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chris200

Engine braking

42 posts in this topic

Does anyone know what has been done to the later model 4rt's to reduce engine braking ?

Is it a mechanical method or ptb programmed differently ?

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There is  a one way reed valve in the crankcase.  Simple but affective. 

OWRV.jpg

Edited by billyt

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Here you go right out the owners manual.  

OWRV.jpg

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Introduced (I think) in/around 2015, for the 2016 MY bike - along with a re-map and revised fuel pump.

 

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I'm reasonably technical but have wondered how does this work.

i always believed the 4t engine braking was due to the extra stroke taking momentum out of the engine- i.e. Power required to push out exhaust gasses and compress the fresh charge.

without lifting a valve or reducing compression how else is engine braking reduced unless my thinking is completely wrong.

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3 hours ago, billyt said:

Here you go right out the owners manual.  

OWRV.jpg

"One way reed valve" - as opposed to ?

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I don't understand this either. Clearly there is a lot of pumping going on in the crankcase of a single cyclinder 4 stroke. A volume equivalent to the swept volume needs to be pumped in and out of the breather every stroke. If the breather isn't big enough there will be a fluctuating pressure in the crankcase. Adding a one-way valve simply means that the breather flow in one direction is enhanced. I can see this being a benefit for stopping the engine pumping oily fumes out through the breather, but I don't see what it does for engine braking.

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That reed used as a "breather" must be mounted to the crank case and any build up pressure will be released by the reeds out.

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Positive pressure under the piston as well as on top?

In a 2t there is positive pressure as the piston comes down the cylinder closes the reed valves and pushes the charge up through the ports above the piston ready for compression......so crankcase reed valve in 4t to dump the possitive pressure [blowby ?] under the piston[and therefore in the crankcase] quickly  as opposed to just ordinary crankcase breather valve which is fine on a road bike but undesireable in a modern trials application? 

As you push an empty syringe plunger downwards without blocking the end it will plunge easily, if you  gradually decrease the size of the hole at the end it will become increasingly more difficult to press the plunger down[a braking effect]

Edited by oni nou

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Very funny Billy.

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9 hours ago, b40rt said:

"One way reed valve" - as opposed to ?

 

4 minutes ago, dan williams said:

Very funny Billy.

Very funny b40rt.

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10 hours ago, jimmyl said:

I'm reasonably technical but have wondered how does this work.

i always believed the 4t engine braking was due to the extra stroke taking momentum out of the engine- i.e. Power required to push out exhaust gasses and compress the fresh charge.

without lifting a valve or reducing compression how else is engine braking reduced unless my thinking is completely wrong.

Air flow restriction [closing the throttle] is a major factor in the engine braking effect

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While the sump reed-valve may reduce engine braking, it isn't the change that may have additionally reduced engine braking in later models.

I say that because the same reed valve has been in place since the very first 4RTs. I believe its primary purpose is for moving oil from the sump to the screen-filter / oil drain cavity that's exposed when the left side cover is removed.

There are other changes in the 2014 and later bikes. One is an orifice in the left crankcase half that vents between the crankcase and left side cover (stator & timing chain). A similar orifice has been used for several years in the CRF250/450R, and went through some sizing changes over the years on those engines. It could be that this is helping reduce engine braking a little because it provides some venting on the upstroke to reduce vacuum -- vacuum created because positive pressure on the down-stroke opened the reed-valve.

Another change is the cam, which has longer open-times for both the inlet and exhaust valves. This could reduce engine braking. See http://www.montesa4rt.nl/  (Tuning | HRC | Camshaft) for the difference between the new cam (HRC cam) and original cam profiles.

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I believe that some of the systems to reduce engine braking vent some of the crankcase pressure back into the intake side of the motor.There is info on the internet as there are aftermarket products available for high performance street motorcycles.

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