I have been working on the plan for the rebuild and thought I would share some Blow down info and pic's. There is a lot of discussion about the exhaust port and altering it so I though I would include some info that pertains to altering the exhaust port and if one should raise it or not. I will stick to the height for now and touch Low Blow Width and Blow Down Time Area.
SO what is blow down. We all have read it in a post or two so for those who are not sure just what it is, the term is defined as: Measured in degrees of crankshaft rotation from Exhaust Port opening to the Transfer Ports opening.It one step in the port timing process. I have taken a sleeve that was made for the pilot and drawn it up on paper so you can see how it related to the numbers. The drawing will show all the port window ( openings). For those with a touch more knowledge in the port windows it will plainly show that this sleeve needs some work. SO what I have done for discussion purposes is taken the drawing and used actual data from the .25 over cylinder used for the cc test and transfed the numbers. I used just plain number from top of cylinder and changed them into degrees and used a correction factor based on the negative deck height. If you do not correct the numbers using the top of cylinder based on rod ratios the data will be flawed by .0961 degrees. I will later install this cylinder and degree for accuracy.
Where this may be useful to some one is when they are having a problem with the
Engine after some one has played with it and altered the static setting and the Engine and becomes non responsive in one way or another based on bolt up parts porting or head work. one can get the basic numbers and see what if any thing can be done.
A prime example of this would be when some one oops ed a head (been there and done that). The Cr is out of whack because of milling and or cambering or both. This brings on the onset of detonation and if your running a cast ( oem Piston). it can get bad quick. The cast piston has a thinner skirt thickness and is subject to fracturing quicker than a forged piston there is other issues there but you get the idea this discussion is about Blow Down degrees.
First let's look at some pics and get a handle on what it is.
The first pic is of the sleeve and the port windows.
You will see the window and a description of the port openings ( windows). The big one in the middle is the exhaust port. The two on each side are the transfers. The one nearest the exhaust port are referred to as the main front transfer and the one closet to the intake is referred to as the main rear transfer port. The ones on the outer edges of the paper are the intake side boost port as you were looking through the intake where the reeds bolt up.
As mentioned above a quick look and you will notice that the center boost port is too short. They may have done this so it can be angle as necessary The roof angle is key to the timing. This should line up with the top of the transfers. I will show a pics later of this. The other is the height of the exhaust port to the top of the cylinder( top of the paper) It's just to high to start with. This sleeve would be a good candidate for a messed up head and a rev pipe and a high stall clutch as well as high rpm power, forged piston race only.