sicivicdude

That particular number was a simple copy and paste from an air cooled motor. You can't push the charge nearly as fast with a higher head temperature. The same basic rules apply for liquid cooled singles, twins, triples etc etc, just that the number is smaller (thus the charge is more energetic) This thread is really different than head design parameters. This is really dealing with just the squish area as that seems to throw some folks for more of a loop.

It is easy to tear down someone else, not so easy to express it yourself and I can see how forthcoming you are with your ....er..... vast technical knowledge. You simply tore down what I tried to make relatively simple and easy (IER!) to understand for folks who have no understanding about how it works and offered NO information at all about the subject matter. I defined a term incorrectly and was "taken to task" for it. I don't have a problem at all with someone adding to or even correcting the information available so that everyone has a better understanding (including myself) of the "how" and "why" of it but to simply tear it apart because of one ill defined term is quite narrow minded. While the Webster definition is obviously not as specific as what engine builders will deal with, it nonetheless highlights how undefined the terms are across the board. I didn't aid in this ambiguity any by defining the term incorrectly myself but you can see how easy it is to make the mistake.

Not exactly my cup of tea appearance wise but he gets straight 10's across for the board for inventiveness and fab skill, from me!

The original setup was done as a favor to surfrjag. He happened to run up on a deal with Willard to trade and took it. DDQ's head is next on the "chopping block".

"So let's walk through a combustion cycle in a MSV designed head..... It all starts when the transfer ports close (piston coming up)" From the very first post of this thread excluding all of the events that have taken place up to then to get us to the start of the actual compression for the point of explaining how varying squish or quench parameters will affect the combustion process. None the less, I'll indulge. The crankcase on a either reed valved or rotary disced two stroke engine DOES act as the intial "air pump" to get the process started. The reeds also act as the one way check valve in the system to keep it from back flushing. I am agreeing with you there. However, your "simple" explanation that the crankcase does all of the "work" is as flawed as my "simple" explanation of the operation of squish while excluding the operation of the rest of the engine. The point was to explain is as simple terms as possible how changing squish (without changing combustion chamber volume, crankcase volume, port timing, port size, carburetor size, or reed choices) can "tune" how an engine responds to those changes. Internal combustion engines operate as gigantic air pumps. Whether they be 4 stroke engines that open a poppet valve and increase internal displacement (piston falling on the intake stroke), schneurle ported 2 strokes that operate by using the crankcase as their initial air pump, crossflow 2 strokes that operate without the aid of a true tuned pipe (some would argue that the "loop scavenging" of the crossflows acts as a tune pipe but they're not truly balanced so we'll exclude those), Screaming "jimmies" detroit diesel supercharged 2 strokes, or even opposed piston 2 strokes (look up Junkers Jumo 204 or Napier Deltic engines) they ALL operate on pressure differential. Ambient air pressure at sea level is 14.7psi, 1 bar, 760 mmHg nominally. We'll exclude slighty pressure changes due to weather events (although if you want to get REALLLLLLY technical, you should consider rejetting for storms) and call it "nominal". No matter the induction style, pressure differential is how they induct air by moving the air and fuel mixture (except in the case of the diesel!) into the combustion chamber, compressing it, using the force of the explosion to drive the piston down, and exhausting the old mixture. The old "suck, squish, bang, blow" is the really simple version of how it's done. All engine operations are fighting this initial starting pressure (whether they are ultimately forced induction or not) from the moment the air enters the low pressure area around the inlet to the engine (whether that the air filter, airbox inlet, air horn, carburetor horn, turbo inlet, whatever) everything that happens from that point on is a function of restriction versus actual flow. Volumetric efficiency is that measure. That's what I was talking about when I mentioned 14.7 psi as being the starting point. The ability of the engine to actually fill the volume of the combustion chamber to the swept volume of the engine is the "hard fight" on a naturally aspirated engine. Tuned pipes can overstuff the cylinder at their peak operating range but at all others, they don't help. When you leakdown test your engine, how much pressure do you put in there? 7psi. If you apply a lot more than that, you will blow those crank seals out. When the piston is at BDC, both the exhaust port and transfer ports are open. If the exhaust actually operated at 20-50 psi, you would blow crank seals out every time you got on the pipe. I will agree with you that the pipe operates at greatly elevated pressure HOWEVER, only for a brief section and even then only in certain parts of the pipe. During blowdown, the pressure wave from the cylinder passes through the headpipe and divergent section. While that pressure wave is passing, there is a greatly elevated pressure (I doubt 50 psi but definitely higher than ambient) however once the pressure wave enters the divergent section, the relative pressure (pipe pressure to combustion chamber pressure) swings violently back to vaccum! While the transfer ports are open, the pipe begins to actually "draw" charge through the engine directly which is why boyesen ports and the boost port, properly designed, work as well as they do. This is why I say, your "simple" explanation that the crankcase does all of the work is as flawed as mine about squish parameters without including the rest of the operation. That process can actually continue to draw charge directly through all the way until we get back to the start of this thread. That's why I started this thread at that point instead of "muddying" the conversation with what you are discussing. Now, back to the original purpose, do you have any specific information to add about squish or quench areas?

You cannot even begin to compare a 2 stroke "Jimmy" to a schneurle ported 2 stroke gasoline engine. The supercharger HAS to be there or the engine will not operate (not, will not operate properly, just flat out will not run). In a TRUE schneurle ported engine (weedeater for instance) the process you are describing DOES take place (the charge is slightly pressurized by the falling piston) HOWEVER this process does not explain how the engine continues to operate with a boost port or boyesen ports (direct pathways backwards into the intake tract) or how it doesn't pass the entire charge out of the engine while the exhaust port is open. "Higher exhaust pressure"? Do you mean the exhaust wave pulse returning from the tuned pipe? Because again, a scheurle ported 2 stroke engine will still operate without a tuned pipe (chainsaws and weedeaters are fine examples of this) just not to the same tune level as an engine with a tuned pipe. The incorrect term was explained well enough. Pre-ignition or detonation. einstine (Einstein actually) did say that, however, there is very little "simple" about what is going on inside a combustion chamber, even a "simple" 2 stroke engine. The entire point of the thread was to give everyone who don't know any better a little "primer" on how certain parameters effect power output. For someone who knows NOTHING, this is a relatively simple (uh oh, there we go with simple again) method to understand something relatively complicated. Just so there is no confusion, the moderator you all are getting your "information" from is Awk08, the single largest cheerleader for Ken O'Connor Racing out there. The "nut hugging", is 95% him and he does have a chip on his shoulder against me because of some off hand comment made to a fellow KOR groupie.

Check out 2 stroke engine designs before the 1961 grandprix when Suzuki got their hands on tuned pipe technology. They still ran.... Granted, not with nearly as much power output but they run just the same. In fact, if you took your banshee and put a straight pipe on it (no stuffing at all) it would still run (like shit, but run just the same). If you'll read back through, this is for a general understanding of how the engine works and not specific information. Go ahead, tear down the post because I used an incorrect term and not add anything at all about your understanding of how they work. Please, enlighten us as to how they work. I'm sure there are many members who would appreciate you straightening it all out.

I know perfectly well how a 2 stroke works. The entire point of the thread is for people who really don't know any better to be able to read through and get an idea of what the principle is behind the squish area. The entire point of every spark ignited engine is to match the flame front to the piston crown BASICALLY at TDC and push the piston down as the explosion spreads and the pressure rises dramatically. Any preignition on a gasoline engine is detrimental to the power output and engine longevity and should be avoided. I'm not even going to get into diesels deep other than to say that while it may have been the case years ago, when mechanically injected engines ruled the road, that you never had preignition events, each power stroke now can actually have multiple "ignition events" including some well before TDC to "warm up" the compressed air. Those would actually be classified as preignition because the fuel is injected while the piston is still rising and not in order to raise the pressure of the charge in order to push the piston down. Your statement about NEVER wanting preignition ever is absolutely not true. Top fuel engines (I know, we're REALLY stretching the entire point of this thread with this example but I'm making a point that there are very few "laws" of engine building) inject enough fuel to lower the AFR to about 1.5:1. The majority of the fuel entering the cylinder does NOT atomize and enters as a pure liquid. They run ~50 degrees BTDC of ignition timing in order to detonate (and I use that term literally and correctly) the nitromethane that does enter the combustion chamber as an atomized gas. The pressure created by this initial explosion is actually what sets the main explosion off. Liquid nitromethane is an a monopropellant and an explosive under the right conditions. Without detonation, a top fuel engine would not be able to put out the power it does. 14.7 psi is the absolute pressure of air at sea level. When the engine is pulling in air, that's how much it can "grab" by dropping the pressure inside the cylinder to a relative vacuum. When you start speaking to volumetric efficiency of an engine, atmospheric pressure plays a HUGE role in that figure. When discussing the amount of squish a given engine can run, the calculation for volumetric efficiency plays a crucial part in figuring out how much volume is inside the chamber when trying to figure out how fast you can "push" the charge with the piston. Ever heard that as altitude rises compression drops? Ever wonder why? It's not because the physical size of the engine changed....

It just means that an spark ignited engine is supposed to be "fired" by the spark plug. Other things can start the ignition event other than the ignition system. Basically, all other sources of ignition (also know as preignition @ sprinklerman) are negative and cause a loss of power or engine longevity.

Agreed, I doubt "Webster" was a gearhead.... Let's just agree that all references to "predetonation" listed apply to all forms of negative parasitic ignition events that are not initiated by the ignition system.

I see what you are referring to but take a look at the Merriam Webster (arguably the final point in all definition arguments) definition: Detonation: 1 : the action or process of detonating 2 : rapid combustion in an internal combustion engine that results in knocking According to their definition, we are both incorrect and the detonation is actually the event you refer to as "pre-ignition" and I am referring to as "predetonation"..... How about we chock that up to a grey area as far as the terms are concerned? Is it safe to say that we can assume that all references in the information listed above to "predetonation" are referring to "detonation" as in the harmful "preignition" of the fuel and air mixture before the normal timed ignition event by the spark plug?

Clearly, that "article" was written regarding blaster heads (where I cracked this nut to begin with) which are fundamentally flawed from the factory. The banshee head is a MUCH better design as far as combustion chamber size, shape, break area, and squish area are concerned. I will be doing a full write up on a stock banshee head maybe tonight so I can tailor the article more directly towards banshee engines.

I'm assuming that you are referring to my seemingly incorrect grammar. If not, please elaborate on your concern. It can also be pre-detonation as the "pre" is a prefix (note prefix DOESN'T use the hyphen), however it is not incorrect to use the term as "predetonation" either. You MUST use the hyphen before anything starting with an "e" but it NOT required before a consonant. Let's not get bogged down in grammar class, however. The point is, there is A LOT going on inside of a combustion chamber while an engine is running. Really, the two threads I've posted are just a "primer" to the real processes anyway in an effort to avoid confusion.

Provided for all those who feel the need to learn WHY and not just be told how.....

Another thing I feel I need to elaborate on is the "overstuffing" of the engine. Volumetric efficiency suffers as engine rpm rises. The same air and fuel has less and less time to make the journey into the engine as the engine turns faster. To this end, anything that can be done to augment the amount or speed of the scavenging can be used to greatly augment the power output at rising engine rpm. We know them as pipes but what they really are is an exhaust driven supercharger. Some are more efficient than others and some have different engine rpm's that they operate at better than others. Generally, pipes operate to "help" the engine not lose power as the time interval for scavenging decreases. However, some pipes actually operate SO efficiently that they can actually put more volume into the combustion chamber at high engine rpm than the engine naturally traps at lower rpm (without the help of the pipe). Drag pipes are a fine example. If one simply calculates the theoretical volumtric efficiency of a drag engine at a given rpm, the output would be dismal. The charge wouldn't have enough time (at the high rpm's) to get the old charge out and new charge completely in before the engine "closed up" and the next combustion cycle began. The tuned pipe is actually so efficient at its job that it can suck the old charge out and even some of the new charge out too and then, due to its design, push that new charge back in before the exhaust port closes. If the pipe is good enough at its job that it does manage to get more charge into the cylinder than can be trapped at lower rpm's, it is overstuffing the engine. It's not uncommon to see highly tuned race engines exceed 110% of their "theoretical" volume. A 100cc combustion chamber (the area above the exhaust port) could potentially contain 110 cc's of fresh charge before the piston compresses it into the combustion chamber and even begins the squish phase of the stroke... This is another reason why head designs for "most" engines are kept "safe" instead of to the raggedy edge of their possible design. Change elevation with the exact same engine (no chagnes at all) and even rejetting may not be able to help predetonation that could come on from the increased volumetric efficiency.