Is freeporting good or bad?

[Chainsaw Jim]

I just measured the gap on my 372 and it is 1.8mm

 

[drf255]

.005" is about 10° at BDC probably. It won't have time to let much OUT.

Out or in?

I may not be seeing it right. The piston's moving upwards, creating vacuum in the case. Air takes path of least resistance, so it fills though intake port. Piston starts to freeport, suddenly gets a vacuum leak, which is a lot of degrees because the piston speed is nearly zero at TDC and around it. There not enough space to overcome flow in the intake port, and the freeporting is gone before the intake skirt covers the intake roof.

I just don't think there is enough time/area in a free ported area to do anything when you imagine the engine in a dynamic state.

 

[drf255]

I wonder if the draw the muffler has out is pulling charge out of the case when it free ports or not.

I was seeing that too, like an exhaust header. Venturi effect.

I feel that a lot of the flow in these motors at high RPM is just happening and has a lot of inertia. The port timing is just changing how the wave flows. I don't think that an air wave is stopping 32,000 times a minute (piston passes each port twice per revolution).

As for freeporting, perhaps the exhaust inertia balances out the intake vacuum for a net flow of zero.

Most of the engines I've done with it ran very well, but I think it was not because of the freeporting alone.

I think saying a saw with freeporting runs faster is like saying loud saws run faster. It's not the actual factor you're seeing that caused the saw to run faster, it's the side effect of all the other mods.

 

[KG441c]

Well ?? Is she passing gas or not is the question???

 

[EvilRoySlade]

I guess run a cat muffler on a free ported saw. The hot cat should ignite any unburnt crankcase charge making things real exciting.

 

[redfin]

Id be more concerned with a sloppy piston catching the floor.

 

[CR888]

I've read a lot about freeporting being a bad thing. I've also heard old school saw builders mention it like it's a big secret for hopping up saws.

'Freeporting' is a wonderful thing if its done by a good reputable builder. lt costs enough these days to buy good saws so if the right builder offers up his services for zip its great. In the Christmas give away thread there was some freeporting going on. Some say you pay for what you get therefore freeporting is not that good but IMO done buy the right guy freeporting is welcome and good for your saw. l have paid in full for all the port work and mods to my saws, with CAD it can cost ya heaps. l say more builders should offer freeporting to the masses not just occasionally in silly Christmas threads.

 

[wcorey]

Similar to what mdavlee suggests but piped.
With a tuned pipe the back pulse stuffs excess charge back into the combustion chamber just before the exhaust closes.
Well, just after that the freeport is starting, the intake is done intaking, maybe it can stuff whats left into the crank case and/or add pressure.
Particularly on a reed valve saw.
Who knows what goes on in there at those velocities...

 

[bwalker]

I agree with that in that it would also reduce some spitback too. Probably on a smaller capacity crankcase such as a 440/460 hybrid it may help alil

Spit back is typically caused by over zealous intak port timing in my expiereance. I would not be happy with any free port.

 

[blsnelling]

Free porting is never a desirable thing. It's not necessarily a deal breaker though. A very free flowing muffler is a good idea when you do have an exhaust port the free ports.

 

[nk14zp]

.005" is about 10° at TDC probably. It won't have time to let much out.

40%

 

[Grande Dog]

Atomized fuel spitting out the muffler instead of being burnt in the combustion chamber doesn't sound like a performance mod to me

Howdy,
We just need to add a igniter to the muffler for an afterburner. When I first developed Big-Bore kits, we had issues with the over sized pistons fitting the crankcase cavity at BDC on the Stihl saws. On most of their crankcases it wasn't an issue when both case halves formed close to a perfect circle but, there was a large enough amount that we had to do something. First off we shortened the piston skirt 2mm. No more possibility of hitting the crankcase but, we breached the bottom of the exhaust port. Basically by testing, the only thing we could put a number on was fuel consumption. Although there was no actual HP increase, the general perception was faster acceleration most notable on the 066 / 660. So we ended up raising the bottom of the exhaust port 1mm, and called it a day.
Regards
Gregg