Compression Ratio

[traxxas-modifier]

hi guys

heres the specs of T135

Displacement 134.4cm3
Bore & stroke 54.0X58.7mm
Compression ratio 10.9 : 1

my question is,
how is the Compression Ratio computed?

 

[Mr.Rockies]

first,you need to know octane umber of your fuel.from there you decide the compression.the higher octane the higher compression or it will knocking.and on some case could make the valve like this,dare?

 

[justkir]

:shock:

 

[dehanz]

hi guys

my question is,
how is the Compression Ratio computed?

Quite simple actually. Compression ratio is the ratio of the cylinder's volume when the piston is at BDC divide by the volume when the piston at TDC. Let say the former volume is 100cc and the latter is 10cc. 100/10 equal to 10:1 compression ratio. You can calculate it by measuring everything, but the most accurate way is by performing liquid test.

The higher the compression ratio, meaning the AF mixture is compressed more and more power produced to crank. There are many factor limiting the compression ratio that you can have on your engine, one of it is the fuel being used. Racing fuel have higher octane rating, meaning you can have your engine with higher compression ratio, more than you can have when using pump fuel.

The picture that Mr. Rockies's provide not necessarily caused by high compression ratio setup. I believe the valve bent because it hit the piston head. The root cause might be inacurate measurement of the piston to valve clearance. Might also because of the valve spring failed? or the timing chain snap?

If you are going to have your bike's engine modified to high compression, i would recommend you to get a big valve head with 5-angle valve seat cut, like the one Mr Rockies is selling. That should provides adequate valve sealing. I'm hoping my explaination is clear enough. Cheers!

 

[rudduan]

i have that kind of valve with my previous bike...hehehehe and the parts that holds the valve also break...luckily my piston was ok.... 8) 8)

 

[traxxas-modifier]

thanks for the info guys.

so how did the 10.9:1 compression ratio computed?
anyone care to show the solution how it was computed?

thanks

 

[Mr.Rockies]

@dehanz

yes its not straight cause compression but as the valve material isnt good and excesive use then shaft bent

 

[stroker]

first,you need to know octane umber of your fuel.from there you decide the compression.the higher octane the higher compression or it will knocking.and on some case could make the valve like this,dare?

YOU SURE?? :shock: :shock:

Most bent valves cases (especially exhaust valve) are victim of overrev + valve float. The other cases are: a mistimed cam, oversized valves hitting each other and another finally, the lack of enough valve-to-piston clearance.

Im almost 100% sure if you hear knocking sound and next you get a bent valve- that's the sound of the valve knocking on the piston top.

Maybe that's just me.. I got a box of bent valves.

 

[Mr.Rockies]

same like me,box of bent valve.

 

[traxxas-modifier]

so mr rockies and stroker.

how to compute the compression ratio of 10.9?
how was it solved?

i would like to know what will be my new compression ratio
if i shaved off 1mm in the cylinder block from stock135cc block.

 

[dehanz]

so mr rockies and stroker.

how to compute the compression ratio of 10.9?
how was it solved?

i would like to know what will be my new compression ratio
if i shaved off 1mm in the cylinder block from stock135cc block.

ok, now i understand what your question really is. You are referring to 135LC right? i never measures stock engine. So maybe others can help?

 

[Mr.Rockies]

@traxxas

if you want to know precisely how high your compression after shave then use compression gauge,attach on your plug hole then start to kick starter.just like this

 

[CryptonX]

one note... high compression = low mid rpm
low compression = high rpm
right rockies?

 

[stroker]

so mr rockies and stroker.

how to compute the compression ratio of 10.9?
how was it solved?

i would like to know what will be my new compression ratio
if i shaved off 1mm in the cylinder block from stock135cc block.

1mm?? I'm sure you dont wanna do it. Share more details of your engine. Bore? Cam? Big valve?

 

[stroker]

where
b = cylinder bore (diameter)
s = piston stroke length
Vc = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed, prior to ignition. Because of the complex shape of this space, it usually is measured directly rather than calculated.

 

[stroker]

Bear in mind that the compression ratio you computed with these formulas, are static compression ratio. Which is merely a rough gauge.

The real deal is the dynamic compression ratio. That's what's most important. It depends on a lot more other factors.

The compression gauge is actually a worse tool to analyse CR. Cos it is more of a blow-by gauge. Whereby it factored in the blow-by past the piston rings, leakages and CR. So a low reading might not necessarily be a low CR. It could be weak rings, usage of a single compression ring, or some leakage pass the valve seats, gaskets etc..

Cheers.

 

[dehanz]

Bear in mind that the compression ratio you computed with these formulas, are static compression ratio. Which is merely a rough gauge.

The real deal is the dynamic compression ratio. That's what's most important. It depends on a lot more other factors.

The compression gauge is actually a worse tool to analyse CR. Cos it is more of a blow-by gauge. Whereby it factored in the blow-by past the piston rings, leakages and CR. So a low reading might not necessarily be a low CR. It could be weak rings, usage of a single compression ring, or some leakage pass the valve seats, gaskets etc..

Cheers.

Yes, you are right about using the compression gauge to identify condition of the piston ring and the cylinder itself. But the method also applicable to identify what kind of power the engine is making. Higher compression ratio will result in higher pressure reading on the compression gauge. This is one of the way to 'calculate' the dynamic compression ratio.

 

[Mr.Rockies]

one note... high compression = low mid rpm
low compression = high rpm
right rockies?

High compression=good accel=need high octane otherwise knocking

Low compression,you mean add extra gasket?

 

[Mr.Rockies]

one note... high compression = low mid rpm
low compression = high rpm
right rockies?

High compression=good accel=need high octane otherwise knocking

Low compression,you mean add extra gasket?

 

[CryptonX]

one note... high compression = low mid rpm
low compression = high rpm
right rockies?

High compression=good accel=need high octane otherwise knocking

Low compression,you mean add extra gasket?

well look many bikes that take place to dragsters have low compression and they kick ass at high rpm's and all the bikes i know with high compression have power only at low mid rpm's