Port Heights change due to Squish band change????

[rolloman]

After decking my cylinder to improve the squish band I got to thinking about how in affect this also was lowering all of my port heights without doing any cutting to the ports, I think. Is anything done by builders in order to compensate for this?

 

[brokenbudget]

After decking my cylinder to improve the squish band I got to thinking about how in affect this also was lowering all of my port heights without doing any cutting to the ports, I think. Is anything done by builders in order to compensate for this?

this is a very talked about subject. lots and lots of threads on this very topic. after you have this question answered you'll have about a 100 more the search function is a good start. sit back, get comfy and start reading:hmm3grin2orange:

 

[mdavlee]

It will add a little intake timing and lower the exhaust which helps bring up compression. I think .030 was 4-5° if I remember right.

 

[chainsman1]

this will lower the max rpm and add torque,if you want more top-end you will have to raise the transfer and exhaust ports

 

[bertrand]

if you do bring it down of 0.030inch the minimum to do is to raise the exhaust and transfert port of the same amount to go back to where you were earlier. Your intake duration will be increased but it wiil just be a good thing because you will need it

 

[rolloman]

That's probably what I will do this week after I degree it to get the port timing numbers. I'll keep you all posted on the progress and how it is running. I'm interested in the numbers plus it will be a learning process for me on using the degree wheel. I have run the saw and it cut very good and I want to compare the way it cut with the present numbers to what the new numbers end up if I raise the ports.

 

[troutfisher]

Here's a thread that might help you get going......

http://www.arboristsite.com/chainsaw/113678.htm

 

[rolloman]

After degreeing I got

.............. Open / Duration
Exhaust: 105* 151*
Transfers: 123* 112*
Intake: 275* 160*

Blowdown = 18*

After the replies that I got I'm gonna start tomorow raising the exhaust to original and raise the transfers to get 20* of blowdown.

 

[SpaayDawg]

After degreeing I got

.............. Open / Duration
Exhaust: 105* 151*
Transfers: 123* 112*
Intake: 275* 160*

Blowdown = 18*

After the replies that I got I'm gonna start tomorow raising the exhaust to original and raise the transfers to get 20* of blowdown.

Let me start by saying that I don't know everything yet either, however there are some flaws in what you have above.
The duration of say the exhaust port is = 360-(start of opening)x2, so for your example 360-(105x2) = 360-210 = 140. Is 151 where the piston top has cleared the floor of the port? It's still open isn't it?
Also, blowdown is the degrees between the start of exhaust opening and the start of transfer opening, the way to increase blowdown is to raise the exhaust port roof, not the transfer(s). Raising the transfers will actually decrease your blowdown.

 

[rolloman]

I keep typing and it keeps timing out. This is a test.

 

[rolloman]

Thanks SpaayDawg, for double checking the numbers. I see how you came up with your number and it looks correct. That’s what’s confusing me because the way I did it which was suggested in a different thread also looks correct to me but we both get a different number. I have to admit I’m not that great with math, and since this is my first porting and timing job I appreciate any feedback that comes my way. The formula I used was Port Opens = 180 – (duration/2) Please excuse my long windedness but I feel the need to explain my method in great detail in order to get it right for myself. Since I needed to measure the duration I put the piston at tdc and continued rotation until the top of the piston traveled down to the port roof and the port just started to crack open. I adjusted the piston stop that I had installed to touch the piston top. I also set the degree wheel pointer to 0*. The next thing was I continued rotation until the piston traveled down opening the entire port. I continued rotation past bdc until the piston traveled back up closing off the entire port and came to rest at the same spot I earlier had set the piston stop and degree wheel to 0*. I then took the degree wheel reading which would be the total port duration.

The reading was 151*. I then had my number to plug into the above formula. So Port Opens = 180 – ( 151 / 2 ) = 180 – ( 75.5 ) = 104.5* So far I have got the total port duration for a 360* cycle. Divided it in two to find the duration during just a 180* cycle. Subtracted this duration from 180* to find the first part of the cycle where I first started rotating from tdc to port just starting to open where I first set the degree wheel to 0* and set the piston stop. I can’t find anything myself wrong with either way thus confusing myself. Please heeeelp anyone!!!!!!!!!!! I’m getting grayer by the minute.
I had already discovered my statement about the blowdown was incorrect and got that matter straight in my mind so that I didn’t make a porting mistake.

 

[MechanicMatt]

what are the effects of raising the exhaust but not the transfers?? The saw Im asking about is a husky 50 that has open transfers??? I haven't devised a way to get in there and raise them yet, just wondering what would happen if i left them alone??

 

[rolloman]

I finally see the problem. I didn't check the math. 360 - 210 = 150 not 140.

 

[troutfisher]

Sounds like you measured exhaust duration correctly, and 151* sounds pretty low. 160*-165* is a lot more common numbers to see. What saw are you working on?

 

[rolloman]

Husky 359. The 151* duration was the measurement after removing the .020" base gasket and decking the cylinder base .010". I raised the exhaust and transfers back up and the new exhaust numbers are the following.

Exhaust Opens: 98.5*
Exhaust Duration: 163*

 

[troutfisher]

That's better. How about the transfers? Maybe around 120* would be good...........

 

[troutfisher]

Does it have a green gas cap? What are you doing with the muffler on it? All this port work won't do much with a plugged up cat muffler on it.

 

[SpaayDawg]

I finally see the problem. I didn't check the math. 360 - 210 = 150 not 140.

That is correct, sorry if I confused you more. The 151 was misleading because you said it opened at 105, then I made an error in the math.

 

[rolloman]

No problem SpaayDawg were all allowed one mistake. Haahaa.

No epa junk here. Muffler completely gutted. Has triangular tube same dimension as the port that goes from flange all the way thru to the mufflers outside front.

Transfers:

Open: 121*
Duration: 118*

 

[troutfisher]

Rolloman what is the saw going to be used for? Work saw or racing?