Street Ryderz
Well-Known Member
That 24x20 flange area is 23.9mm equivalent dia and big enough already, the lead engineer at Aprilla told me that from the window/piston face to the exhaust flange a 1% increase in dia was except able so in the case of 23mm it would be 23.23mm with 1% not that much of a difference but that dia can flow 80 cfm.That opens my eyes a little bit more to porting, I've really only cleaned the ports up on mine. I wish my calipers fit down to exhaust port opening. At the exhaust flange/opening its 24x20mm on these 49mm steel sleeve cylinders I've been using. I'm just curious about porting transfer ports, I'm hesitant to mess with them with all the reading I've been doing.
I don't suggest any one mess with the transfers or the port timing unless they understand what the timing changes will do/effect and many people go ahead and cut the exhaust and intake thinking leaving the transfers alone they will be ok but that changes both exh and intake durations and the blow down period.
Transfers mostly rely on the cross section just after opening for their volume and in most cases are all ready very big and here's why, lets say the transfers have a width of 20mm each and just after opening as the velocity is highest have an open height of 4mm that's 9.08mm equivalent dia and combined is then 18.16mm not bad for the stock port volumes but at full open it's double that and therefore kinda big but again it's that cross section at partial open that's more important in considering the volume.
When it comes to correcting the transfers exhaust side angle this also increases the cross section of the track/feed and allows a bit of a radius to be made between the back wall and roof, the roof angle is also important and at this displacement and area 15 degrees works best.
The blow down for our engines is best around 20-22 degrees and when raising the exh timing this will effect the blow down's timing/duration so the transfer may very well need to be raised also if going with more exh timing/duration, the effect of these being mistimed is poor cylinder fill, lack of pipe aid, slow to build rpm's under load and very narrow if any real power band.
There are alot of other things that go along with all of this and I'm not going to write a book here LOL but trust Blair and Jennings published works are very informative in all of these area's and give you the formula's and explanations/examples needed to get a better understanding of it all. BTW I cheat when it comes to the math and use online calculators intended for the task.