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

Jan Thiel a écrit:

ktuningteam a écrit:
question about that, in its concept fos claimed to be indifferent to the length of the tail pipe after the counter-cup, right now am developing this exhaust and after very many tests only in that part, have made significant changes in performance, changing length and diameter, including the silencer !, and viewing restriction option and more tube diameter has not been the best, in the end we opted to go by reducing the diameter at losing power after long tube that was cut to find the optimum point . I realize that the development of exhaust is very laborious, and although you can have a base through formulas, the end result is always the truth dyno.
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I fully agree with you, the tailpipe diameter and length still make a considerable difference, as does the silencer!

Thank you for your comment!
It is really difficult, I think I can finish crazy! I few more tests, but I change things and also all the changes in the rest of the engine affecting everything had already tried before.

But I could take some finding.
Someone agrees with this I've seen ?:
  countercone long, more rpm after full power, shorter means a more pointed and hard drive curve, intermediate part shorter max more power, longer shifts the power towards the middle of the curve, the less conical front improves acceleration , more conical more maximum power but worse acceleration.
  Finding balance all leads me to find a good exhaust.

I don't think there is the perfect pipe only ones that suit the rider/track/engine better than others. If you ever do design the perfect pipe it won't fit on the bike

A perfect racing pipe would be one that automatically changes its shape according to power delivery requirements to maximize supercharging at all rpms. Unfortunately pipes are rigid structures. A movable reflector cone would help but still has its limitations. As Frits says the pipe needs to be designed around the intended purpose and not just for peak HP. All engines have a usable power rpm band (sweet spot) that needs a pipe that works within this band. We race engines with a CVT but even the best CVT engine setup requires an rpm range of about 1000 rpm within to tune. Also, as Jan stated, as the EGT rises during wide open throttle down the straight the peak HP/ RPM changes and the cvt must be tuned to follow the increased rpm. A direct drive cart requires a pipe that is asked to work over a huge rpm range. not so easy! If it was easy everybody would be a pipe builder...

Hi,
in part 4, page 2, I asked about putting my 2004 RSW250 on Avgas.

I did it and the results are amazing!

I manufactured 17.5:1 torodial heads, went to the dyno and found the optimum ignition advance (+1.5 dgrees) and optimum jetting (down approx #13 main jet) and gained 4hp over super-unleaded fuel from 10,000rpm onwards.

I was VERY happy. I raced it for 4 meetings without any of the jetting hassles of unleaded that I have suffered for years. I only changed the clip by 0.5 over all the race meetings. On unleaded, I would be swapping cips every other session!

I then chanced upon even more power by fitting some 02-99 pipes that were rusting away in my workshop, They gained me another 3 hp from 10500rpm onwards. So I measued the pipe dimensions and they appeared the same as the 102A pipes I had been running with unleaded. The only difference between the pipes was that the silencers on the 02-99 were 24.7mm i.d and 300mm long compared to the 102A pipes that were fitted with 26mm i.d silencers 240mm long.

Its an incredible improvement due to a silencer change only.

Comments welcome!!!!

and here she is in full flight......

Well I tried to upload it.........any tips?

Captain Scarlet a écrit:

Well I tried to upload it.........any tips?

Putting pictures on this forum is too difficult, a pity!

No Jan, putting pictures is quite easy if you do like that:

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Thank you Frits for your explanations ans sorry for my vocabulary mistake yes definitly restritor and not stringer as i said!!
Regards
Pierre

pierre95 a écrit:

Thank you Frits for your explanations ans sorry for my vocabulary mistake yes definitly restritor and not stringer as i said!!

Restrictor!
No worries, Pierre, I am only joking. And I am sure that your English is a lot better than my French.
It is me who should be apologizing for writing in English on this marvellous French forum.

TSSSS!!! Frits, tu lis le français mieux que certains français...

Jan / Frits, can you remember any Samba pipe codes for pipes that were designed for the RSW (or RSV) to run on leaded fuel?

as we are talking pipes again, I have some questions. I understand any program to calculate exhausts is just a starting-point, but one needs to understand what's happening in order to make some changes in the right direction. I bought an exhaust calculator programe from Coker racing products (SA), and playing with input gives some idea's of what's happening. But when I compare them with the FOS (http://www.underdogsracing.com/fospipe/ from which Frtis said it was ok?), there are some things contradicting  

I understand what determines the length of a pipe : more revs or lower exhaust timings = shorter pipe and both agree on that. What's rather confusing is :

1/ when do you need a bigger dwell-diameter (centre part between cones) ?
FOS : bigger dwell is for higher revs and lower timings
CRP : exactly the oposite. bigger dwell is for lower revs and bigger timings.

lower revs could mean more time to suck gases in so a bigger volume might be better, and higher timings would mean ports  are longer open so again bigger volume to contain more gases, so why is it the oposite in FOS ?

2/ the total length of a pipe doesn seem to change much between a pipe for high peak-power or a wide spread powerband (litlle longer ?), but for high peak slightly longer first part, shorter dwell and shorter end cone ?

3/ the header (first shallow part) does the suction during blowdown, while the diverging cone does the suction from the transferports ?

JanBros a écrit:

I bought an exhaust calculator programe from Coker racing products (SA), and playing with input gives some idea's of what's happening. But when I compare them with the FOS (http://www.underdogsracing.com/fospipe/ from which Frtis said it was ok?), there are some things contradicting.

Jan, I did not say that this (or any) calculator is OK.
Maybe (I don't remember) I said that the calculated results for a particular engine were in line with my ideas, but that's all.
Besides, I tried to keep my exhaust concept so simple that you can do the calculations yourself, without resorting to a 'fospipe' program on the internet. As I have said before, I do not take responsibility for any such programs and I hope you'll understand that I can't comment on the differences between two programs that I do not know.

[Vous devez être inscrit et connecté pour voir ce lien] is  meets my excell sheet which are based on simple calculations representation from Frits.

I perfectly understand that Frits, and I wasn't enquiring about specific programs, they were actualy general questions as to know in what direction one should change his pipe trying to make it better.

I simply refered to 2 programs as they were contradicting and was wondering which one makes the most sence. if both would have given the same result I could have distracted the most likely conclusion myself.

to make it a more general question : when do you need/make a pipe bigger in diameter ? with higher or lower revs? with more or less timing ?

For real good scientifically based two stroke data and calculations I strongly recommend TORQSOFT.NET
I'm afraid for most of the other so called experts that this is the best one can get in this particular field. Just have a go and try it. The man behind it is a rare combination of science and practical experience. Let me explain  clearly that I have NO commercial connection and or interest with this person whatsoever. Please wake up two stroke youngsters , show some guts and do try TORQSOFT.NET...

JanBros a écrit:

I perfectly understand that Frits, and I wasn't enquiring about specific programs, they were actualy general questions as to know in what direction one should change his pipe trying to make it better.
I simply refered to 2 programs as they were contradicting and was wondering which one makes the most sence. if both would have given the same result I could have distracted the most likely conclusion myself.
to make it a more general question : when do you need/make a pipe bigger in diameter ? with higher or lower revs? with more or less timing ?

Vary the revs, the timing, the cylinder capacity etc. in the FOS exhaust concept and you will have your answers, Jan .
I can understand that you want a little more than just numeric answers, that you are looking for coherence between the various engine parameters. If you bring enough variation in your input values, you will soon discover the coherence in the output values. But seemingly simple questions are not always simple to answer. For example:

JanBros a écrit:

the header (first shallow part) does the suction during blowdown, while the diverging cone does the suction from the transferports ?

During blowdown the pressure ratio between cylinder and pipe is supercritical and lowering the low-side pressure even more will have no effect at all on the blowdown outflow; it will only be a waste of exhaust gas energy. The header serves (amongst other things) to delay the onset of suction until the pressure ratio has dropped to below supercritical.
Furthermore the length and diameter of the gas column in the header, together with the pipe volume, are the decisive factors for the frequency of the Helmholtz resonance in the pipe.
Understanding the influence of the pipe length is relatively simple; understanding the influence of the diameters is far from simple, but just as important.

Frits Overmars a écrit:

Vary the revs, the timing, the cylinder capacity etc. in the FOS exhaust concept and you will have your answers, Jan .
I can understand that you want a little more than just numeric answers, that you are looking for coherence between the various engine parameters. If you bring enough variation in your input values, you will soon discover the coherence in the output values.

tnx for your answer. As could be expected you just don't give a simple answer, you give food for thought, and that's fine. whenever I try to help people with technical issues (on a "slightly" lower level), I to always try to give them just enough info in a way that I believe that they can distract the correct answer themselves. As I think that something you found out/understood by yourself is better and something you'll never forget again.

my problem is I've already spent hours changing data and couldn't figure it out, that's why I asked. guess a few more hours are needed

@ Bob, I'm familiar with the Torqsoft website, but spending 50 euro for each pipe is just a bit too much.

If you're getting better results what is 50 euro. Show guts and try just 1 !

Bob van der Zijden a écrit:

If you're getting better results what is 50 euro. Show guts and try just 1 !

because the pipes are need for moped-races like "de zwarte cross" and 50 euro would be like 1/3rd of the price of the entire moped.
and because I wouldn't learn much from it unless I order 2 or 3 pipes so I can compare them.

I think if you are like me the 50 euro is a smart choice to get a good exhaust.
Lucf does the same thing and considering the price iTS cheap to get so close to optimal.
It depends on your experience.
I tried to understand the exhaust fenomenon and i still dont get it. To me its an enigma.
Both options (lucf and torqsoft) give good peace off mind.

JanBros a écrit:

as we are talking pipes again, I have some questions. I understand any program to calculate exhausts is just a starting-point.

Jan, you may call it that way, but my experience is completely different and so many users of my calculations I did from 2003 till now. This was the respons in 2007 from a professional pipe bilder in GP motocross for many years.

Hi Luc, First of all my sincere thanks for the very quick response (and of course for the new calculation and the long-sought answer to my question about compression Dmax.exhaust). The problem with the chamber was that I wanted to create as one of them and only then noticed that the exhaust port height was wrong (better late than never shall we say). The "old" chamber and outlet run super (only have to listen a little close to the rest of the set as curve and carburetor setting but that seems to belong to the tune ...).
It has failed me yet, to make the exhaust you send me to get it better (I do not know if this is something to be proud of but I report it here anyway), luckily it with some minor adjustments succeeded him / her something more adapted to get my needs (one small extra cone in the end cone and a tailpipe such as Honda this applies to the last RS125-jes) .O yes, I write this as fast but I spent weeks to to arrive at the result. With other words, the hassle on various forums about whether or not a pipe from the computer, I have you convinced that you can do quite a distance to come close to a very good base setup. Once again, many thanks for the quick and detailed response, then he writes exhaust the following about a 50cc: "I have therefore mainly have experience exhausts with 125cc, hence my interest in your program (out of sheer desperation I myself but started writing a program manages and testing. But it turns out that you can not just grab a good basis for a 50cc pipe like a 125cc motocross ".Groet, Sjef

And there are many more: [Vous devez être inscrit et connecté pour voir ce lien]

Kind regards Luc