had many more parts missing. This is complete apart from pistons and disc valves.

I will inspect the rotor tomorrow for markings and report back.

My feeling is that the ignition boxes and wiring are specialist low volume and not Nippondenso. Just the wiring sheathing, etc. looks more bespoke/one-off, although I could well be wrong,

Best Regards

Ian

Your probably right with the bespoke ignition, Frits and Jan would know better than me but I don't think programmables were around in 92. Quickshifts started to appear in 93 that were a separate bolt on item. Though would be interesting to see circa mid 90's programming hardware/software, any DOS/Windows 2.0/3.0/95 experts out there?

Frits Overmars a écrit:

I can't help you with the electrical parts, Ian. But to my knowledge no Aprilia race engine has ever left the factory with flat top pistons. The impossibly high compression ratio you mention may still be correct: before leaded fuel was banned, this ratio has been as high as 19.5.

Indeed, flat pistons were tested, but never used on track, or sold.
Inserts have a letter, followed by a number stamped on them.
MW for leaded fuel, and AC for unleaded.

Hi Jan

Thanks for that info and the inserts are marked MW25, so indeed leaded as suspected. I am familiar with the AC designation as this was also used by BRC on their tandem twin engines that we raced for a few years.

Lozza, I checked the rotor and it is only marked with what I presume is a part number: 032000-2201 and seperately 10P.

The owner found a picture of a system off a 1998-2000 RSV, which looks identical (apart from it has the dash with it), so at least I can now assume that it is the standard issue Aprilia Kit of the time.

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I'll post up some more images on Monday, but for now I'm busy getting ready for a race meeting at Donington Park this weekend.

Best Regards

Ian

Frits Overmars a écrit:

I can't help you with the electrical parts, Ian. But to my knowledge no Aprilia race engine has ever left the factory with flat top pistons. The impossibly high compression ratio you mention may still be correct: before leaded fuel was banned, this ratio has been as high as 19.5.

In my dyno testing and motor building days of the 2 strokes.... I've never got the high compressions to work.

It hurt top end horsepower and killed the over-rev.

I'm curious as to why such high compression ratio's?... or what made those high ratio's work?