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Post by hotrodsuperxl on Feb 22, 2015 18:03:19 GMT -5
Ok , so in nutshell i had a good xl500 crankshaft, crankcase and a new rod in my parts bin and i had a like new used sxl piston and cylinder..... not sure how all this would play out i assembled parts and tested the assembly , and it all worked out.... everything cleared and the piston did not bottom out on top of cylinder even with the base gasket removed... what did change is the compression and the overall stroke is .062" inch longer than the stock SXL STROKE if i remember correctly... did a lil porting to matching since the intake transfer port openings in the XL500 crankcase are a bit wider , didnt go crazy , just did a lil port maching and blending at the SXL cylinder base , that was it...saw got assembled minus and fires up and runs rather well .... but i need to check my piston clearance still, but it RUNS! AND RATHER WELL
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Post by hotrodsuperxl on Feb 23, 2015 1:53:33 GMT -5
So with this combination what have i gained? Well we know i increased the stroke of the crankshaft, i increased compression , but i didnt gain any displacement because theres only so much volume from the top of the exhaust port to the top of combustion chamber, and that didnt change.... im thinking i probably gained some torque with the slight increase in stroke and compression, which i cant measure just yet because my compression gauge bottoms out at 200 psi. If anything i know that i should be uncovering more of the intake transfer ports as the piston moves down toward the crankshaft, due to increased stroke.... any other thoughts on this combination? Or questions, please share...
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Post by hotrodsuperxl on Feb 23, 2015 2:03:52 GMT -5
Just for those wondering the difference between the SXLAO and the xl500 crankshaft stroke .
1)super xl has a stroke of 1-3/8" or 1.375"
2) xl-500 has a stoke of 1-7/16" or 1.4375"
The difference between the two is .0625" or 1/16"
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Post by sweepleader on Feb 23, 2015 10:57:04 GMT -5
You are perhaps right about functional displacement, no change. Longer stroke is a real thing however and that means more torque which means more horsepower. Looking at the ratio of the strokes, 1.375/1.4375, and there is a 4.5% increase. That would seem to indicate a 4.5% increase in torque to me. Now add the increased compression due to reduced volume in the combustion chamber and the increased crankcase compression. It would seem there should be a serious power increase. It might be as much as 10%. Without knowing what the chamber volume is or was it would be tough to offer any numbers there. Perhaps it might be a similar improvement (that is where I got ~10%, 4.5% + 4.5%). A compression gauge would not tell you much as actual running/operating pressures would need to be measured, too many variables to account for just pulling the rope. Horsepower = (torque x rpm) divided by 5252 so I think the hp increase would be the same as the torque, percentage wise.
I don't see how it could be bad as long as the piston does not hit the head or crank. It will move farther running at high speed than it does turning by hand...
Keep us up to date.
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Post by hotrodsuperxl on Feb 25, 2015 18:44:40 GMT -5
Im going to check my piston to cylinder clearance this weekend... im sure its possible the clearance might be too tight ... if so i will reinstall the base gasket and recheck.
Another thing i forgot to mention is that i beveled the top edge of the piston , ever so gently , on a little less than a 45 degree angle.... in an effort to help increase airflow as the ports open .... a trick ive seen other 2 stroke guys use, they say it helps airflow and increases duration of the ports without modifying ports....
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Post by Clasec on Mar 12, 2015 23:37:22 GMT -5
If I'm figuring it right should be 60.80cc.
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Post by hotrodsuperxl on Mar 15, 2015 2:35:27 GMT -5
Here is the the thing about adding displacement in a 2 stroke..... the volume of the combustion chamber and cylinder did not change by adding stroke.... if i were to take the cylinder and stand it up so the cylinder base was pointed up and measure the cc's of the cylinder from the spark plug to the opening of the exhaust , theres only so much volume the.....the placement of the ports, and the diameter of the bore determine displacement ..... i suppose i should cc my cylinder and see what i end up with .....
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Post by hotrodsuperxl on Mar 15, 2015 2:46:14 GMT -5
Heres another thought as well, im not running a base gasket, not sure what a base gasket measures in thickness , lets use .025 just in theory ... what does that do ? Raises compression, reduces clearance between piston an combustion chamber , moves the ports towards the crankshaft by .025 which changes port timing.... take into account the stroke difference in crankshaft.... its an interesting combination for sure
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Post by sweepleader on Mar 15, 2015 19:45:34 GMT -5
I would say that if all else remains the same, just increase the stroke, the piston would go higher in the cylinder by 1/2 the stroke change. That would increase the compression and displacement a little, but not like on a 4 stroke which has no exhaust port as you noted. Still, it would be something. If you CC'd the cylinder with the base up and the plug in, then again through the plug hole with the piston at TDC, you could figure the compression ratio change. I guess you would have to use both cranks to compare. Interesting information...
Dan
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Post by Clasec on Mar 19, 2015 23:26:37 GMT -5
You will also have to take in consideration the pin placement on the SXL piston. If the rod for the XL500 and SXL are the same length the pin will be lower in the piston on the SXL versus an XL500. It would raise the piston higher in the cylinder using the XL500 crank. That would also raise the compression somewhat.
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Post by hotrodsuperxl on Mar 22, 2015 13:39:43 GMT -5
I really need to tear the saw back apart and take some measurements.... i also have an old xl500 piston to compare to the super xl piston..... at some point it might be nice to just put a xl-500 piston and cylinder in there and be done, but for what that would cost, its most likely not worth it.
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