Piston port

[Jackson]

What does piston port do for a motor that does not have any in it ??

[Mark75H]

Adding piston porting to a reed valve motor does not seem to make any measurable power increase.

The theory is that reeds could be letting in mixture earlier in the stroke than straight piston port allows.

[Bill Van Steenwyk]

Some years ago when embarking on trying to improve the performance of a Yamato 80, one of the first things Harry ZAK did after the bolt on's like pipes, improved ignition, improvement of flow thru the factory reed valves, was to finger port the sleeves and pistons. This was done BEFORE any port work on the sleeves themselves such as raising/eyebrowing the exhaust's etc. Quite dramatic improvements in performance were seen by this modification. I seem to remember about 3-4 MPH improvement in boat speed just from this one modification. That being said, the improvment was not done prior to the installation of two expansion chambers, so the improvment might not have been as dramatic without the chambers. As with a lot of modifications, changes were also made in the pipe lengths after initial testing which improved the performance even more. In other words, a lot of times when you change one thing, if you change something else to take more advantage of the first, dramatic improvment can be realized.

Sometimes "theory" belongs in books from whence it comes. A lot of performance improvement would have never taken place if the only thing ever "turned" were pages of books or listening to those who are "experts".

Those who are afraid or for other reasons never experiment usually don't run up front, all other things being equal. There is much information available about two-stroke tuning and performance. There are also computer programs available to help.

[Skoontz]

I only can share the experiences I had when racing Bultaco motorcycles. Up to 1980, all Bull's used piston ports. Then 1981, they installed reed valves to try and improve perfection. They failed miserably, although the use of the reed valve on the piston port motor produced 15 more lbs of torque from 5,000 to 8800 than the piston port motors. At 8800, through 9600 they tended to peak and loose power. The reeds would break causing the crank case to fill with gas to the point where raw fuel would spit out the stinger of the pipe. They took out the reed valves in '82, and had planned on doing more R&D, but after a strike the factory went BK and thus ended the company.

I know they would have played with that reed valve until they saw improvement and always wondered the how what and why's of the PP vs the reed.....

[Bill Van Steenwyk]

I have never owned a strictly piston port motor, except perhaps in a garden type tool or chain saw. In other words, never one that I had an opportunity to try to improve performance of. That being said, some of the fastest motors I have ever seen run, ONCE THEY GOT ON PLANE AND GOT GOING, were strictly piston port motors. I will clarify by saying the strictly piston port engines for their time, and against competitive engines in the same time frame, were very viable race engines. The down side was they were very touchy carburetion wise (jetting) to allow them to plane off without loading up, and also props used had to be right on. As far as I know, and based on conversations with those who ran them, by having the fixed piston port to allow fuel to enter the crankcase and then the cylinders, there was very little flexibility on the low end of the RPM range. Motors with rotary and reed valves were and are more "flexible" and you don't have to be as closely "right on" insofar as props and jetting are concerned in the lower RPM ranges when first starting off when the boat breaks over on plane and the engine takes the full load.

If you will remember the early road race bikes, Yamaha, Suzuki, Kawasaki, etc., were piston port engines in some of their models, and all did well and had lots of power from a start to top speed, BUT they all had gearboxes to assist in keeping the engine in the piston port narrow power band. A lot of the early Kart engines were also piston port, but they used slipper clutches that allowed the engine to maintain RPM in the range where max power was developed or at least a lot more than lower in the range. When the engine RPM got high, the clutch locked up and then was basically direct drive like could be compared to an outboard lower unit. In the early 70's Stan Leavendusky built an "M" engine for Eileen made from a 100CC Kart engine complete with a slipper clutch and chain drive. She had to lube the chain by pushing on a pump with her knee a couple of times a lap to squirt oil on the chain. We used a pump and hose/bag assembly from an early windshield washer aftermarket assembly filled with oil. Had to start it with an electric starter and battery that was not nearly as compact as they are now, and really never got it propped out right, but it was a fun project and got lots of attention at the races in the early 70's.

Guess the best way to summarize is most of the race engines used today use some type of valve to assist in getting the air/fuel mixture into the crankcase and then to the cylinders, and don't depend on a fixed opening such as in a piston port engine, whether it be in Stock, Mod, PRO or OPC, so that should tell you something. Finally, there are still some engines out there with finger ports in the sleeves and ports in the piston to assist in getting extra fuel into the cylinders, but this type "help" is used in conjunction with the other types of induction valving mentioned earlier.

There are lots of very sharp folks that post here and I am sure you will get some more answers and helpful hints.

[88workcar]

What does piston port do for a motor that does not have any in it ??

sss

[Fast Fred]

the power Band of a piston port Moda is narrow, vary narrow. older style dirt bikes.

[Jackson]

I was going to cut a port hole in my piston below the last ring, in line with the boost port like a nissan 25 has . Do you think that it will work??? Thats what iam calling piston port. I put the piston in the block and mark the top of the piston where the boost is at and then moved my marks down below the rings about 3/8in" And then cut a hole a little bit smaller than the boost port. What do you think??? I have not done this to all pistons and have not put motor together yet. Before I did so I wanted somebodys EXPERENCE on this.

[mac19f]

"Adding piston porting to a reed valve motor does not seem to make any measurable power increase."

So why were there so many 3rd port Quincy engines built? I know OF Christner verified all his modifications on a dyno. Was he selling an unnecessary modification?

The 3rd port modification replaced the "mumps" extra reed valve modification so they took out the extra reeds and added a piston port.

I have 3rd ported engines and can say fairly comfortably there is a difference at top end. The 3rd ports add nothing perceptible to the low end. The benefits only show up at very high intaking velocities.

[Frank Volker]

The Quincy flatheads (loopers) had both reed valves and piston ports--or "3rd" ports" as we called them. The 3rd port addition increased hp over the entire band from about 6500 to 9200 RPM when tested on an FA (15 cu in) engine. I seem to recall an improvement of around 5-8%.


Frank