Calculation of compression ratio

[TS1-200]

6.5:1

[sean brady scooters]

with any two stroke fitted with an expansion chamber ,compression ratios vary with engine revs ..even more so with a powervalve fitted .

[TS1-200]

According to the table, if I reduce the exhaust port number the corrected figure gets higher, if I increase the chamber to 23cc it brings the corrected figure to 6.05:1 & the uncorrected figure to
11.04:1, I just don`t want to take any chances

[TS1-200]

Must be something I`m missing,what is the x 2.2/7 bit

[Darrell Taylor]

Ah, looks like I`ve done it wrong I`ve measured from Bdc to top of exhaust port, I`ll do it again, my combustion chamber is 21cc 60mm stroke 70mm bore, the measurement from top of the exhaust to top of barrel is 30mm

bit of a how long is a piece of string question really,more info is needed to provide proper guidance

1.whats the engines intended use and fuel to be used
2. whats the current squish clearance
3. whats the piston height(the piston height dictates the port duration so measuremnt to top of cylinder could be misleading)
4.whats the rod length
5.whats the transfer height
6. what pipe is being used
7.what cylinder is it ?ts1200(reed valve and piston port require slight differances)
8. what ignition system (has it a retard feature )
9. what carb size

at the moment if the piston height is 0 then a 30mm port is approx 196*
i presume the 60 stroke crank may have a longer rod so a packer used
as a simple set up id start by reducing the packer/gaskets to get the durations down then using more head gaskets set the compression ratio

[Darrell Taylor]

yes,good calculator eden i use it myself now and again for calculating inlet duration and i use the crown to top of barrell function,i regularly see cylinder port maps that never include the piston height and are often using inset heads of 1.5mm with another 0.7mm squish on a 40mm stroke so are miles out
just asking ts1200 if his 30mm ex height has allowed for piston height

[TS1-200]

Thanks eden for the help, maths was never my strong point, but I`ve managed to work it out I think
stroke 60mm bore 70mm exhaust port height 29mm cylinder head volume 21cc = 6.68 correct ?
as Darrell points out, there are variables, the squish is 1.5mm so a head gasket is a no no
it is a long rod 115/6 yam - 34mm delly - Broadhurst MB3 pipe (shortly to be NK) standard TS1 reeds
GP gears - 19:46 the scooter is mainly used for normal rally work generally not worked too hard, usual sort of hard accelleration now & again 55-65 cruising, bit of motorway work 70ish. Question is 6.68 a bit high, will be fitting an EGT guage. The reason for the re-build is a holed piston after the base gasket disintegrated, and the only piston I could get has a 1mm lower compression height than the the original one, so I have done away with the base gasket and had .5mm taken from the head to get the squish clearance of 1.5mm. hence my dilemna. I`m going down the pub now, for a friendly chat & a pint.

[sean brady scooters]

6.5 would be better

[Darrell Taylor]

the comp ratio is ok but the port spec is way off for your requirements
199* is full race and some , dropping the cylinder 1.5mm to193* and adding 1.5mm head gasket to regain your original cr will go someway to improving the setup
the uncorrected figures for cr are the ones to watch for the suitability to the fuel
the corrected ratio is constantly changing with operatng rpm ,scavenging efficiency and plugging efficiency so what starts out as a 230cc potential trapping can drop to 150cc with a bad transfer/pipe design low rpm/small carb motor and go up to 280cc on an efficient higher rpm motor
lets presume were designing a head for a stock cylinder low durations or a rapido classic and you want 6.5 corrected
try this in the calculator to see the problem , 70 bore 60 stroke 38.5 ex height 15cc head vol

[Muppet]

Wow thats gonna have to rev; then when it does what happens to the geometric compassion ratio?