Kymco SC10AS 100cc and beyond:

[190mech]

I'll re check those numbers after work today and see what errors I made..Is the squish band 12.5 mm wide?

[90GTVert]

Sorry, I was out most of yesterday or I would have posted earlier. I'll show my math here as I check so anyone can see if I mess something up or get it right. Just woke up so who knows how it will go. lol


First, figuring out the displacements. I'm going to use the measured stroke of 51.8mm. Advertised stroke is usually so close to what I measure with the Minarelli engines that I've done that I assume the slight error is mine and just go with advertised stroke. In this case it's significantly off.

Displacement = ( (Bore Radius Squared x 3.14159) x Stroke ) / 1000
Displacement = ( (25² x 3.14159) x 51.8) / 1000)
Displacement = ( (625 x 3.14159) x 51.8) / 1000)
Displacement = (1963.49375 x 51.8) / 1000
Displacement = 101708.97625 / 1000
Displacement = 101.70897625cc or rounded to 101.71cc

To be sure it's all covered, I did the same process using 52.6mm advertised stroke and that gave me 103.27977125cc. So your displacement is either about 101.71cc or 103.28cc. I'd assume the former and will use it for future calculations.


Next, I'll do the corrected displacement. Displacement above the exhaust port basically. The reason that you get the same numbers for corrected and different numbers for uncorrected is that you only have 1 number to work with for corrected. You have 2 different strokes listed, so that will change the uncorrected ratio any time you change the stroke.

You'll notice I used 27.5mm here and not 28mm listed for exhaust port depth. You should know the actual stroke above the exhaust port for this measurement. Not just the depth. You want to know how much volume the piston is displacing, not just how much volume is above the piston to the cylinder deck. I think my old equations were a little off because I wasn't accounting for this when I wrote the displacement thread. Corrected cylinder displacement and volume would be 2 different specs unless you have 0 deck, where the piston sits right at the cylinder deck. Then it gets easier and both are the same number. In this case, the piston sits 0.5mm below the cylinder deck, so I subtracted 0.5mm from the 28mm exhaust port below deck number to get an actual stroke above the port of 27.5mm. The other 0.5mm above deck is part of the trapped volume that gets calculated later, the same for both corrected and uncorrected numbers.

Corrected Displacement = ( (Bore Radius Squared x 3.14159) x Stroke Above Exhaust Port ) / 1000
Corrected Displacement = ( (25² x 3.14159) x 27.5) / 1000
Corrected Displacement = ( (625 x 3.14159) x 27.5) / 1000
Corrected Displacement = (1963.49375 x 27.5) / 1000
Corrected Displacement = 53996.078125 / 1000
Corrected Displacement = 53.996078125cc or rounded to 54.00cc

BTW, I'm rounding to 2 decimal places for easier "handling" of the numbers. I don't think any of us are capable of measuring in a way that would make beyond hundredths necessary.


Now to find piston crown displacement.

Piston Crown Displacement = Calculated Volume - Actual Volume
Piston Crown Displacement = ( ( (Bore Radius Squared x 3.14159) x Piston Depth Below Deck As Tested ) / 1000 ) - 7.67cc
Piston Crown Displacement = ( ( (25² x 3.14159) x 5.15) / 1000 ) - 7.67cc
Piston Crown Displacement = ( (1963.49375 x 5.15) / 1000 ) - 7.67cc
Piston Crown Displacement = (10111.9928125 / 1000 ) - 7.67cc
Piston Crown Displacement = 10.1119928125cc - 7.67cc
Piston Crown Displacement = 2.4419928125cc or rounded to 2.44cc


Next I'll calculate volume above the piston to deck. I didn't show all steps for the calculations that have already been done numerous times.

Volume Above Piston To Deck = ( (Bore Radius Squared x 3.14159) x Piston Depth Below Deck ) / 1000
Volume Above Piston To Deck = (1963.49375 x 0.5) / 1000
Volume Above Piston To Deck = 981.746875 / 1000
Volume Above Piston To Deck = 0.981746875cc or rounded to 0.98cc


Now I can find the trapped volume.

Trapped Volume = ( Combustion Chamber Volume + Head Gasket Volume + Volume Above Piston @ TDC To Deck ) - Piston Crown Displacement
Trapped Volume = (7.67 + 0 + 0.98) - 2.44
Trapped Volume = 8.65 - 2.44
Trapped Volume = 6.21cc


Finally, I can get to finding the compression ratios; beginning with uncorrected compression ratio.

Uncorrected Compression Ratio = (Displacement + Trapped Volume) / Trapped Volume
Uncorrected Compression Ratio = (101.71 + 6.21) / 6.21
Uncorrected Compression Ratio = 107.92 / 6.21
Uncorrected Compression Ratio = 17.37842190016103 or rounded to 17.38:1

That sounds extremely high, but you've got a huge cylinder volume and a small combustion chamber with very little volume above deck. The combustion chambers on most of my 90cc stuff are 10cc and above and yours is 7.67cc, which is more similar to what I'd expect with a 70cc engine.

Now corrected.

Corrected Compression Ratio = (Corrected Displacement + Trapped Volume) / Trapped Volume
Corrected Compression Ratio = (54 + 6.21) / 6.21
Corrected Compression Ratio = 60.21 / 6.21
Corrected Compression Ratio = 9.695652173913043:1 or rounded to 9.70:1


So to make long math short; CR = 17.38:1 and CCR = 9.7:1 by my calculations.

I'm pretty much on with what Scott came up with. CCR is probably off because of the 0.5mm stroke difference mentioned earlier that I'm guessing he used.


All of those sound really high for something that doesn't spark knock like crazy to me. 195psi is a high compression test reading too though. I'm curious. Is it a coincidence that your combustion chamber measurement and volume above piston for the crown displacement are both 7.67cc or did a number get entered somewhere it shouldn't have? It happens, but seems unlikely to get to the hundredth of a cc exact on both of those so I was wondering if maybe we had an input error to throw things off. EDIT : Nevermind. I just went back and looked and realized you said both numbers just happened to be the same. Hmmm.


John's numbers sound like reasonable figures for something that runs well... I'm just not sure what is done different in the math.

Nov 14, 2018 18:55:38 GMT -5 190mech said:

I got 12.77 uncorrected,7.34 corrected..Double check me though!!!

BTW, don't feel bad oldgeek. I started this post at 5:30-5:45AM and it's now 7:30AM. Are we having fun yet?

[oldgeek]

Guys, as always thanks for all the help. I really spent more like 4-5 hours ciphering all them numbers and decimal points. I may have to take the day off to recuperate my brain.

I guess it is now understood what needs to be done to the cylinder head. Now I gotta figure out how to do it. I think 190mech 's clamping fixture sketch is the way to go. If I understand you correctly John, the fixture is attached to a turntable and rotated slowly while my dremel is stationary?

[gsx600racer]

Just remember if you reshape the dome(make it deeper) the spark plug will be sticking out more into the cylinder. You might have to shim the plug up to flush it with the dome, if that matters.

[190mech]

Right,slowly spin the head and hold a cutting tool where you want it..If your squish band width is the proper 12.5mm wide,I'd try deepening the bowl at the edges of the squish shooting for a flat top bowl like in the RSA125 drawing that Frits Overmars drew,your plug depth wouldnt change..Also folks like the torroidal combustion chamber in some engines,may be something to try if you need to go deeper..

[gsx600racer]

Nov 15, 2018 13:02:26 GMT -5 190mech said:

Right,slowly spin the head and hold a cutting tool where you want it..If your squish band width is the proper 12.5mm wide,I'd try deepening the bowl at the edges of the squish shooting for a flat top bowl like in the RSA125 drawing that Frits Overmars drew,your plug depth wouldnt change..Also folks like the torroidal combustion chamber in some engines,may be something to try if you need to go deeper..

I thought he was making the dome deeper to bump down the compression vs changed the dome shape you supplied. 2nd I thought the plug in the head was at an angle. After going back and looking the at head it has the plug straight in this case.

[oldgeek]

190mech unfortunately the band is less than 8mm. If it were 12.5mm the chamber would have a very deep profile. I will get some better pictures.

gsx600racer the picture of the head is deceiving, the plug does come in at the usual angle like a stock head. I need roughly 3.3 more cc of volume, so shimming the plug out a bit is a good idea to help achieve the extra volume, as long as I still have enough threads left to hold the plug in.

[190mech]

gsx600racer,You are right!Didnt think about the angled plug hole that'll be scarfed away..

oldgeek,ARRRG!So,we have too narrow squish,shallow chamber bowl,angled plug..WOW!What a challenge..Even if its not 'textbook',deepening the bowl is the only way,,would still start at the outer edge like this;



Sorry for the crappy drawing,easier for me to draw than type...

Other options are "head hunting",or weld it up and start over...

[gsx600racer]

Nov 15, 2018 17:43:13 GMT -5 190mech said:

gsx600racer,You are right!Didnt think about the angled plug hole that'll be scarfed away..

oldgeek,ARRRG!So,we have too narrow squish,shallow chamber bowl,angled plug..WOW!What a challenge..Even if its not 'textbook',deepening the bowl is the only way,,would still start at the outer edge like this;



Sorry for the crappy drawing,easier for me to draw than type...

Other options are "head hunting",or weld it up and start over...

190mech, No worries. I will always value your input.

My thought process was if the top of the dome was removed, it would expose more of the plug base/threads. If this engine decides to run hot the exposed plug would/could act like a "glow plug" like in a RC engine situation and cause preignition or diesel effect.

As usual, I will be following the progress.

[190mech]

Its obvious oldgeek has something going with this build,,first under square build and already with enough guts to shread CVT parts in a few miles looks promising..Those darn stupid simple 2 strokes keep us in suspense all the time!!

[pitobread]

Make a wooden circle that has the bolt pattern of the head in it

Use a router bit with a corner rad with a bearing guide.

Voila.

[oldgeek]

Nov 15, 2018 20:25:15 GMT -5 pitobread said:

Make a wooden circle that has the bolt pattern of the head in it

Use a router bit with a corner rad with a bearing guide.

Voila.

Yup, I was just thinking about the router and template, I have done that to mina case throats. I like your idea of a wooden template better than the metal one I have made in the past. Wood will be much easier to work with. I was looking at a nice dewalt trim router today, just find a bit with the right radius. I already have some small bearings that will slide perfectly onto a 1/4 bit shank.

I am on a headhunt for now, hope to find something ready made to use or something that will mod better than what I currently have. Looking for a 50mm head with the proper cylinder stud spacing (68mm) from any 2T motor preferably with a spark plug that does NOT come in at an angle.

[oldgeek]

Nov 15, 2018 18:47:24 GMT -5 190mech said:

Its obvious oldgeek has something going with this build,,first under square build and already with enough guts to shread CVT parts in a few miles looks promising..Those darn stupid simple 2 strokes keep us in suspense all the time!!

I got good mentors!

Also some of the CVT destruction has to do with launching a 300lb payload repeatedly.

I remember a few years back letting a 125lb friend that was used to "slower" scoots ride my 50+ MPH 70cc Streetrace. I gave him my phone for the ride, with a speedo app on it. I remember him coming back, handing my phone back to me and his hand was shaking! LOL I can't remember how fast he had it, but I was jealous cause I had never had it that fast.

[190mech]

Head hunting,50 bore,scoots=hyper race stuff(39-44 stroke),ATV 90's(later years had 50X45),motorcycles=many of the old 100's were 50X50,maybe the old Hodaka 100 as it had close stud spacing..
Most all will need cutting as youve got a long stroke build!

[oldgeek]

I bet something like this would work, but then I would have to add radiator and pump and........

www.twdeeparts.com/product-page/dio-water-cool-head-48-50-54mm


www.twdeeparts.com/product-page/dio-water-cooled-cylinder-head-level-10