372 Cylinder head.

[Darryll]

Your head volumes do not line up with mine at all. So what you are sayin is Fusion is calculating my combustion chamber wrong and I should go back to using a plate and a syringe?

 

[Ketchup]

Great discussion. A precision cut head adds a lot of opportunity to experiment with chamber shape and volume. I’m excited to see people talking about it. And I like pretty pictures too.

Cutting my own heads is something I have barely experimented with. I’m not sure why I’m wading in here where I’m not qualified but here goes…
Do we include squish in chamber volume or not? Is there a standard?

If Jensen had a documented practice, perhaps that should be the standard, since it will have informed many people’s calculations over the years.

If not, then I think we should include measured squish as part of chamber volume. All other measurements are taken cold and assumed to be constants. Removing or altering squish measurements because they may change during operation seems like guesswork. We can have more productive conversation if we are all doing the same calculations.

I also think there is value in having an analog method of checking our work. That should also be standardized. Using the same fluid
consistently and noting plug brand are important factors.

Thanks for posting.

 

[Darryll]

Great discussion. A precision cut head adds a lot of opportunity to experiment with chamber shape and volume. I’m excited to see people talking about it. And I like pretty pictures too.

Cutting my own heads is something I have barely experimented with. I’m not sure why I’m wading in here where I’m not qualified but here goes…
Do we include squish in chamber volume or not? Is there a standard?

If Jensen had a documented practice, perhaps that should be the standard, since it will have informed many people’s calculations over the years.

If not, then I think we should include measured squish as part of chamber volume. All other measurements are taken cold and assumed to be constants. Removing or altering squish measurements because they may change during operation seems like guesswork. We can have more productive conversation if we are all doing the same calculations.

I also think there is value in having an analog method of checking our work. That should also be standardized. Using the same fluid
consistently and noting plug brand are important factors.

Thanks for posting.

Your statement that you don't feel qualified to comment tells me everything I did not know about this forum. I don't think any qualification is required. A well functioning forum should be a platform where like minded people can kick around differing views without degenerating into an argument. It should also welcome everyone's perspective.

You raise good points. It's highly probable that my view is out of step with others and thats how I roll sometimes and I'm ok with that. Should I just give in and accept for there sake of keeping it simple that we include the squish? Yes maybe. To my mind that is an error and Scarr has made no argument sufficient to change my mind but I'm kinda done too.

For me personally the measure I take gives me a reference that has worked perfectly as I have done saws this way now for myself and others for a long time and every time I get the result I'm expecting. I brought a compression tester and it did not suit the way I build. All my saws are mapped out in great detail before I do anything. I work to a port map that has calculated port area timing giving me all the information I need to cut too and the compression I will have on completion. My chambers are cut specifically to give me either 9.5 or 10:1 at a given exhaust height. All my machine work is done before I start porting. A compression tester does not fit in that methodology at all. I have no way to make calculation using a tester but I certainly can use compression ratio. Despite the conversation here I will not be changing a thing.

You make a good point when you suggest we have a standard. I went back the other day and reread the chapter in the Jennings book after I implied here that he excluded squish as that was my interpretation. In fact he does not make any comment at all. Which by default more likely implies that he too included it. Being the pig headed bastard that I am he is wrong too lol.

That set aside Scarr and I are getting quite different numbers on presumably the same cylinder. We both think the other is wrong and frankly I don't think we will get to the bottom of that. My spread sheet is good and I trust Fusion 100%. I don't trust a syringe 100%.

So to answer your question I'd happily for the sake of sharing information include squish if that is the standard we believe works best.

I did not want to get bogged down in this detail I was kinda hoping we might get into a broader discussion on cylinder head design. Its not every day a guy turns up with a CNC and the ability to design and build a rather precisely machined head. I was thinking more along the lines of having a general discussion of a design that we all think could work well and I'd build it and report back the result. A group think type of thing.

Honestly I'm not getting that vibe here at all. That's a shame.

 

[jacob j.]

I'm really enjoying watching you guys talk about custom heads and playing around with squish bands and chamber shape, volumes, etc.

I probably won't get a lathe myself and dive into it but I enjoy the mechanics and engineering that go into it.

I have some two-piece head saws done by others - a few run really well and one or two not so well.

 

[Darryll]

I'm really enjoying watching you guys talk about custom heads and playing around with squish bands and chamber shape, volumes, etc.

I probably won't get a lathe myself and dive into it but I enjoy the mechanics and engineering that go into it.

I have some two-piece head saws done by others - a few run really well and one or two not so well.

Well I don't think we have covered any interesting ground yet. I'd like to to discuss everyone's thoughts on the 500I deign. But if everyone is fearful they are going to get flamed there will be no discussion.

I feel like an outsider who mistakenly jumped in the lion pit. I'm trying to kick the bastard off my foot so I can get the f#@k out of here lol.

 

[Nutball]

How much does the thermal expansion of the hot piston and cylinder affect squish?

 

[srcarr52]

Well I don't think we have covered any interesting ground yet. I'd like to to discuss everyone's thoughts on the 500I deign. But if everyone is fearful they are going to get flamed there will be no discussion.

I feel like an outsider who mistakenly jumped in the lion pit. I'm trying to kick the bastard off my foot so I can get the f#@k out of here lol.

Sorry if you felt I jumped on you. I wanted to back up or add some extra case studies to your original premise but your numbers didn’t match up with anything I’ve had. So I went down the road of figuring out why.

The heads you’ve machined look great, and nicely machined. I’ve put fins on heads before but never made them that thin as you have to take pretty small cuts to make a nice cut on something unsupported like that.

 

[Ketchup]

Your statement that you don't feel qualified to comment tells me everything I did not know about this forum. I don't think any qualification is required. A well functioning forum should be a platform where like minded people can kick around differing views without degenerating into an argument. It should also welcome everyone's perspective.

You raise good points. It's highly probable that my view is out of step with others and thats how I roll sometimes and I'm ok with that. Should I just give in and accept for there sake of keeping it simple that we include the squish? Yes maybe. To my mind that is an error and Scarr has made no argument sufficient to change my mind but I'm kinda done too.

For me personally the measure I take gives me a reference that has worked perfectly as I have done saws this way now for myself and others for a long time and every time I get the result I'm expecting. I brought a compression tester and it did not suit the way I build. All my saws are mapped out in great detail before I do anything. I work to a port map that has calculated port area timing giving me all the information I need to cut too and the compression I will have on completion. My chambers are cut specifically to give me either 9.5 or 10:1 at a given exhaust height. All my machine work is done before I start porting. A compression tester does not fit in that methodology at all. I have no way to make calculation using a tester but I certainly can use compression ratio. Despite the conversation here I will not be changing a thing.

You make a good point when you suggest we have a standard. I went back the other day and reread the chapter in the Jennings book after I implied here that he excluded squish as that was my interpretation. In fact he does not make any comment at all. Which by default more likely implies that he too included it. Being the pig headed bastard that I am he is wrong too lol.

That set aside Scarr and I are getting quite different numbers on presumably the same cylinder. We both think the other is wrong and frankly I don't think we will get to the bottom of that. My spread sheet is good and I trust Fusion 100%. I don't trust a syringe 100%.

So to answer your question I'd happily for the sake of sharing information include squish if that is the standard we believe works best.

I did not want to get bogged down in this detail I was kinda hoping we might get into a broader discussion on cylinder head design. Its not every day a guy turns up with a CNC and the ability to design and build a rather precisely machined head. I was thinking more along the lines of having a general discussion of a design that we all think could work well and I'd build it and report back the result. A group think type of thing.

Honestly I'm not getting that vibe here at all. That's a shame.

This is a forum where anyone can discuss and contribute. It’s a good place. I just don’t want anyone taking my comments as those of an expert. I want to learn and get better though, so here I am.

No need to change your process, though it would be interesting to know what your Compression Ratios land at with Squish calculated in. That may be the root of the the discrepancy between your and @Srcarr52’s numbers, but it could also be something else.

Examining a 500i chamber sounds great. I’m traveling until Monday, but after that I can pull a jug and post up some photos and measurements. From memory the dome is small, the radius large and the band slightly larger than average.

How much does the thermal expansion of the hot piston and cylinder affect squish?

My napkin math is this:
Aluminum should expand roughly 0.00023cm per degree Celcius. If we start at 30C and raise temp to 175C, 145C*0.00023cm= 0.03335cm (or 0.013…”).

I think this gets at the difficulty of measurements at WOT operation. Materials and geometry will effect where and how much things shift. There are cooler and hotter areas. The idea that a properly set squish compresses down to nearly zero is very interesting. So is the idea of the piston crown being under near constant compression, thus making squish larger during operation.
The expansion or compression of case internals like the con rod and bearings should also be considered. And the tilt of the piston.

The heads you’ve machined look great, and nicely machined. I’ve put fins on heads before but never made them that thin as you have to take pretty small cuts to make a nice cut on something unsupported like that.

I wonder if cooling fin blanks could be attached rather than cutting them from billet. I was looking at some that had fluid filled copper tubing integrated to carry heat farther.

 

[Darryll]

Sorry if you felt I jumped on you. I wanted to back up or add some extra case studies to your original premise but your numbers didn’t match up with anything I’ve had. So I went down the road of figuring out why.

The heads you’ve machined look great, and nicely machined. I’ve put fins on heads before but never made them that thin as you have to take pretty small cuts to make a nice cut on something unsupported like that.

I don't know why our numbers don't line up Scarr, they should. I'm sure if we were having a beer in my shop we would sort it out in a few minuets. For the sake of being reasonable I will concede my stock cylinder values may well be off because of my process of modeling them in Fusion etc. There is certainly room for error there. I went back through some old records last night and to be fair my syringe values weren't far away from yours. But due to my rather sloppy record keeping they may have been taken after I did a cylinder cut. Back then I was not nearly as refined as I am now.

I would like to discuss the pro's and cons of combustion chamber shapes. I have a few observations with the few experiments I have run and I'm happy to share that if others are willing to share there thoughts.

I use the 500i as an example as its a little different and I don't think those engineers are fools. I'm a little keen to replicate that and fit it too my 372. I have a few cylinders turning up maybe next week which I intend to butcher in the name of science.

I am sure we will eventually agree on something.

 

[Darryll]

The heads you’ve machined look great, and nicely machined. I’ve put fins on heads before but never made them that thin as you have to take pretty small cuts to make a nice cut on something unsupported like that.

Yes painfully small! .1mm. I rough the top out with an adaptive clearing using a 3mm cutter. I broke a few before I dialed in the best feeds and speeds. It takes about 1.5hrs to rough out and those heads took about 2 hrs to finish cut. I have since brought some tapered cutters so that will bring the time down to less than an hour to finish I expect. The fins are about 4mm at the base and 2mm on top.

I was very surprised how cool it run. I really could not tell the difference from stock. And that was with the poor bastard detonating its self to death and me abusing it with a big bar. The second day when I corrected the ratio and solved the detonating it run much better.

 

[stumpyfine]

I'll have a look. The best O Rings I could find here in NZ were rated to 200C. They showed absolutely no sine of heat and sealed perfectly. I have since got these FKM O rings and they are rated to 260C I'm pretty confident they will last a good while. I brought a heap of them and its so easy to replace them so I could careless if they do fail every 100hrs or so. I'd say that my machined head sitting on the machined cylinder did the line share of the work. There was a very small amount of evidence that the gas leaked to the O Ring in two places. Further more I could see the head was bedding into the cylinder and in few more hours I'd say it would be pretty much a perfect seal. Those surfaces were milled not turned in a lath. If you are turning them its likely you will have a better finish than I got.

After all the work you have done I hope it goes like hell.

You lost me when you were talking about putting a plate in the exhaust? what was that about?

Thanks, there is many different ways to modify machinery. Having done it with varying success most of my life.(if in doubt use a bigger hammer). I find these discussions very helpful, especially when personal like you do this R&D,saves us doing it and also produces more refinement.
What I mean about the exhaust plate is that when we set the cylinder with the bottom of the transfers level with the piston at BDC. The exhaust can open too early. I am working on fixing an approx 2mm plate in the top of the port to correct this.Still deciding about ring clearance to the edge of the plate. Will get some pics up.
Are you able to machine a billet cylinder as well? Then we could optimize all the ports. NZ cylinders can plate for about $400 for an open cylinder.

 

[Darryll]

My napkin math is this:
Aluminum should expand roughly 0.00023cm per degree Celcius. If we start at 30C and raise temp to 175C, 145C*0.00023cm= 0.03335cm (or 0.013…”).

I think this gets at the difficulty of measurements at WOT operation. Materials and geometry will effect where and how much things shift. There are cooler and hotter areas. The idea that a properly set squish compresses down to nearly zero is very interesting. So is the idea of the piston crown being under near constant compression, thus making squish larger during operation.
The expansion or compression of case internals like the con rod and bearings should also be considered. And the tilt of the piston.

I think all of those are factors to some degree. On my own personal saws I push things too the absolute limit. I cut a saw down to .3mm clearance some years ago and that rattled like a baby's toy. At .4mm it was fine. I had considered heat and inertia etc but not the piston rocking but of course it does. My belief that the inertia overcomes compression actually comes from racing RC cars. They spin around 36,000rpm. The piston and liner is what we call an ABC or Aluminum, brass, chrome. When cold those engines were seized and I used to heat them up with a heat gun to 150c before starting them. In no way did those piston rock in the bore! We could not run them with zero squish, they too needed clearance. Now that may well be solely a heat expansion cause but we all assumed it to be inertia. I'm sure some clever bastard could mathematically solve that equation and it is highly possible that there is more than enough force to overcome inertia. Maybe Scarr knows of this?

 

[Darryll]

Thanks, there is many different ways to modify machinery. Having done it with varying success most of my life.(if in doubt use a bigger hammer). I find these discussions very helpful, especially when personal like you do this R&D,saves us doing it and also produces more refinement.
What I mean about the exhaust plate is that when we set the cylinder with the bottom of the transfers level with the piston at BDC. The exhaust can open too early. I am working on fixing an approx 2mm plate in the top of the port to correct this.Still deciding about ring clearance to the edge of the plate. Will get some pics up.
Are you able to machine a billet cylinder as well? Then we could optimize all the ports. NZ cylinders can plate for about $400 for an open cylinder.

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!

 

[Darryll]

.The exhaust can open too early.

Actually a better response from me would be to suggest finding another piston. I'm sure someone here could point you in the right direction. I'm not a Stihl guy sorry.

 

[stumpyfine]

Yep have modded several that way. And swapped pistons to alter compression height. IMHO as far as aftermarket cylinders go,if it's asian I buy the cheapest and take what comes, The one meteor I used had hard plating, the ports were fairly average and the squish band tapered with a flat piston, so basically no squish. Would much rather put on a used oem
I have an Italian precision tooling top end for a 660, I would rate the casting and machining as good as or better than OEM.
I fully agree with you about optimizing the chamber shape and size.
The main reason for wanting to lower the exhaust is on the 660 it is too high and lowering the jug means grinding the transfers to a shape that, IMHO is loosing velocity.

 

[Nutball]

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

 

[Darryll]

So I have a couple of examples here of chamber cross sections.

This is a near approximation of a 50mm 372 36mm stroke. The combustion chamber volume is in the box simply add the decimal point after the first digit. I don't have a cylinder here so I just made that up from my memory if its too far wrong let me know and I'll change it.



Inspired some what by the Stihl 500I a possible contender. Once again for the sake of any calculations assume a 50mm 372.

I'm interested in any thoughts of the pros and cons of each shape. More than happy to modify volumes etc if we want to drill deep into the subject.

 

[Darryll]

 

[Moparmyway]

Your statement that you don't feel qualified to comment tells me everything I did not know about this forum. I don't think any qualification is required. A well functioning forum should be a platform where like minded people can kick around differing views without degenerating into an argument. It should also welcome everyone's perspective.

You raise good points. It's highly probable that my view is out of step with others and thats how I roll sometimes and I'm ok with that. Should I just give in and accept for there sake of keeping it simple that we include the squish? Yes maybe. To my mind that is an error and Scarr has made no argument sufficient to change my mind but I'm kinda done too.

For me personally the measure I take gives me a reference that has worked perfectly as I have done saws this way now for myself and others for a long time and every time I get the result I'm expecting. I brought a compression tester and it did not suit the way I build. All my saws are mapped out in great detail before I do anything. I work to a port map that has calculated port area timing giving me all the information I need to cut too and the compression I will have on completion. My chambers are cut specifically to give me either 9.5 or 10:1 at a given exhaust height. All my machine work is done before I start porting. A compression tester does not fit in that methodology at all. I have no way to make calculation using a tester but I certainly can use compression ratio. Despite the conversation here I will not be changing a thing.

You make a good point when you suggest we have a standard. I went back the other day and reread the chapter in the Jennings book after I implied here that he excluded squish as that was my interpretation. In fact he does not make any comment at all. Which by default more likely implies that he too included it. Being the pig headed bastard that I am he is wrong too lol.

That set aside Scarr and I are getting quite different numbers on presumably the same cylinder. We both think the other is wrong and frankly I don't think we will get to the bottom of that. My spread sheet is good and I trust Fusion 100%. I don't trust a syringe 100%.

So to answer your question I'd happily for the sake of sharing information include squish if that is the standard we believe works best.

I did not want to get bogged down in this detail I was kinda hoping we might get into a broader discussion on cylinder head design. Its not every day a guy turns up with a CNC and the ability to design and build a rather precisely machined head. I was thinking more along the lines of having a general discussion of a design that we all think could work well and I'd build it and report back the result. A group think type of thing.

Honestly I'm not getting that vibe here at all. That's a shame.

Well I don't think we have covered any interesting ground yet. I'd like to to discuss everyone's thoughts on the 500I deign. But if everyone is fearful they are going to get flamed there will be no discussion.

I feel like an outsider who mistakenly jumped in the lion pit. I'm trying to kick the bastard off my foot so I can get the f#@k out of here lol.

So I have a couple of examples here of chamber cross sections.View attachment 393160

This is a near approximation of a 50mm 372 36mm stroke. The combustion chamber volume is in the box simply add the decimal point after the first digit. I don't have a cylinder here so I just made that up from my memory if its too far wrong let me know and I'll change it.

View attachment 393161

Inspired some what by the Stihl 500I a possible contender. Once again for the sake of any calculations assume a 50mm 372.

I'm interested in any thoughts of the pros and cons of each shape. More than happy to modify volumes etc if we want to drill deep into the subject.

Happy to see that you are still here and asking questions. I am reading and learning, I have ported a few 500i’s after spinning the squish and bases, and absolutely love them, very interested in watching your work and progress. Please keep us posted as you go !!!!

Selfishly, I’m very interested in your thoughts on a tapered squish band and matching it with the piston crown ………????

 

[Darryll]

Selfishly, I’m very interested in your thoughts on a tapered squish band and matching it with the piston crown ………????

Interesting comment?? I don't recall mentioning that I have been building saws with a tapered squish on here. I have a lot to say about that and happy to share at some point. I think now is a good time for me to be quite and listen.

You could start by sharing how much you took of and the results you got with the 500?