[Technique] Honda RS 125 Exhaust dimensions and other characteristics

Hi everybody...

So I build the exhaust with the measures that Frits gave me. The only diference is that it has more 10mm of Stage 1 diffusor lenght, just after the exhaust flange.

I don't know why but It does not make mutch power, it overrevs to mutch and the maximun power is at about 13k to 14k RPM.

The mixture is not very lean at high revs, and with the ignitech CDI I am having 10º advance at 12,5k , 1º advance at 13k and 0 at 14k. I tried more and less advance at about 12 12,5k but it does not seem to run better. It works better at lower rotations compared with the older kit I had (the older exhaust was about the same lenght as the new and exhaust timing is about the same), but does not have the same sudden power as the older one that was very good at about 12000 rpm.

What can be causing the exhaust to overrev more than expected? The exhaust temperature is hotter than is expected, I can't think of any cause for that....

I thought my engine would actualy have maximun power lower in rpm than a common Honda Rs 125.

Maybe I am building a new longer exhaust soon, Has this does not fit very whell my purpose overrenving this way.

thanks in advance

romeuh80 a écrit:

...What can be causing the exhaust to overrev more than expected? The exhaust temperature is hotter than is expected, I can't think of any cause for that....

Maybe the fact that your ignition timing is far too late?
Take a look at this, Romeuh:
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Thank you Frits

This was used in Honda RS 125 with standard gasoline?

That map has far more advance than the one Seb4lo posted.

Will try that soon...

romeuh80 a écrit:

That map has far more advance than the one Seb4lo posted.

It was lower indeed, but that was also because your max power RPM was at about 11750, not 13000!
As stated elsewhere, a rule of thumb is 28 deg when starting to be "on pipe", 14 deg at max power RPM, then falling quickly past that to assist overrev.
Plus, when testing timing curves, better to start on a safe side and rising.

romeuh80 a écrit:

This was used in Honda RS 125 with standard gasoline?

It is a map from the Aprilia works racer, with maximum torque at 12500 rpm and maximum power at 13000 rpm. The fuel quality did not play a role here, because the Aprilia engine never detonated at full power.

Vagelis a écrit:

Your max power RPM was at about 11750, not 13000! As stated elsewhere, a rule of thumb is 28 deg when starting to be "on pipe", 14 deg at max power RPM, then falling quickly past that to assist overrev. Plus, when testing timing curves, better to start on a safe side and rising.

Quite right, Vagelis. Romeuh could subtract 1000 rpm from all values on the graph, so for example at 12000 rpm he would have 15° ignition advance instead of 20°.
What you say about ignition timing falling quickly past max power rpm to assist overrev, is Honda practice.
We found it better to keep a little more advance, even at maximum rpm, as the graph shows.

Frits Overmars a écrit:

What you say about ignition timing falling quickly past max power rpm to assist overrev, is Honda practice.
We found it better to keep a little more advance, even at maximum rpm, as the graph shows.

Well, since he's using a honda pipe..

Frits, when setting the overrev range of the timing curve, is more power everything you look for?
Eg, I imagine honda started from, say, 10 deg flat past max HP, and started removing ADV until they have found best power. Say they ended up with 0 deg at the end of overrev.
With the aprilia, you have found that it's best to leave it at 9 to 6 deg.
Has this anything to do with how more ADV might affect other conditions (eg acceration after coming back on full throttle)? Or it is just an engine/pipe design difference?

The purpose of overrev is, you'll never guess, to raise the maximum revs that the engine will permit. If a rider can negotiate a series of corners without having to change gear, then that may be more beneficial to lap times than just engine power. How high that power is at the far end of the rev scale, is less important.
Being able to rev higher also guarantees a better engine pick-up after an upchange. This pick-up is indeed influenced by conditions before the gear change (pipe temp, mixture strength (power jet!)).
That Honda uses a different advance curve, probably comes from a different engine/pipe design, like you say.

Vagelis a écrit:

romeuh80 a écrit:

That map has far more advance than the one Seb4lo posted.

It was lower indeed, but that was also because your max power RPM was at about 11750, not 13000!
As stated elsewhere, a rule of thumb is 28 deg when starting to be "on pipe", 14 deg at max power RPM, then falling quickly past that to assist overrev.
Plus, when testing timing curves, better to start on a safe side and rising.

You are right, but that other pipe I build had lower temperature in calculation and was about 805 mm of lenght. The one Frits posted has 801 mm, and I add more 10 mm to it, I don't want too mutch Rpm because I searching for some more releability using the standard road components, like the rod for example, is the stock one for de NSR 125.

Thanks

I believe that only 10 extra mm's won't change the rpm behaviour much. Perhaps one hundred rpm?
You can deside the target rpm, find out the proper length for your engine using Frits model : [Vous devez être inscrit et connecté pour voir ce lien] , then add that length to the pipe.
I would use something around 580m/s for wave speed, but you should ask himself for an accurate estimation. :) (if that even makes sense)

I tested, and there was some improvement, but still, not that mutch power. I am a bit tired of working on the engine by now, later maybe try some improvement, meanwhile I will just ride for some time...

Thanks

Can Anyone tell me if the NX E-1 head for Honda RS 125 has always the same shape an volume?

Here is the photo

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E-1 will not give you any informations about the volume as all the NX4 head for unleaded fuel are E-1 (sorry, i don't have leaded fuel heads)

the only information that you could have is with the plug-seating surface :
97 to 00 heads have an elevated seating
01 and after have a concave plug seating (because they are toroidal heads)

the only thing you can do it's just checking your volume because second hand heads could have been modified

Stephane a écrit:

E-1 will not give you any informations about the volume as all the NX4 head for unleaded fuel are E-1 (sorry, i don't have leaded fuel heads)

the only information that you could have is with the plug-seating surface :
97 to 00 heads have an elevated seating
01 and after have a concave plug seating (because they are toroidal heads)

the only thing you can do it's just checking your volume because second hand heads could have been modified

Ok, the plug seating information helps :).

Many thanks.

What volume would you use for unleaded fuel, domed top piston and a fast, long straights track without tight corners.

well ...

how is your engine : full stock ? or have you made some changes ?

and what kind of unleaded fuel (street or racing fuel ?)

Stephane a écrit:

well ...

how is your engine : full stock ? or have you made some changes ?

and what kind of unleaded fuel (street or racing fuel ?)

I don't have exacly an Honda RS engine. Let's say I would have 2 engines, one with some small cylinder porting changes like the ones that came in the old manuals and the other full stock.

Street Fuel!

Thanks

my engine is a 2001, ported as are the sand kits / longer rod (109 Samarin rod) / V force / air box / 11,4 cc head.

with 75% avgas, the advance was incraese of 1,5° and it revs a lot : on a inertia dyno i have to close the throttle after 14000 because there is more over rev

so it would have been great with a little less compression for very fast track : with 100% street fuel i'd have use a stock head (11,7 cc) or a little more


for the 100% stock engine, i'd just change ign timing on a dyno


if it's a power jet model : try all the PWJ plugs (to change de cuting point of PWJ) because it is very important for the over rev

Thanks

You use about 0,6 or 0,7mm squish clearance right? Giving a compression ration of about: 12,6:1 with domed piston and the 11,4cc head

i'm actually at 0,68 mm

i'll send you a MP to give you a link to head volumes Vs driving conditions & fuel

Stephane a écrit:

i'm actually at 0,68 mm

i'll send you a MP to give you a link to head volumes Vs driving conditions & fuel

Ok, I will try to read the link, my French is really realy poor, like zero

Thanks

It's in english ;-)

Stephane a écrit:

It's in english ;-)

I have seen the link and saved some pdf´s.

I have this picture! Is this the thruth about stock Ignition Advance?

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I may have more a question or two to come. Many thanks

Hi Frits,

Thanks to share with us your knowledge.
Regarding the formulas you gave us with " FOS exhaust concept " and particularly for the calculation of D tail restrictor can you tell me what 's the value to be taken for " CRANK", is it the stroke??

Best regards.
Bernard

Hi Frits,
I have found the answer in an other topic: it's HP at the crank.

Thanks again.

Bernard

 

Frits Overmars a écrit:

Aqui, Romeu: the most recent Honda pipe.
Lenght dimensions are taken through the center, starting at the piston. Diameter dimensions are internal.

PS:   your English is fine; a lot better than my Portuguese and my French.
PS2: when are you going to change your nickname from RomeuH80 into RomeuH125? :lol:

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Hi Frits,
thank you so much, that you are sharing so much information with us!
What is the needed exhaust and transfer timing for that pipe? Do you also have temperatur information. (EGT)

Regards