Piston Modification?

C

cajones2

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Oct 3, 2008
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Cedar Rapids, IA
While looking through the reed intake and looking into the case, i noticed that at Bottom Dead Center, the piston skirt obstructs some "flow" coming in throught the reeds. Can you cut the skirt to match the profile to get maximum charge coming into the case? Is there much gain in doing so? How would you do that if it was a good idea?

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Thanks!
Craig
 
i have a magazine article on that mod i think it was the sept. 1997 issue of snow tech magazine i will check on that
 
I could be wrong but I don't think you would see any gain as the charge comes in when the piston is moving up, not at bottom dead center. The movement of the piston towards Top Dead Center is what pulls the charge/fuel in. At bottom dead center the fuel/charge has been forced through the transfer ports to above your piston, ready for compression.
 
jaydaniels -> I was thinking that same thing about it being at BDC. But....if the skirt was cut as it starts to move upward, the charge may enter the crankcase a bit sooner and with a bit more volume because you have removed that "obstruction". Basically the piston skirt will clear the sleeve bottom sooner if the piston is cut, allowing for unobstructed inward flow of the charge. I don't know how significant or insignificant that would be, but it has to help I would think. Thoughts?
 
cajones2 said:
jaydaniels -> I was thinking that same thing about it being at BDC. But....if the skirt was cut as it starts to move upward, the charge may enter the crankcase a bit sooner and with a bit more volume because you have removed that "obstruction". Basically the piston skirt will clear the sleeve bottom sooner if the piston is cut, allowing for unobstructed inward flow of the charge. I don't know how significant or insignificant that would be, but it has to help I would think. Thoughts?
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Yeah, I see what you mean. I'm not really sure how much gain you would see and then you may have some issues with reliability. The pistons wouldn't be equal or balanced and might rock in the cylinder. May break the other skirt or somethig. I'm sure someone on this site has tried this though and might have some input. Mr.Viper or Turk really know there stuff and would more than likely be able to answer this right away.
 
balancing for sure would be way off. skirt life on the intake side of the piston would probably be in the crapper. not enough support to keep the piston from rocking so much back and forth on the wristpin....if you know what i mean. i see where you are coming from tho, w/your idea.
 
sure its a mod, and yes it gains power, you wouldnt shorten the intake skirt up until its flush with the cylinder bottom, but shortening the skirt will gain a little bit. There is also a mod to be done that gains more power because your actually helping to fill the crankcase with more mixture and increasing the velocity of it up thru the trans fer ports. Look at a cylinder at the rear and study the intake ports vs the piston, youll find it.
 
mrviper700 said:
sure its a mod, and yes it gains power, you wouldnt shorten the intake skirt up until its flush with the cylinder bottom, but shortening the skirt will gain a little bit. There is also a mod to be done that gains more power because your actually helping to fill the crankcase with more mixture and increasing the velocity of it up thru the trans fer ports. Look at a cylinder at the rear and study the intake ports vs the piston, youll find it.
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Ok I have looked at it, All I can figure would be "porting" or removing some of the material from the intake port down by the case for more airflow, ??

Am I close?
 
hanker is closer, but theres still something your overlooking, keep goin you guys are on the right track.
 
Ok guess number 2. optimizing the transfer ports and timing to let more air/fuel in without letting the exhaust back in.......You would modify the skirt and the piston. Mr viper...... Am I closer??
 
i think i know the awnser on it but still can't figure out how that little trick works to helping to fill the crankcase with more mixture and increasing the velocity of it up thru the trans fer ports
 
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I have to confess, I had a little help. You mess with the hole in the side of the piston on the intake side.... What is the reasoning behind doing this....
Closer yet ??
 
I don't believe mine has the holes. Granted I swapped the stock pistons out with the BB kit, but I can check them. My vmax-4 and vmax 600 had them. I drilled out the hole on the vmax-4 using a 1/2" drill bit. I think the pistons that have the hole are on those machines that have cylinder reeds instead of case reeds.
 
DV8, cheater.......................lol :rofl:


your very close now, you would open and shorten the bottom of the cylinder skirts, change the height of the rear booster port, then add a booster hole to the rear of the piston( I am not giving out the height because it took a long time to get it perfect), but you cant just drill out the hole, because the timing wont be right to shut off the port, it has to be raised(offset hole or slot). The sx/sxr engine dont have the pistons drilled from factory anyways, I add one to them. The srx/viper do come with a boost hole but you need to change the height to gain, agin a offset hole or slot is used. Along with advancing the timing on the rear boost port(height) to get the gain.

The sx/sxr engine gains alot between the 3 things I just mentioned as these engines are really under tuned stock. They also greatly respond to a larger reed cage. that little redhead engine is highly underated when the appropiate mods are done, they flat out run hard even with the single pipe, triples add an extra 15-20hp depending upon brand of pipes. It is not unheard of to have a 148-150hp 700cc redhead mod engine. :2strokes:
 
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now that the cat is out of the bag ....do u need to play with the rear boost height on srx too(Along with advancing the timing on the rear boost port(height) to get the gain. ))i got that hole done but still can't figure how it could help ... i must forgot to do something for that hole to make it work .... my friend still yelling at me that hole don't do nothing at all lolllll....so i try to learn and to teach him stuff...
 
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