Chainsaw Repair

What is free porting? What's wrong with it?

When the piston skirt comes above the bottom of the exhaust port at the top of its stroke thereby venting the crankcase to the atmosphere. This is free porting!

My question is . . . What ore the downfalls, or possibly advantages of FREEPORT?? What happens inside the two stroke engine?

I would think that free porting would be the end of the world as far as engine efficiency and performance is concerned?

I happen to have a Domar 115 with a Meteor crowned piston that free ports about .010". As far as I can tell, it is a nice running chainsaw.

I've had other two strokes that free ported at the top of the stroke and they seemed to run fine?

IIRC & IMHO:   The situation that 660 is describing used to be called or probably still is called piston overlap or in 660s case negative piston overlap;

Piston overlap

Piston overlap isn't a common term, but it is a measurement that is built into every engine. It refers to the amount of overlap the pistons bottom edge has over the exhaust port @ TDC. Pistons skirts are designed to be long enough to prevent the exhaust port from opening at TDC. Typical overlaps are in the 1mm or .040" range. Normally piston overlap isn't an issue, until you install a 2 mm stroker crank. Small exhaust port openings @ TDC are acceptable in the .005" to .010" range, but excessive openings may lead to detonation.


So in the above if the piston overlap is negative as 660 describes you get short circuiting;

Short circuiting

Controlling factors: exhaust port shape and width, transfer port angles

Refers to the fresh scavenging flow that enters the cylinder and the possibility that a portion of that fresh fuel air mixture may by-pass its normal looping flow and exit out the exhaust port. Fresh fuel air mixture that has short circuited and remains unburned is power lost.  High output engines like the Husqvarna 400cc uses transfer port angles to prevent short circuiting. When modifying an engine, it is wise to be aware of the possibility.

IMHO, there are no advantages in a short circuit or free porting situation only losses, however the looses may be so minimal that they are insignificant in effect to overall performance if controlled to the .005 to .010 range.

Most probably however I have seen pistons designed for world class race motorcycles which had piston skirts of unequal length to get that last little bit out of the engine. Keep in mind that there are a million and two other factors that impact the performance of a 2-cycle engine and that the .005-.010 number I quoted is for somewhat larger engines than a chainsaw typically uses.

Intake port duration

Controlling factors: crankcase compression, carb size and type, intake manifold, air cleaner

The intake port consumes the fresh fuel air mixture the engine uses to produce its power. The port duration determines the power and RPM potential of the engine. The problem with piston port intake systems is the timing is symmetrical. The sooner you open the port, the later it closes.

An example of this is an old piston port 125cc race motorcycle that may have a port opening around 90' BTDC (the piston is on the up stroke) and has had plenty of time to build up vacuum to suck in the fuel air mixture. The problem is on the down stroke. The engine has had 90' of time to build up pressure inside the crankcase and it wants out. With the intake port still open, a back flow through the intake port at low and possibly the mid RPM range. The back flow will stop when the incoming fuel air mixture velocity is greater than the back flow. Any engine with that much intake timing would have terrible bottom end power, some engines may not even be able to start running.

The later the engine closes the intake port, the shorter the time (in degrees) the engine has to build up pressure to send the fuel through the transfer ports. This is "Crankcase Pressure Time". Increases in intake port timing decreases the CPT. Some engines have such low intake port timing and high CPT that small changes with intake port timing can make a significant difference. The intake port must be a balance of the need for power @ a given RPM vs. torque required for a satisfactory low & mid range power.


As a general rule: timings with openings less than 68' BTDC make good torque, openings from 68' to 75' BTDC have good overall power and openings from 76' to 82' BTDC can have substantially better top end with a corresponding loss of bottom end.

Joe aka 1manband is exploring this torque vs. high-end right now in another thread.

So depending on where in the rev band you are seeking advantage will either increase or decrease the impact of Intake Port Duration on the performance of the engine.

Free porting .010" isn't that big of a deal because it's very low time area thus there is hardly any flow.  But as it gets larger the time area increases rapidly and will let more exhaust enter back into the crankcase.  At the same time that exhaust that is flowing in could have been more a/f charge.  So freeporting can get out of control in a hurry.