Port timing question

[edisto]

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.

TDC is not relevant...unless you want to figure out what "5 degrees worth of metal" really is.

Like tdi-rick said...the math is helpful in terms of translating degrees into something you can measure on the port with a ruler.

I just made an Excel spreadsheet that, given stroke, connecting rod length (center to center) and squish calculates the distance from the top of the cylinder that the top of the piston will be. At least I hope it does. I haven't checked it against an actual setup. It basically makes 2 tables, 1 for every degree, and one for every half-degree. It really only needs to go to 180, but I ran it to 360 anyway.

Unfortunately, I just found out that it is not a file format that I can attach here. If anyone would like to see it they can PM me their e-mail address.

 

[rmihalek]

Make a pencil mark on the piston when the piston is just at the top of the exhaust port. Rotate flywheel 5 degrees. Make another mark on the piston. That's "5 degrees worth of metal" and you still don't need TDC.

 

[Mastermind]

Make a pencil mark on the piston when the piston is just at the top of the exhaust port. Rotate flywheel 5 degrees. Make another mark on the piston. That's "5 degrees worth of metal" and you still don't need TDC.

Simple, too simple for me to have thought of it...thanks I'll file that for future reference.

 

[edisto]

Make a pencil mark on the piston when the piston is just at the top of the exhaust port. Rotate flywheel 5 degrees. Make another mark on the piston. That's "5 degrees worth of metal" and you still don't need TDC.

For me, it's easier to do the math. It took me about 5 minutes to set up the spreadsheet, and all I have to do is input stroke and rod length to get the piston travel for any engine, and add squish to get the distance from the top of the cylinder.

I guess i don't understand what you mean by TDC not being relevant...especially if you want to change timing. You can't know the timing without finding TDC. Or are you advocating increasing duration by arbitrary amounts?

 

[mtngun]

I have to do is input stroke and rod length

Ah, but tracking down the rod length spec ......

 

[edisto]

Ah, but tracking down the rod length spec ......

Exactly.

 

[AUSSIE1]

Ah, but tracking down the rod length spec ......

With a little effort you can get a rod length with the barrel removed.

 

[rmihalek]

For me, it's easier to do the math. It took me about 5 minutes to set up the spreadsheet, and all I have to do is input stroke and rod length to get the piston travel for any engine, and add squish to get the distance from the top of the cylinder.

I guess i don't understand what you mean by TDC not being relevant...especially if you want to change timing. You can't know the timing without finding TDC. Or are you advocating increasing duration by arbitrary amounts?

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. Rotate the degree wheel to zero without turning anything else (ie zero the degree wheel). Now, slowly rotate the flywheel so that the piston goes down through BDC and back up JUST until the top of the piston closes the exhaust port. What does the degree wheel read? If it reads 180, then your exhaust duration is 180 degrees: 90 coming down from TDC and 90 coming up from BDC.

However, I guess it depends on what you mean by "arbitrary amounts." For example, in my previous post, I said you could add 10 degrees to the exhaust duration by removing 5 degrees worth of metal from the top of the exhaust port. That was, on one hand, a completely arbitrary number, yet, on the other hand, may be the perfect number for one particular saw.

My experience is that AS member Timberwolf has the most complete grasp of how all the variables interact to produce maximum horsepower. There are very many factors that come into play when determining how to modify the port timing. I have a Timberwolf modified Husky365 with a first generation Bailey's 78cc big bore kit on it and the saw is un-freakin'-believable. It'll pull full comp chain, a 28 inch bar with an 8 tooth sprocket through red oak and be screaming. I'm not kidding.

BTW, I am not against using equations, in fact, I think they are absolutely required for maximum HP. However, if an AS member posts information about porting a 372 or a 440, and others want to duplicate those numbers, I am trying to show that you don't need the equations to calculate port duration.

If more than a few members post that they have good results with 170 degrees duration on a Dolmar big bore cylinder exhaust, then the method I described will allow you to grind away enough metal to get 170 degrees.

Clear as mud? Please keep the discussion going!

 

[Woodplayer]

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

Aussie1, it was my understanding that blowdown is the difference in degrees between the exhaust port opening and transfer ports opening, which in this case would be 121-105=16° blowdown. I think you may have inadvertantly doubled this value in your calculation.

 

[blsnelling]

Aussie1, it was my understanding that blowdown is the difference in degrees between the exhaust port opening and transfer ports opening, which in this case would be 121-105=16° blowdown. I think you may have inadvertantly doubled this value in your calculation.

That is correct.

 

[AUSSIE1]

Aussie1, it was my understanding that blowdown is the difference in degrees between the exhaust port opening and transfer ports opening, which in this case would be 121-105=16° blowdown. I think you may have inadvertantly doubled this value in your calculation.

Yeah your right I have.
It's a wonder it went this long before it was picked up.

 

[edisto]

Clear as mud? Please keep the discussion going!

I see what you are saying, but if you already have a degree wheel on...why not find TDC? Use BDC to get mean area, and see what changes you are making. I can't imagine anyone changing duration before they have played with port width, and with a different size/shape to the port, someone else's duration numbers aren't going to be to useful.

 

[bjorn773]

Would anyone care to elaborate on blowdown for a newbie? I understand it's the number of degrees between the exhaust opening and the transfers opening. But what does it really mean in terms of power? I've been advised to stay around 25 degrees.

 

[Mastermind]

On a lot of saws 25* is hard to come by.

 

[blsnelling]

If they open too soon, you start blowing exhaust down the transfers and hindering flow of the fresh charge trying to come up through them. You would be fine with 20 on a Husky, but don't try that on a 7900. They need all the blowdown they have. It's just part of how they work.

 

[bjorn773]

So again we have a balancing act. We're trying to achieve max flow without stepping over the line and creating a backflow through the transfers. It sounds like in the end it boils down to trial and error, or experience based on previous trial and error.

 

[edisto]

So again we have a balancing act. We're trying to achieve max flow without stepping over the line and creating a backflow through the transfers. It sounds like in the end it boils down to trial and error, or experience based on previous trial and error.

It's all a balancing act.

To increase blowdown in terms of duration, you are shortening the powerstroke by having the exhaust open early. Given that the point is to get the cylinder close to ambient before the transfers open, widening the exhaust port will help this without a change in direction. Duration is only part of the equation.

Controlling resonances is tricky, but the advantages are well beyond what you can do with just porting. I diffuser on the exhaust will create negative pressure in the cylinder, "drawing" exhaust out and charge through the transfers. Unfortunately, without a baffle and stinger after the diffuse, i.e., a tuned pipe, a diffuser can draw too much, and pull charge into the exhaust. The positive pressure wave from the baffle cone stuffs that lost charge back into the cylinder from the exhaust.

Tuning the intake length is more difficult still, but if you get it right, a positive pressure wave enters the crankcase before the intake closes, preventing charge from heading back out the intake as crankcase pressure goes up (on piston ported intakes) and increasing crankcase pressure to move charge through the transfers.

Change the intake and exhaust ports, and you change resonances, which requires changes in the intake and exhaust.

 

[bjorn773]

Thanks for the insight. I will err on the side of caution and stick with the suggested numbers established by guys here. I can see where a person may scrap several cylinders before finding the ultimate combination of durations etc. My wallet isn't thick enough for that

 

[edisto]

Thanks for the insight. I will err on the side of caution and stick with the suggested numbers established by guys here. I can see where a person may scrap several cylinders before finding the ultimate combination of durations etc. My wallet isn't thick enough for that

Neither is mine. Stick with widening to start out with (keep 2mm of piston skirt on either side for a seal), keep the widened part rounded, read up on chamfers to ease the rings back in, keep the work symmetrical, don't mess with the transfers, and you should be fine.

 

[bjorn773]

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?