[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)

josh20 a écrit:

Frits FOS uses Squish Gap and the Squish Center Gap (What i guess is the inner point at the beginning of the Bowl?) to see if the Gap is Parallel or opens Conical.

You guessed right, Josh.

Citation :

So my conclusion is at the moment, the most common told 25-50m/s MSV is a static Value and at least a bit useless... because in the enginge its very dynamic.

Right again.

Frits Overmars a écrit:

josh20 a écrit:

Frits FOS uses Squish Gap and the Squish Center Gap (What i guess is the inner point at the beginning of the Bowl?) to see if the Gap is Parallel or opens Conical.

You guessed right, Josh.

Citation :

So my conclusion is at the moment, the most common told 25-50m/s MSV is a static Value and at least a bit useless... because in the enginge its very dynamic.

Right again.

Thank you Mr.Overmars,
i hope you dont mind if i ask another question(s)?

How would it affect the Detonationrisk at Partial Load if the Head is modified to more "Race-specs"?
Because my 500 is rather an Countryroad-Hunter than a Racebike, so it runs often on low load/partial load.
i have the feeling it could be risky. As running long on low throttle openings is not what powerfull Twostrokers like in general...
And would a variable Tailpipe (like your Idea that you posted here) could be usefull for this "low Load"case?
I´m thinking to build some kind of exhaust pressure reduction system in connection to throttle opening, like Cruising around= bigger Tailpipe / Full Throttle = normal Tailpipe.

Best Regards
Josh

josh20 a écrit:

Thank you Mr.Overmars, i hope you dont mind if i ask another question(s)? How would it affect the Detonationrisk at Partial Load if the Head is modified to more "Race-specs"? Because my 500 is rather an Countryroad-Hunter than a Racebike, so it runs often on low load/partial load. i have the feeling it could be risky. As running long on low throttle openings is not what powerfull Twostrokers like in general...
And would a variable Tailpipe (like your Idea that you posted here) could be usefull for this "low Load"case? I´m thinking to build some kind of exhaust pressure reduction system in connection to throttle opening, like Cruising around= bigger Tailpipe / Full Throttle = normal Tailpipe.

Josh, it's very kind of you to address me as Mr. Overmars, but just 'Frits' is good enough for me, 'Mr. Overmars' was my beloved father.
I have no idea what 'Race-specs' are supposed to be; it could mean anything. Maybe you mean a high comression ratio; that would work in a four-stroke, although I would not recommend it for a two-stroke. I'd rather concentrate on an effective, tight squish gap.
My variable tailpipe, shown below, was designed with two objectives in mind: to reduce the torque dip at 2/3 of max.torque-rpm, and to avoid part-throttle detonation. I like your suggestion of using it in connection to throttle opening.
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Frits Overmars a écrit:

Josh, it's very kind of you to address me as Mr. Overmars, but just 'Frits' is good enough for me, 'Mr. Overmars' was my beloved father.
I have no idea what 'Race-specs' are supposed to be; it could mean anything. Maybe you mean a high comression ratio; that would work in a four-stroke, although I would not recommend it for a two-stroke. I'd rather concentrate on an effective, tight squish gap.
My variable tailpipe, shown below, was designed with two objectives in mind: to reduce the torque dip at 2/3 of max.torque-rpm, and to avoid part-throttle detonation. I like your suggestion of using it in connection to throttle opening.

If its ok for you with Frits, i stay with that...

With "Race-Specs" i meant not high compression...i´ve read already much about this in your Posts (within the low-comp Racer from Langtuning(?) and so on)

I meant more that usually for Roaduse built Twostroker like an TZR/TDR/RGV250 or something like this, is more on an Conservative side of Headdesign... a bit bigger on Gap,a bit lower on Comp,a bit smaller on Area and so on because its for roaduse with maybe not very experienced riders,sometimes bad fuel,lots of low load riding and it should last longer...
If you give such an engine a head thats designed for high msv,thight gaps etc. (Like for Race Use), could this lead to more Detonation when just cruising around? Because you ride often just on the leaner needle,Piston cooling airflow tru the engine is on the lower side and so on.
I guess it would, and there could a variable Tailpipe help to lower the risk of detonation when you just cruise around.

So in my mind there is a Second Tailpipe that is open/closed via a roller/valve on part throttle openings till maybe 3/4 Throttle, where i would shut it for maximum power. And opens again when you have to close the Throttle more than the 3/4...

Does this make sense or complete nonsense?

Best Regard
Josh

Your idea of a second tailpipe makes sense. But your fear of using a combustion chamber with about 50% squish area and a tight squish gap is unfounded. On the contrary: it will promote faster burning and it will make any engine more deto-resistant.

Frits Overmars a écrit:

Your idea of a second tailpipe makes sense. But your fear of using a combustion chamber with about 50% squish area and a tight squish gap is unfounded. On the contrary: it will promote faster burning and it will make any engine more deto-resistant.

Thank you Frits,
i think my fear comes from the Bikes common Nickname "Ping King"...and because it comes out of the Factory with an Squishgap/Area Combination that is kinda weird...2,5mm Gap and around 68% Area with 89x79 Bore/Stroke. The Honda Guys arent stupid i guess, so i was a little afraid.
But it was an "Agricultural" Engine, maybe its better for this kind of use and for beeing more failsafe.

I will test my new head and see what happens.
So long, thanks for your Input all over in this Forum.

Best Regards
Josh

josh20 a écrit:

Frits Overmars a écrit:

Your idea of a second tailpipe makes sense. But your fear of using a combustion chamber with about 50% squish area and a tight squish gap is unfounded. On the contrary: it will promote faster burning and it will make any engine more deto-resistant.

i think my fear comes from the Bikes common Nickname "Ping King"...and because it comes out of the Factory with an Squishgap/Area Combination that is kinda weird...2,5mm Gap and around 68% Area with 89x79 Bore/Stroke. The Honda Guys arent stupid i guess, so i was a little afraid.

I wasn't familiar with the Ping King nickname but squish dimensions of 68% area x 2,5 mm gap give a squish band volume of no less than 15,6 cc; over 1/3 of the total combustion chamber volume!
That chamber must look like a pancake wearing a bowler hat, and with the giant 2,5 mm gap you can forget about any squish at all.
Ping King indeed. I cannot for the life of me understand why Honda would build and sell something like that.

Frits Overmars a écrit:

I wasn't familiar with the Ping King nickname but squish dimensions of 68% area x 2,5 mm gap give a squish band volume of no less than 15,6 cc; over 1/3 of the total combustion chamber volume!
That chamber must look like a pancake wearing a bowler hat, and with the giant 2,5 mm gap you can forget about any squish at all.
Ping King indeed. I cannot for the life of me understand why Honda would build and sell something like that.

Build and sold over almost 10 Years...
You may understand now why i´m afraid. I thought about it over and over again,but didnt understand the reason. And simply thinking these guys are not very clever couldnt be true...
For Scale: Headbolts are 8mm Diameter
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Josh, if my understanding is correct this is what you obtained by molding inside the original head of your CR500 engine?

Frits, don't know if you know but your drive.google site with all your pics and info isn't working anymore.

Thanks Jan, I'll look into it.

carlovitch1 a écrit:

Josh, if my understanding is correct this is what you obtained by molding inside the original head of your CR500 engine?

Yes it is. Its the Factory Head of the CR500 1992/1993-2001 with around 66ccm, nearly 20mm Squishbandwidth and 2,5mm Gap. Molded at TDC with Candlewax.
But now i prefer some kind of 2-Component Silikonkautschuk to get good negatives of transfers/heads.See below(Transfers are TZR...) [Vous devez être inscrit et connecté pour voir cette image]

Good evening everyone, I hope you and families are all with good health.

I have an historical question for Jan and Frits: I read that Jorg Möller did very well when he worked for Van Veen then Morbidelli.
Do you know what he did to the engines, did he developed a special idea to make his engines work so well compared to those days competition?

carlovitch1 a écrit:

I have an historical question for Jan and Frits: I read that Jorg Möller did very well when he worked for Van Veen then Morbidelli.
Do you know what he did to the engines, did he developed a special idea to make his engines work so well compared to those days competition?

Jörg Möller worked on all aspects of the engine, and he did a fine job, first at Van Veen, then Morbidelli and finally Minarelli. He was particularly good at designing crankshafts, in cooperation with the German Hoeckle company who produced his designs.
De biggest step forward while working at Van Veen, was after he'd had the opportunity to study a cylinder from the Yamaha 125-4 (pictures) that was lent to him by my friend Ferry Brouwer, who was a Yamaha works mechanic then. By the way, Jörg is celebrating his 76th birthday today.
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Thanks for your quick answer, Frits ! And for these nice pictures of this incredible machine.
Did I see 9 speeds in the gear box? The powerband must have been super narrow...

In case Jörg is reading us, then happy birthday to you !

Frits Overmars a écrit:

carlovitch1 a écrit:

I have an historical question for Jan and Frits: I read that Jorg Möller did very well when he worked for Van Veen then Morbidelli.
Do you know what he did to the engines, did he developed a special idea to make his engines work so well compared to those days competition?

Jörg Möller worked on all aspects of the engine, and he did a fine job, first at Van Veen, then Morbidelli and finally Minarelli. He was particularly good at designing crankshafts, in cooperation with the German Hoeckle company who produced his designs.
De biggest step forward while working at Van Veen, was after he'd had the opportunity to study a cylinder from the Yamaha 125-4 (pictures) that was lent to him by my friend Ferry Brouwer, who was a Yamaha works mechanic then. By the way, Jörg is celebrating his 76th birthday today.
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Frits, thx for the very interesting pictures ! That yamaha RA31A whas a technical marvel ! Almost unbelievable, that yamaha already had such a well thought of engine design. In an age where aircooled engines where still being used, they had it all. Gear driven waterpump, separated cylindres and cylinder heads, modern crankshaft, a very sofisticated autolube-lubrication system : i see an oil pump and 12 oil lines !

Unfortunately, they stopped in 1969... The successor, the yz 623 was like an agricultural engine, no oil pump, aircooled, no rotary valves... They had all that top of the notch technology that they aquired when they bought Showa Works Ltd, and then they shelved it...


For Jorg Möller : and there I was thinking that Jorg Möller was the second best two stroke genius of that era (after Jan of course) but after 50 years we hear that he found the 2Stroke gospel by looking at yamaha...


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