stroker_2

Many Thanks Mopar

No problem. I assumed the exhaust port data needed to be ATDC hence I have quoted it that way (104 deg atdc). Therefore deckheight has already been accounted for. Should the programme require deckheight for onscreen drawing sure just put in zero.

bore - 70 mm stroke - 70 mm rod - 125 mm exh opens at 104 deg atdc eng speed - 5000 revs/min trapped compression ratio 7.0 squish area ratio 0.5 (i.e 50%) squish clearance - 1.2 mm If you could throw a screen dump of output in your post would be much appreciated.

mopar, could you do me a favor and calculate the msv using tsr squish programme if I give you required dimensions. It would be great if you could put a screen grab of the output in a posting. stroker

I was possibily a bit to brief in my previous post. What I wish to point out is that when there is any doubt about the compression ratio of an engine it is best to actually measure the trapped compression ratio. This is the compression ratio calculated using the trapped swept volume of a cylinder. i.e the volume of the cylinder from the point of exhaust port closure to the tdc position. As an example, if the exhaust port on a standard stroke(54mm) banshee opened at 27mm from tdc, the calculated trapped compression ratio would be exactly half of the static (also known as geometric compression ratio). In a two-stroke the cranking compression is directly related to the trapped compression ratio! If you raise the TCR, the cranking compression ratio will also rise. The static compression ratio has no relevance to the cranking compression. Factors that can cause the cranking pressure to appear different to what was expected include. 1/ Gauge accuracy 2/ altitude 3/ warm or cold engine. 4/ squish clearance 5/ dome shape of piston 6/ cylinder bore 7/ exhaust port timing 8/ worn rings 9/ worn bore Trapped compression ratio is sometimes known as dynamic compression ratio. However the name recommmended by the SAE is trapped compression ratio. Does not matter to much what it is called as long as you realize that they are the same thing. Below is a small sample table for the calculated trapped compression ratio. CCV is the combustion chamber volume in CC. Exhaust Port Opens from tdc CCV 26mm 27mm 28mm 13 7.43 7.68 7.93 14 6.97 7.2 7.43 15 6.58 6.79 7.01

Measure up the actual combustion chamber volume with the pistons at tdc and the heads bolted down. Also measure your exhaust port opening timings from tdc. Now calculate your "trapped" compression ratio. http://www.torqsoft.net/compression-ratio.html

What is your new booster duration? Also what is your main transfer duration?

I have been running avgas over the years in many forms of two stroke motor sport. It is a good fuel and can be mixed with any percentage of pump gas you like. My favorite mix is 50/50. If you are not running to the limits of compression I find an engine just feels that little bit more powerful with a mix.

Just curious but when you talk of alky, is it methanol or ethanol?

A longer rod will lower the port timings. Using the torqsoft piston position calculator for a rodlength of 110mm, stroke 54mm and an exhaust port opening of 89 deg atdc the piston position is calculated as 29.89 mm from tdc. Now if a longer rod of 115 mm is used the angle calculated for the piston position of 29.89 mm is 89.31 deg atdc. As the exhaust port now opens 0.31 degrees later, the power at lower speeds will be increased. http://www.torqsoft.net/piston-position.html