[Technique] Honda RS 125 Exhaust dimensions and other characteristics

Hi everyone

I need to know some information about Honda RS125 engine. The thing is the following:

My tuned bike is a Honda NS1-80cc or just NSR75cc, it have a Honda CRM 125 engine on it, with NS1 electric system (cdi and cables) adapted, tuned cylinder, tuned head for around 13:1 compression ration, using a YZ 125 97-01 piston, mikuni tm 35, and hand made exhaust made for maximum power around 11750 rpm....

I have some problems with low rpm power when exhaust return is undesirable, by about 6000 rpm because I still have the exhaust valve voluntarily stuck fully open. Meanwhile, I buyed a Honda RS 125 NF4 cylinder and NX4 E-2 Head with 10.6cc of volume or so at a very good price and excellent condition. It fits with few modifications on the CRM 125 engine, when my original cylinder gets tired, I will replace it with the rs125 cylinder.

My idea is to get a good 38/39mm carburettor and a 110mm center to center KTM 125 conrod and fit it on the CRM 125 crank for improvements in leverage, lower crank compression a bit and the other advantages on a bigger conrod like less suffering from acceleration.

One of my doubts is the exhaust I will have to build one in a way to fit the cylinder and my frame et cetera like I did for the CRM125 cylinder, the Objective is drag races, 600/800 meters, sometimes 1 kilometre or more.

I don't know exactly how much length and diameters it should have, so I am thinking about this:

Header diameter of the RS125 is 40mm right!?
Total length maybe about 820mm or 830mm;
Mid section about 115/120mm in diameter;
Stinguer size about 22mm of diameter.

Can you people provide me with some sizes of real Honda RS 125 exhausts?

Would be nice some ignition maps, sizes, crank compression used, et cetera about the RS 125.

Do you think that the NF4 cylinder and this head will work properly together with about 0,9 or 1mm of squish clearance?

Sorry for my bad English.
Thanks allot.
Regards

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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ok that's for drag ....

what fuel is allowed ?

here is a baseline ignition timing on an NF4 with Avgas and 10.6cc head ( mounted head and volume including plug hole )

RPM_ADV 300 2000 8000 9000 11000 11500 12000 12300 13000 13500

ADVANCE 10 25 23 17 14 8 5 5 1 0

you have certainly found that the NX head is 10.6 but you need to mesure it bolted on the barrel ... and you won't find that .... at least 12 cc with 1mm squish

SEB4LO a écrit:

ok that's for drag ....

what fuel is allowed ?

here is a baseline ignition timing on an NF4 with Avgas and 10.6cc head ( mounted head and volume including plug hole )

RPM_ADV 300 2000 8000 9000 11000 11500 12000 12300 13000 13500

ADVANCE 10 25 23 17 14 8 5 5 1 0

you have certainly found that the NX head is 10.6 but you need to mesure it bolted on the barrel ... and you won't find that .... at least 12 cc with 1mm squish

With 10,6cc head, 1 mm of squish and dome volume of 2,64cc I will get 13:1 compression ration.....this is using the YZ 125 piston, I should use one RS piston, but the prices are high here

98 "octanas" normal gasoline is used.....not avgas.....

Thankś to Frits Overmars for exhaust dimensions.

Hello frits, have you the dimensions for stock NX4 2004 pipe?
Thank you.

Frits, about two years ago, I saw some kind of scheme about the RS125 exhaust that said to don't remove a small piece at the tail pipe entrance, like a restrictor. It said the bike would lose around 1 hp.

Should I build a restrictor like that or just connect the tail pipe with the baffle and it's ok?

I am thinking about filling with alloy and subsequently reshape one of my spare NSR125 heads for the actual tuned CRM 125 cylinder, I want to build it with a toroidal shape and I am thinking to use some math functions and integration to be able to calculate the head volume and keep it the shape I want. Does it exist some easier way to do that?


I prefer to do as much as I can calculating and tuning the engine.

Thanks

romeuh80 a écrit:

Frits, about two years ago, I saw some kind of scheme about the RS125 exhaust that said to don't remove a small piece at the tail pipe entrance, like a restrictor. It said the bike would lose around 1 hp. Should I build a restrictor like that or just connect the tail pipe with the baffle and it's ok

The final diameter of 23 mm in the drawing above is the restrictor. After that you should continue with a larger diameter for tailpipe and silencer; I would advise at least 26 mm diameter.

ridley a écrit:

Hello frits, have you the dimensions for stock NX4 2004 pipe?

I suppose I must have them somewhere at home. But it will be a while before I'll be back there.

Ok Fritz, thank you.

Frits Overmars a écrit:

romeuh80 a écrit:

Frits, about two years ago, I saw some kind of scheme about the RS125 exhaust that said to don't remove a small piece at the tail pipe entrance, like a restrictor. It said the bike would lose around 1 hp. Should I build a restrictor like that or just connect the tail pipe with the baffle and it's ok

The final diameter of 23 mm in the drawing above is the restrictor. After that you should continue with a larger diameter for tailpipe and silencer; I would advise at least 26 mm diameter.

ridley a écrit:

Hello frits, have you the dimensions for stock NX4 2004 pipe?

I suppose I must have them somewhere at home. But it will be a while before I'll be back there.

How exacly do we calculate the proper size for stinguer diameter this days?

Until this day, I have mainly used values explain in the Blair Book, desing and simulation of 2 stroke engines.

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I have got this for you, Romeu. It is an extremely simplified exhaust concept. Factors like time*areas, compression ratio, ignition timing and mixture strength are not taken into consideration. All you need to do is enter an appropriate value for the speed of sound. 550 m/s will put you in the right ballpark and from there you can optimize the calculation by adjusting this speed value.
I emphasize that this type of exhaust calculation is only meant to help beginning tuners on their way; it is not anything like 'the real deal'.
(although, if you are lucky, the outcome might be identical to the real deal from time to time ).

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[quote="Frits Overmars"]I have got this for you, Romeu. It is an extremely simplified exhaust concept. Factors like time*areas, compression ratio, ignition timing and mixture strength are not taken into consideration. All you need to do is enter an appropriate value for the speed of sound. 550 m/s will put you in the right ballpark and from there you can optimize the calculation by adjusting this speed value.
I emphasize that this type of exhaust calculation is only meant to help beginning tuners on their way; it is not anything like 'the real deal'.
(although, if you are lucky, the outcome might be identical to the real deal from time to time ).


This is different from any explanation I have seen before.

But, aint that for your FOS system or will work properly on the normal modern 2 stroke system?

Thanks

It is supposed to be universal, Romeu.

So, if I apply that formula to my engine data with the rs parts I will get:

Dx = (407-195) / 7100 * sqrt(124.8*12500) <=> Dx = 37,47 mm

Lmax = (550 * 195 * 88)*12500 = 755mm -> To short probably.....

Lmax(600) = (600 * 195 * 88)*12500 = 823 mm -> maybe better

header diameter = ((0.36*56)/(0.34*823)+1)*37.47 <=> hd = 40.2

restrictor = 3.06 * sqrt(44?) = 20.29 ???

It seems to have good accuracy, but tail pipe restrictor seems to be too low....

How mutch horse power and max power RPM for the ultimate Honda RS 125 with the exhaust you showed first?
Regards.

I guess my question is wright since Frits ain't answer me.

But, do anyone knows how much horse power and max power RPM are the latest Honda RS125 making?

In Portuguese we say, "Quem cala consente!"

I guess about 53 horse power at 13000rpm with JHA kit.....

Regards

Does anyone have the cdi map of NSR 125 or even NS1 80??

I can supply the photos of cdi if needed, the NS1 cdi externaly looks the same as the NSR125, but the code is CI627 and latest NSR125 is CI626, the old NSR 125 ones are CI572....

I have both CI627 and CI572, the diferences I noted some time ago is that my bike overrev better with the CI627, low and mid-range seemed identical.

idealy I would love to buy one progammable CDI ignition system, but they are expensive. I would like to work with the actual parts for now...

Thanks in advance

I found this on the internet, it is supposely an improvement exhaust for NF4 125cc versions from 93/94´

What you 2 stroke tuners think? Too big mid section? To small stinguer diameter? Its a bit longer that the exhaust from the RS 125 year 2011 that frits posted.

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Here is the url: [Vous devez être inscrit et connecté pour voir ce lien]
I build an exhaust, with slightly modifications from the one that frits posted, here it is, it´s not very nice, but is made by hand with home tools and no money

Here before welding

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It has some last hour arrangements to fit perfectly the Honda NS1 80cc frame with the CRM 125 engine (not easy) :S

It will be running soon as possible

you won't do anything good with a production bike CDI , you need an ignitech unit

You won't do anything good with that 1992/93 pipe design either. Here is Honda's latest
(but I have a strange feeling I already posted this same drawing not too long time ago...)

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Frits Overmars a écrit:

You won't do anything good with that 1992/93 pipe design either. Here is Honda's latest
(but I have a strange feeling I already posted this same drawing not too long time ago...)

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If you say, I believe, can you say something more why do you disaprove that old exhaust design?

romeuh80 a écrit:

If you say, I believe, can you say something more why do you disaprove that old exhaust design?

Build both pipes, test them and you will know for yourself. At bit of experience will not hurt.

If I have the time I might do it

People, about Honda RS 125 cylinder, NF4 cylinders have very different secondary transfer ports shape of those from the NX4, the ones from the NX4 are wider, maybe different heights, and angles.

Would it be a good difference in power with this evolution? What is the usual port timing from stock NX4, and another famous kit's, if anyone can provide.

Thanks

Test that two exhaust pipes and give feedback to us :)
Did you use frits overmars formulas?

hi
obvisously you do not understand who frits overmars IS
frits and jan thiel are at the cutting edge of 2/s engine
design & develoment they are not to mistaken for God
but they are his second in command ignore their advice
at your peril of complety wasting your time trying something
they have already wasted their time to prove or disprove and
then passing the results on to you We would have to live to 200
years of age to gain the knowledge they have and are willing to share
REGARDS
TZ125

SEB4LO a écrit:

you won't do anything good with a production bike CDI , you need an ignitech unit

The ignitech is on the way. In this case one DC CDI-P with 6 pin connector for NSR 125

I have searched a lot about the ignitech, I found both sucesses and failures....

One problem now is to measure the Base Advance timing (necessary for the software), since the stator and pickup are mounted into the cover....but I have some unconvencional ideias to do it.

What is your experience with the Ignitech SEB4LO?