Port timing question

[bjorn773]

I subtacted from the intake. Should it be 79?

 

[edisto]

I've been doing some grinding on my cylinder. I think I've about got it ready for chamfering the ports. I raised the exhaust from 107 to 102 and the transfers from 130 to 125. I left the intake alone at 77. This leaves me 23 blowdown. I took a vernier caliper thru the plug hole and measured .115" for 10 degrees of crank rotation. This is .0115" per degree. So, with my .021" gasket, this should put my exhaust right about 100 degrees, transfers 123, and intake 75. Is my math correct?

Careful with the transfers...you change symmetry you can completely screw up scavenging. It tends to matter less at high rpms though.

Did you widen the exhaust, or just increase duration? I certainly wouldn't recommend the latter as a first step.

I'm not sure where you are going with the .1115" per degree, unless you are measuring exactly at the point of piston travel that you are interested in. Piston travel can be approximated as a linear function at some points in the cycle, but is decidedly nonlinear 60 degrees before and after TDC and BDC.

 

[MostShady1]

opcorn:

 

[mtngun]

Piston travel can be approximated as a linear function at some points in the cycle, but is decidedly nonlinear 60 degrees before and after TDC and BDC.

What edisto said.

Two ways to check your port timing while you are working on the jug.

-- port map and computer calculations. Not super accurate, but ball park. You can play with the computer parameters to see how raising/lowering a port or squish will change durations.

-- just bolt the jug on (no rings on piston, no clips on piston) and check with a degree wheel. If the degree wheel is already installed and dialed in (just leave the degree wheel on until the job is done), it only takes a few minutes to check timing.

Good luck.

 

[Justsaws]

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


.

:hmm3grin2orange:

Good reading folks.

 

[bjorn773]

Careful with the transfers...you change symmetry you can completely screw up scavenging. It tends to matter less at high rpms though.

Did you widen the exhaust, or just increase duration? I certainly wouldn't recommend the latter as a first step.

I'm not sure where you are going with the .1115" per degree, unless you are measuring exactly at the point of piston travel that you are interested in. Piston travel can be approximated as a linear function at some points in the cycle, but is decidedly nonlinear 60 degrees before and after TDC and BDC.

I have already milled the cylinder .030 to achieve .016 squish when assembled with my .021 gasket. I have widened the intake and exhaust to 70% of bore diameter. The transfers at the base have been worked to match the crankcase and gasket. The transfer ports in the cylinder have been beveled toward the intake side. I reassembled and measured with a degree wheel. At that point, the numbers were:
Exh 107
Int 77
Tran 130

I believe it was Brad, may have been in another post, who recommended raising the exh to no more than 100, raising the transfers an equal amount to maintain roughly 25 blowdown and leaving the intake alone. I went back in and raised the exh to 102 and transfers to 125. That's where I'm at now.

As far as the .0115 per degree goes, I did not measure at the point of port opening. You make an excellent point about it not being linear, I didn't think about that. I was merely trying to approximate where my final timing would be with the gasket. I should have just bolted it together with the gasket and measured. I will do that next.

 

[edisto]

I have already milled the cylinder .030 to achieve .016 squish when assembled with my .021 gasket. I have widened the intake and exhaust to 70% of bore diameter. The transfers at the base have been worked to match the crankcase and gasket. The transfer ports in the cylinder have been beveled toward the intake side. I reassembled and measured with a degree wheel. At that point, the numbers were:
Exh 107
Int 77
Tran 130

I believe it was Brad, may have been in another post, who recommended raising the exh to no more than 100, raising the transfers an equal amount to maintain roughly 25 blowdown and leaving the intake alone. I went back in and raised the exh to 102 and transfers to 125. That's where I'm at now.

As far as the .0115 per degree goes, I did not measure at the point of port opening. You make an excellent point about it not being linear, I didn't think about that. I was merely trying to approximate where my final timing would be with the gasket. I should have just bolted it together with the gasket and measured. I will do that next.

Gotcha.

Is that 70% of bore along the circumference, or actual port width (the chord). If it's the former, that's a port width of about 62% of the bore diameter, which is reasonable, if it is the latter, the chamfers become critical, and you probably are getting on the ragged edge in terms of short-circuiting.

Milling the base can necessitate readjustment of timing, but it is absolutely critical that you maintain symmetry of flow from the transfer ports.

 

[bjorn773]

The 70% is along the circumference of the bore, not straight across. I marked 70% of bore diameter out on tape, then stuck it to the inside of the cylinder.

 

[knockbill]

You can certainly know the timing (aka duration) without knowing TDC. Here's how: with the degree wheel on the crank, lower the piston from TDC to the top of the exhaust port.

Clear as mud? Please keep the discussion going!

i've always been interested and read these porting threads, but have trouble with teh math (formulas),, it seems a lot of you guys are engineers, not just mechanics, and some are both,,, me, i'm happy with a muffler mod that gets the firewood done!!!
heres a question, you referred to "knowing timing without knowing tdc" but then in your explanation, you say "lower piston from tdc"... what am i reading wrong???
thanks...

 

[bjorn773]

Gotcha.

Is that 70% of bore along the circumference, or actual port width (the chord). If it's the former, that's a port width of about 62% of the bore diameter, which is reasonable, if it is the latter, the chamfers become critical, and you probably are getting on the ragged edge in terms of short-circuiting.

Milling the base can necessitate readjustment of timing, but it is absolutely critical that you maintain symmetry of flow from the transfer ports.

By symmetry of flow from the transfer ports are you referring to maintaining the same blowdown or not changing velocity in the ports by widening too much or both?

 

[edisto]

By symmetry of flow from the transfer ports are you referring to maintaining the same blowdown or not changing velocity in the ports by widening too much or both?

Basically that one port has to be the mirror image of the other in terms of flow. If one flows better than the other, or there are any differences in timing, or you alter the angle, you'll impair scavenging.

Widening the transfer ports does decrease velocity, which usually drops power at the high end and adds some to the middle, as long as the flow is symmetrical.

Apparently, asymmetry in the transfer ports is not very noticeable at high rpms though...you make less power than you could, but the saw will run fine.

 

[Mastermind]

Basically that one port has to be the mirror image of the other in terms of flow. If one flows better than the other, or there are any differences in timing, or you alter the angle, you'll impair scavenging.

Widening the transfer ports does decrease velocity, which usually drops power at the high end and adds some to the middle, as long as the flow is symmetrical.

Apparently, asymmetry in the transfer ports is not very noticeable at high rpms though...you make less power than you could, but the saw will run fine.

All the more reason for me with my two left hands to leave the upper transfers alone...for now. Thanks guys for this thread...most informative.

 

[stinkbait]

So, is there a set of durations that work well with all saws? What type of durations do you look for if porting for torque vs speed?

 

[edisto]

So, is there a set of durations that work well with all saws? What type of durations do you look for if porting for torque vs speed?

Stock durations work well with all saws. Lots of gains to be had by just widening the intake and exhaust.

 

[blsnelling]

stock durations work well with all saws. Lots of gains to be had by just widening the intake and exhaust.

+1 You can't go wrong just widening. You may or may not see gains if you start playing with port timing. It depends on the saw to some extent. Of course there are rules of thumb, but it varies.