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redswr

anyone running Powroll engines?

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http://www.powroll.com/P_YAMAHA_YZFWR426F.htm

i started a thread a long time ago about a big bore set up for my 99 and this was posted... thing is i don't know if the cylinder on mines already been changed out to the 426 or not or if it's been bored. How can i tell if anythings been changed? Can i tell from the outside if it's the 426 top end?

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I don't think you can tell from the outside. Maybe, just maybe, a compression test will show higher readings if it's a big bore, but I'm just guessing. Maniac

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the comp ratio should be the same provided teh big bore kit doesn't change the stroke... shouldn't it? regardless of displacement?

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If the cylinder is changed to a 426 is should be marked with "426cc" on the front right side, if it is a 400 the marking is "399cc".

You can't tell from the outside if it has been bored out, you must remove the cylinder head and check the bore.

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the comp ratio should be the same provided teh big bore kit doesn't change the stroke... shouldn't it? regardless of displacement?

Compression ratio is calculated using displacement, so no.

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Compression ratio is calculated using displacement, so no.

I was using a compression ratio calculator online... I entered a 426's specs and came out with a 12.5 : 1 CR. then entered the big bore kit dimensions of the same stroke with larger bore and displacement... same 12.5 : 1 ... Could you please elaborate? Here is the calulator I was using... a link to it anyhow.

http://www.csgnetwork.com/compcalc.html

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CR= volume at BDC / volume at TDC

If you change anything that alters volume (bore, stroke, piston shape, gasket ect.) you'll change the CR.

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but what im asking is... bored out to 444 a 426 will have the same 12.5 to 1 because BOTH those numbers in the CR equation are increased proportionately, are they not? IE the volume at BDC is larger on the big bore kit... but proportionately so is the volume at TDC... the length of the stroke hasn't changed so your likely to have the same CR in a big-bored bike as it was when stock unless you change something else... wouldn't you?

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I tend to agree. If the stroke doesn't change and the head volume has not changed (different head, domed or scalloped piston) then the comp ratio should be the same.

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Sounds reasonable.

I guess I didn't understand what you were saying, and didn't think it all the way thru...

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