I'm getting 6.24 and 7.02 respectively, assuming the other numbers are right. This is best done by direct measurement of cc 's of oil in the combustion chamber and then summing the piston crown hump displacement.
Take the power wire of the Dyna and stick a comp gauge on it...... if your getting 100-110-120 psi wouldn't that show it ?
1978 KZ650 b-2
700cc Wiseco kit 10 to 1.
1980 KZ750 cam, ape springs, stock clutch/ Barnett springs.
Vance and Hines Header w/ comp baffle and Ape pods, Dyna S and green coils, copper wires.
29MM smooth bores W/ 17.5 pilots, 0-6s and 117.5 main
16/42 gearing X ring chain and alum rear JT sprocket....
That would be unreliable due to the new rings and only 200 miles on it. I'm checking my valve adjustments this morning. I will do the wire test. I have some heavy gauge wire I can use. I can measure at the side where the piston is flat and then measure the center of the dome if that would help.
I can measure at the side where the piston is flat and then measure the center of the dome if that would help.
Not sure it would.
A cc syringe and plate glass with hole in it over combustion chamber sealed with grease and filled with oil and no bubbles gives the combustion chamber volume, usually 1 cc within factory spec unless it has been skimmed or had a valve job.
Then, unless a perfectly flat top piston with no valve pockets, you calculate the piston dome displacment which comes from direct oil cc measurement.
You might be able to get a rough measure by tipping the bike on an angle, filling a bore through the plug hole with oil at TDC, noting the volume, then go to BDC and see how much you add, using the bottom of the spark plug thread as your fill mark.
Add the first two then divide by the first. Probably accurate within less than half a point of compression ratio I guess.
Usually the piston dome is above the deck height and subtracted from the combustion chamber volume and gasket width to give V(tdc) , in this case it will be much less, that's why we are making assumptions by back calculating from factory specs and some sort of direct measurement might be neccesary.
1980 Gpz550 D1, 1981 GPz550 D1. 1982 GPz750R1. 1983 z1000R R2. all four aces
Hind sight is 20-20...LOL....
it sure would have been nice if you had measured with a steel rule how far the piston drops from TDC in 180° !! HA ha
I would not be surprised to find out you do not have a 750 crank from what everyone is saying.
I think you "might" be able to drop a straight wire in a plug hole and mark it off against a top fin and rotate the engine to see how far it goes up or down.... etc.
A negative 8mm would be fairly easy to see if you can keep the wire straight up and down ....and always compared to the same fin and the same spot the top of the piston.
Seems like a goofy backyard mechanic way to do it, but what the heck... lol
8mm, that's .315" or 5/16" of an inch from a 903 at 66mm / 2.6" stroke.
So if you measured only 2.3" I guess it is a 750 crank.
2016 Triumph T120 Bonneville
Far North East Metro Denver Colorado
Tyrell Corp wrote: I'm getting 6.24 and 7.02 respectively, assuming the other numbers are right. This is best done by direct measurement of cc 's of oil in the combustion chamber and then summing the piston crown hump displacement.
If you take the stock displacement and CR you can calculate V(TDC) that includes chamber volume + Gasket+ deck height - piston dome
Then you can determine the additional piston dome volume for the 10.25 : 1 pistons
Again this is just estimation, the real measurements would be needed, but it should be in the ball park, and that doesn't look too good.
Someone familiar with these engines can comment about the normal head volume and so forth, I haven not had a 1015 apart.
In Swest's case its the reduction in cylinder displacement that really knocks the CR.
If this is the case with your engine Swest you may need to deck the block and plane the head if you want to run it that way.
If I knew what I was doing all the time life wouldn't be any fun.
'80 KZ650 E 700cc, dyna ignition and coils, frame up restoration, daily driver
'81 KZ1300 A3 full restoration, custom big bore pistons, 1400cc 6 cylinder super bike
"77 KZ650 B1 - Barn Find, work in progeress
"74 Yamaha DT 400 Enduro...