372 Cylinder head.

[Moparmyway]

Last one I took 0.065 off the base. It gave me the opportunity to shape the transferrs nicely without having too much grinding to do. Different cuts from a flat squish because of the tapered piston crown, which is the same angle as the squish band, so much different numbers than any flat squish machining.

 

[Darryll]

Last one I took 0.065 off the base. It gave me the opportunity to shape the transferrs nicely without having too much grinding to do. Different cuts from a flat squish because of the tapered piston crown, which is the same angle as the squish band, so much different numbers than any flat squish machining.

Nice work.
.065 is a pretty typical number. What angle did you cut the squish too? Yes there is some difference to flat machining. Which inherently gives us the first "con" to doing it. For that reason I'm sitting on the fence as to the benefit.

I simply program the angle and cut it, same with the piston and it comes out perfect. I can see you have a great setup and I doubt you have any trouble at all doing the same. Yes it is more work but not bad.

Mathematically there is a bit more going on. My experience is the saws run differently. Has that been your experience? If so what is your observation? If they run different do you have some thoughts as too why?

I think there is more potential to be had if the overall measurement of the piston from top edge to the bottom remained stock or I'd like a piston a little taller even. I'm sure you understand why. Hence I'm seriously considering making my own pistons.

 

[Moparmyway]

Great minds think alike ….. I’ve already suggested to the “piston makers” around here that a taller piston for the 500i would be better.

My experience is that they do run better with a tapered squish (piston same angle as the band)…….. how much better is the question ?

I’m unsure of what angle I’m going to settle on, but I’ve decided somewhere less than 15 degrees is the target.

Why: I have my thoughts on this, but I think, (out of respect for this thread, it’s best if we’re going to discuss this further or dive into it), we start it’s own thread.

 

[srcarr52]

Ok so much like a KX power valve. So you are doing this because your exhaust port timing has got too high?

Yes I have seriously considered making my own cylinder. NZ cylinders replated my sons KTM 50 for me and I have spoken to them about doing a 372 cylinder.

Where I am at now is I can get pretty much the numbers I want and all the benefits by fitting my own head. I have 2 52mm cylinders arriving soon and I am hoping they are the same cylinders I was getting off Mikes Chainsaws a few years back. They were some no name Chines cylinder but they were actually very good and had real Nikasil plating. I hate those Hyway cylinders with a passion but begrudgingly brought one to run the experiment I just did and I'm pretty pissed that by midday on the second day I felt the saw go off song. I knew It was possibly self destructing so I did what all good responsible saws owners should do. I lent on it harder and abused it as much as I could before it died. To its credit it kept running all day. I kept using it because it was still more powerful everywhere than my 592.

Assume a 372:
At this point in the game I am moving away from overly large transfer tunnels. Transfer entry is more important. The Exhaust is what it is mine are pretty much 100 and 63-65%, If someone has a better recipe let others know. The intake is the real reason I started building heads. To build a saw to pull you need more compression than stock. So you cut the squish 1mm and set the clearance to say .5mm. Often this means you cut about 1.5mm from the base. This lowers your intake too 83 or maybe 85. Thats fine your saw will run great. But you will have an exhaust about 103-105 now that will make some nice torque but your intake is now a bit low. So you cut the exhaust back up too 100. Now your intake has about the right port area timing. You may widen it a bit but often I don't. This will run strong and last good. Not the fastest saw out there but a bloody good work saw.

Now when I build my head I have a blank sheet of paper. I can do almost what ever the hell I want. That Hyway piece of *s-word I just built went like hell. Heads work!

You’d have to run the numbers but it probably has something to do with squish velocity causing detonation. As the squish clearance gets really small the velocity picks up very fast. I’d have to go back and read up on that theory. As far as chainsaws go I realized we couldn’t get tight enough squish on the typical piston rock on typical bores to really care about the maximum threshold for squish velocity.

I also cut the squish at a slight angle.

 

[stumpyfine]

Fill the transfers with JB Weld after a good prep, and shape them how you want.

Yep that's another way to do it.

 

[Darryll]

My experience is that they do run better with a tapered squish (piston same angle as the band)…….. how much better is the question ?

I’m unsure of what angle I’m going to settle on, but I’ve decided somewhere less than 15 degrees is the target.

Why: I have my thoughts on this, but I think, (out of respect for this thread, it’s best if we’re going to discuss this further or dive into it), we start it’s own thread.

Do we have to start different thread for each sub topic?

I have settled on 13 degrees.

When I get this thing sorted I might post something then.

 

[Moparmyway]

13 degrees leaves lots more crown area than 6 or 7 degrees. 13 also leaves a larger chamber area on some cylinders. I’ve been leaning towards a more narrow squish band lately, increasing the angle as I’ve gone. It’s difficult to compare the differences between angles, but I suspect that there’s a sweet spot somewhere that will work well for the chamber and the effectiveness of the transferrs. In my mind, the loop scavenging is enhanced by the angle the piston has, and possibly the angle of the squish band too, directing the charge upward as it flows outwards from the upper transferrs

What has been the influencing factors for your angle selection and why did you settle on 13 degrees ?

Was 20 degrees too much ?

What’s the highest angle you tried ?

I just found out that Shawn angles his squish too. @srcarr52 ….. what have you observed regarding squish band angle and do you match the crown ? Have you tried different angles and why have you settled on the angle you use ?

 

[Nutball]

Would a large radius between the cylinder and squish help scavenging if the piston was shaped to match?

 

[wcorey]

Would a large radius between the cylinder and squish help scavenging if the piston was shaped to match?

Do you mean the radius between the chamber and band? If so sounds like what you describe is basically a popup that closely follows that radius.

Past consensus seemed to conclude that from a scavenging point of view, a popup creates more disruptive turbulence than it does beneficial direction. Or at least it would be very difficult to design/predict what would be a shape that would enhance scavenging. Whatever it was, it would be very unlikely to be a linear ring such as that turned on a lathe.
Some of the old outboards used to have pretty curious and extreme popups and dams...

The question brought up of the effect the combustion chamber and squish shape have on scavenging is an interesting one.
I have to wonder on the proportion of that effect on performance as opposed to in the combustion part of the cycle...

 

[wcorey]

Different cuts from a flat squish because of the tapered piston crown, which is the same angle as the squish band, so much different numbers than any flat squish machining.

Hey Kev,

I seem to recall intriguing conversations with you about that tapered piston/squish many years ago, I'm sure you must have at least some solid conclusions by now... Yes...?

 

[Darryll]

13 degrees leaves lots more crown area than 6 or 7 degrees. 13 also leaves a larger chamber area on some cylinders. I’ve been leaning towards a more narrow squish band lately, increasing the angle as I’ve gone. It’s difficult to compare the differences between angles, but I suspect that there’s a sweet spot somewhere that will work well for the chamber and the effectiveness of the transferrs. In my mind, the loop scavenging is enhanced by the angle the piston has, and possibly the angle of the squish band too, directing the charge upward as it flows outwards from the upper transferrs

What has been the influencing factors for your angle selection and why did you settle on 13 degrees ?

It may be confusing to look at, The far left vertical line is a centre line. The 11.6mm is from the top of the combustion chamber too where the squish meets the cylinder wall including .5mm If I add more angle it would mean a greater cut on the piston while maintaining a combustion chamber of a given volume. This is an older design I found for the purpose of demonstrating my thinking here. I'm not sure of the ratio but I expect it was 9.5:1. As I see it when we are reconfiguring an existing cylinder head we are confined to the dimensions of that design. It is very limiting. Now that I am making my own heads I'm still restricted by the stock piston. If I start making my own pistons then things can get pretty interesting I guess and 20 degrees is absolutely possible.

Was 20 degrees too much ?

The first cylinder I cut to 10 degrees and I did not like it at all. I had the same port numbers as I typically do and it was peaky and had no guts. The last 2 I did to 13. I do think 13 had more effect. I lowered my exhaust to 104 for them both. A combination of that and the angled squish I believe is giving me a nice spread of power with plenty of top end. I did bump the ratio in the last one to 10:1. There's always a but. None of them have been particularly exciting to run, the last was better but not in your face. To the point that I thought they were slow. Then I put them up against other saws including the 592 and they sure as hell are fast. The delivery is super smooth, particularly the last one I did.

The 52mm Hyway in this thread had a square squish because I think they light up faster but there power bands are narrower maybe. I'm still trying to work it out. Anyhow I did some other stuff to it and it was really aggressive to use. It was quicker than the 51.4mm buy a margin everywhere but to be honest at the end of the day when I was feeling a bit stuffed I far preferred the 51.4mm. It has tons of grunt its faster than the 592 on top but not quite as much torque and it's nice to start with good throttle response and the delivery is super smooth.

What’s the highest angle you tried ?

13 degrees.

I'm going in the opposite direction to you and Scarr and wanting to try a 60% area squish band. I have no reason too other than I can and I want to know what effect that will have. Srcarr52 raised some very interesting points in his last post and I think his suggestion of having the squish band a degree or two different from the piston makes sense with such a wide squish band.

It is nice to bounce these thoughts around with you. I certainly don't have all the answers, I'm just another guy in a shed carving up cylinders for fun.

 

[Darryll]

Do you mean the radius between the chamber and band? If so sounds like what you describe is basically a popup that closely follows that radius.

Past consensus seemed to conclude that from a scavenging point of view, a popup creates more disruptive turbulence than it does beneficial direction. Or at least it would be very difficult to design/predict what would be a shape that would enhance scavenging. Whatever it was, it would be very unlikely to be a linear ring such as that turned on a lathe.
Some of the old outboards used to have pretty curious and extreme popups and dams...

The question brought up of the effect the combustion chamber and squish shape have on scavenging is an interesting one.
I have to wonder on the proportion of that effect on performance as opposed to in the combustion part of the cycle...

That was a good response but now you have sent me off on another line of thought.

 

[Moparmyway]

Hey Kev,

I seem to recall intriguing conversations with you about that tapered piston/squish many years ago, I'm sure you must have at least some solid conclusions by now... Yes...?

Hi Bill !!
It’s been a while, but you’re right about discussing it in person at Dans. About the only conclusion I’ve come to is that I don’t know all that I want to about these angles, and I wish I had more time to play around with them.

I’ve produced some very strong runners with these angles …….but was it the angles or something else that made them slightly better than other saws with the same displacements ?

I’m very very curious about your thoughts here Bill, as I believe the value that your perspective brings is just about priceless

 

[Moparmyway]

That was a good response but now you have sent me off on another line of thought.

Indeed !

I’m not sure if Bill intended to make us think outside of the box here or not. I do know that he’s probably the most intelligent machinist I’ve ever met.

Id like to propose that the “standard” popup has its disadvantages when comparing it with a tapered crown, which is essentially a popup without any obstruction.

The standard popup also doesn’t assist with directing the upper transferrs initial opening discharge upwards.


In my pea brain, this is what drove me to start spinning with the angles

 

[Ketchup]

@Darryll, this guy’s spreadsheet is pretty interesting. His videos are like stereo instructions but he has some cool perspectives. Does your sheet calculate optimal Squish area?

 

[Darryll]

@Darryll, this guy’s spreadsheet is pretty interesting. His videos are like stereo instructions but he has some cool perspectives. Does your sheet calculate optimal Squish area?

No its not at that level. But I have downloaded this guys spreadsheets and played with them. I don't recall seeing that sheet though. If his predictions are correct I'm heading in the right direction at 60% though.

 

[Darryll]

Indeed !

I’m not sure if Bill intended to make us think outside of the box here or not. I do know that he’s probably the most intelligent machinist I’ve ever met.

It doesn't take much to set me off on a tangent unfortunately. I do prefer it though when we share our observations of changes we make. I'm sure by now we all know a good saw from and average one. Any conversation that causes me to ponder a thought is a valued one. Its even more valuable if it causes me to think I'm wrong. That saves me wasting time perusing something I believed to be true.

Id like to propose that the “standard” popup has its disadvantages when comparing it with a tapered crown, which is essentially a popup without any obstruction.

The standard popup also doesn’t assist with directing the upper transferrs initial opening discharge upwards.

I would agree with that too. I think it is the cut to the piston that has done more to affect the power delivery than the angle of the squish band. My pistons perfectly match my squish band to try and keep the flow uniform at TDC. I have done some other odd things too pistons that have had far greater effect than the angled cut but not relevant to this topic that suggest to me small things matter.

What do we think of the second drawing I posted on the first page? Which of the two shapes do you think will work best and why?

 

[Ketchup]

@Mastermind had a thread up with a good picture of a chamber cross section. I can’t remember the exact name of the thread. Hopefully he will share it here. It looked a lot more like the second drawing, but larger dome radius relative to bore. I think it was an 066 cylinder.

 

[wcorey]

I’m not sure if Bill intended to make us think outside of the box here or not. I do know that he’s probably the most intelligent machinist I’ve ever met.

Ok, enough of that, I don't want to be held to those standards. Sounds to me like you just don't get out much, lol.
There's a few just in this thread alone that I have a difficult time keeping up with.

It continually amazes and frustrates me how simple the mechanics of a 2stroke engine are while the dynamics of operation are so complex I can only digest it in small compartmentalized bites. And that the specifics of the chainsaw application seem to have their own uniqueness that often defies conclusions based on most other platforms.
Unfortunately I find it much easier to come up with questions and loose conjecture rather than meaningful answers...

 

[wcorey]

On the subject of compression...

I went through a long and painful journey with a somewhat aggressively ported 394 where it started out fairly well behaved and progressively devolved over maybe 10 tanks into an untune-able be-otch.
Wouldn't make it through a cut without significantly leaning out but when richened up a bit would be blubbering and wouldn't clean up under load. Was at first using it as a test mule for a dyno project so most of the runtime after some initial break in was in a series of many short runs so it took a while to notice the issue. It also was very quick/easy to start so the odd fact that it was building significant compression as it broke in was escaping me.
After many carb swaps, jet drilling, timing changes, etc I'd get frustrated and shelve it for a year at a time. I finally realized it was getting difficult to pull over to the point where I couldn't do a reasonable compression check because I couldn't pull it through more than one cycle or so with no ignition. It started out around 220-230psi if memory serves and at the end was pulling over 280 on like one and a half compression cycles. Was getting concerned about ruining the recoil and/or snapping the end of the crank off.

So evidently it was overheating so significantly that at the end it would lean out just on short piss revs. Surprisingly it didn't have detonation issues even on my usual 87 ron/mon mix...
I didn't want to alter the port timing or squish clearance so ended up grinding out the chamber to I think around 200psi, tuning issue disappeared but seemed to then suck the life out of it. WTF...
Again I shelved it in frustration and it's been sitting there a few years now...