Port timing question

[luvsaws]

Maybe, there is more then 1 way to skin a cat.

The OP is just trying to understand the basic's, you guys throwing out fancy forumla's is just going to confuse him. He needs to just learn some basics first, then progress futher once he gets that.

Well said I am trying to figure it out also,but dang them formula's are well over my head right now

 

[tdi-rick]

[snip]
I've found stroke, but I've never been able to find rod length.

You must have missed my request for the 7900 rod length on RS a month or so back then
No one seemed to know either....

Al and I had this conversation only the other day as it's not easy to measure.
He's worked out a work around for getting the crankpin diameter and i don't think he'll mind me posting it here. Once you have that it's straight forward to get the rod length by measuring the gudgeon/little end diameter then the rod between the two.

You couldn't get a 6"/150mm rule in there?
I hold a pair of rules like a pair of chopsticks with the ends at the crankpin and then use another to measure the gap at various points while slightly spreading till I have a consistent reading. It sounds like a bit of ginning around but it works surprisingly well.

 

[Terry Syd]

Taking your time

You can avoid raising the exhaust port too far by filing the last part of the port job. You can shine a bright light through the sparkplug hole and watch for when the piston top cracks open (a low light environment helps).

With the degree wheel on you can see the timing that the port opens. As you raise the exhaust port those last few degrees, you can work you way to that new timing with the jug on the saw. With the light and the degree wheel you can get the exact timing you want. It takes a bit longer than hitting it with a Dremel, but you won't make a - 'opps, too far'.

Take the ring/s off the piston and mount the jug back on the saw. You have to PULL the file towards you so that you will not chip the cylinder lining. That requires a special file - no problem, you can make one.

Take a new chainsaw file and snap off the tang end of the file in a vice. Grind the sharp edges off the broken end and put some sort of soft coating on the end (I use electronic tool dip, but some duct tape will work). Removing the sharp edges and putting some insulation on the end of the file is required - you will occasionally hit the other side of the cylinder and you do not want to score the cylinder liner.

Take your time and you will get both the timing and the shape exactly the way you want it.

To chamfer the edges of the port, put a bit of emery cloth/sandpaper on the end of your finger and shape the chamfer the way you want. If the sandpaper won't stay on the end of your finger, put a drop of contact cement on the end of your finger to hold the sandpaper.

 

[bjorn773]

It would appear I've opened Pandora's Box

 

[Tzed250]

It would appear I've opened Pandora's Box

The box was opened decades ago...different people get in and kick things around from time to time...


.

 

[troutfisher]

I think all that math comes from the fact that the piston speed changes depending on where it is in the stroke. 20 degrees of crank rotation will move the piston a lot at the ehxaust port, but the same 20 degrees of roatation moves the piston very little at TDC. The crank is turning a steady rpm, while the piston is speeding up and slowing down and stopping and speeding up again. Those big formulas will help you figure out how long those ports are open.

 

[ronT2]

Parrisw has it, They are the same thing. if you know when the exhaust opens after TDC, then you know duration. If you know duration, you know when the port opened.

say the exhaust opens at 90*. There is 180* from TDC to BDC, so if it opens at 90*, then it's open for the 90* on the way down and another 90* on the way back up...duration is 180*

if you know duration is 160*, then it's open for 80* on the way down and 80* on the way back up. It must have opened at 100* after TDC.

if you know one, you can figure the other. Saying the exhaust opens at 105* after TDC is the same as saying there is 150* of exhaust duration.

Thanks Troutfisher, the last time I had a degree wheel on one of my saws I think I started to see what you’re saying. I noticed that the port openings and closings were equidistant from TDC. Then after Will explained it it started to click a little more, the math started to make sense. It’s not like timing a V8 since there are no cams, chains/belts and sprockets that all have to be timed relative to TDC or your valve openings and closings could happen at any time during a crank rotation. I have a spare 365 cylinder hangin around that might be good enough for an experiment. I might try playing around with the power curve on it. I’ll try timing both ways, hopefully learn a couple things….old habits die hard though.

 

[ronT2]

you need to know, and understand, mean time area of each port and how they relate to each other befor you can determen port timing.

scott

I’m listening if you care to elaborate a little more. opcorn:

 

[bjorn773]

Ok, back to the original question... how do these numbers look. I checked the wheel with a piston stop in both directions and split the difference to zero the wheel. Here are my new numbers:

Intake 69* BTDC
Exhaust 105* ATDC
Transfer 121* ATDC

This is a 346xp OE

 

[AUSSIE1]

Ok, back to the original question... how do these numbers look. I checked the wheel with a piston stop in both directions and split the difference to zero the wheel. Here are my new numbers:

Intake 69* BTDC
Exhaust 105* ATDC
Transfer 121* ATDC

This is a 346xp OE

69 + 69 = 138 deg Duration for inlet
105 + 105 = 210 from 360 = 150 for exhaust
121 + 121 = 242 from 360 = 118 for transfers
150 - 118 = 32 deg blowdown

 

[bjorn773]

Do those look right for a bone stock 346xp?

 

[tdi-rick]

I’m listening if you care to elaborate a little more. opcorn:

Here's a good explanation of time/area. It's a follow on from Jennings forty years ago and yeah, more equations
http://www.macdizzy.com/cylinder_map5.htm

and this is the original chapter from Jennings http://microcarproject.tripod.com/html/port_timing_alteration.htm

 

[AUSSIE1]

Do those look right for a bone stock 346xp?

I haven't seen a 346 in the flesh let alone know the No's but you look like you are onto it.

 

[ronT2]

I’d say they look pretty darn close. Below are the timing #s from Brads thread on porting an NE346 and what I found when I checked my 2150. Measurements on both saws were taken without base gaskets.

Stock NE 346XP
Squish: .013"
Intake: 76°
Exhaust: 108°
Transfers: 123°

2150
Squish: .018"
Intake: 71°
Exhaust: 101°
Transfers: 126°

Sorry about hijacking your thread but it has been a good one.

 

[ronT2]

Here's a good explanation of time/area. It's a follow on from Jennings forty years ago and yeah, more equations
http://www.macdizzy.com/cylinder_map5.htm

and this is the original chapter from Jennings http://microcarproject.tripod.com/html/port_timing_alteration.htm

Yeh I have two versions of Gordon Jennings’ book on my computer, talk about exhaustive and more scary formulas.

Guess I’m looking for a condensed version.

I’ve been to macdizzy.com before searching for info on a couple 2-stroke four wheelers we have… tons of great info there. I don’t remember stumbling across the webpage you linked to though. The one at tripod is all new to me. I’ll check them out for sure, thanks.

 

[bjorn773]

I’d say they look pretty darn close. Below are the timing #s from Brads thread on porting an NE346 and what I found when I checked my 2150. Measurements on both saws were taken without base gaskets.

Stock NE 346XP
Squish: .013"
Intake: 76°
Exhaust: 108°
Transfers: 123°

2150
Squish: .018"
Intake: 71°
Exhaust: 101°
Transfers: 126°

Sorry about hijacking your thread but it has been a good one.

Mine were with the base gasket installed. I'll have to see how they compare without the gasket. I can't imagine more than a degree or so?

 

[tdi-rick]

Yeh I have two versions of Gordon Jennings’ book on my computer, talk about exhaustive and more scary formulas.

Guess I’m looking for a condensed version.

I’ve been to macdizzy.com before searching for info on a couple 2-stroke four wheelers we have… tons of great info there. I don’t remember stumbling across the webpage you linked to though. The one at tripod is all new to me. I’ll check them out for sure, thanks.

Supposedly all pre-school stuff compared to Blair.
I have a few links to Bell's book too if you want one. Although well over twenty years old now, a little more modern than Jennings.

 

[edisto]

Yeh I have two versions of Gordon Jennings’ book on my computer, talk about exhaustive and more scary formulas.

Guess I’m looking for a condensed version.

I’ve been to macdizzy.com before searching for info on a couple 2-stroke four wheelers we have… tons of great info there. I don’t remember stumbling across the webpage you linked to though. The one at tripod is all new to me. I’ll check them out for sure, thanks.

Jennings is the condensed version.

If you search porting, you'll be able to pick up some rules of thumb that let you do some porting without having to do any math, but 2-strokes are all about balancing trade-offs. For example, you might open the intake to improve flow, but change the resonance and actually decrease charge, or you could open up the transfers, and lose at the top end and pick up in the midrange because you have lost velocity (or lose at both because the scavenging pattern is no longer balanced).

 

[Log Hogger]

I'm just learning the basics

Maybe, there is more then 1 way to skin a cat.

The OP is just trying to understand the basic's, you guys throwing out fancy forumla's is just going to confuse him. He needs to just learn some basics first, then progress futher once he gets that.

I'm just learning the basics, and I've saved those formulas to my HD already, they're interesting and potentially useful. Just because someone doesn't know much about saws doesn't mean they're afraid of a little math.

Like you said, there's more than one way to skin a cat. I probably won't use the formulas, but I like to have a full toolbox.

 

[rmihalek]

Just remember that the top of the piston is the "gate" for the exhaust port, so when removing material from the cylinder, you want to be removing it from the top of exhaust port.

The skirt of the piston is the "gate" for the intake port. Remove material from the bottom of the intake port to increase intake timing.

Now remember that when you're removing the metal, the cylinder is in your hand and could be, at times, upside down. So, you want to be sure you're working on the correct surface.

I also feel that TDC is not relevant when porting. If you want to increase exhaust duration from 160 to 170 degrees, then all you need to do is take 5 degrees worth of metal off the top of the exhaust port and you're done. You'll get 5 degrees added to the duration when the piston is coming down from TDC and the other 5 degrees when it's coming back up from BDC.