Porting cylinder w/o crankcase

[computeruser]

What kind of results could be obtained porting a cylinder but not matching it/timing it to a particular crankcase? I’m assuming you aren’t going to be able to go 10/10 this way, but how good a port job could be done if someone had just a cylinder and a new piston to work from?

 

[Wilhelm]

Hello B

No reason why one couldn't get gains porting just having the P&C, as long as one knows what to do.

Will there be something left on the table, certainly.

My understanding of woods porting is that porting the cylinder is only part of the whole deal.
Builders do offer cylinder work only, but they tend to have the particular saws case to match what needs worked over.

I have no idea how much can be gained by just porting the cylinder.

 

[Nutball]

It depends on the references and experience the builder has. If they know the general dimensions of a particular engine, they can get very good results porting and machining without the bottom end.

 

[beaglebriar]

What kind of results could be obtained porting a cylinder but not matching it/timing it to a particular crankcase? I’m assuming you aren’t going to be able to go 10/10 this way, but how good a port job could be done if someone had just a cylinder and a new piston to work from?

In every instance that I've had cylinder work done by another builder they have had a complete crankcase assembly to work from. You can get everything very close that way. Basically the squish will need to be double checked on the actual saw but most will leave it a few thousandths big to accommodate for differences. I've had to sand the squish a bit to get where it needed to be but nothing major.

 

[kneedeepinsaws]

What about just widening ports slightly as a safe approach? I would think gains would be seen doing this?
Also increasing the intake near the carb mount to increase the venturi effect? Matching the exhaust by increasing the outlet flange to match the gasket and or muffler?

 

[drf256]

Really need a builder case to port a jug. I even use a builder case when I have an entire saw, provide I have one. I do for most Stihl models. It enables me to not have to clean the jug between every grinding step and not scratch up the slug in the end use saw. But then again, to each their own.

Factory squish bands are rarely truly flat, so someone telling me factory squish is 40 doesn’t mean I can take 20 from the band and 40 from the base and be 20 squish. It will almost always be wrong.

I’ve been taking measurements of base to band depths of most models I have done. If you need those figures for a particular model, they can get you close. Just let me know.

Port height can be figured out fairly accurately by using the Torquesoft software for piston height vs degree. There is a more complicated program for figuring out intake floor etc... Maybe someone can chime in on that one. Sounds like “Berlinetta” of something like that, the name escapes me right now. Both are available free online.

The other issue will be “can” vs “will” someone do it. There are many reasons builders will not port a jug without doing the assembly. An undesired result may occur, and then the finger pointing starts as to who’s fault that is.

Just my $.02

 

[cus_deluxe]

Sounds like “Berlinetta” of something like that, the name escapes me right now. Both are available free online.

i think lambretta is the one ur thinking of.

 

[drf256]

i think lambretta is the one ur thinking of.

Bingo

 

[srcarr52]

Really need a builder case to port a jug. I even use a builder case when I have an entire saw, provide I have one. I do for most Stihl models. It enables me to not have to clean the jug between every grinding step and not scratch up the slug in the end use saw. But then again, to each their own.

Factory squish bands are rarely truly flat, so someone telling me factory squish is 40 doesn’t mean I can take 20 from the band and 40 from the base and be 20 squish. It will almost always be wrong.

I’ve been taking measurements of base to band depths of most models I have done. If you need those figures for a particular model, they can get you close. Just let me know.

Port height can be figured out fairly accurately by using the Torquesoft software for piston height vs degree. There is a more complicated program for figuring out intake floor etc... Maybe someone can chime in on that one. Sounds like “Berlinetta” of something like that, the name escapes me right now. Both are available free online.

The other issue will be “can” vs “will” someone do it. There are many reasons builders will not port a jug without doing the assembly. An undesired result may occur, and then the finger pointing starts as to who’s fault that is.

Just my $.02

All the equations to calculate port opening/duration from port heights are given in A. Bell's book "Two-Stroke Performance Tuning". They are pretty simple and can be automated using Excel or freeware like Libre Office Calc.

I've made my own Excel sheet to do all the calculations, it can even calculate strato port duration, exhaust free porting and theoretical cranking compression if I measure the chamber volume. This makes it easy for me to see what I can achieve with a cylinder before I ever cut on it, which is really helpful when changing stroke length. Once I settle on my numbers I just measure the port openings to the squish band till I achieve the target heights. No need to ever reassemble the saw until it's done.

I agree with the "can" vs. "will". I have to be certain the person is able to measure the squish clearance accurately and assemble the saw without issues. I also have people send the intake and muffler along so I can port match those. Now I probably wouldn't do this for saws that require modifications to other parts of the saw, like clearancing for the cylinder to sit flat. But I have done multiple 394/5's this way without problems and their owners are very happy.

 

[fdhamlin]

All the equations to calculate port opening/duration from port heights are given in A. Bell's book "Two-Stroke Performance Tuning". They are pretty simple and can be automated using Excel or freeware like Libre Office Calc.

I've made my own Excel sheet to do all the calculations, it can even calculate strato port duration, exhaust free porting and theoretical cranking compression if I measure the chamber volume. This makes it easy for me to see what I can achieve with a cylinder before I ever cut on it, which is really helpful when changing stroke length. Once I settle on my numbers I just measure the port openings to the squish band till I achieve the target heights. No need to ever reassemble the saw until it's done.

I agree with the "can" vs. "will". I have to be certain the person is able to measure the squish clearance accurately and assemble the saw without issues. I also have people send the intake and muffler along so I can port match those. Now I probably wouldn't do this for saws that require modifications to other parts of the saw, like clearancing for the cylinder to sit flat. But I have done multiple 394/5's this way without problems and their owners are very happy.

Any chance you could share that excel spreadsheet with the less fortunate?

 

[srcarr52]

Any chance you could share that excel spreadsheet with the less fortunate?

Sorry, I've shared it in the past and spent too much time explaining it and answering questions.

 

[fdhamlin]

Sorry, I've shared it in the past and spent too much time explaining it and answering questions.

I understand! I've been down that path myself.

 

[kneedeepinsaws]

Sorry, I've shared it in the past and spent too much time explaining it and answering questions.

Cant give away your work for free.
The less fortunate can always pick up a book on porting and a book on excel and in a few months of their time make their own.

 

[drf256]

All the equations to calculate port opening/duration from port heights are given in A. Bell's book "Two-Stroke Performance Tuning". They are pretty simple and can be automated using Excel or freeware like Libre Office Calc.

I've made my own Excel sheet to do all the calculations, it can even calculate strato port duration, exhaust free porting and theoretical cranking compression if I measure the chamber volume. This makes it easy for me to see what I can achieve with a cylinder before I ever cut on it, which is really helpful when changing stroke length. Once I settle on my numbers I just measure the port openings to the squish band till I achieve the target heights. No need to ever reassemble the saw until it's done.

I agree with the "can" vs. "will". I have to be certain the person is able to measure the squish clearance accurately and assemble the saw without issues. I also have people send the intake and muffler along so I can port match those. Now I probably wouldn't do this for saws that require modifications to other parts of the saw, like clearancing for the cylinder to sit flat. But I have done multiple 394/5's this way without problems and their owners are very happy.

Shaun,

How do you arrive at squish without a builder case? Have you saved measures for ballpark on a certain model?

I know the back of a caliper can be used to figure out depth next to the cylinder wall, but I am not sure on the accuracy. It’s tough to get close with a depth mic and be on the money.

 

[srcarr52]

Shaun,

How do you arrive at squish without a builder case? Have you saved measures for ballpark on a certain model?

I know the back of a caliper can be used to figure out depth next to the cylinder wall, but I am not sure on the accuracy. It’s tough to get close with a depth mic and be on the money.

I have to rely on the owner to measure squish.

 

[Mastermind]

In my notes I have very detailed measurements (how much from squish, and base, from squish band to top of ex, and transfer ports, etc) and on most current models would be able to port the jug without having the rest of the saw. I rarely will agree to do it unless I'm doing a group of like models though.

I prefer to be able to check the clearance when it's all done. People do make mistakes.

I just did a group of 5 MS261s using my notes. The finished squish clearance varied only .002" over the 5 units. It's more likely that I varied in touching off when setting up for the machine work than that there was any real variance in the factory clearances.

 

[computeruser]

Thanks for the replies. This insight is helpful to my understanding of this whole porting business.

I ended up making arrangements with a builder to port a couple 044 cylinders when he sets up to do a batch of other 044s in the coming months. Then I can set about searching for 044 carcasses to build later and at my leisure, since the part of the work that depends on someone else's schedule and availability will be done already. I recognize both the upsides and downsides of this approach versus the builder having the rest of the saw on-hand at the time of porting, and at this time it makes sense for me to go the a la carte route.

Now the hunt for carcasses resumes!

 

[cuinrearview]

Thanks for the replies. This insight is helpful to my understanding of this whole porting business.

I ended up making arrangements with a builder to port a couple 044 cylinders when he sets up to do a batch of other 044s in the coming months. Then I can set about searching for 044 carcasses to build later and at my leisure, since the part of the work that depends on someone else's schedule and availability will be done already. I recognize both the upsides and downsides of this approach versus the builder having the rest of the saw on-hand at the time of porting, and at this time it makes sense for me to go the a la carte route.

Now the hunt for carcasses resumes!

You should check over in Lowell Brandon. Word is there's a guy over there that hoards Stihls.

 

[drf256]

Thanks for the replies. This insight is helpful to my understanding of this whole porting business.

I ended up making arrangements with a builder to port a couple 044 cylinders when he sets up to do a batch of other 044s in the coming months. Then I can set about searching for 044 carcasses to build later and at my leisure, since the part of the work that depends on someone else's schedule and availability will be done already. I recognize both the upsides and downsides of this approach versus the builder having the rest of the saw on-hand at the time of porting, and at this time it makes sense for me to go the a la carte route.

Now the hunt for carcasses resumes!

Spend a bit extra and have someone Hybridize an 046 P&C for you. Not certain I’d ever port a 50mm jug again on an 044 if it was for me. You’ll need to relieve the top cover and get a muffler spacer when the final assembly time comes.

Power output will be much greater in the same size saw. Just my $.02