hi guys.I had bought an os max 46 ax engine.I made some mistake while breaking in the engine which caused d connecting rod to break. I got the broken rod replaced with a new one . But still d engine is under performing.It is giving 11800 rpm with a 11x7 prop(which i ideally think shud be around 13000 rpm).I tried using imported nitro fuel which didnt work out either. What might be the possible way to increase its performance?
Pls help.  

shakee93, its been almost 2 years since you posted this. Even then, the only reason I can think of is, the new connecting rod which you replaced would have a longer length than the old one. Even an insignificant length of 1mm can make a differenc in the overall performance.
So how does the Length  of Rod affect the RPM?
   The longer the rod, the smaller the Clearance Volume above it. This means that, the compression increases ie increases power/torque/hp but reduced RPM.

What is meant by a ""longer connecting rod?""

If, as seems, you are referring to the increased distance from small end to big end, then yes, it will create a higher Compression ratio

But, the effect of that will be a higher RPM!

This is why:

Power = k. PLAN. (BME Pressure, Length X Area, RPM)

Although the L (length X area = swept volume) will decrease, the increase in CR will lead to an increase in BMEP, more significant.

If Power increases, all other factors being same, the same prop will rotate faster.

I think when he broke his CR some other damage too took place, which, not being rectified, is causing this problem. But, was the RPM measured before the CR broke?

In any case, @11,800 rpm, and @ 25 C, with a 11*7 prop, that engine will produce 841 watts. And @ 13,000 rpm, 1125 watts. That is a very significant difference!

sanjayrai,

okay, So..

Power = P.L.A.N.k

What I thought was,

1. The Power has to remain constant.
2. BMEP i.e. 'P' increases due to increase in CR.
3. So, RPM i.e. 'N' decreases.

Correct me, if I am wrong.

Also, Sanjayrai55, can you tell me how you got thoe values of power ( "@11,800 rpm, and @ 25 C, with a 11*7 prop, that engine will produce 841 watts. And @ 13,000 rpm, 1125 watts. That is a very significant difference!" )

BMEP = Brake Mean Effective Pressure. A function of fuel calorific value, Compression Ratio and other factors eg Catalysts (Nitromethane)

If you increase that, you are changing a fundamental characteristic of the engine (like L, which affects the Swept Volume) and this will therefore lead to a change in power

The effect of the power increase is the engine (a Prime Mover) will be able to rotate the prop (Load) faster as more energy/unit time (Power) is supplied to it

Hope it is clear; I am a little rusty, learnt in 4th year Mech Engg in IIT-Bombay - 1978 

Thanks a lot sanjayrai. I am still doing my Engineering in VIT University.

Looking forward to learn from people like you.