Yt 175 crancase volume Tuning

[sublunacy]

Im playing with some ideas and the crancase volume has me all charged up at the moment. My reasoning behing this is that Yami uses this same case for 100cc up to 225cc(not the blasty). Thats pretty much all I need to say about that. It leaves the 100cc configuration to be a low pressure charge for low end, and leaves the large 225cc cylinders high pressure.
The yt175 falls right in the middle of this. Do I have this right? I:I

Im wanting to optimise my intake track and Crankcase volume to go with my aftermarket pipe and stock engine. Hoping later to port the engine during a rebuild, but I will not worry to much as I need to Begin somewhere.

I however would like to Banter about the subject and maybe someone can help me get this straight. :-/

 

[Rubbersalt]

Modern engines seem to have about a 1:1.4 ratio on volume.

The YT175 and YT125 have the same bottom end. High crankcase volume= low pressure easy to move air/fuel. low volume= high bottom end compression and resistance to the ports. You can only open the ports so much on an engine before having to raise them and alter port timing. Lower pressure=smaller ports, higher pressure=larger ports.

Lower ports give more power. Unfortunately, we cant have ultra low ports that flow effectively, and you can only go so wide.. so we raise them. Taller ports open longer, give more time for air fuel to enter/leave. Porting is a mind job to make a beast perfect.

I use software to help develop mine. Getting ready to port another blaster to push 10,560rpm peak HP then fall off drastically at 11k.

Dont worry too much about crank case volume, worry about porting/pipe/carb size/reeds/squish band

 

[best]

It has been a few years but I did a lot of work with case stuffing and port height.
Case stuffing (reducing volume) tends to help bottom end power and can reduce top end power if done too far.
Increasing port height is like increasing the duration on a cam on a 4-stroke, it pushes the power peak to a higher RPM.
More RPM generally means more power but your pipe has to match.

It is all kind of a balancing act with a 2-stroke.
The porting has its own resonant frequency, which is calculable (see http://www.vintagesnow.com/SledU_Folder/Two-stroke%20Tuner%27s%20Handbook.pdf )
which has to match the tuning of the pipe and the crank case and intake have their own resonant frequencies.
If all of these items are tuned together (same resonant frequencies) these cause a strong burst of power at that RPM.

Generally the porting sets the stage how everything else should be tuned.
The stock Blaster porting is for the mid-range, 5-7000 rpm. This favours a longish pipe and intake and a stuffed crank or case with smaller ports.
As you raise the rpm tuning you would raise the intake and exhaust ports, shorten the pipe and open the case and ports up.

Generic advice I know, and maybe you know all this stuff already, but I thought it might help those who don't.
If any of this seems wrong, please chip in to discuss it!

 

[Larry's Shee]

Have seen many poo-poo that book, but yet none provide any better. First read that about 1975 . Not much has changed since. Be thankful you don't have to figure intake timing into the equation on a Blasty. I:I