Kawi2strokes.com Forum

With your "hot" top end, what is your cold cranking psi again h2B?

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I am just a man working in a tin shed.

165psi and they deto like hell on pipe (stock iggy yet) so i have to run stock heads (140psi on same gauge) need new heads!

With that exh duration you don't need new heads. Just take 1/2 cc out of the combustion chamber and re measure. 155psi will be ok with that duration. If you have a STOCK CDI set spark at 2.9 mm btdc at first. Keep the squish good and do not be afraid to give it some fuel.

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I am just a man working in a tin shed.

Agreed! 155psi should be good but heads are a oldschool doublesquish design. So i think i will go for a more modern design when i send them for machining. Will do a test with 165psi first as a got some racefuel i can blend in to prevent deto..

Measured comp.ratio today with a syringe, cyl. and head bolted together with gasket on the bench, piston at tdc. (deckhight 0,6mm.) and got 16,15:1 cr!! (Displacement 259,95cc+17,154cc comp.vol. diveded with 17,154 =16,15:1) So now i know why i have some deto.. 3cc more headvolume will be 13,9:1cr should be alright or?? What's stock cr. anyway?

You're measuring the whole stroke. 2-stroke compression is normally "effective compression", measured from the top of the exhaust port, and stock is 7:1.

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If it surges, that's normal, upshift.

Correct Jim! And my "trapped" cr. is 8,55:1 way to much! And when bike is "on pipe" the return pulse will "close" ex.port and than i got 16,15:1..

It doesn't quite work like that, but yes, when you're on the pipe, pressures go up.

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If it surges, that's normal, upshift.

16.54 is roughly where the hottest over the counter Denco drag heads were, so it will run great for 9 seconds on race fuel. I have to say I believe it IS, very important to know the comp ratio proper. So yes the "corrected" ratio may have some value, but I have not been able to find it yet. How does this work on a power valve engine I ask? My KTM has a compression ratio of "oh well 6.5 to 8.2 :1 " Measure your cylinder with a stock head and squish. You have had a rebore or two as well. I had an engine 20 yrs ago with the denco heads and 202 exh duration, no idea what "trapped" compression. I was running it on methanol , which is 140 octane when jetted correctly. The engine would detonate after about 5 minutes of use. I measured it at 16.8 CR........ sh!t !!! Let it down to 14:1 and it ran for 5 seasons of roadracing, two sets of rings, thats it . Oh , as a by product it made MORE hp and it was much much easier to push start. Maybe I am missing something ? Here is a variable comp ratio motor. Lower graph is stock, brand new out of the box, upper line is after work.

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I am just a man working in a tin shed.

Measured my stock heads tonight at 28,5cc vs. 23cc for the 16:1 heads. Installed cc on my 780 cyl. is 25cc that means cr. is 11,4:1 and just 6,2:1 trapped.. That in combination with a huge squish (about 1,7mm.) will defently hurt lowend! Have the 16:1 heads at a local machine shop and they are not used to this kind of work but will try to help me Will try to get a pic. up here tomorrow as i/they need some advise how to cut em.. And measure procedure was same as Jorgens only i used a syringe from the local pharmacy, don't know how accurate they are..