Port timing question

[parrisw]

Ok...so now you know the duration but what is it relative to?

I'm going to see if I can get this right, but probably wont, Mr Troutfisher told me how he does it.

There is 360° of revolution, 180° up 180° down. Measure duration. say 160°, half of duration is on the down stroke, half is on the up stroke, so 80° each way. So 360° minus 160° is 200° divide that in half gives your opening timing of 100°.

 

[Tzed250]

The reason for indexing the degree wheel to top dead center is so all of the ports can be measured relative to one another.


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[ronT2]

I'm going to see if I can get this right, but probably wont, Mr Troutfisher told me how he does it.

There is 360° of revolution, 180° up 180° down. Measure duration. say 160°, half of duration is on the down stroke, half is on the up stroke, so 80° each way. So 360° minus 160° is 200° divide that in half gives your opening timing of 100°.

Thanks for the explination Parris. It might take me a while to get my head around it but I'll work on it.

 

[ronT2]

I am attempting to get a grasp on port timing. I printed out a template degree wheel that I found by searching here. I glued it to a clear cd and have it attached via a spark plug boot to my crank. Sounds hokie, but it seems to work fairly well. Anyway, here are my numbers.

Please be kind, I'm a virgin at this stuff.

Exh begins opening @85*ATDC
Exh fully open @18*BBDC or 162*ATDC
Int begins opening @82*BTDC
Int fully open @28*BTDC
Transfer opens @68*BBDC or 112 ATDC

Do these sound like logical numbers?
Am I interested in the opening numbers primarily or the duration?

Here is a link to a thread that Brad had for porting a 346. It should give you an idea of what numbers you should be looking for. I was surprised to find out how close the intake and exhaust timing on my 2150 were to the 346.

http://www.arboristsite.com/showthread.php?t=118591&highlight=port+timing

 

[parrisw]

Thanks for the explination Parris. It might take me a while to get my head around it but I'll work on it.

Ya, I know. I still scratch my head.

 

[troutfisher]

Ok...so now you know the duration but what is it relative to?

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.

 

[Mastermind]

I'm getting it!!!! Thanks guys, I'm slow and stupid but I can be taught.

 

[troutfisher]

I like to think in durations. If you raise the exhaust 1 degree, you add 2 degrees to duration (it opens earlier and closes later). If you raise 4*, you add 8* to duration, and so on.......

 

[parrisw]

I like to think in durations. If you raise the exhaust 1 degree, you add 2 degrees to duration (it opens earlier and closes later). If you raise 4*, you add 8* to duration, and so on.......

How do you mark the ports for the amount you want raised? Say you want to go another 5° earlier on ex? I've seen people use a long fine felt pen through the spark plug hole to mark it.

 

[Mastermind]

I've heard guys speak of 1 degree = x amount of stroke travel, I guess that would be a way to figure it out...someone who knows more will surely chime in.

 

[tdi-rick]

I've heard guys speak of 1 degree = x amount of stroke travel, I guess that would be a way to figure it out...someone who knows more will surely chime in.

It depends on the rod length (and stroke)

There is a formula for determining duration from the actual measured heights of the ports, so you'd just change 'E' (or F for inlet) in the formula the amount you want to raise the port to get the change in duration.
Just measure accurately and use a scientific calculator and you're set.

Exhaust and transfers.

Where T=R+L+C-E
R= stroke/2 in mm
L= rod length ctr to ctr
C= deck clearance (squish for us) in mm
E= top of the exhaust or transfer port from the top of the cylinder

Note that this is designed around two piece heads and for measuring from the top of the cylinder down, so you need to accurately measure your squish clearance and use that as your deck measurement. (all measurements in mm ! )

Inlet port duration is

Where R= stroke/2 in mm
L= rod length
C= deck height (squish)
H= piston height on inlet side
F= inlet floor depth ie. measurement from the top of the cylinder to the floor of the inlet port
P=R+L+C+H-F

All from Bell.

 

[Mastermind]

OK I'm drowning, this is way over my head...

 

[parrisw]

OK I'm drowning, this is way over my head...

LOL, me too. I saw those formula's, and then ran away. Didn't even read them, you don't need to know that stuff to build a good saw.

 

[tdi-rick]

LOL, me too. I saw those formula's, and then ran away. Didn't even read them, you don't need to know that stuff to build a good saw.

MM did ask the question

 

[Tzed250]

LOL, me too. I saw those formula's, and then ran away. Didn't even read them, you don't need to know that stuff to build a good saw.

No,

but you might need to know them to build the best saw...


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[Mastermind]

No,

but you might need to know them to build the best saw...


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Point taken, rep sent...if it will let me!

 

[parrisw]

No,

but you might need to know them to build the best saw...


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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.

 

[tree monkey]

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

 

[tdi-rick]

[snip]

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.

Will, you asked about how far for say, 5* up on the exhaust and how you mark it and MM mentioned x amount of stroke = y*.
All I was trying to do was show that you can predict how far to grind without trying to guess from just a few measurements, so you can do it with the jug in your hand from the bottom rather than a pen through the plug hole and a degree wheel.
More than one way to skin a cat

As for basics, as tree monkey said most of us don't even want to be changing the port timing, big can-o-worms for the unwary/unknowing although there are some basic rule of thumbs.

 

[parrisw]

Will, you asked about how far for say, 5* up on the exhaust and how you mark it and MM mentioned x amount of stroke = y*.
All I was trying to do was show that you can predict how far to grind without trying to guess from just a few measurements, so you can do it with the jug in your hand from the bottom rather than a pen through the plug hole and a degree wheel.
More than one way to skin a cat

As for basics, as tree monkey said most of us don't even want to be changing the port timing, big can-o-worms for the unwary/unknowing although there are some basic rule of thumbs.

True enough. Sorry,

I dont' really get that formula though. I guess I could try to learn it.

Just went and looked at it again. Just seems too confusing???? to me. First of all, how do you get the specs of rod length and stroke? I've found stroke, but I've never been able to find rod length.