Cylinder head porting refers to the process of modifying the intake and exhaust ports of an internal combustion engine to improve the quality and quantity of the gas flow. Cylinder heads, as manufactured, are usually sub-optimal due to design and manufacturing constraints. Porting the heads provides the finely detailed attention required to bring the engine to the highest level of efficiency. More than any other single factor, the porting process is responsible for the high power output of modern engines.
This process can be applied to a standard racing engine to optimize its power output as well as to a production engine to turn it into a racing engine, to enhance its power output for daily use or to alter its power output characteristics to suit a particular application.
Daily human experience with air gives the impression that air is light and nearly non-existent as we move slowly through it. However, an engine running at high speed experiences a totally different substance. In that context, air can be thought of as thick, sticky, elastic, gooey and heavy . Pumping it is a major problem for engines running at speed so cylinder and head porting helps to alleviate this.
When a modification is decided upon through careful calculations and mesurements, the ports are carefully reshaped with die grinders. Sometimes the ports must be welded up or similarly built up to add material where none existed.
Porting a Two Stroke is a complacated process, more than just putting a 4 stroke cylinder head on my SF 600 flow bench and checking flow at various mesurements of lift, and grinding on the cylinder port until it has good port velocity and good flow numbers. There is no short cuts to porting a two stroke cylinder, its time comsuming, takes lots of math.Do it Wrong, and you will be buying another cylinder, or be very disapointed with all the results, not to mention all the time and energy you wasted.
I will attempt to show some of the process and the mechanical, disciplines I use in porting a cylinder, and reconfigure head design, I will start with a 200cc Blaster engine. and a stock cylinder head..
This engine is a customers, it was here because he felt it was down on power.The squish .045 and the deck was -.015 and the head gasket .032 for a total of .092 piston to head clearance. this needs to be logged before any grinding or machining can be started. Before the cylinder is removed map the cylinder with graph paper to measure port area. check port timing. Check to see if the piston is level or below the ports,at BDC This Cylinder is above and will need to be corrected later. Check Blowdown timing.The concept I'll use here is that of "Blowdown time"(BD), which is the time, in degrees of piston travel, between the time of exhaust port opening and transfer port opening. A typical race engine has a BD interval of 35deg. A stock engine will usually have a BD of 25-30deg. in stock form, pretty zoomy for a stock engine, This is why you get into trouble if you start raising the exhaust port too much without raising the transfers as well. long blowdown period is needed to clear the cylinder at high rpm's, where the time allowed for scavenging the burnt gasses out the exhaust port is short. ( port area is a significant factor in this process, and really must be taken into consideration in designing a cylinders port configuration, but I want to keep this at a basic level, But...the longer the exhaust is open, especially at lower rpm, the more chance, actually a certainty, there is of the fresh charge simply "short-circuiting" and instead of staying in the cylinder to burn, it heads right out the open exhaust port and you lose power. So..if you raise the exhaust by too much, which will certainly hurt low to mid rpm performance. it does make more power at a higher rpm now, but the real reason it feels like it hits hard ,is because it is making less power below those rpm's than it did before.
Back to the transfers, higher ports also give the cylinder more time to fill at high rpm, boosting power by giving more time and area which is needed in the shortened time period that occurs at high rpm. Too low a transfer height and there is not enough time left for complete cylinder filling after the scavenge phase is complete, which also limits power. The need to have exhaust and transfer ports in proportion to each other in order to make the best power possible for your application. It is critical!
The directional flow of the transfer ports is also very important. This is controlled by the shape and angle of entry, both horizontally and vertically, and the boost port adds to this as well. This is the real tricky part, because it is fairly straightforward (but hugely time consuming) to simply raise the transfers heights, but preserving or improving the directions that they discharge in is difficult. That is why that job is best left to an experienced and steady hand.Without a right-angle handpiece you'll not be able to do any of this work at all. One of the latest trends in transfer flow is to use flat domed pistons and transfer ports aimed straight across the cylinder at each other, instead of the "old school" upward pointing streams. This new process simply fills the cylinder from the bottom up with a colliding swirl of gas that purges the cylinder of spent gasses as it rises with the piston. It also cuts down on the amount of charge that can in some cases simply blow right out the exhaust port by making the two (or more) transfer flows "stall" when they collide instead of swirling around in front of the exhaust port or shooting straight for it! The theory is that this gives greater cylinder filling, and I believe it. Even with conventional domed pistons I think it helps. Angle the transfers toward the rear wall, and away from the exhaust port. Very important, widening and raising intake ports will also raise compression ratio. The old myth of cutting the cylinder head to raise compression will produce more power, is not altogether true, The first big problem is that cutting the head decreese the distance from the top of the head to the spark plug, (putting the piston closer to the fire) This condition will short live a set of rings. Before whacking a cylinder head, measure the distance from the deck of the head to the spark plug, you will need to use spark plug spacers, they come in different thickness, and can be purchased from any speed shop. (index rings) You will also need to remove metal at the spark plug threads in the head , so that the plug will not be shrouded.
MSV - Maximum Squish Velocity rates the maximum velocity of the fuel air traveling across the squish band just before the piston reaches TDC. If MSV is to low the flame front will not burn the fuel air mixture effectively. If MSV is to high, detonation will occur and cause engine damage. This is an area of topic that is highly contested among many engine builders, Tighter squish bands allows more fuel to be turned into energy thus more power, others state,this type of squish band causes the flame front propagates too fast, it will increase cylinder pressures too fast and induce auto-ignition or an un-controlled burn. Pushing the limits of MSV. MSV changes as piston speed increeses.. You ask why use a squish head, the squish band serves two functions, one cool the outer edge of the piston as the air is being compressed by the piston coming to TDC in this process the air heats up and is transferred the the cylinder wall and the top of the piston. the area that is being squished is so far away from the flame and in such a narrow band the it will not burn . thus cooling the outer area of the piston.At TDC. The next function is to force air/fuel to the center of the combustion chamber,at the correct time that the plug fires. What is the right clearance?,
28 m/sec is considered Ideal, (this is a mesurements in time) , this is usually about 30thou clearance. for those that think less is better below this the pumping inefficiency robs the engine of power.you need the "gap" to expel the mixture across to the combustion chamber. contry to popular belief retarding the timing increases the power for two reasons ,it allows the power stroke to be still active with greater angle on the crank ,and it delivers hotter gas to the pipe giving faster sound wave reflection allowing higher effective revs(more power)this all depends on piston speed, i.e. bore stroke RPM. Ignitition system (Built in Retard) I use software to configure this! you will need to know rod stretch,in order to set the final squish.
I cut three to four angles in a cylinder head in the name of MSV, I feel that there is a lot more to be learned in this area, I have cut some funky angles in a cylinder head due to time or because of the piston height, and compression ratio's, thinking I was wasting my time, only to find out the motor screened! In my drag racing and 4 stroke background, all the horse power is in the cylinder head, I have thought this of two strokes for years, I am tickled that there are people out there that are doing research in this area.
Check this gnarley head out. This is the same head This is the head I will use for this motor
The "Porting and Polishing" Myth
It is popularly held that enlarging the ports to the maximum possible size and applying a mirror finish is what porting is. However that is not so. Some ports may be enlarged to their maximum possible size (in keeping with the highest level of aerodynamic efficiency) but those engines are highly developed very high speed units where the actual size of the ports has become a restriction. Often the size of the port is reduced to increase power. A mirror finish of the port does not provide the increase that intuition would suggest. In fact, within intake systems, the surface is usually deliberately textured to a degree of uniform roughness to encourage fuel deposited on the port walls to evaporate quickly. A rough surface on selected areas of the port may also alter flow by energizing the boundary layer, which can alter the flow path noticeably, possibly increasing flow. This is similar to what the dimples on a golf ball do. Flow bench testing shows that the difference between a mirror finished intake port and a rough textured port is typically less than 1%. The difference between a smooth to the touch port and an optically mirrored surface is not measurable by ordinary means. Exhaust ports may be smooth finished because of the dry gas flow and in the interest of minimizing exhaust by-product build-up. A 300 - 400 Grit finish followed by a light buff is generally accepted to be representative of a near optimal finish for exhaust gas ports.
The reason that polished ports are not advantageous from a flow standpoint is that at the interface between the metal wall and the air, the air speed is ZERO This is due to the wetting action of the air and indeed all fluids. The first layer of molecules adheres to the wall and does not move significantly. The rest of the flow field must shear past which develops a velocity profile (or gradient) across the duct. In order for surface roughness to impact flow appreciably, the high spots must be high enough to protrude into the faster moving air toward the center. Only a very rough surface does this.
I glass bead my cylinders after porting, main reason is to get a rough texture conformty in each port. I polish the exhaust port and the transfer divider. I angled the transfers away from the exhaust port, on this cylinder I raised the intake and widen it. Flattened the upper intake wall of the transfer ports, to flow across the piston. The boost port I narrowed with epoxy to increase port velocity for low and mid range..
I did not have to raise the exhaust port, I widened it to 1.5mm each side, tapered the exhaust port from the inside to the outside, to help in scaveging. I radised the short side of all the transfer ports. The head squish I cut at 10 degree's and 11 degrees total squish band width 8.88 mm ,20 degree cut 10mm wide and the rest is a secret? .032 squish clearence.Removed .095 thousands from the top of the cylinder . Deck hight 0. installed a .030 index ring on the spark plug. .050 base spacer, exhaust open 88 degrees ATDC, Intake open 121 degrees ATDC, Blowdown 33 degrees.kicking comp. 210lb.
The cylinder in the first picture was an off the shelf port job,from another engine builder. Using TSR software, The grinding looked good,I didn't see anything wrong with the cylinder. I use the same software, in the end it is only a tool not the absolute. The problem here is that it wasn't built for this engine, .095 squish, band clearence, Exhaust port opened at 84 degrees ATDC, Intake opened at 114 degrees ATDC, port time area not suited for MX,
The most important aspect of porting is, Port to your Engine and the type of riding you do.
I personally don't do shelf port work, I use dummy engines with various stroke cranks, to match what the customer's engine specs are, If I don't have a dummy motor, then I want there's.There is a lot of artical's on the web about basic porting 101, a good book to get is 2 Stroke Tuners Hand Book by Gordon Jennings, Eric Gore, has one on his web site.Mcdizzy.com has a great web site, full of information . Gordon states in the first section of his book "Don't guess, Know what you are doing! Air Flow has everything to do with a two Stroke engine. The more you understand, The faster you will go!! "And I said That."
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