Ultimate Squish Band Video - In Depth Explanation.

If you would like to support my channel and my projects, I have added a few ways for you to do so. I have created a Patreon account for those of you who would like to support me in that way. Regardless of the amount, every bit helps and will be put to use. / joel_a
I have also created an Amazon wish list consisting of both items that will help me out and gift cards because they always come in handy. This will be a constantly changing list. www.amazon.ca/...
If you would like to send me something directly, or if you have a project idea, a product or tool that I should have, please feel free to let me know in the comments.
Actual Description
As I was editing this video I realized that I left a few details out, but as I said in the video... I needed to eventually finish this video.
I didn't get into squish area, squish area relative to bore, how reduced squish volume reduces possibility if detonation due to less gasses and many other things.

[@timbirkmann9898]

I'm not the type of guy that writes comments on a regular basis. But for this video I couldn't resist and definetely have to make an exception! Thanks for uploading such incredible and easy digestable information! It's just crazy. You're looking through the whole world wide web to find valuable information about a very specific topic and then there's this one guy sitting in his garage and spitting it out like it's nothing. You're a true gem for everybody who's watching. Very sadly common people aren't really interested in these in-depth knowledge. Keep up the good work, you rock!

[@JoelArseneaultYouTube]

Wow ! Thanks for the very kind comment. I appreciate it. There is so much more to it and to two cycle engines, but this is just a little taste of the complexity. It is usually easier and safer to explain things in layman's terms / the very very basics ... as soon as you start getting into detail, you open yourself up for attacks when you inevitably say something wrong or say something that is misinterpreted. There are people who know way more about this topic than me.

[@g.g4816]

Great video

[@JoelArseneaultYouTube]

Thanks !

[@mikemccarthy6127]

Very interesting video. They way I understand it, as the squish band gets tight, it accelerates the mixture faster towards the spark plug as you describe. If it speeds up too much it can become sonic flow. At that point a shock wave develops as the pressure wave can't transmit as fast as the air is traveling. The thermodynamics change across a shock wave causing the extreme heating you also mention. It make sense that this can result in detonation. My guess is that you want to stay a fair amount away from this calculated air speed because turbulence in the cylinder will result in pockets of the flow going sonic sooner than others. I'd bet that if one area detonates, that front travels faster than the spark ignited flame front and all gains are lost. I'd also guess the squish band calculations you also mention take this into account. I also enjoyed the discussion of the heat transfer in a hot vs cold engine. That makes sense too. I'll have to watch the video again to let it all sink in.

[@mikemccarthy6127]

I have likely interchanged the preignition and detonation terms in my comment. I just went further down the rabbit hole on that topic. Here's the link for anyone else interested: resources.savvyaviation.com/detonation-and-pre-ignition/#:~:text=Detonation%20causes%20a%20very%20rapid,also%20much%20harder%20to%20detect. Again thanks for the explanation of squish band. I'm sure it was a lot of hours to create the content.

[@JoelArseneaultYouTube]

This is basically correct, except that many many things come into play beyond the squish being tight. Squish speed is something that you need to consider carefully and most of the the other calculations that you do will be to stay under a chosen speed. If you have a good design, you can get away with 30-40 m/s before detonation becomes an issue. I'm not sure about "sonic flow". I have never heard about this in my travels, and 40 m/s is only about 90 MPH. I may be misinterpreting this information because things work a LOT different under pressure. At fairly light loads the pressure can be hundreds of psi. At these pressures the speed of sound and pressure waves work very differently than at atmospheric pressures. This is one reason why I like the idea of a tapered squish as it gradually accelerates the charge and creates an escape path for the gases, instead of just smashing the fuel and air. In a non tapered squish, you are trapping your fuel and air around the edges with the pressure from the gasses closer to the combustion chamber. You are 100% correct. Any detonation will cause a intense pressure spike and the fuel will all burn before the spark happens. Best case scenario, you lose some power, worst case you burn a hole through your piston and ruin a bunch of stuff. For most mortals, these calculations are way to complex and I have to use a program to figure this stuff out. As I said above, velocity is one of the main concerns. Thanks for the comment !!

[@mikemccarthy6127]

@@JoelArseneaultKZbin Hi Joel thanks for long reply. I know the speed of sound near atmospheric pressure and temp air is about 1100 ft/sec or 340 m/sec, so way higher than 30-40 m/ sec. I'll see how ambitious I am later and maybe try to estimate what the sonic speed of air is at elevated temp and pressure if I can find some information. The speed of sound in air increases with temp regardless of pressure (at least you can express it that way, P, V and T are related). Compressing the air in the cylinder adds energy so the temp rises along with the pressure. This tells me the speed of sounds is even higher than 340 m/ sec when the squish band dynamics are in play. The mixture is also ignited and starting to burn by the time the squish band is in full effect at TDC is it not? That would indicate that maybe shock wave heating due to squish band air velocity is not an issue and something else is going on. What do you think? I'd guess that the pressure wave from the burning mixture traveling from the spark plug towards the squish band at TDC may also slow the flow out of the squish band down or even reverse it from what I was originally imagining in my head. I'm guessing detonation definitely causes a shock wave. I've heard it can disrupt boundary later conditions in the combustion chamber and result increased heat transfer and overheating of combustion chamber surfaces. Interesting stuff!

[@JoelArseneaultYouTube]

​@@mikemccarthy6127 The spark happens before TDC, but it happens before TDC because ignition of the mixture and propagation is delayed, and at higher speeds, that delay translates to more degrees of crank rotation = lost pressure / power. As you say, temp goes up as pressure goes up, and as the gap decreases, the resistance to flow increases, so more heat is created in moving the mixture. You can calculate change in piston position at 30º BTDC, how much time it takes to get to TDC, figure out piston speed at 7000 RPM and then factor in flame propagation speed... this would tell you where the piston is when the flame reached the squish, assuming you have 50% squish. It's all very interesting, but also very time consuming (for me anyway) These things are all taken into account when you make other calculations (using calculators). If your head, plug, piston crown etc is 10º hotter or cooler, this will change the threshold of where detonation will occur. Also, a rough surface can create a hotspot and / or cause more resistance to flow and therefore cause more heat = detonation at lower velocity. This is why I focus on getting the combustion chamber and squish polished, dimensionally accurate and tapered. There are many variables that aren't taken into account in a "normal" squish calculator, and so you can get away with more velocity and make more power IF you have everything working in your favor. I've heard a lot of contradictory information regarding boundary layers, and recently watched something on "Super Fast Matt" KZbin channel that just added to the confusion. He did a video about aerodynamics. I think you'd like it.

[@mikemccarthy6127]

@@JoelArseneaultKZbin Hey Joel. It is all theoretical and hard to quantify or observe but still super interesting. I think almost all flow bench testing of cylinder heads (and cylinders) is done with air only and not with fuel/ atomized liquid because it is a huge mess and hard to observe among other things. I wonder if the pressure front of the ignited mixture expanding/ advancing at TDC as the squish band gap minimizes causes a potential sonic condition of the unburned mixture. The pressure wave pushes/ compresses the unburned mixture ahead if it into the squish band as the gap narrows and accelerates the flow. I'd guess the motion of the piston is not the issue but just the minimum static gap in the squish band because the piston is approaching zero velocity at TDC. Conversely, maybe the piston motion does contribute to the flow velocity in the squish band. The pressure profile vs time has to be initiated so it can do useful work in the piston on the downstroke instead of just overloading the parts at TDC. Maybe the piston starting to drop in the bore just after TDC pulls the compressing unburned mixture into the squish band to hyperaccelerate it in front of the advancing pressure wave of the burning mixture. I'm not sure when the mixture is completely burned but would guess it is well after TDC and maybe after the exhaust port is uncovered. I can probably Google that but let me know what you think. I can see how optimizing the squish band can better match the pressure vs time curve to the geometry of the moving piston/rod/crank and gain power. I suppose jet skis vs snowmobiles vs MX bikes all require different numbers based on the machine and intended application. I'm a 50+ yo who still rides a YZ250. I broke some ribs recently so am sidelined for a few weeks. I have been passing some of the time watching KZbin. Thank you for publishing your knowledge here. I am no expert but have been nonprofessionally wrenching on bikes and sleds my whole life for myself and others up in MN. It was a pleasure bantering with you about this. I'll check out the Super Fast Matt content if I can find it.

[@UpAllNight91]

Thats a great video and explanation of how stuff works. Can't believe this video doesn't have more views but maybe to come. Thanks again!

[@JoelArseneaultYouTube]

Hey, thanks for the positive comment. Much appreciated.

[@Jacobskibros]

I can't wait to see what's going down next! An interesting idea. If you can fine a mastercraft wetjet..... The old pickle fork kind that kind of looks like a snowmobile. Put the 650 in that or a nasty snow ski engine. Seems fitting for you to have one of those!

[@JoelArseneaultYouTube]

LoL ... Yeah. I always have my eyes open for something cool... I have BIG plans for the 650 that I'm about to announce pretty soon... and it's not building a new pipe for it... altho I am also doing that.

[@adrianwall9130]

Wow Joel that helps me as I am rebuilding a Kawasaki sts 900 engine, I have no access to a lathe, which would make my job alot easier, thanks for the video cheers

[@JetSkiBrothers]

I am digging this video, even though it’s just talking it’s stuff that is interesting to learn about. Oh dang you got a tig nice nice 👍 oh that KZbin money haha.

[@JoelArseneaultYouTube]

Yes, a LOT of talking, haha. It's hard to believe, but as I was editing this I thought of about 10 things that I left out of the explanation... oh well. This was the longest video I've ever done, haha. I'm pretty excited about the TIG... probably take a week to arrive as I ordered it from the manufacturer and it's coming from the other side of the country. Being able to weld aluminum will come in handy for building random jet ski bits. KZbin money is kinda funny. It's all about doing videos for the love of it. Obviously I hope that my channel grows and it helps fund some of my projects, but I definitely can't count on $$ from YT to save my butt, lol.

[@JetSkiBrothers]

@@JoelArseneaultKZbin exactly the KZbin money is not very dependable. Any way just wanted to let you know that the engine is supposed to be picked up from my house tomorrow. And if everything goes the way it’s supposed to you will get it on the 10th but as I am sure you know nothing ever goes exactly to plan. I have sent you a email with sort of a test video to see if you are able to download edit and or use it. I have other videos that I want to send but want to make sure you are actually able to download and use the test one first.

[@MrJmoney1980]

Wait until you start fooling with exhaust acoustics lol that’s the fun stuff

[@JoelArseneaultYouTube]

@@MrJmoney1980 Yes, pipe design is intense. When you start fooling with it, there's almost no aspect of your engine that is left untouched. I've been involved in building and modifying exhaust systems for decades (on and off) yet every time I start into a new project I need a refresher course. I have read through the two stroke tuners hand book probably 5 times and I'll have to read it a few more times before building my own pipe(s) Fortunately, there are a lot of videos talking about pipe design and some really good software out there to help with design.

[@JoelArseneaultYouTube]

@@JetSkiBrothers Awesome. I will check it out and let you know !

[@zacksatawa1605]

Awesome video!

[@JoelArseneaultYouTube]

Thanks Zachary ! Glad you enjoyed it and I appreciate the support !

[@kraftzion]

I came across an axiom put forth by Michael Forrest that you don't gain hp by increasing your squish velocity until you retard your ignition timing. It makes sense in theory but in practice I don't think most igniton timing on 2 strokes are that close to the correct point. I haven't seen this put into practice or mentioned anywhere but on his channel. Have you ever had to retard your ignition after a squish velocity increase?

[@JoelArseneaultYouTube]

I'm not going to try to pretend I'm an expert on this topic or the topic of ignition timing.... However, timing advance / retardation is linked to rpm, but more importantly, combustion chamber pressure and on a two stroke, it is a way to manipulate your exhaust pipe (exhaust) temperature. Temp defines the speed of the pressure wave in the pipe, so manipulating the temp can change the effective tuning of the pipe. I would like to hear a in depth explanation of this theory. It has been a while since I have done much reading on the topic, but is my understanding that squish velocity is a byproduct of squish design, and the gain in power is due to the placement of the combustion and the reduction of volume allowing the peak pressure to occur at the "right time", more completely and thus more efficiently transmitting energy to the crankshaft. If I had some time to think about it, perhaps I could give you a better answer. Two stroke engines clearly do pull timing at the top of the rev range, but if he is making this statement, I'm guessing that he is talking about peak hp/ torque.

[@2_be_determined]

Thanks for this, now I need a lathe smh 🤦

[@JoelArseneaultYouTube]

Haha ! A lathe is a very handy tool, and once you have it you find many other uses for it. With all of the stuff you are into I'm sure it would come in handy. With everything that I said.... There are usually more gains to be had elsewhere, like exhaust systems and porting - especially porting matching your specific pipe and your rpm range. The more you modify an engine, the more you need to work for every .001% gain, if you know what I mean. You can take a chainsaw engine and easily put on a pipe, a few other things and double the power. After that though, you're doing hours of work to get just a little more.

[@kraftzion]

The part of the piston not exposed to the high pressure event is not doing any work at that moment in time is it?

[@JoelArseneaultYouTube]

Thanks for the comment. I may have said something in my video that caused you to think this or may have even said something incorrect .... but. Your question is based on some incorrect assumptions. The pressure is fairly even over the entire piston cylinder wall and head surface. Basically, pressure can not raise unless it is contained, and these parts are the container. Pressure on all parts (head, piston, cylinder spark plug) will be equal to the pressure on all other pars and equal to the pressure in the air space as well. High pressure always moves to fill low pressure areas until it is equalized. There are some exceptions to this like a shockwave from a explosion. These waves travel faster than the speed of sound and I believe pressure will equalize at the speed of sound. Combustion inside of an engine is not considered an explosion and these pressure waves travel at the speed of sound. It gets a little more complicated than that because "the speed of sound" changes at high temperatures. At 68F the speed is 767mph but at 752F the speed of sound is 1150 mph. Cheers.

[@kraftzion]

@@JoelArseneaultKZbin Excellent explanation. Thanks!

[@JoelArseneaultYouTube]

@@kraftzion You're welcome.

[@michpatriot9097]

So this is why my engines run their best on vaporized Aluminium for that split second before the expensive noises..now I understand.

[@JoelArseneaultYouTube]

Haha... people actually used to believe that years ago. The best a two cycle would run is on the edge of burning up (due to a lean mix). It is now understood that that is not the case and despite the engine, you will make more power before leaning out to melt down. To some extent what you are saying is true... not sure if that is sarcasm or not, but an engine ultimately can handle X amount of pressures, those pressures translate to heat (in some areas) and depending on what the weak link is in your engine, molten aluminum, on your piston or head, may be the weak point of your engine. WIth that being said, the whole point of the squish band is to remove gasses from that area and contain them in the combustion chamber where they are unlikely to melt aluminum, even in extreme circumstances. If you melt a piston over the combustion chamber, it is due to early ignition. When you start pushing an two cycle engine really hard, and have a properly designed combustion chamber, you are more likely to see the piston give way around the exhaust port as the extreme pressures and heat pass over this area on the way out, and this part of the piston doesn't get cooled as much as the rest of the piston as it is not in contact with the cylinder wall for very long. I have run a few two cycle engines on a cold day and had them go "squeak" and then stop haha. It's never a good feeling.

[@98armando98]

what do you think of a 2.5mm squish on a yamaha banshee , is it too much?

[@JoelArseneaultYouTube]

Yes... but as you may have noticed from this video, there's a lot more to it than just gap. Lots of stuff has to be taken into consideration. You need to be sure you are measuring against the outer circumference, or you may have a false reading. If you are running good fuel, good oil and not doing anything crazy with timing (or anything else) you can easily remove .030" - .040. about .8 - 1mm . Even just on supreme pump gas you could run 1.5mm if there's no issues (increased timing, crazy porting, damaged head or piston, excess carbon build up etc. ) I love the Banshee... I use to work for Yamaha 1995 -2001, got to ride a few.

[@JoelArseneaultYouTube]

LOL... just looked at your channel... If you are running a modded 450 that changes things a little - velocity of the squish changes as the displacement goes up - but 2.5 is still a LOT ... it's a risky thing to play with unless you are willing to do a lot of testing, but I would still say 1.5 would be more reasonable.

[@98armando98]

@@JoelArseneaultKZbin perfect i am changing doomes so can achieve 0.9 mm maximun

[@JoelArseneaultYouTube]

@@98armando98 Nice. What a beast !

[@adrianwall9130]

Hi Joel I am out of work as well so I know exactly what you are going through,

[@JoelArseneaultYouTube]

Sorry to hear that. I hope you're doing alright. For me it's a seasonal thing, so I'm usually prepared for it. This time around, with COVID and trying to make video content, I'm not nearly as prepared financially. I'm sure it will work out in the end.

[@joescissorhands141]

"aluminum's heat-dissipation has a bad side, though: this shedded heat means cooler combustion-event, ergo less pressure/torque from the engine" (paraphrased to hell of course :P ) Wouldn't this be a non-issue, though? IE, isn't this lost heat (and its "power potential") isn't it inherent to the trade-off wherein aluminum dissipates heat better *therefore* allows higher compression ratios to be run with same level of risk? IE it's not a 'bad tradeoff' or undesirable, it just "is".. You say 1-2deg or 1-1.5deg slope IIRC for squish bands...why not more? Seems small enough cylinders don't even have squish bands (25cc weed whip in-mind...should mention my engine interests are exclusive to chainsaws, I learn Re motors generally but it's only to that specific end, I cut trees for work :P ) I ask this because, in cutting my first squish band ('cut', I sanded it :P ), it turns out I gave it like 7deg of inward slope, and if I'm being honest I actually realized slope was developing, fixed it and went too-far creating inverse slope, then went over-did it creating 7deg of inward (I do small runs at a time, plating and sides of piece are all taped appropriately, and use 2" sanding discs inside my 54mm cylinder, so just happens to be lucky I have this sanding-kit....and I do a piece of sandpaper on a pure-flat surface for cylinder bottoms! Am, of course, doing obsessive measurements the entire time to ensure I'm on-point and not un-truing anything!! Thanks for thoughts on differing squish band slopes....and thanks moreso for the awesome video, subscribed & am gg see what else you have!!

[@joescissorhands141]

Man thank you so much for this, I've had to watch it twice (so far) to properly intake it but you really put out a lot of great stuff here (are you canadian? The amount of self-deprecating humor, however mild, adn the down-playing of your own smarts, are a uniquely canadian thing I've noticed -- not a bad thing mind you just something i've noticed, candians are my favorite not even exaggerating lol) Sorry to hear things aren't as active as you'd like, thought it was supremely cool how candid and matter-of-fact you were on that topic ($/work), it's slow for me right now (nobody wants you doing tree work the week of xmas LOL, and - if you don't do holidays much yourself - makes for a lotttt of engine-play!!!) Would love hearing your thoughts on expected-problems (or expected-losses) that'd be incurred if I go ahead and use that jug I cut a 7deg slope in its band, I look at it and I see that if I go further 'to correct it' then i'm not only risking my rings being wayyy over the plating but at this point my comb.chamber is already small and so far as I understand it you want bigger chambers for high-end power, smaller chambers for lower end torque/power....so, since it's a chainsaw that I want cutting around 11k rpm, seems I want a big chamber (while still having proper squish &compression ratio of course, but I'd initially thought "oh neat the smaller chamber gives that much more oomph on a given squish-level", not realizing that smaller area also meant less peak-pressure generation IE less power realization potential....gah just being able to speak about it with more, accurate terms helps, thanks again to your video here, so eager to dig into your collection after a couple more intakes of this one!! Best wishes/holidays/etc to you & yours man!!!

[@JoelArseneaultYouTube]

I'm trying to answer quick, so I hope I don't sound rude. Combustion chamber temp vs compression ratio. This is true to some extent, but the heat from compression and the heat from combustion are two very different things. WIth a precision made combustion chamber and good engine management / monitoring, this wouldn't be much of a factor. In a Yamaha R6, for example, they can rev it 17,500 rpm with a 12.4:1 ratio, and still run it on pump gas. While the heat form an increase of compression ratio is 100% a thing that needs to be calculated for, factors like fuel ratio, timing and efficiently using the exhaust to get rid of heat and intake charge to cool, are much larger factors. IF the heat from the combustion is turned into useable work, then the head and piston top will not get as hot. One way to deal with this is ceramic coatings and this has been done. Consistency is key here as any hot spot in the squish or chambre will force you to use that as your limiting factor instead of pushing the whole volume closer to its limits. The reason you don't want to go with more angle is that this will do two undesirable things 1. Increase the volume of that area, and 2. Introduce areas where the fuel has enough space to ignite in the squish. I know my video may be hard to follow to follow to some extent, but the small squish volume is necessary to ensure that the fuel and air do not detonate. It is impossible for a fuel air mix to combust if it doesn't have enough volume. You are a brave man doing this by hand ! Good to hear you are messing with stuff. Squish looks different in different engines. Some older two cycles did not have any, but most do have some sort of piston / head design to promote flow is directed to the plug area as the piston reaches TDC. If I were doing this by had, and I have before, I would use strips of sticky back sandpaper on the top of the piston, put the piston in the cylinder and spin it by hand so that the head takes the actual shape of the piston. Trying to taper such a small head is going to be very tricky. Cheers . I see you posted another comment. I will go look at that now.

[@JoelArseneaultYouTube]

@@joescissorhands141 Yes, I am Canadian, haha. I'm not sure if I said something wrong, or if I just confused you, but generally, you want smaller volume for higher revving engines. The reasoning being that it takes time to fill volume, the more volume the more time. With an engine that revs 11k, if you have a large volume head, the piston would already be several degrees into the power stroke before peak pressure was reached, resulting in a much lower peak pressure and less torque. I don't know how to direct you, but there are sites / calculators for both squish to chambre ratios and ideal compression ratios based on bore and volume. If you don't already watch his videos, go back and watch all of the 2stroke stuffing videos. He makes me look like a chump. Thanks for the holiday wishes and all of the kind comments! Take care !

[@sidecarbod1441]

5:05 This explanation of ignition advance is just plain wrong for a 2 stroke engine, a 4 stroke will run more advance as the revs rise but there are other factors at work here and they relate 'heavily' to 2 strokes. The whole point of igniting the charge BTDC is to allow the peak cylinder pressure to be at around 14-20 degree ATDC, this gives the best mechanical advantage for creating torque at the crankshaft. One of the main things that determines flame speed is the initial cylinder pressure BEFORE the plug fires, the higher this pressure the greater the flame speed. Now with a 2 stroke when it 'comes on the pipe' due to resonance in the inlet and exhaust system the initial cylinder pressure goes up along way compared to when it's 'off the pipe' This increase in initial pressure means that we need to retard the ignition in order to ensure that peak cylinder pressure still occurs at the right time. A clever system will advance the timing at low RPM up to the point where the engine hits its powerband, the timing will then start to retard from that point onwards.

[@JoelArseneaultYouTube]

Making videos is probably a lot harder than it looks. In the first 30 seconds I pointed out that I was struggling quite a bit. This 44 minute video took me about 6 hours to record. Knowing something and being able to say it to a crowd, or to a camera are two very different things. I also said that the piston goes down at relatively the same speed and said a lot of other things that are totally incorrect, to some extent, but are either not part of what I was trying to explain or just slip ups. I did say "to a certain point". I am aware that there are several factors that change flame speed. The idea that I was trying to get across to the audience here was simply that things happen so quickly during these processes that it can be tricky to explain and that the standard explanation or GIF you see is a major simplification. A clever system, not even a very modern on, like on Yamaha snowmobiles form the late 90s early 00s, map out the timing based on several inputs.

[@JoelArseneaultYouTube]

I let the clip play to catch more of my mistakes, and at 5:40 I explained why I said what I said.

[@sidecarbod1441]

@@JoelArseneaultKZbin I'm sure that making videos is harder than it looks, I've not made any!

[@JoelArseneaultYouTube]

@@sidecarbod1441 It is a very humbling process... and looking at all of your quirks and bumblings while editing is also difficult, haha. I'm getting better at it, but for me it's not at all a natural talent. Some people are just really good at it.