Porting 394 cyl

[Tzed250]

Real easy to make an engine that will rev to the moon...but wouldn't pull the skin off of a bowl of pudding. Take care...

 

[Four Paws]

Though you gain torque buy leaveing exhaust below stock the RPM loss will mean slower cutting times unless you run longer bars.

Widen the port out and gain your time-area back and you won't have to lose any RPM with the benefit of added torque!

**more thoughts**

I like a torquey saw. Compression is king - get that squish tightened up, and keep the exhaust low and wide. Transfers are the most crucial part of a cylinder IMO and the easiest thing to mess up. If you are going for a mild increase in performance, don't touch the transfers until you have a few saws under your belt and have a good understanding of what you need to do to them.

 

[parrisw]

Don't sweat it, if you raise the exhaust 0.5mm or about 20 thou and take the gasket out you will be good. Start first getting the width of the port you want then blend in the roof to give a nice oval with slightly flattened area top center.

Any good high temp automotive sealer will work in place of the base gasket, Permatex MotoSeal 1, Yamabond, Hondabond ect or just red or copper silicone sealer.

So if I widen first? Then blend the top in. Where is the ex raised to, that would be the highest point, in the top of the ex port? So that would be the center, I guess?

 

[timberwolf]

I think you have the picture.

As far as keeping the exhaust port wide and low, yes that is good for a work saw and time area can be gained by having extra width, however on some saws the piston skirt limits how much width you can get, If your limited to 60% of bore or less, then you may need to raise exhaust a little to get enough blowdown. Letting transfers drop below stock hieght helps get enough blowdown, but low durration transfers can limit RPM if they don't have enough capacity and time to flow charge up from base into the jug.

When a saw lacks blowdown, it almost feels like a saw with too high an exhaust port, it revs, then when you hit the wood it dies, but the difference is it will find torque down lower in RPM where a saw ported to high will have less and less the more RPM is dragged down.

 

[blsnelling]

I've gotten where I don't raise the exhaust at all on most of my port jobs. Going as wide as safely possible and as square as is reasonable on the roof nets the biggest gains IMHO. I do the same on the intake as well. I angle the transfers a little more towards the intake but not much more. The lower transfers on a Stihl cylinder I taper all the way to the base of the jug. I also spend a fair amount of time porting and lightening the piston, keeping in mind that the flow is from the inside of the windows out.

 

[parrisw]

I think you have the picture.

As far as keeping the exhaust port wide and low, yes that is good for a work saw and time area can be gained by having extra width, however on some saws the piston skirt limits how much width you can get, If your limited to 60% of bore or less, then you may need to raise exhaust a little to get enough blowdown. Letting transfers drop below stock hieght helps get enough blowdown, but low durration transfers can limit RPM if they don't have enough capacity and time to flow charge up from base into the jug.

When a saw lacks blowdown, it almost feels like a saw with too high an exhaust port, it revs, then when you hit the wood it dies, but the difference is it will find torque down lower in RPM where a saw ported to high will have less and less the more RPM is dragged down.

Thanks for all your help, very much appreciated. I really don't want to ruin this cylinder, 394 cyl's aren't easy to come by, and new their $$$$$. So if I widen until 1.5mm or .100" to the piston skirt? Also what is going to be my most accurate means of measurment? I have dial indicators, vernier calipers, and micrometers. Can I just measure from the bottom of the jug, to the top of the stock ex port, then just add .020"?

Anybody have any pics of a good ported ex port that has a nice blended top?

 

[parrisw]

I've gotten where I don't raise the exhaust at all on most of my port jobs. Going as wide as safely possible and as square as is reasonable on the roof nets the biggest gains IMHO. I do the same on the intake as well. I angle the transfers a little more towards the intake but not much more. The lower transfers on a Stihl cylinder I taper all the way to the base of the jug. I also spend a fair amount of time porting and lightening the piston, keeping in mind that the flow is from the inside of the windows out.

Thanks Brad. I'll defiantly keep that in mind.

On the intake, the stock plastic intake is the same size as the port. Does this get ported out as well. if so there isn't much grinding room on the plastic intake.

 

[timberwolf]

Make a port map (google search), wrap it around the piston and measure. You can also use machienists ink to mark the piston through the exhaust port to see how much room you have on the skirts, this helps keep the port centered on the skirt. for the height of the port you can measure from head or base, port maps too can help with this as well.

On intake for a basic worksaw porting job, I would not mess with plastic other than to blend and smooth.

 

[Wet1]

Very good reading guys. I look forward to picking up a book or two on the topic and giving porting a try myself. If you guys could give some before and after pics with what you're describing, I'm sure it would be very helpful.

 

[blsnelling]

Thanks Brad. I'll defiantly keep that in mind.

On the intake, the stock plastic intake is the same size as the port. Does this get ported out as well. if so there isn't much grinding room on the plastic intake.

Don't take the intake port opening any larger than the plastic intake. I still widen the port at the cylinder wall and taper it back.

 

[parrisw]

Don't take the intake port opening any larger than the plastic intake. I still widen the port at the cylinder wall and taper it back.

Yes thanks thats what I was thinking, since the plastic intake has a small ridge around it to help the gasket seal, that would get ground off if you did anything to it.

How wide do you go on the intake, same idea as the ex? As wide as you can?

 

[timberwolf]

You need to be a bit more conservative on intake width for a work saw, there is thrust on the intake skirt, so you need some support or piston wear will become a factor. Rings don't fully cross into the intake port, so you have more flexibility in the port shape and don't need as much of an oval shape as on exhaust side. Also on saws that the ring does drop slighty into the intake port, piston speed is pretty low near BDC so there is more time for ring to be guided back into place, and you don't have gas pressure bulging the ring out like you do on exhaust port side.

Sounds like Jennings "Two Stroke Tuners Handbook" would be a good read for you, lots of info on port shapes and ring considderations, a bit dated, none the less ideas are good though new technology has taken us past some of these limits.

http://www.chuckbunnell.com/kart/jennings/jennings.html

 

[parrisw]

You need to be a bit more conservative on intake width for a work saw, there is thrust on the intake skirt, so you need some support or piston wear will become a factor. Rings don't fully cross into the intake port, so you have more flexibility in the port shape and don't need as much of an oval shape as on exhaust side. Also on saws that the ring does drop slighty into the intake port, piston speed is pretty low near BDC so there is more time for ring to be guided back into place, and you don't have gas pressure bulging the ring out like you do on exhaust port side.

Sounds like Jennings "Two Stroke Tuners Handbook" would be a good read for you, lots of info on port shapes and ring considderations, a bit dated, none the less ideas are good though new technology has taken us past some of these limits.

http://www.chuckbunnell.com/kart/jennings/jennings.html

Thanks allot for the info. I really appreciate it!!!

 

[parrisw]

http://www.chuckbunnell.com/kart/jennings/jennings.html

the link doesn't work?

 

[timberwolf]

Must be your browser settings, I rechecked it and it works for me, if you can't get into that one just google the book title, it's on the web a dozen places or more.

 

[blsnelling]

It was down for me last night as well but is back up this morning.

 

[Crofter]

Yes thanks thats what I was thinking, since the plastic intake has a small ridge around it to help the gasket seal, that would get ground off if you did anything to it.

How wide do you go on the intake, same idea as the ex? As wide as you can?

Not unless you are doing a whole lot of other things that increases the operating rpm greatly (like bigger carb and additional transfers) Having too much intake time /area will result in lower gas velocity and hurt the torque game at medium and low rpm You lose the ram effect if you go too big and could wind up actually trapping a smaller charge in the base at lower rpm. Consider that the descending piston is already starting base compression while the flow is continuing inward nearing the end of intake duration. The great big hole seems like a good idea at the time of max flow but consider the venturi diameter in comparison and you might rethink what the bottleneck is. Lots of fun things to think about eh?

 

[timberwolf]

Key to a strong saw, be it a torque monster work saw or a high reving racer is to have exhaust, transfers and intake all working together and optimized for the same RPM. Problem though is this is not that easy, no simple formula, rule of thumb or Zen like insight is going to make it happen. I see only 4 ways to get at it; lots and lots of testing (trial and error style with a heap of scrap metal left over), take someone else's proven design, lots of math and computer work, or blind luck

Working from time area I think can get you close, but time area numbers worked out from bikes won't be right for a boxed muffler. Also some builders have a pretty good idea of target port velocities and with the aid of computer programs I think that can work too. I would bet this is along the line of how Marcel Vincent arived at some of the saw designs.

Which port here needs more time area? As it is no matter what RPM the motor turns not all ports will be ideal and power lost.

 

[parrisw]

Key to a strong saw, be it a torque monster work saw or a high reving racer is to have exhaust, transfers and intake all working together and optimized for the same RPM. Problem though is this is not that easy, no simple formula, rule of thumb or Zen like insight is going to make it happen. I see only 4 ways to get at it; lots and lots of testing (trial and error style with a heap of scrap metal left over), take someone else's proven design, lots of math and computer work, or blind luck

Working from time area I think can get you close, but time area numbers worked out from bikes won't be right for a boxed muffler. Also some builders have a pretty good idea of target port velocities and with the aid of computer programs I think that can work too. I would bet this is along the line of how Marcel Vincent arived at some of the saw designs.

Which port here needs more time area? As it is no matter what RPM the motor turns not all ports will be ideal and power lost.

Thanks, but I really don't understand that graph at all.

Too dam much to think about!!! I'm starting to loose faith in this project.

 

[AUSSIE1]

Don't loose faith.
There has been enough info here for you already.
Do your exhaust and inlet as described.
Remove base gasket checking squish.
Do a muffler mod.
Put it back together and see how you like it.
You can always go back in if you want more.

Al.