Re: Project catastros (rx rebuild)

thats odd i thought its around 140-150 psi , also please perform the test again after the running in there will be a boost in compression compared to the present reading

Re: Project catastros (rx rebuild)

Stock rx has a ratio of 7:1. That with the static formula for compression translates to 102psi.

With the old 4sp head on the same set up showed a reading of 110 psi.
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Re: Project catastros (rx rebuild)

what is this formula can you explain

Re: Project catastros (rx rebuild)

At sea level static compresion ratio is, cylinder pressure (psi) / 14.7.



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Re: Project catastros (rx rebuild)

where did you get this formula the method you are using is outright wrong , one of the reasons being that it doesn't take into consideration the precompression provided to the charge in the crank case of a 2stroke engine , or the presence of fuel in the charge mixture,

Re: Project catastros (rx rebuild)

I am not using any formula. I am a lawyer, not an engineer. My experience with complicated tech stuff is limited. So i gather whatever knowledge i can. Its a basic formula. Not a specific one.

In any way, compression on the stock head is less than the moded one. Thats all i need to know and all that matters.

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Re: Project catastros (rx rebuild)

basically the combustion chamber volume and squish has been altered which is giving these high values . once he completes the running in properly then it will be much more interesting to know the comp value.

Sarin please take the liberty to educate for the betterment of us all here

Re: Project catastros (rx rebuild)

Sarin, I understand that calculating the compression ration in a 2 stroke motor is not quite handy and too close to practical values because of the simultaneous operation of scavenging of gases and compression of the charge. But fortunately science formulation is not too exhaustive over this calculation and can be calculated over this simple mathematically equilibrium.

Compression ratio in a 2 stroke motor = {Displacement (132)} + {Cylinder head volume (stock head volume is approx 20cc)} + {Head gasket volume approx 2cc}
DIVIDED BY
{Cylinder head volume (stock head volume is approx 20cc)} + {Head gasket volume approx 2cc}

The above caluculation gives a stock compression ratio of approx 7:1 which is close to 110 to 120 psi depending upon the height above sea level.

Bibhu claims that he has got the head mod done which is modifying the squish to shave off the extra volume + performace head gaskets which. I think a soup bowl mod.
This formulates to a higher compression ratio which eventually gives higher compression.

Experts can please share their views.
Cheers

Re: Project catastros (rx rebuild)

[MENTION=59927]augmentor[/MENTION] : Thank you for the wonderful explanation.
[MENTION=65331]Mr.Boss[/MENTION] : Thank you for the idea for Installing the float in the 2t tank. Here are the pics.


Hole drilled for the float.


Float unit from max100R


Unit installed in the tank. Installed an oring on it and sealed it with epoxy sealant.

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Re: Project catastros (rx rebuild)

@Bibhu, you dont need to be an engineer to use a simple formula, and as far as i know there are no laws prohibiting the same, all you need is the intelligence of a 12 year old and a calculator. and also most of the guys here who are experts have no engineering background what they know they have learnt it the same way you are now,

nowhere in my post i have mentioned that the compression of a stock head is more than the modded one ,all i have said that the formula /equation that you have used to calculate the compression pressure of the stock engine is wrong and that a good stock engine should have about 140 psi


it is wrong for the following reasons"


1): the equation (Comp ratio=psi/14.7) will work only for pure air with nothing else added to it , for example take an empty syringe pull its plunger back as far as it goes , now close its open end with your finger tip push the plunger back , you will observe that the plunger will stop moving after a small distance , this above distance and the original volume will give you the compression ratio and the above equation can be used to calculate the psi , now try the same procedure after filling a quarter of the syringe with water you will be surprised

2): in a 2stroke engine the charge that gets sucked through the carburetor first passes through the crankcase where it is pressurized by the supercharging action caused by the movement of the crank and connecting rod and it is this pressurized charge that is transferred into the cylinder through the transfer ports (the reason why the ports are called transfer ports and not inlet ports) the equation does not take into account this pressurization of the charge before it is transferred to the cylinder.

3): i personally own an rx 100 which i myself had rebuilt recently , the compression test done on it before i started the rebuild showed a compression of 140psi , it wasn't ported or running any fancy stuff but a pile of crap which i fooled myself into purchasing , if the equation was right then it should have had only a compression of (6.7x14.7) 98.5psi can anybody please explain why it is having this difference


the above are my reasons for saying that the equation is wrong , i know that the above will not be clear to many ,confusing to most and flawed at best, so i a requesting to all , if you have any thing to support or condemn my reasons please feel free to post the same


@augmentor : i agree to everything you have posted anney , but can you give me the reason why yamaha has given the compression as 9.9:1 for the 135 4speed and 10.1:1 for the 5 speed respectively in their factory supplied workshop and owners manuals and not as 7:1



@yamdoot : totally agree with you doc once the rings and piston beds in he should see a minimum ao 5-10 psi increase in the measured values


PS: Bibhu i don't mean to offend you or anybody but please be a bit more specific in terms to things like this as this thread is among the few where somebody has taken the pains to do the entire rebuild by themselves document them and then post it here. and im pretty sure that this thread is being followed by good number of members and non members also and that this will become a bible for many more and it is for their sake im again requesting the same

Re: Project catastros (rx rebuild)

Thank you for your input. Here is the thing, I am rebuilding my bike, not doing PHD in automobile engineering. If I were doing that, I wld research how to calculate compression among other things.

I am yet to see any "experts" you speak of, in this and other great technical 2-stroke forums, coming out and actively helping poor enducated clueless people like me. So lets not go there. You want to impart knowledge, thats fine by me. By all means, please make thread on compression calculations, I will be the first one to subscribe to it.

But the way you do it is rather offensive. Making sarcastic remarks on someone's education and inteligence is not an "enlightened" thing to do. Just saying "no offence" does not justify offending someone in the first place. Your argument may be technically sound, but moraly wrong and out of place.

Yes, I am aware that people are following this thread. I do not wish to lead anyone down a wrong path, anyone who has approched me for help in this forum, I have helped them to the best of my abilities without any personal gain. So I dont need to be reminded of the fact that people are watching.

All I wanted to convey was I installed a high compression head and it works. I did not set out show my technical superiority or show off any "fancy stuff" I have. Infact unlike most rx builds I dont even use all oe parts because I am on a budget. So this discussion ends here. I will get back to documenting my build.

Again, I thank you for enlightning me.
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Re: Project catastros (rx rebuild)

I was so sure that this will be picked up . Yes I have read the shop manual earlier and I did have the compression values in my mind before I mentioned them here in this thread. Having this evidence, I still stuck to my calculated values which is approx 7:1.

As you agreed about this simple equation, I did apply it practically and had a quest to find out the head volume and miscellaneous factors to complete equation. The end result was astonishing.

I am not sure why Escorts came up with values of this considerable difference in the manual which was written in dinosaurs age with hell mistakes right from the electrical values, electrical identifications, flow charts and other standard maintenance practices. And I presume that this is one of them.
Having said these mistakes, they never bothered to issue a revision to it at any later stage.

Anyways at the end would just like to say that - we are here to learn and spread that knowledge which shall eventually make a difference and a subject to talk about or debate, but never a topic of distortion or detoriate.

Thankyou again to all for sharing their valuable ideas, atleast this way it makes one to scratch his head and struggle to remember what one studied back in university days . Sorry OT.

Re: Project catastros (rx rebuild)




Fiber reed petals installed on old reed cages.

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Re: Project catastros (rx rebuild)



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Re: Project catastros (rx rebuild)


90/100 coil plate installed.

New oil pump installed. Motor mounted.

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