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2T dome shape

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Read the whole squish thread over in the Yamaha 2T forum this morning. Thought I'd post a Q or 2 over here, capturing a wider range of experience.

When setting squish, it will typically be necessary to machine the chamber's dome to restore the stock compresssion ratio, if not going to race gas. Several people mentioned ending up having to shim the spark plug to get it back even with the top of the chamber after doing that.

Would there be any benefit to giving the chamber more of a toroid (mmmm... forbidden donut) shape, keeping the plug at full depth closer to the center of the chamber but having the chamber go higher into the head around the plug?

Is the optimum chamber shape generally considered to be as close to part of a sphere (constant radius in cross section) as possible from the end of the squish band up to the top?

What radius is best for the transition from the squish band to the dome? Just break the edge? 1mm? 5mm? More?

Is the goal to have the squished charge separate and stay down on the piston, or is it better to give it a little radius so it can more easily stay attached and follow up into the dome?

Which way the charge goes seems like it would be highly rpm dependent - would be more easily kept attached to the head at low speeds, so would make a difference there, whereas it's going to just go straight at high speeds.

Interested in general theory, but just for context, I have a 2002 YZ250 and am mostly a low-mid rpm trail rider. I will definitely be getting the squish set correctly when I do my upcoming top end, but I have access to a lathe and am a nerd, so I'm thinking about doing it myself rather than just paying RB or EG.

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there is some free head programs out there. problem is, the info it spits out is useless unless you know what parameters you need. now if you ran your motor through a simulator and it said you need , for example, 25 m/s msv, 48% squish area, .9mm squish clearance, 10* angle, then you would have some info you could use. or maybe try graham bells book. it should have some solid info regarding heads.

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Family surprised me by picking Bell's book out of my Amazon wish list for Christmas. It's what I'm sitting on the couch reading. The section on squish is pretty general, similar info to what I've found here.

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you must think every engine requires the same dome dimension ? ??? thats why the book cant say exact numbers, because not every engine is the same. it does give good general info like squish 50% of bore, thats a good general number for a performance 2stroke. that doesnt meen 50% will be exactly perfect for any/every particular engine, but it is a good starting point

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No, of course not every engine is the same. In my original post, I didn't even ask about percent squish. That is covered pretty well, at least directionally, in the literature.

Bell discusses wider and narrower squish bands and percent squish. He doesn't talk much about the details of combustion chamber shape, though. Pretty much either squish or not, with squish chambers always approximately spherical in the illustrations and either offset away from the exhaust or centered.

There are photo examples in the book with large radii and very small radii at the transition from squish to chamber, with no discussion of the difference.

No discussion of chamber shape variations, such as spherical, "top hat", toroidal, etc.

Have read in Gordon Jennings' book regarding chamber design, and he is pretty dismissive of any strategy but spherical for the chamber. On the other hand, it was written in something like 1973.

Lots of good discussions on two-stroke road race bike and sled forums. Lots of toroidal heads out there. One interesting comment was that having too small a radius/large an angle at the transition from squish to chamber zones can reduce the ability of the flame front to turn the corner and move out into the squish band.

Edited by FRECNDY

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blairs book should answer your question. it talks about all the different heads. most the bikes ive deal with use hemi chamber with central located plug. i thought you were interested about how to modify your existing head

Edited by harryhandshake

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Remember typically you'll be constrained by things like piston dome profile, not exposing the plug's thread (unless you want to fit a top hat insert) and whatnot. By the time you've decided on the clearance then things like the squish band area and combustion chamber shape may be a compromise. Online calculators are ok, but plan on spending plenty of time measuring combustion chamber volumes with a burette and doing sums to work out trapped compression. I've always felt that a wee bit of radius is a good thing, but I'm minded that thumper combustion chambers are full of sharp corners (like valve cutouts on piston crowns) so I'm not really sure about anything any more!

I do feel that working towards a minimum possible surface area for the chamber is good as it keeps the heat energy where you want it.

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you have minimal options when talking about modifying your existing setup. changing anything about the dome of the piston is really out of the question and your limited to what you can do with the head. if theres a cool head available for your bike you could buy one along with a blank dome and basically cut it any way you want but modifying the piston dome is still out of the question. ive bought blank domes and cut them to my own specs so you do have that option. if your just wondering theoretically how well a different head design besides hemishpherical would work on your bike then pick up blairs book like i said. talks about offset chamber, bathtub, hemi with dished piston, just to name a few

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thats one of his books and a pretty good one. he wrote design and simulation of the 2stroke a few years later and is a better book. has all the previous books info plus alot more new stuff. i think its around $150. at one time i think one of those books had cd's included of his 2stroke programs but im not sure if the new copies still have the cds included. pretty sure it was the basic design of 2stroke but maybe both books had a cd.

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Messing with alot of 2 strokes and getting the current KTM's dialed, I can say that dome shape is an interesting topic, The YZ goes one way, a TM (Moto) goes another, KTM is bowl, semi hemi, or trapezoidal ..yada yada....

Piston dome shape leads to interesting stuff.

I have tried the semi Hemi in place of the trapezoid in a KTM, and vice versa in a Gas Gas and YZ.. ... The overall results are not earthshaking but burn efficiency was a factor., \

The radius transition from squishband to bowl wall does effect power as does the squish angle. Changing the radius to a angle cut helped a few roadrace GP motors but did not provided expected results in a small bore offroad bike,.

Cool topic.

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