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Old 09-25-2014, 11:07 AM   #1
motobene OP
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Altering Combustion Chamber (Head) Profiles

I was considering recently fitting some thick base gaskets to lower the compression on my `11 300 Raga beyond that done by fitting the orange Stars low-compression head insert. I thought it best to retain the same port timing in my bid to tame my 300 further.

One of the things fitting thick base gaskets does is violate the engine tuner's rule of the 1mm squish... that is, the gap between the piston crown and the outer perimeter of the head. This factors into how the flame propagates after the air-fuel mixture fires in ways I do not understand. I do recall the awful detonating common to TY350s, some said because the squish 'band' was way too open. One way we'd fix that was removing the I think 1mm copper head gasket and simply running no gasket and just a little sealer. That brought squish much closer to 1mm, and reduced the detonation.

Another thing thicker base gaskets do is change the port timing by raising all the ports. Not by much, but by a little.

So an option is take the silver head insert in the photo and custom profile it. When I re-profile the stock silver head insert, can I cut an angle into some portion of the squish area, leaving say half at 1mm gap then tapering on an angle upward. I've got to get additional volume from somewhere. Or do I just enlarge to dome bigger than the orange insert, and then just blend in the transition to the squish area with a bigger radius? How big a radius? Anyone know?

There are some odd differences in the three inserts I have. Look here how the silver standard head insert has such a small blend radius between the dome and squish areas:



Three of five is not all that correct. Others are simply OEM or called "machinable." So the orange is as low as it gets.

Hmm. I see the low-compression insert combustion chamber dips down away from the plug, which may always be in the same position. I could dip deeper, enlarge the dome, and increase the transition radius.

Perhaps I should not be so picky on shapes because who knows, really? I'll point Stoodley to this thread. Of anyone, he should know.

motobene screwed with this post 09-25-2014 at 11:16 AM
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Old 09-25-2014, 02:05 PM   #2
Thats_a_Five
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Motobene,
Figuring out things like this is the stuff of highly sophisticated flow dynamics programs and some good old experience. I am certainly no expert on either.I have done some porting and polishing on some Datsun motors.Whatever is done on intake manifolds, port shape size, shape, and location as well as piston done shape and head shape all have an effect on the swirl of the incoming charge and exhaust scavenging.The swirl has an impact on flame propagation in the charge. Very generally speaking, the smoother the surfaces and transitions better the swirl will be.Abrupt edges and corners create high and low pressure areas and uneven density of the fuel / air charge.
I don’t know if you follow Nascar racing but years ago when they implemented restrictor plates, the teams experienced huge drops in horsepower. By focusing on the shape of the intake manifolds, port shapes, etc, the teams were able to regain some of that lost power or adjust the shape of the power curve.The specifics of what they did are highly guarded information to this day.
Best of luck to you.
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Old 09-25-2014, 05:59 PM   #3
robertlhammond
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I would check out 'The Two Stroke Cycle Engine: In development, operation, and design' by John Heywood & Eran Sher.

I loaned it to a friend a few years ago but it appeared to be a decent book on the two stroke engine. Heywood wrote the bible on the ICE...Internal Combustion Engine Fundamentals.

Regards,

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Old 09-26-2014, 08:08 AM   #4
motobene OP
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Originally Posted by robertlhammond View Post
I would check out 'The Two Stroke Cycle Engine: In development, operation, and design' by John Heywood & Eran Sher.

I loaned it to a friend a few years ago but it appeared to be a decent book on the two stroke engine. Heywood wrote the bible on the ICE...Internal Combustion Engine Fundamentals.

Regards,

Rob
Thanks!
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Old 09-26-2014, 08:09 AM   #5
motobene OP
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Regarding a walking bible, Jon Stoodley replied this morning by email:

"Sounds like fun! I'm working on a Trial next two days but I'll contribute after that. I have a lot of experience with heads, both 2 and 4 strokes."

Some patience, then we'll get insights
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Old 09-26-2014, 02:05 PM   #6
thegraydog
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The use of the S3 insert requires the S3 exterior head as well?
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Old 09-27-2014, 06:33 AM   #7
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The use of the S3 insert requires the S3 exterior head as well?
Yes. The vast majority of the time all anyone needs is a one-piece cast head. If you like to tweak compression ratios, however, the Stars head is a brilliant concept, being an outer part with an inner insert. An o-ring seals the face of the insert on the cylinder, and another o-ring in the head seals the coolant off from the outside on the neck of the insert containing the plug threads. There is no tradition core of passages for the water cooling. Water simply flows through the cavity between the insert and the head.

The inserts are all-machined aluminum and round, making it possible to chuck it up in a lathe without having to make a jig to hold the head.

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Old 09-28-2014, 08:58 AM   #8
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O.K.!
I have a few minutes to spare today so this will be short, but wanted to throw a few things out just to keep the ball rolling.

Chris and I are the worst kind of motorheads as, when we change something in an engine and it works well we are still not satisfied until we completely understand WHY it worked well (truth is, it helps us reproduce success).

A little history on the modern two-stroke head, known as a "hemi" head. The hemi head configuration for internal combustion engines has been around since the beginning of the last century. Other chamber designs were tried, in the 70's Bultaco's had a "bathtub" chamber (trough chamber) and in the 80's Yamaha used a trapezoidal design and in the 60's PUCH had a two cylinder two-stroke engine with a single combustion chamber that straddled both the piston crowns (both pistons rose and fell together, you could buy one at the Sears store under the "Allstate" label and we nicknamed it the "twingle" engine).

The guy who really made the head design changes that elevated the two-stroke engine from industrial status to full-bore racing powerplant was Sir Harry Ricardo, a brilliant mechanical engineer from England. Sir Harry, along with Dr. Schnurle from Germany that developed the modern cylinder porting arrangement (the two-stroke you now own today operates on "The Schnurle Loop Principal") came up with a new way that totally improved what was, up to then, an inefficient, low RPM engine. Sir Harry designed the modern "hemi-squish" head that just about every two-stroke racing engine now uses. We could talk about Walter Kaaden and two-stroke exhaust systems, but that and Dr. Schnurle are different subjects altogether....:)

I'm probably going to oversimplify here and onward, but the squishband concentrates the air/fuel mixture in the center of the head near the ignition source, thereby reducing the amount of unburned fuel out near the outside of the cylinder boundry. What the squishband also does is to force the A/F mix into the center of the head under pressure that causes turbulence and, if done correctly, increases the combustion "quality" of the mixture that, in turn, affects the rate of combustion pressure rise and also has an effect on optimum ignition timing.

The measurement of that "pressure" of the air/fuel mix outward from the squishband is called "Maximum Squish Velocity" or just "MSV" for short. It can have an effect on power delivery and in the case of Chris's Trials engine, the difference between an abrupt or smooth off-idle response, for instance. MSV is not the ONLY thing that affects response, but it always is a consideration.

So, this is just to get started but I've got to get some Trials stuff handled but will have more time tomorrow undoubtedly. This topic is cool and every time I talk with somebody about it I learn something new.

The question to Chris: what is your engine doing (or not doing) that you want to modify and under what ambient conditions do they occur/not occur?

Jon
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Old 09-28-2014, 09:27 AM   #9
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It would have been easier to buy the 125 and put the 225 kit on it! Interesting thread though, and we might all learn something. Thanks, Jon for the input.
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Old 09-29-2014, 07:45 AM   #10
motobene OP
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Originally Posted by JSE View Post
O.K.!

Chris and I are the worst kind of motorheads... (it's a geek problem)

The question to Chris: what is your engine doing (or not doing) that you want to modify and under what ambient conditions do they occur/not occur?

Jon
Thanks Jon! I knew you'd pull through!

OK, here is what's specifically up:

This forum has seen me try various of top-end combinations and seeking optimizing power for my riding style and my skill level. It's not really much 'a struggle', since I have attained what outside observers describe as engine characteristics which are "so smooth." Being the 'worst kind of motorhead', I'm simply drifting in optimization and exploring what more ideal means in an overlay of multiple axes of engine attributes. Many riders are going the other way.. wanting more abrupt bottom-end hit, but I find that harder for me to deal with. I suspect most of us are already over powered and taking more points as a result. Some riders use stock gearing a ride in 2nd gear as a way to constrain the power. I ride most of the time in 1st these days, and also slower than I used to. I use 2nd and 3rd only when I need to rush at something tall.

I currently sit on axes of preferences for lower gearing (43/10), and smoother, less-powerful and less-abrupt output via a low compression head, the S3 flywheel weight, 1/2 the available range of timing retardation, Michael's 2-1/2" longer titanium header pipe for bias of power more toward bottom and mid, and the use of the Kokusan module off my `10 Econo (which feels like it has a flatter general ignition advance curve).

Now that I have described these changes, in the reader's mind might be the impression that this 300 Raga is a rather flat-running bike. While it is more mellow than it was, it still has lots of torque and power! Ray Peters, who is way higher skill level than we mere mortals, once rode this bike at 4,000 feet and complained of it being too quick on the power. That was before I added the S3 flywheel weight, but I suspect he might still find it a bit quick compared to his 300 4-stroke Beta.

So... what is it I'm after? The answer is some movement more toward what the bike felt like at Sipapu... at 9,500 feet. It still did not lack there for power to get up onto obstacles. This log at Sipapu was popped up onto via weighting and unweighting in time with throttle modulation with only a tiny bit of clutch:



When I returned from altitude and rode the bike at the ranch, I was shocked at the abrupt power. After a brief period of reprogramming I absorbed that, but the memory of the smoother power at altitude made me want to drift the specs more in that direction. I thought to do that via more head volume, lowering compression more than the orange head insert: Buying a profile gage and machining out the silver head insert to an more exaggerated version of the orange head insert.

Your first treatise, Jon, steers me toward leaving the squish area alone and exaggerating the depth from the transition radius inward. I need to model this in CAD and attempt to approximate the model on a manual lathe. That means modeling also a profile gage and mating it to the modeled head in CAD. I don't have a profile gage like that. Any recommendations for one?

As for a specific combustion shape, I realize this is so complex and non linear that over time and despite modern computer modeling of the head side of the combustion chamber profile with respect to flow dynamics, the shapes have remained generic and very simple, at least on our trials machines. Were we to be able to observe and know what specifically was going on in there within a bandwidth of RPMs, a more ideal chamber might have complex geometry here and there.

I really look forward to your further comments on this, Jon.

motobene screwed with this post 09-29-2014 at 07:51 AM
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Old 09-29-2014, 07:55 AM   #11
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It would have been easier to buy the 125 and put the 225 kit on it! Interesting thread though, and we might all learn something. Thanks, Jon for the input.
I would have had someone not backed out on selling me one when I started back in early `12. Instead I bought the near-new Econo 280 from Steve Higgs, for which I remain quite grateful.

I have ridden several of the 125s-turned 225. I like them, but find the short-stroke 125 crank really spins up fast with the 250 top end on it. That bike could be quite a great ride with the addition of the S3 flywheel weight, but I don't have one. My choices were made and I went the route of 280 and 300. I wish I had a 250 around as well, as I'd have put that top end on the 300 already and tried it in competition.
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Old 09-29-2014, 07:59 AM   #12
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As Spock would say, "Fascinating".

This is a dying art. Literally. There were only 2 guys here locally that I would ever trust with reshaping 2-stroke combustion chambers, both sadly gone now. Their experiences spanned decades, modifying engines used in Roadracing, Flattrack, Desert Racing, Arena and Motocross, and even some Trials applications. After their passing, I have not had a 2-stroke head shaved since, knowing the chamber profile will change as well. I may hire Ginder to do the head on our Team TY80 after the MWVTA event in Roswell here in a couple weeks, since I heard recently he offers this service. Point being: I'm following this thread closely.
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Old 09-29-2014, 08:15 AM   #13
motobene OP
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Originally Posted by brewtus View Post
As Spock would say, "Fascinating".

This is a dying art. Literally. There were only 2 guys here locally that I would ever trust with reshaping 2-stroke combustion chambers, both sadly gone now. Their experiences spanned decades, modifying engines used in Roadracing, Flattrack, Desert Racing, Arena and Motocross, and even some Trials applications. After their passing, I have not had a 2-stroke head shaved since, knowing the chamber profile will change as well. I may hire Ginder to do the head on our Team TY80 after the MWVTA event in Roswell here in a couple weeks, since I heard recently he offers this service. Point being: I'm following this thread closely.
Cool! Ginder certainly has done a lot of tweaking over the years.

The typical 'shaving' is chucking the head up and taking material off the face of the mating surface with the cylinder. Raising compression or closing up the squish is often the reason.

There is a quick experiment you can do on any head that uses a flat copper head gasket as many of the older Yamahas do. Y

Clean off both surfaces with Scotchbrite pad, then and coat both surfaces with a very thin layer of silicone 1 and immediately assemble, torque, and let stand a hour at least. Ride the bike to directly experience what 'shaving' of an amount equal to gasket thickness will do. If you like it, you can forgo shaving and just leave the gasket out. Or determine by seat-of-pants how much shaving short of the thickness of the gasket you desire.

I ran my TY350s in the `80 with no head gaskets, no problems.
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Old 09-29-2014, 08:20 AM   #14
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Originally Posted by motobene View Post
Clean off both surfaces with Scotchbrite pad, then and coat both surfaces with a very thin layer of silicone 1 and immediately assemble, torque, and let stand a hour at least. Ride the bike to directly experience what 'shaving' of an amount equal to gasket thickness will do. If you like it, you can forgo shaving and just leave the gasket out. Or determine by seat-of-pants how much shaving short of the thickness of the gasket you desire.

I ran my TY350s in the `80 with no head gaskets, no problems.

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Old 09-29-2014, 08:45 AM   #15
JSE
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Originally Posted by motobene View Post
There is a quick experiment you can do on any head that uses a flat copper head gasket as many of the older Yamahas do. Y

Clean off both surfaces with Scotchbrite pad, then and coat both surfaces with a very thin layer of silicone 1 and immediately assemble, torque, and let stand a hour at least. Ride the bike to directly experience what 'shaving' of an amount equal to gasket thickness will do. If you like it, you can forgo shaving and just leave the gasket out. Or determine by seat-of-pants how much shaving short of the thickness of the gasket you desire.

I ran my TY350s in the `80 with no head gaskets, no problems.
You need to be careful in doing this and it's probably not safe with all engines. The 80's TYs were safe as the squish clearance generally ran from .084" to .093" stock, which is the major cause of the dreaded "Mono rattle" and detonation at high temps. Before you do this you need to measure the pre-modification squish clearance so you have a baseline to draw from. The standard soft solder method works and if you want to be more accurate (always my goal) you will use one at the front and one at the back of the piston crown. This will compensate for "piston rock" and the more piston skirt clearance you start with, the less accurate your measurement will be with the one piece of solder method.

Generally not a problem with machined squish bands (although I have seen some), but very common with as-cast squishband heads is that the clearance will be quite different at various places. The two solder method will give you a direct reading of any variance in squish clearance.

For air-cooled Trials bikes you probably want to aim for about 1.4mm squish clearance and for liquid-cooled Trials engines, 1mm seems to work well.

Jon
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